Hospital Physician: Hematology/Oncology

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm that arises from a reciprocal translocation between the Abelson (ABL) region on chromosome 9 and the breakpoint cluster region (BCR) of chromosome 22, t(9;22)(q34;q11.2) (the Philadelphia chromosome), resulting in the generation of the BCR-ABL1 fusion gene and its protein product, BCR-ABL tyrosine kinase. BCR-ABL is a constitutively active fusion kinase that confers proliferative and survival advantage to hematopoietic cells through activation of downstream pathways.

CML is divided into 3 phases based on the number of myeloblasts observed in the blood or bone marrow: chronic, accelerated, and blast. Most cases of CML are diagnosed in the chronic phase (CP), which is marked by proliferation of primarily the myeloid element.

The advent of tyrosine kinase inhibitors (TKIs), a class of small molecules targeting the tyrosine kinases, particularly the BCR-ABL tyrosine kinase, led to rapid changes in the management of CML and improved survival for patients. Patients diagnosed with CP-CML now a have life-expectancy that is similar to that of the general population, as long as they receive the appropriate TKI therapy and adhere to treatment. As such, it is crucial to identify patients with CML, ensure they receive a complete, appropriate diagnostic work-up, and select the best therapy for each individual patient. The diagnosis and work-up of CML are reviewed in a separate article; here, the selection of TKI therapy for a patient with newly diagnosed CP-CML is reviewed. 

Case Presentation

A 53-year-old woman who recently was diagnosed with CML presents to review her treatment options. The diagnosis was made after she presented to her primary care physician with fatigue, early satiety, left upper quadrant abdominal pain, and an 8-lb unintentional weight loss over the prior month. On physical exam her spleen was palpated 8 cm below the left costal margin. Laboratory evaluation showed a total white blood cell (WBC) count of 124,000/μL with a left-shifted differential including 6% basophils, 3% eosinophils, and 3% blasts; hemoglobin and platelet count were 12.4 g/dL and 801 × 10³/µL, respectively. Fluorescent in-situ hybridization for BCR-ABL gene rearrangement using peripheral blood was positive in 87% of cells. Bone marrow biopsy and aspiration showed a 95% cellular bone marrow with granulocytic hyperplasia and 1% blasts. Cytogenetics were 46,XX,t(9;22)(q34;q11.2), and quantitative real-time polymerase chain reaction (RQ-PCR) to measure BCR-ABL1 transcripts in the peripheral blood showed a value of 98% international standard (IS). Her Sokal risk score was 1.42 (high risk). In addition, prior review of her past medical history revealed uncontrolled diabetes, coronary artery disease requiring placement of 3 cardiac stents 2 years prior, and chronic obstructive pulmonary disease (COPD) related to a 30-pack-year history of smoking.

• What factors must be considered when selecting first-line therapy for this patient?

Selection of the most appropriate first-line TKI for newly diagnosed CP-CML patients requires incorporation of many patient-specific factors. These factors include baseline karyotype and confirmation of CP-CML through bone marrow biopsy, Sokal or EURO risk score, and a thorough patient history, including a clear understanding of the patient's comorbidities. In this case, the patient's high Sokal risk score along with her history of diabetes, coronary artery disease, and COPD are all factors that must be accounted for when choosing the most appropriate TKI. The adverse effect profile of all TKIs must be considered in conjunction with the patient's ongoing medical issues in order to decrease the likelihood of worsening her current symptoms or causing a severe complication from TKI therapy.

Imatinib

The management of CML was revolutionized by the development and ultimate regulatory approval of imatinib mesylate in 2001. Imatinib was the first small-molecule cancer therapy developed and approved. It acts by binding to the adenosine triphosphate (ATP) binding site in the catalytic domain of BCR-ABL, thus inhibiting the oncoprotein's tyrosine kinase activity.¹

The International Randomized Study of Interferon versus STI571 (IRIS) trial was a randomized phase 3 study that compared imatinib 400 mg daily to interferon α (IFNα) plus cytarabine. More than 1000 CP-CML patients were randomly assigned 1:1 to either imatinib or IFNα plus cytarabine and were assessed for event-free survival, hematologic and cytogenetic responses, freedom from progression to accelerated phase (AP) or blast phase (BP), and toxicity. Imatinib was superior to the prior standard of care for all these outcomes.² The long-term follow up of the IRIS trial reported an 83% estimated 10-year overall survival (OS) and 79% estimated event-free survival for patients on the imatinib arm of this study.³ The cumulative rate of complete cytogenetic response (CCyR) was 82.8%. Of the 204 imatinib-treated patients who could undergo a molecular response evaluation at 10 years, 93.1% had a major molecular response (MMR) and 63.2% had a molecular response 4.5 (MR4.5), suggesting durable, deep molecular responses for many patients (see Chronic Myeloid Leukemia: Evaluation and Diagnosis for discussion of the hematologic parameters, cytogenetic results, and molecular responses ussed in monitoring response to TKI therapy). The estimated 10-year rate of freedom from progression to AP or BP was 92.1%.

Higher doses of imatinib (600-800 mg daily) have been studied in an attempt to overcome resistance and improve cytogenetic and molecular response rates. The Tyrosine Kinase Inhibitor Optimization and Selectivity (TOPS) trial was a randomized phase 3 study that compared imatinib 800 mg daily to imatinib 400 mg daily. Although the 6-month assessments found increased rates of CCyR and a MMR in the higher-dose imatinib arm, these differences were no longer present at the 12-month assessment. Furthermore, the higher dose of imatinib led to a significantly higher incidence of grade 3/4 hematologic adverse events, and approximately 50% of patients on imatinib 800 mg daily required a dose reduction to less than 600 mg daily because of toxicity.⁴ 

The Therapeutic Intensification in De Novo Leukaemia (TIDEL) -II study used plasma trough levels of imatinib on day 22 of treatment with imatinib 600 mg daily to determine if patients should escalate the imatinib dose to 800 mg daily. In patients who did not meet molecular milestones at 3, 6, or 12 months, cohort 1 was dose escalated to imatinib 800 mg daily and subsequently switched to nilotinib 400 mg twice daily for failing the same target 3 months later, and cohort 2 was switched to nilotinib. At 2 years, 73% of patients achieved MMR and 34% achieved MR4.5, suggesting that initial treatment with higher-dose imatinib subsequently followed by a switch to nilotinib in those failing to achieve desired milestones could be an effective strategy for managing newly diagnosed CP-CML.⁵

Toxicity

Imatinib 400 mg is considered the standard starting dose in CP-CML patients. The safety profile of imatinib has been very well established. In the IRIS trial, the most common adverse events (all grades in decreasing order of frequency) were peripheral and periorbital edema (60%), nausea (50%), muscle cramps (49%), musculoskeletal pain (47%), diarrhea (45%), rash (40%), fatigue (39%), abdominal pain (37%), headache (37%), and joint pain (31%). Grade 3/4 liver enzyme elevation can occur in 5% of patients.⁶ In the event of severe liver toxicity or fluid retention, imatinib should be held until the event resolves. At that time, imatinib can be restarted if deemed appropriate, but this is dependent on the severity of the inciting event. Fluid retention can be managed by the use of supportive care, diuretics, imatinib dose reduction, dose interruption, or imatinib discontinuation if the fluid retention is severe. Muscle cramps can be managed by the use of a calcium supplements or tonic water. Management of rash can include topical or systemic steroids, or in some cases imatinib dose reduction, interruption, or discontinuation.⁷  

Grade 3/4 imatinib-induced hematologic toxicity is not uncommon, with 17% of patients experiencing neutropenia, 9% thrombocytopenia, and 4% anemia. These adverse events occurred most commonly during the first year of therapy, and the frequency decreased over time.^3,6 Depending on the degree of cytopenias, imatinib dosing should be interrupted until recovery of the absolute neutrophil count or platelet count, and can often be resumed at 400 mg daily. However, if cytopenias recur, imatinib should be held and subsequently restarted at 300 mg daily.⁷ 

Dasatinib

Dasatinib is a second-generation TKI that has regulatory approval for treatment of adult patients with newly diagnosed CP-CML or CP-CML in patients with resistance or intolerance to prior TKIs. In addition to dasatinib's ability to inhibit ABL kinases, it is also known to be a potent inhibitor of Src family kinases. Dasatinib has shown efficacy in patients who have developed imatinib-resistant ABL kinase domain mutations. 

Dasatinib was initially approved as second-line therapy in patients with resistance or intolerance to imatinib. This indication was based on the results of the phase 3 CA180-034 trial which ultimately identified dasatinib 100 mg daily as the optimal dose. In this trial, 74% of patients enrolled had resistance to imatinib and the remainder were intolerant. The 7-year follow-up of patients randomized to dasatinib 100 mg (n = 167) daily indicated that 46% achieved MMR while on study. Of the 124 imatinib-resistant patients on dasatinib 100 mg daily, the 7-year progression-free survival (PFS) was 39% and OS was 63%. In the 43 imatinib-intolerant patients, the 7-year PFS was 51% and OS was 70%.⁸

Dasatinib 100 mg daily was compared to imatinib 400 mg daily in newly diagnosed CP-CML patients in the randomized phase 3 DASISION trial. More patients on the dasatinib arm achieved an early molecular response of BCR-ABL1 transcripts ≤10% IS after 3 months on treatment compared to imatinib (84% versus 64%). Furthermore, the 5-year follow-up reports that the cumulative incidence of MMR and MR4.5 in dasatinib-treated patients was 76% and 42%, and was 64% and 33%, with imatinib (P = 0.0022 and P = 0.0251, respectively). Fewer patients treated with dasatinib progressed to AP or BP (4.6%) compared to imatinib (7.3%), but the estimated 5-year OS was similar between the 2 arms (91% for dasatinib versus 90% for imatinib).⁹ Regulatory approval for dasatinib as first-line therapy in newly diagnosed CML patients was based on results of the DASISION trial. 

Toxicity

Most dasatinib-related toxicities are reported as grade 1 or grade 2, but grade 3/4 hematologic adverse events are fairly common. In the DASISION trial, grade 3/4 neutropenia, anemia, and thrombocytopenia occurred in 29%, 13%, and 22% of dasatinib-treated patients, respectively. Cytopenias can generally be managed with temporary dose interruptions or dose reductions.

During the 5-year follow-up of the DASISION trial, pleural effusions were reported in 28% of patients, most of which were grade 1/2. This occurred at a rate of approximately ≤ 8% per year, suggesting a stable incidence over time, and the effusions appear to be dose-dependent.⁹ Depending on the severity of the effusion, this may be treated with diuretics, dose interruption, and in some instances, steroids or a thoracentesis. Typically, dasatinib can be restarted at 1 dose level lower than the previous dose once the effusion has resolved.⁷ Other, less common side effects of dasatinib include pulmonary hypertension (5% of patients), as well as abdominal pain, fluid retention, headaches, fatigue, musculoskeletal pain, rash, nausea, and diarrhea. Pulmonary hypertension is typically reversible after cessation of dasatinib, and thus dasatinib should be permanently discontinued once the diagnosis is confirmed. Fluid retention is often treated with diuretics and supportive care. Nausea and diarrhea are generally manageable and occur less frequently when dasatinib is taken with food and a large glass of water. Antiemetics and antidiarrheals can be used as needed. Troublesome rash can be best managed with topical or systemic steroids as well as possible dose reduction or dose interruption.^7,9 In the DASISION trial, adverse events led to therapy discontinuation more often in  the dasatinib group than in the imatinib group (16% versus 7%).⁹ Bleeding, particularly in the setting of thrombocytopenia, has been reported in patients being treated with dasatinib as a result of the drug-induced reversible inhibition of platelet aggregation.¹⁰

Nilotinib

The structure of nilotinib is similar to that of imatinib; however, it has a markedly increased affinity for the ATP‐binding site on the BCR-ABL1 protein. It was initially given regulatory approval in the setting of imatinib failure. Nilotinib was studied at a dose of 400 mg twice daily in 321 patients who were imatinib-resistant or -intolerant. It proved to be highly effective at inducing cytogenetic remissions in the second-line setting, with 59% of patients achieving a major cytogenetic response (MCyR) and 45% achieving CCyR. With a median follow-up time of 4 years, the OS was 78%.¹¹ 

Nilotinib gained regulatory approval for use as a first-line TKI after completion of the randomized phase 3 ENESTnd (Evaluating Nilotinib Efficacy and Safety in Clinical Trials-Newly Diagnosed Patients) trial. ENESTnd was a 3-arm study comparing nilotinib 300 mg twice daily versus nilotinib 400 mg twice daily versus imatinib 400 mg daily in newly diagnosed, previously untreated patients diagnosed with CP-CML. The primary endpoint of this clinical trial was rate of MMR at 12 months.¹² Nilotinib surpassed imatinib in this regard, with 44% of patients on nilotinib 300 mg twice daily achieving MMR at 12 months versus 43% of nilotinib 400 mg twice daily patients versus 22% of the imatinib-treated patients (P < 0.001 for both comparisons). Furthermore, the rate of CCyR by 12 months was significantly higher for both nilotinib arms compared with imatinib (80% for nilotinib 300 mg, 78% for nilotinib 400 mg, and 65% for imatinib) (P < 0.001).¹² Based on this data, nilotinib 300 mg twice daily was chosen as the standard dose of nilotinib in the first-line setting. After 5 years of follow-up on the ENESTnd study, there were fewer progressions to AP/BP CML in nilotinib-treated patients compared with imatinib. MMR was achieved in 77% of nilotinib 300 mg patients compared with 60.4% of patients on the imatinib arm. MR4.5 was also more common in patients treated with nilotinib 300 mg twice daily, with a rate of 53.5% at 5 years versus 31.4% in the imatinib arm.¹³ In spite of the deeper cytogenetic and molecular responses achieved with nilotinib, this did not translate into a significant improvement in OS. The 5-year OS rate was 93.7%  in nilotinib 300 mg patients versus 91.7% in imatinib-treated patients, and this difference lacked statistical significance.¹³

Toxicity

Although some similarities exist between the toxicity profiles of nilotinib and imatinib, each drug has some distinct adverse events. On the ENESTnd trial, the rate of any grade 3/4 non-hematologic adverse event was fairly low; however, lower-grade toxicities were not uncommon. Patients treated with nilotinib 300 mg twice daily experienced rash (31%), headache (14%), pruritis (15%), and fatigue (11%) most commonly. The most frequently reported laboratory abnormalities included increased total bilirubin (53%), hypophosphatemia (32%), hyperglycemia (36%), elevated lipase (24%), increased alanine aminotransferase (ALT; 66%), and increased aspartate aminotransferase (AST; 40%). Any grade of neutropenia, thrombocytopenia, or anemia occurred at rates of 43%, 48%, and 38%, respectively.¹² Although nilotinib has a Black Box Warning from the US Food and Drug Administration for QT interval prolongation, no patients on the ENESTnd trial experienced a QT interval corrected for heart rate greater than 500 msec.¹²

More recent concerns have emerged regarding the potential for cardiovascular toxicity after long-term use of nilotinib. The 5-year update of ENESTnd reports cardiovascular events, including ischemic heart disease, ischemic cerebrovascular events, or peripheral arterial disease occurring in 7.5% of patients treated with nilotinib 300 mg twice daily compared with a rate of 2.1% in imatinib-treated patients. The frequency of these cardiovascular events increased linearly over time in both arms. Elevations in total cholesterol from baseline occurred in 27.6% of nilotinib patients compared with 3.9% of imatinib patients. Furthermore, clinically meaningful increases in low-density lipoprotein cholesterol and glycated hemoglobin occurred more frequently with nilotinib therapy.¹² 

Nilotinib should be taken on an empty stomach; therefore, patients should be made aware of the need to fast for 2 hours prior to each dose and 1 hour after each dose. Given the potential risk of QT interval prolongation, a baseline electrocardiogram (ECG) is recommended prior to initiating treatment to ensure the QT interval is within a normal range. A repeat ECG should be done approximately 7 days after nilotinib initiation to ensure no prolongation of the QT interval after starting. Close monitoring of potassium and magnesium levels is important to decrease the risk of cardiac arrhythmias, and concomitant use of drugs considered strong CYP3A4 inhibitors should be avoided.⁷

If the patient experiences any grade 3 or higher laboratory abnormalities, nilotinib should be held until resolution of the toxicity, and then restarted at a lower dose. Similarly, if patients develop significant neutropenia or thrombocytopenia, nilotinib doses should be interrupted until resolution of the cytopenias. At that point, nilotinib can be reinitiated at either the same or a lower dose. Rash can be managed by the use of topical or systemic steroids as well as potential dose reduction, interruption, or discontinuation.

Given the concerns for potential cardiovascular events with long-term use of nilotinib, caution is advised when prescribing it to any patient with a history of cardiovascular disease or peripheral arterial occlusive disease. At the first sign of new occlusive disease, nilotinib should be discontinued.⁷

Bosutinib

Bosutinib is a second-generation BCR-ABL1 TKI with activity against the Src family of kinases that was initially approved to treat patients with CP-, AP-, or BP-CML after resistance or intolerance to imatinib. Long-term data has been reported from the phase 1/2 trial of bosutinib therapy in patients with CP-CML who developed resistance or intolerance to imatinib plus dasatinib and/or nilotinib. A total of 119 patients were included in the 4-year follow-up; 38 were resistant/intolerant to imatinib and resistant to dasatinib, 50 were resistant/intolerant to imatinib and intolerant to dasatinib, 26 were resistant/intolerant to imatinib and resistant to nilotinib, and 5 were resistant/intolerant to imatinib and intolerant to nilotinib or resistant/intolerant to dasatinib and nilotinib. Bosutinib 400 mg daily was studied in this setting. Of the 38 patients with imatinib resistance/intolerance and dasatinib resistance, 39% achieved MCyR, 22% achieved CCyR, and the OS was 67%. Of the 50 patients with imatinib resistance/intolerance and dasatinib intolerance, 42% achieved MCyR, 40% achieved CCyR, and the OS was 80%. Finally, in the 26 patients with imatinib resistance/intolerance and nilotinib resistance, 38% achieved MCyR, 31% achieved CcyR, and the OS was 87%.¹⁴ 

Five-year follow-up from the phase 1/2 clinical trial which studied bosutinib 500 mg daily in CP-CML patients after imatinib failure reported data on 284 patients. By 5 years on study, 60% of patients had achieved MCyR and 50% achieved CCyR with a 71% and 69% probability, respectively, of maintaining these responses at 5 years. The 5-year OS was 84%.¹⁵ These data led to the regulatory approval of bosutinib 500 mg daily as second-line or later therapy. 

Bosutinib was initially studied in the first-line setting in the randomized phase 3 BELA (Bosutinib Efficacy and Safety in Newly Diagnosed Chronic Myeloid Leukemia) trial. This trial compared bosutinib 500 mg daily to imatinib 400 mg daily in newly diagnosed, previously untreated CP-CML patients. This trial failed to meet its primary endpoint of increased rate of CCyR at 12 months, with 70% of bosutinib patients achieving this response compared to 68% of imatinib-treated patients (P = 0.601). In spite of this, the rate of MMR at 12 months was significantly higher in the bosutinib arm (41%) compared to the imatinib arm (27%; P = 0.001).¹⁶ 

A second phase 3 trial (BFORE) was designed to study bosutinib 400 mg daily versus imatinib in newly diagnosed, previously untreated CP-CML patients. This study enrolled 536 patients who were randomly assigned 1:1 to bosutinib versus imatinib. The primary endpoint of this trial was rate of MMR at 12 months. A significantly higher number of bosutinib-treated patients achieved this response (47.2%) compared with imatinib-treated patients (36.9%, P = 0.02). Furthermore, by 12 months 77.2% of patients on the bosutinib arm had achieved CCyR compared with 66.4% on the imatinib arm, and this difference did meet statistical significance (P = 0.0075). A lower rate of progression to AP- or BP-CML was noted in bosutinib-treated patients as well (1.6% versus 2.5%). Based on this data, bosutinib gained regulatory approval for first-line therapy in CP-CML at a dose of 400 mg daily.¹⁷ 

Toxicity

On the BFORE trial, the most common treatment-emergent adverse events of any grade reported in the bosutinib-treated patients were diarrhea (70.1%), nausea (35.1%), increased ALT (30.6%), and increased AST (22.8%). Musculoskeletal pain or spasms occurred in 29.5% of patients, rash in 19.8%, fatigue in 19.4%, and headache in 18.7%. Hematologic toxicity was also reported, but most was grade 1/2. Thrombocytopenia was reported in 35.1%, anemia in 18.7%, and neutropenia in 11.2%.¹⁷

Cardiovascular events occurred in 5.2% of patients on the bosutinib arm of the BFORE trial, which was similar to the rate observed in imatinib patients. The most common cardiovascular event was QT interval prolongation, which occurred in 1.5% of patients. Pleural effusions were reported in 1.9% of patients treated with bosutinib, and none were grade 3 or higher.¹⁷

If liver enzyme elevation occurs at a value greater than 5 times the institutional upper limit of normal, bosutinib should be held until the level recovers to ≤2.5 times the upper limit of normal, at which point bosutinib can be restarted at a lower dose. If recovery takes longer than 4 weeks, bosutinib should be permanently discontinued. Liver enzymes  elevated greater than 3 times the institutional upper limit of normal and a concurrent elevation in total bilirubin to 2 times the upper limit of normal is consistent with Hy's law, and bosutinib should be discontinued. Although diarrhea is the most common toxicity associated with bosutinib, it is commonly low grade and transient. Diarrhea occurs most frequently in the first few days after initiating bosutinib. It can often be managed with over-the-counter antidiarrheal medications, but if the diarrhea is grade or higher, bosutinib should be held until recovery to grade 1 or lower. Gastrointestinal side effects may be improved by taking bosutinib with a meal and a large glass of water. Fluid retention can be managed with diuretics and supportive care. Finally, if rash occurs, this can be addressed with topical or systemic steroids as well as bosutinib dose reduction, interruption, or discontinuation.⁷

Similar to other TKIs, if bosutinib-induced cytopenias occur, treatment should be held and restarted at the same or a lower dose upon blood count recovery.⁷

Ponatinib

The most common cause of TKI resistance in CP-CML is the development of ABL kinase domain mutations. The majority of imatinib-resistant mutations can be overcome by the use of second-generation TKIs including dasatinib, nilotinib, or bosutinib. However, ponatinib is the only BCR-ABL1 TKI able to overcome a T315I mutation. The phase 2 PACE (Ponatinib Ph-positive ALL and CML Evaluation) trial enrolled patients with CP-, AP-, or BP-CML as well as patients with Ph-positive acute lymphoblastic leukemia who were resistant or intolerant to nilotinib or dasatinib, or who had evidence of a T315I mutation. The starting dose of ponatinib on this trial was 45 mg daily.¹⁸ The PACE trial enrolled 267 patients with CP-CML: 203 with resistance or intolerance to nilotinib or dasatinib, and 64 with a T315I mutation. The primary endpoint in the CP cohort was rate of MCyR at any time within 12 months of starting ponatinib. The overall rate of MCyR by 12 months in the CP-CML patients was 56%. In those with a T315I mutation, 70% achieved MCyR, which compared favorably with those with resistance or intolerance to nilotinib or dasatinib, 51% of whom achieved MCyR. CCyR was achieved in 46% of CP-CML patients (40% in the resistant/intolerant cohort and 66% in the T315I cohort). In general, patients with T315I mutations received fewer prior therapies than those in the resistant/intolerant cohort, which likely contributed to the higher response rates in the T315I patients. MR4.5 was achieved in 15% of CP-CML patients by 12 months on the PACE trial.¹⁸ The 5-year update to this study reported that 60%, 40%, and 24% of CP-CML patients achieved MCyR, MMR, and MR4.5, respectively. In the patients who achieved MCyR, the probability of maintaining this response for 5 years was 82% and the estimated 5-year OS was 73%.¹⁹

Toxicity

In 2013, after the regulatory approval of ponatinib, reports became available that the drug can cause an increase in arterial occlusive events including fatal myocardial infarctions and cerebral vascular accidents. For this reason, dose reductions were implemented in patients who were deriving clinical benefit from ponatinib. In spite of these dose reductions, ≥90% of responders maintained their response for up to 40 months.¹⁹ Although the likelihood of developing an arterial occlusive event appears higher in the first year after starting ponatinib than in later years, the cumulative incidence of events continues to increase. The 5-year follow-up to the PACE trial reports 31% of patients experiencing any grade of arterial occlusive event while on ponatinib. Aside from these events, the most common treatment-emergent adverse events in ponatinib-treated patients on the PACE trial included rash (47%), abdominal pain (46%), headache (43%), dry skin (42%), constipation (41%), and hypertension (37%). Hematologic toxicity was also common, with 46% of patients experiencing any grade of thrombocytopenia, 20% experiencing neutropenia, and 20% anemia.¹⁹

Patients receiving ponatinib therapy should be monitored closely for any evidence of arterial or venous thrombosis. In the event of an occlusive event, ponatinib should be discontinued. Similarly, in the setting of any new or worsening heart failure symptoms, ponatinib should be promptly discontinued. Management of any underlying cardiovascular risk factors including hypertension, hyperlipidemia, diabetes, or smoking history is recommended, and these patients should be referred to a cardiologist for a full evaluation. In the absence of any contraindications to aspirin, low-dose aspirin should be considered as a means of decreasing cardiovascular risks associated with ponatinib. In patients with known risk factors, a ponatinib starting dose of 30 mg daily rather than the standard 45 mg daily may be a safer option resulting in fewer arterial occlusive events, although the efficacy of this dose is still being studied in comparison to 45 mg daily.⁷

In the event of ponatinib-induced transaminitis greater than 3 times the upper limit of normal, ponatinib should be held until resolution to less than 3 times the upper limit of normal, at which point it should be resumed at a lower dose. Similarly, in the setting of elevated serum lipase or symptomatic pancreatitis, ponatinib should be held and restarted at a lower dose after resolution of symptoms.⁷ 

In the event of neutropenia or thrombocytopenia, ponatinib should be held until blood count recovery and then restarted at the same dose. If cytopenias occur for a second time, the dose of ponatinib should be lowered at the time of treatment reinitiation. If rash occurs, it can be addressed with topical or systemic steroids as well as dose reduction, interruption, or discontinuation.⁷

Case Conclusion

Given the patient's high-risk Sokal score, ideal first-line treatment is a second-generation TKI in order to increase the likelihood of achieving the desired treatment milestones and improving long-term outcomes. Her history of uncontrolled diabetes and coronary artery disease raises concerns for using nilotinib. Furthermore, her history of COPD makes dasatinib suboptimal because she would have little pulmonary reserve if she were to develop a pleural effusion. For this reason, bosutinib 400 mg daily is chosen as her first-line TKI. Shortly after starting bosutinib, she experiences diarrhea that occurs approximately 3 or 4 times daily during the first week on treatment. She is able to manage this with over-the-counter loperamide and the diarrhea resolves shortly thereafter.

After 3 months of bosutinib therapy, quantitative real-time PCR (RQ-PCR) assay on peripheral blood is done to measure BCR-ABL1 transcripts, and the result is reported at 1.2% IS. This indicates that the patient has achieved an early molecular response, which is defined as a RQ-PCR value of ≤10% IS. She undergoes RQ-PCR monitoring every 3 months, and at 12 months her results indicate a value of 0.07% IS, suggesting she has achieved a MMR. 

Conclusion

With the development of imatinib and the subsequent TKIs, dasatinib, nilotinib, bosutinib, and ponatinib, CP-CML has become a chronic disease with a life-expectancy that is similar to the general population. Given the successful treatments available for these patients, it is crucial to identify patients with this diagnosis, ensure they receive a complete, appropriate diagnostic workup including a bone marrow biopsy and aspiration with cytogenetic testing, and select the best therapy for each individual patient. Once on treatment, the importance of frequent monitoring cannot be overstated. This is the only way to be certain patients are achieving the desired treatment milestones that correlate with the favorable long-term outcomes that have been observed with TKI-based treatment of CP-CML. 

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm that arises from a reciprocal translocation between the Abelson (ABL) region on chromosome 9 and the breakpoint cluster region (BCR) of chromosome 22, t(9;22)(q34;q11.2) (the Philadelphia chromosome), resulting in the generation of the BCR-ABL1 fusion gene and its protein product, BCR-ABL tyrosine kinase. BCR-ABL is a constitutively active fusion kinase that confers proliferative and survival advantage to hematopoietic cells through activation of downstream pathways.

CML is divided into 3 phases based on the number of myeloblasts observed in the blood or bone marrow: chronic, accelerated, and blast. Most cases of CML are diagnosed in the chronic phase (CP), which is marked by proliferation of primarily the myeloid element.

The advent of tyrosine kinase inhibitors (TKIs), a class of small molecules targeting the tyrosine kinases, particularly the BCR-ABL tyrosine kinase, led to rapid changes in the management of CML and improved survival for patients. Patients diagnosed with CP-CML now a have life-expectancy that is similar to that of the general population, as long as they receive the appropriate TKI therapy and adhere to treatment. As such, it is crucial to identify patients with CML, ensure they receive a complete, appropriate diagnostic work-up, and select the best therapy for each individual patient. The diagnosis and work-up of CML are reviewed in a separate article; here, the selection of TKI therapy for a patient with newly diagnosed CP-CML is reviewed. 

Case Presentation

A 53-year-old woman who recently was diagnosed with CML presents to review her treatment options. The diagnosis was made after she presented to her primary care physician with fatigue, early satiety, left upper quadrant abdominal pain, and an 8-lb unintentional weight loss over the prior month. On physical exam her spleen was palpated 8 cm below the left costal margin. Laboratory evaluation showed a total white blood cell (WBC) count of 124,000/μL with a left-shifted differential including 6% basophils, 3% eosinophils, and 3% blasts; hemoglobin and platelet count were 12.4 g/dL and 801 × 10³/µL, respectively. Fluorescent in-situ hybridization for BCR-ABL gene rearrangement using peripheral blood was positive in 87% of cells. Bone marrow biopsy and aspiration showed a 95% cellular bone marrow with granulocytic hyperplasia and 1% blasts. Cytogenetics were 46,XX,t(9;22)(q34;q11.2), and quantitative real-time polymerase chain reaction (RQ-PCR) to measure BCR-ABL1 transcripts in the peripheral blood showed a value of 98% international standard (IS). Her Sokal risk score was 1.42 (high risk). In addition, prior review of her past medical history revealed uncontrolled diabetes, coronary artery disease requiring placement of 3 cardiac stents 2 years prior, and chronic obstructive pulmonary disease (COPD) related to a 30-pack-year history of smoking.

• What factors must be considered when selecting first-line therapy for this patient?

Selection of the most appropriate first-line TKI for newly diagnosed CP-CML patients requires incorporation of many patient-specific factors. These factors include baseline karyotype and confirmation of CP-CML through bone marrow biopsy, Sokal or EURO risk score, and a thorough patient history, including a clear understanding of the patient's comorbidities. In this case, the patient's high Sokal risk score along with her history of diabetes, coronary artery disease, and COPD are all factors that must be accounted for when choosing the most appropriate TKI. The adverse effect profile of all TKIs must be considered in conjunction with the patient's ongoing medical issues in order to decrease the likelihood of worsening her current symptoms or causing a severe complication from TKI therapy.

Imatinib

The management of CML was revolutionized by the development and ultimate regulatory approval of imatinib mesylate in 2001. Imatinib was the first small-molecule cancer therapy developed and approved. It acts by binding to the adenosine triphosphate (ATP) binding site in the catalytic domain of BCR-ABL, thus inhibiting the oncoprotein's tyrosine kinase activity.¹

The International Randomized Study of Interferon versus STI571 (IRIS) trial was a randomized phase 3 study that compared imatinib 400 mg daily to interferon α (IFNα) plus cytarabine. More than 1000 CP-CML patients were randomly assigned 1:1 to either imatinib or IFNα plus cytarabine and were assessed for event-free survival, hematologic and cytogenetic responses, freedom from progression to accelerated phase (AP) or blast phase (BP), and toxicity. Imatinib was superior to the prior standard of care for all these outcomes.² The long-term follow up of the IRIS trial reported an 83% estimated 10-year overall survival (OS) and 79% estimated event-free survival for patients on the imatinib arm of this study.³ The cumulative rate of complete cytogenetic response (CCyR) was 82.8%. Of the 204 imatinib-treated patients who could undergo a molecular response evaluation at 10 years, 93.1% had a major molecular response (MMR) and 63.2% had a molecular response 4.5 (MR4.5), suggesting durable, deep molecular responses for many patients (see Chronic Myeloid Leukemia: Evaluation and Diagnosis for discussion of the hematologic parameters, cytogenetic results, and molecular responses ussed in monitoring response to TKI therapy). The estimated 10-year rate of freedom from progression to AP or BP was 92.1%.

Higher doses of imatinib (600-800 mg daily) have been studied in an attempt to overcome resistance and improve cytogenetic and molecular response rates. The Tyrosine Kinase Inhibitor Optimization and Selectivity (TOPS) trial was a randomized phase 3 study that compared imatinib 800 mg daily to imatinib 400 mg daily. Although the 6-month assessments found increased rates of CCyR and a MMR in the higher-dose imatinib arm, these differences were no longer present at the 12-month assessment. Furthermore, the higher dose of imatinib led to a significantly higher incidence of grade 3/4 hematologic adverse events, and approximately 50% of patients on imatinib 800 mg daily required a dose reduction to less than 600 mg daily because of toxicity.⁴ 

The Therapeutic Intensification in De Novo Leukaemia (TIDEL) -II study used plasma trough levels of imatinib on day 22 of treatment with imatinib 600 mg daily to determine if patients should escalate the imatinib dose to 800 mg daily. In patients who did not meet molecular milestones at 3, 6, or 12 months, cohort 1 was dose escalated to imatinib 800 mg daily and subsequently switched to nilotinib 400 mg twice daily for failing the same target 3 months later, and cohort 2 was switched to nilotinib. At 2 years, 73% of patients achieved MMR and 34% achieved MR4.5, suggesting that initial treatment with higher-dose imatinib subsequently followed by a switch to nilotinib in those failing to achieve desired milestones could be an effective strategy for managing newly diagnosed CP-CML.⁵

Toxicity

Imatinib 400 mg is considered the standard starting dose in CP-CML patients. The safety profile of imatinib has been very well established. In the IRIS trial, the most common adverse events (all grades in decreasing order of frequency) were peripheral and periorbital edema (60%), nausea (50%), muscle cramps (49%), musculoskeletal pain (47%), diarrhea (45%), rash (40%), fatigue (39%), abdominal pain (37%), headache (37%), and joint pain (31%). Grade 3/4 liver enzyme elevation can occur in 5% of patients.⁶ In the event of severe liver toxicity or fluid retention, imatinib should be held until the event resolves. At that time, imatinib can be restarted if deemed appropriate, but this is dependent on the severity of the inciting event. Fluid retention can be managed by the use of supportive care, diuretics, imatinib dose reduction, dose interruption, or imatinib discontinuation if the fluid retention is severe. Muscle cramps can be managed by the use of a calcium supplements or tonic water. Management of rash can include topical or systemic steroids, or in some cases imatinib dose reduction, interruption, or discontinuation.⁷  

Grade 3/4 imatinib-induced hematologic toxicity is not uncommon, with 17% of patients experiencing neutropenia, 9% thrombocytopenia, and 4% anemia. These adverse events occurred most commonly during the first year of therapy, and the frequency decreased over time.^3,6 Depending on the degree of cytopenias, imatinib dosing should be interrupted until recovery of the absolute neutrophil count or platelet count, and can often be resumed at 400 mg daily. However, if cytopenias recur, imatinib should be held and subsequently restarted at 300 mg daily.⁷ 

Dasatinib

Dasatinib is a second-generation TKI that has regulatory approval for treatment of adult patients with newly diagnosed CP-CML or CP-CML in patients with resistance or intolerance to prior TKIs. In addition to dasatinib's ability to inhibit ABL kinases, it is also known to be a potent inhibitor of Src family kinases. Dasatinib has shown efficacy in patients who have developed imatinib-resistant ABL kinase domain mutations. 

Dasatinib was initially approved as second-line therapy in patients with resistance or intolerance to imatinib. This indication was based on the results of the phase 3 CA180-034 trial which ultimately identified dasatinib 100 mg daily as the optimal dose. In this trial, 74% of patients enrolled had resistance to imatinib and the remainder were intolerant. The 7-year follow-up of patients randomized to dasatinib 100 mg (n = 167) daily indicated that 46% achieved MMR while on study. Of the 124 imatinib-resistant patients on dasatinib 100 mg daily, the 7-year progression-free survival (PFS) was 39% and OS was 63%. In the 43 imatinib-intolerant patients, the 7-year PFS was 51% and OS was 70%.⁸

Dasatinib 100 mg daily was compared to imatinib 400 mg daily in newly diagnosed CP-CML patients in the randomized phase 3 DASISION trial. More patients on the dasatinib arm achieved an early molecular response of BCR-ABL1 transcripts ≤10% IS after 3 months on treatment compared to imatinib (84% versus 64%). Furthermore, the 5-year follow-up reports that the cumulative incidence of MMR and MR4.5 in dasatinib-treated patients was 76% and 42%, and was 64% and 33%, with imatinib (P = 0.0022 and P = 0.0251, respectively). Fewer patients treated with dasatinib progressed to AP or BP (4.6%) compared to imatinib (7.3%), but the estimated 5-year OS was similar between the 2 arms (91% for dasatinib versus 90% for imatinib).⁹ Regulatory approval for dasatinib as first-line therapy in newly diagnosed CML patients was based on results of the DASISION trial. 

Toxicity

Most dasatinib-related toxicities are reported as grade 1 or grade 2, but grade 3/4 hematologic adverse events are fairly common. In the DASISION trial, grade 3/4 neutropenia, anemia, and thrombocytopenia occurred in 29%, 13%, and 22% of dasatinib-treated patients, respectively. Cytopenias can generally be managed with temporary dose interruptions or dose reductions.

During the 5-year follow-up of the DASISION trial, pleural effusions were reported in 28% of patients, most of which were grade 1/2. This occurred at a rate of approximately ≤ 8% per year, suggesting a stable incidence over time, and the effusions appear to be dose-dependent.⁹ Depending on the severity of the effusion, this may be treated with diuretics, dose interruption, and in some instances, steroids or a thoracentesis. Typically, dasatinib can be restarted at 1 dose level lower than the previous dose once the effusion has resolved.⁷ Other, less common side effects of dasatinib include pulmonary hypertension (5% of patients), as well as abdominal pain, fluid retention, headaches, fatigue, musculoskeletal pain, rash, nausea, and diarrhea. Pulmonary hypertension is typically reversible after cessation of dasatinib, and thus dasatinib should be permanently discontinued once the diagnosis is confirmed. Fluid retention is often treated with diuretics and supportive care. Nausea and diarrhea are generally manageable and occur less frequently when dasatinib is taken with food and a large glass of water. Antiemetics and antidiarrheals can be used as needed. Troublesome rash can be best managed with topical or systemic steroids as well as possible dose reduction or dose interruption.^7,9 In the DASISION trial, adverse events led to therapy discontinuation more often in  the dasatinib group than in the imatinib group (16% versus 7%).⁹ Bleeding, particularly in the setting of thrombocytopenia, has been reported in patients being treated with dasatinib as a result of the drug-induced reversible inhibition of platelet aggregation.¹⁰

Nilotinib

The structure of nilotinib is similar to that of imatinib; however, it has a markedly increased affinity for the ATP‐binding site on the BCR-ABL1 protein. It was initially given regulatory approval in the setting of imatinib failure. Nilotinib was studied at a dose of 400 mg twice daily in 321 patients who were imatinib-resistant or -intolerant. It proved to be highly effective at inducing cytogenetic remissions in the second-line setting, with 59% of patients achieving a major cytogenetic response (MCyR) and 45% achieving CCyR. With a median follow-up time of 4 years, the OS was 78%.¹¹ 

Nilotinib gained regulatory approval for use as a first-line TKI after completion of the randomized phase 3 ENESTnd (Evaluating Nilotinib Efficacy and Safety in Clinical Trials-Newly Diagnosed Patients) trial. ENESTnd was a 3-arm study comparing nilotinib 300 mg twice daily versus nilotinib 400 mg twice daily versus imatinib 400 mg daily in newly diagnosed, previously untreated patients diagnosed with CP-CML. The primary endpoint of this clinical trial was rate of MMR at 12 months.¹² Nilotinib surpassed imatinib in this regard, with 44% of patients on nilotinib 300 mg twice daily achieving MMR at 12 months versus 43% of nilotinib 400 mg twice daily patients versus 22% of the imatinib-treated patients (P < 0.001 for both comparisons). Furthermore, the rate of CCyR by 12 months was significantly higher for both nilotinib arms compared with imatinib (80% for nilotinib 300 mg, 78% for nilotinib 400 mg, and 65% for imatinib) (P < 0.001).¹² Based on this data, nilotinib 300 mg twice daily was chosen as the standard dose of nilotinib in the first-line setting. After 5 years of follow-up on the ENESTnd study, there were fewer progressions to AP/BP CML in nilotinib-treated patients compared with imatinib. MMR was achieved in 77% of nilotinib 300 mg patients compared with 60.4% of patients on the imatinib arm. MR4.5 was also more common in patients treated with nilotinib 300 mg twice daily, with a rate of 53.5% at 5 years versus 31.4% in the imatinib arm.¹³ In spite of the deeper cytogenetic and molecular responses achieved with nilotinib, this did not translate into a significant improvement in OS. The 5-year OS rate was 93.7%  in nilotinib 300 mg patients versus 91.7% in imatinib-treated patients, and this difference lacked statistical significance.¹³

Toxicity

Although some similarities exist between the toxicity profiles of nilotinib and imatinib, each drug has some distinct adverse events. On the ENESTnd trial, the rate of any grade 3/4 non-hematologic adverse event was fairly low; however, lower-grade toxicities were not uncommon. Patients treated with nilotinib 300 mg twice daily experienced rash (31%), headache (14%), pruritis (15%), and fatigue (11%) most commonly. The most frequently reported laboratory abnormalities included increased total bilirubin (53%), hypophosphatemia (32%), hyperglycemia (36%), elevated lipase (24%), increased alanine aminotransferase (ALT; 66%), and increased aspartate aminotransferase (AST; 40%). Any grade of neutropenia, thrombocytopenia, or anemia occurred at rates of 43%, 48%, and 38%, respectively.¹² Although nilotinib has a Black Box Warning from the US Food and Drug Administration for QT interval prolongation, no patients on the ENESTnd trial experienced a QT interval corrected for heart rate greater than 500 msec.¹²

More recent concerns have emerged regarding the potential for cardiovascular toxicity after long-term use of nilotinib. The 5-year update of ENESTnd reports cardiovascular events, including ischemic heart disease, ischemic cerebrovascular events, or peripheral arterial disease occurring in 7.5% of patients treated with nilotinib 300 mg twice daily compared with a rate of 2.1% in imatinib-treated patients. The frequency of these cardiovascular events increased linearly over time in both arms. Elevations in total cholesterol from baseline occurred in 27.6% of nilotinib patients compared with 3.9% of imatinib patients. Furthermore, clinically meaningful increases in low-density lipoprotein cholesterol and glycated hemoglobin occurred more frequently with nilotinib therapy.¹² 

Nilotinib should be taken on an empty stomach; therefore, patients should be made aware of the need to fast for 2 hours prior to each dose and 1 hour after each dose. Given the potential risk of QT interval prolongation, a baseline electrocardiogram (ECG) is recommended prior to initiating treatment to ensure the QT interval is within a normal range. A repeat ECG should be done approximately 7 days after nilotinib initiation to ensure no prolongation of the QT interval after starting. Close monitoring of potassium and magnesium levels is important to decrease the risk of cardiac arrhythmias, and concomitant use of drugs considered strong CYP3A4 inhibitors should be avoided.⁷

If the patient experiences any grade 3 or higher laboratory abnormalities, nilotinib should be held until resolution of the toxicity, and then restarted at a lower dose. Similarly, if patients develop significant neutropenia or thrombocytopenia, nilotinib doses should be interrupted until resolution of the cytopenias. At that point, nilotinib can be reinitiated at either the same or a lower dose. Rash can be managed by the use of topical or systemic steroids as well as potential dose reduction, interruption, or discontinuation.

Given the concerns for potential cardiovascular events with long-term use of nilotinib, caution is advised when prescribing it to any patient with a history of cardiovascular disease or peripheral arterial occlusive disease. At the first sign of new occlusive disease, nilotinib should be discontinued.⁷

Bosutinib

Bosutinib is a second-generation BCR-ABL1 TKI with activity against the Src family of kinases that was initially approved to treat patients with CP-, AP-, or BP-CML after resistance or intolerance to imatinib. Long-term data has been reported from the phase 1/2 trial of bosutinib therapy in patients with CP-CML who developed resistance or intolerance to imatinib plus dasatinib and/or nilotinib. A total of 119 patients were included in the 4-year follow-up; 38 were resistant/intolerant to imatinib and resistant to dasatinib, 50 were resistant/intolerant to imatinib and intolerant to dasatinib, 26 were resistant/intolerant to imatinib and resistant to nilotinib, and 5 were resistant/intolerant to imatinib and intolerant to nilotinib or resistant/intolerant to dasatinib and nilotinib. Bosutinib 400 mg daily was studied in this setting. Of the 38 patients with imatinib resistance/intolerance and dasatinib resistance, 39% achieved MCyR, 22% achieved CCyR, and the OS was 67%. Of the 50 patients with imatinib resistance/intolerance and dasatinib intolerance, 42% achieved MCyR, 40% achieved CCyR, and the OS was 80%. Finally, in the 26 patients with imatinib resistance/intolerance and nilotinib resistance, 38% achieved MCyR, 31% achieved CcyR, and the OS was 87%.¹⁴ 

Five-year follow-up from the phase 1/2 clinical trial which studied bosutinib 500 mg daily in CP-CML patients after imatinib failure reported data on 284 patients. By 5 years on study, 60% of patients had achieved MCyR and 50% achieved CCyR with a 71% and 69% probability, respectively, of maintaining these responses at 5 years. The 5-year OS was 84%.¹⁵ These data led to the regulatory approval of bosutinib 500 mg daily as second-line or later therapy. 

Bosutinib was initially studied in the first-line setting in the randomized phase 3 BELA (Bosutinib Efficacy and Safety in Newly Diagnosed Chronic Myeloid Leukemia) trial. This trial compared bosutinib 500 mg daily to imatinib 400 mg daily in newly diagnosed, previously untreated CP-CML patients. This trial failed to meet its primary endpoint of increased rate of CCyR at 12 months, with 70% of bosutinib patients achieving this response compared to 68% of imatinib-treated patients (P = 0.601). In spite of this, the rate of MMR at 12 months was significantly higher in the bosutinib arm (41%) compared to the imatinib arm (27%; P = 0.001).¹⁶ 

A second phase 3 trial (BFORE) was designed to study bosutinib 400 mg daily versus imatinib in newly diagnosed, previously untreated CP-CML patients. This study enrolled 536 patients who were randomly assigned 1:1 to bosutinib versus imatinib. The primary endpoint of this trial was rate of MMR at 12 months. A significantly higher number of bosutinib-treated patients achieved this response (47.2%) compared with imatinib-treated patients (36.9%, P = 0.02). Furthermore, by 12 months 77.2% of patients on the bosutinib arm had achieved CCyR compared with 66.4% on the imatinib arm, and this difference did meet statistical significance (P = 0.0075). A lower rate of progression to AP- or BP-CML was noted in bosutinib-treated patients as well (1.6% versus 2.5%). Based on this data, bosutinib gained regulatory approval for first-line therapy in CP-CML at a dose of 400 mg daily.¹⁷ 

Toxicity

On the BFORE trial, the most common treatment-emergent adverse events of any grade reported in the bosutinib-treated patients were diarrhea (70.1%), nausea (35.1%), increased ALT (30.6%), and increased AST (22.8%). Musculoskeletal pain or spasms occurred in 29.5% of patients, rash in 19.8%, fatigue in 19.4%, and headache in 18.7%. Hematologic toxicity was also reported, but most was grade 1/2. Thrombocytopenia was reported in 35.1%, anemia in 18.7%, and neutropenia in 11.2%.¹⁷

Cardiovascular events occurred in 5.2% of patients on the bosutinib arm of the BFORE trial, which was similar to the rate observed in imatinib patients. The most common cardiovascular event was QT interval prolongation, which occurred in 1.5% of patients. Pleural effusions were reported in 1.9% of patients treated with bosutinib, and none were grade 3 or higher.¹⁷

If liver enzyme elevation occurs at a value greater than 5 times the institutional upper limit of normal, bosutinib should be held until the level recovers to ≤2.5 times the upper limit of normal, at which point bosutinib can be restarted at a lower dose. If recovery takes longer than 4 weeks, bosutinib should be permanently discontinued. Liver enzymes  elevated greater than 3 times the institutional upper limit of normal and a concurrent elevation in total bilirubin to 2 times the upper limit of normal is consistent with Hy's law, and bosutinib should be discontinued. Although diarrhea is the most common toxicity associated with bosutinib, it is commonly low grade and transient. Diarrhea occurs most frequently in the first few days after initiating bosutinib. It can often be managed with over-the-counter antidiarrheal medications, but if the diarrhea is grade or higher, bosutinib should be held until recovery to grade 1 or lower. Gastrointestinal side effects may be improved by taking bosutinib with a meal and a large glass of water. Fluid retention can be managed with diuretics and supportive care. Finally, if rash occurs, this can be addressed with topical or systemic steroids as well as bosutinib dose reduction, interruption, or discontinuation.⁷

Similar to other TKIs, if bosutinib-induced cytopenias occur, treatment should be held and restarted at the same or a lower dose upon blood count recovery.⁷

Ponatinib

The most common cause of TKI resistance in CP-CML is the development of ABL kinase domain mutations. The majority of imatinib-resistant mutations can be overcome by the use of second-generation TKIs including dasatinib, nilotinib, or bosutinib. However, ponatinib is the only BCR-ABL1 TKI able to overcome a T315I mutation. The phase 2 PACE (Ponatinib Ph-positive ALL and CML Evaluation) trial enrolled patients with CP-, AP-, or BP-CML as well as patients with Ph-positive acute lymphoblastic leukemia who were resistant or intolerant to nilotinib or dasatinib, or who had evidence of a T315I mutation. The starting dose of ponatinib on this trial was 45 mg daily.¹⁸ The PACE trial enrolled 267 patients with CP-CML: 203 with resistance or intolerance to nilotinib or dasatinib, and 64 with a T315I mutation. The primary endpoint in the CP cohort was rate of MCyR at any time within 12 months of starting ponatinib. The overall rate of MCyR by 12 months in the CP-CML patients was 56%. In those with a T315I mutation, 70% achieved MCyR, which compared favorably with those with resistance or intolerance to nilotinib or dasatinib, 51% of whom achieved MCyR. CCyR was achieved in 46% of CP-CML patients (40% in the resistant/intolerant cohort and 66% in the T315I cohort). In general, patients with T315I mutations received fewer prior therapies than those in the resistant/intolerant cohort, which likely contributed to the higher response rates in the T315I patients. MR4.5 was achieved in 15% of CP-CML patients by 12 months on the PACE trial.¹⁸ The 5-year update to this study reported that 60%, 40%, and 24% of CP-CML patients achieved MCyR, MMR, and MR4.5, respectively. In the patients who achieved MCyR, the probability of maintaining this response for 5 years was 82% and the estimated 5-year OS was 73%.¹⁹

Toxicity

In 2013, after the regulatory approval of ponatinib, reports became available that the drug can cause an increase in arterial occlusive events including fatal myocardial infarctions and cerebral vascular accidents. For this reason, dose reductions were implemented in patients who were deriving clinical benefit from ponatinib. In spite of these dose reductions, ≥90% of responders maintained their response for up to 40 months.¹⁹ Although the likelihood of developing an arterial occlusive event appears higher in the first year after starting ponatinib than in later years, the cumulative incidence of events continues to increase. The 5-year follow-up to the PACE trial reports 31% of patients experiencing any grade of arterial occlusive event while on ponatinib. Aside from these events, the most common treatment-emergent adverse events in ponatinib-treated patients on the PACE trial included rash (47%), abdominal pain (46%), headache (43%), dry skin (42%), constipation (41%), and hypertension (37%). Hematologic toxicity was also common, with 46% of patients experiencing any grade of thrombocytopenia, 20% experiencing neutropenia, and 20% anemia.¹⁹

Patients receiving ponatinib therapy should be monitored closely for any evidence of arterial or venous thrombosis. In the event of an occlusive event, ponatinib should be discontinued. Similarly, in the setting of any new or worsening heart failure symptoms, ponatinib should be promptly discontinued. Management of any underlying cardiovascular risk factors including hypertension, hyperlipidemia, diabetes, or smoking history is recommended, and these patients should be referred to a cardiologist for a full evaluation. In the absence of any contraindications to aspirin, low-dose aspirin should be considered as a means of decreasing cardiovascular risks associated with ponatinib. In patients with known risk factors, a ponatinib starting dose of 30 mg daily rather than the standard 45 mg daily may be a safer option resulting in fewer arterial occlusive events, although the efficacy of this dose is still being studied in comparison to 45 mg daily.⁷

In the event of ponatinib-induced transaminitis greater than 3 times the upper limit of normal, ponatinib should be held until resolution to less than 3 times the upper limit of normal, at which point it should be resumed at a lower dose. Similarly, in the setting of elevated serum lipase or symptomatic pancreatitis, ponatinib should be held and restarted at a lower dose after resolution of symptoms.⁷ 

In the event of neutropenia or thrombocytopenia, ponatinib should be held until blood count recovery and then restarted at the same dose. If cytopenias occur for a second time, the dose of ponatinib should be lowered at the time of treatment reinitiation. If rash occurs, it can be addressed with topical or systemic steroids as well as dose reduction, interruption, or discontinuation.⁷

Case Conclusion

Given the patient's high-risk Sokal score, ideal first-line treatment is a second-generation TKI in order to increase the likelihood of achieving the desired treatment milestones and improving long-term outcomes. Her history of uncontrolled diabetes and coronary artery disease raises concerns for using nilotinib. Furthermore, her history of COPD makes dasatinib suboptimal because she would have little pulmonary reserve if she were to develop a pleural effusion. For this reason, bosutinib 400 mg daily is chosen as her first-line TKI. Shortly after starting bosutinib, she experiences diarrhea that occurs approximately 3 or 4 times daily during the first week on treatment. She is able to manage this with over-the-counter loperamide and the diarrhea resolves shortly thereafter.

After 3 months of bosutinib therapy, quantitative real-time PCR (RQ-PCR) assay on peripheral blood is done to measure BCR-ABL1 transcripts, and the result is reported at 1.2% IS. This indicates that the patient has achieved an early molecular response, which is defined as a RQ-PCR value of ≤10% IS. She undergoes RQ-PCR monitoring every 3 months, and at 12 months her results indicate a value of 0.07% IS, suggesting she has achieved a MMR. 

Conclusion

With the development of imatinib and the subsequent TKIs, dasatinib, nilotinib, bosutinib, and ponatinib, CP-CML has become a chronic disease with a life-expectancy that is similar to the general population. Given the successful treatments available for these patients, it is crucial to identify patients with this diagnosis, ensure they receive a complete, appropriate diagnostic workup including a bone marrow biopsy and aspiration with cytogenetic testing, and select the best therapy for each individual patient. Once on treatment, the importance of frequent monitoring cannot be overstated. This is the only way to be certain patients are achieving the desired treatment milestones that correlate with the favorable long-term outcomes that have been observed with TKI-based treatment of CP-CML. 

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm that arises from a reciprocal translocation between the Abelson (ABL) region on chromosome 9 and the breakpoint cluster region (BCR) of chromosome 22, t(9;22)(q34;q11.2) (the Philadelphia chromosome), resulting in the generation of the BCR-ABL1 fusion gene and its protein product, BCR-ABL tyrosine kinase. BCR-ABL is a constitutively active fusion kinase that confers proliferative and survival advantage to hematopoietic cells through activation of downstream pathways.

CML is divided into 3 phases based on the number of myeloblasts observed in the blood or bone marrow: chronic, accelerated, and blast. Most cases of CML are diagnosed in the chronic phase (CP), which is marked by proliferation of primarily the myeloid element.

The advent of tyrosine kinase inhibitors (TKIs), a class of small molecules targeting the tyrosine kinases, particularly the BCR-ABL tyrosine kinase, led to rapid changes in the management of CML and improved survival for patients. Patients diagnosed with CP-CML now a have life-expectancy that is similar to that of the general population, as long as they receive the appropriate TKI therapy and adhere to treatment. As such, it is crucial to identify patients with CML, ensure they receive a complete, appropriate diagnostic work-up, and select the best therapy for each individual patient. The diagnosis and work-up of CML are reviewed in a separate article; here, the selection of TKI therapy for a patient with newly diagnosed CP-CML is reviewed. 

Case Presentation

A 53-year-old woman who recently was diagnosed with CML presents to review her treatment options. The diagnosis was made after she presented to her primary care physician with fatigue, early satiety, left upper quadrant abdominal pain, and an 8-lb unintentional weight loss over the prior month. On physical exam her spleen was palpated 8 cm below the left costal margin. Laboratory evaluation showed a total white blood cell (WBC) count of 124,000/μL with a left-shifted differential including 6% basophils, 3% eosinophils, and 3% blasts; hemoglobin and platelet count were 12.4 g/dL and 801 × 10³/µL, respectively. Fluorescent in-situ hybridization for BCR-ABL gene rearrangement using peripheral blood was positive in 87% of cells. Bone marrow biopsy and aspiration showed a 95% cellular bone marrow with granulocytic hyperplasia and 1% blasts. Cytogenetics were 46,XX,t(9;22)(q34;q11.2), and quantitative real-time polymerase chain reaction (RQ-PCR) to measure BCR-ABL1 transcripts in the peripheral blood showed a value of 98% international standard (IS). Her Sokal risk score was 1.42 (high risk). In addition, prior review of her past medical history revealed uncontrolled diabetes, coronary artery disease requiring placement of 3 cardiac stents 2 years prior, and chronic obstructive pulmonary disease (COPD) related to a 30-pack-year history of smoking.

• What factors must be considered when selecting first-line therapy for this patient?

Selection of the most appropriate first-line TKI for newly diagnosed CP-CML patients requires incorporation of many patient-specific factors. These factors include baseline karyotype and confirmation of CP-CML through bone marrow biopsy, Sokal or EURO risk score, and a thorough patient history, including a clear understanding of the patient's comorbidities. In this case, the patient's high Sokal risk score along with her history of diabetes, coronary artery disease, and COPD are all factors that must be accounted for when choosing the most appropriate TKI. The adverse effect profile of all TKIs must be considered in conjunction with the patient's ongoing medical issues in order to decrease the likelihood of worsening her current symptoms or causing a severe complication from TKI therapy.

Imatinib

The management of CML was revolutionized by the development and ultimate regulatory approval of imatinib mesylate in 2001. Imatinib was the first small-molecule cancer therapy developed and approved. It acts by binding to the adenosine triphosphate (ATP) binding site in the catalytic domain of BCR-ABL, thus inhibiting the oncoprotein's tyrosine kinase activity.¹

The International Randomized Study of Interferon versus STI571 (IRIS) trial was a randomized phase 3 study that compared imatinib 400 mg daily to interferon α (IFNα) plus cytarabine. More than 1000 CP-CML patients were randomly assigned 1:1 to either imatinib or IFNα plus cytarabine and were assessed for event-free survival, hematologic and cytogenetic responses, freedom from progression to accelerated phase (AP) or blast phase (BP), and toxicity. Imatinib was superior to the prior standard of care for all these outcomes.² The long-term follow up of the IRIS trial reported an 83% estimated 10-year overall survival (OS) and 79% estimated event-free survival for patients on the imatinib arm of this study.³ The cumulative rate of complete cytogenetic response (CCyR) was 82.8%. Of the 204 imatinib-treated patients who could undergo a molecular response evaluation at 10 years, 93.1% had a major molecular response (MMR) and 63.2% had a molecular response 4.5 (MR4.5), suggesting durable, deep molecular responses for many patients (see Chronic Myeloid Leukemia: Evaluation and Diagnosis for discussion of the hematologic parameters, cytogenetic results, and molecular responses ussed in monitoring response to TKI therapy). The estimated 10-year rate of freedom from progression to AP or BP was 92.1%.

Higher doses of imatinib (600-800 mg daily) have been studied in an attempt to overcome resistance and improve cytogenetic and molecular response rates. The Tyrosine Kinase Inhibitor Optimization and Selectivity (TOPS) trial was a randomized phase 3 study that compared imatinib 800 mg daily to imatinib 400 mg daily. Although the 6-month assessments found increased rates of CCyR and a MMR in the higher-dose imatinib arm, these differences were no longer present at the 12-month assessment. Furthermore, the higher dose of imatinib led to a significantly higher incidence of grade 3/4 hematologic adverse events, and approximately 50% of patients on imatinib 800 mg daily required a dose reduction to less than 600 mg daily because of toxicity.⁴ 

The Therapeutic Intensification in De Novo Leukaemia (TIDEL) -II study used plasma trough levels of imatinib on day 22 of treatment with imatinib 600 mg daily to determine if patients should escalate the imatinib dose to 800 mg daily. In patients who did not meet molecular milestones at 3, 6, or 12 months, cohort 1 was dose escalated to imatinib 800 mg daily and subsequently switched to nilotinib 400 mg twice daily for failing the same target 3 months later, and cohort 2 was switched to nilotinib. At 2 years, 73% of patients achieved MMR and 34% achieved MR4.5, suggesting that initial treatment with higher-dose imatinib subsequently followed by a switch to nilotinib in those failing to achieve desired milestones could be an effective strategy for managing newly diagnosed CP-CML.⁵

Toxicity

Imatinib 400 mg is considered the standard starting dose in CP-CML patients. The safety profile of imatinib has been very well established. In the IRIS trial, the most common adverse events (all grades in decreasing order of frequency) were peripheral and periorbital edema (60%), nausea (50%), muscle cramps (49%), musculoskeletal pain (47%), diarrhea (45%), rash (40%), fatigue (39%), abdominal pain (37%), headache (37%), and joint pain (31%). Grade 3/4 liver enzyme elevation can occur in 5% of patients.⁶ In the event of severe liver toxicity or fluid retention, imatinib should be held until the event resolves. At that time, imatinib can be restarted if deemed appropriate, but this is dependent on the severity of the inciting event. Fluid retention can be managed by the use of supportive care, diuretics, imatinib dose reduction, dose interruption, or imatinib discontinuation if the fluid retention is severe. Muscle cramps can be managed by the use of a calcium supplements or tonic water. Management of rash can include topical or systemic steroids, or in some cases imatinib dose reduction, interruption, or discontinuation.⁷  

Grade 3/4 imatinib-induced hematologic toxicity is not uncommon, with 17% of patients experiencing neutropenia, 9% thrombocytopenia, and 4% anemia. These adverse events occurred most commonly during the first year of therapy, and the frequency decreased over time.^3,6 Depending on the degree of cytopenias, imatinib dosing should be interrupted until recovery of the absolute neutrophil count or platelet count, and can often be resumed at 400 mg daily. However, if cytopenias recur, imatinib should be held and subsequently restarted at 300 mg daily.⁷ 

Dasatinib

Dasatinib is a second-generation TKI that has regulatory approval for treatment of adult patients with newly diagnosed CP-CML or CP-CML in patients with resistance or intolerance to prior TKIs. In addition to dasatinib's ability to inhibit ABL kinases, it is also known to be a potent inhibitor of Src family kinases. Dasatinib has shown efficacy in patients who have developed imatinib-resistant ABL kinase domain mutations. 

Dasatinib was initially approved as second-line therapy in patients with resistance or intolerance to imatinib. This indication was based on the results of the phase 3 CA180-034 trial which ultimately identified dasatinib 100 mg daily as the optimal dose. In this trial, 74% of patients enrolled had resistance to imatinib and the remainder were intolerant. The 7-year follow-up of patients randomized to dasatinib 100 mg (n = 167) daily indicated that 46% achieved MMR while on study. Of the 124 imatinib-resistant patients on dasatinib 100 mg daily, the 7-year progression-free survival (PFS) was 39% and OS was 63%. In the 43 imatinib-intolerant patients, the 7-year PFS was 51% and OS was 70%.⁸

Dasatinib 100 mg daily was compared to imatinib 400 mg daily in newly diagnosed CP-CML patients in the randomized phase 3 DASISION trial. More patients on the dasatinib arm achieved an early molecular response of BCR-ABL1 transcripts ≤10% IS after 3 months on treatment compared to imatinib (84% versus 64%). Furthermore, the 5-year follow-up reports that the cumulative incidence of MMR and MR4.5 in dasatinib-treated patients was 76% and 42%, and was 64% and 33%, with imatinib (P = 0.0022 and P = 0.0251, respectively). Fewer patients treated with dasatinib progressed to AP or BP (4.6%) compared to imatinib (7.3%), but the estimated 5-year OS was similar between the 2 arms (91% for dasatinib versus 90% for imatinib).⁹ Regulatory approval for dasatinib as first-line therapy in newly diagnosed CML patients was based on results of the DASISION trial. 

Toxicity

Most dasatinib-related toxicities are reported as grade 1 or grade 2, but grade 3/4 hematologic adverse events are fairly common. In the DASISION trial, grade 3/4 neutropenia, anemia, and thrombocytopenia occurred in 29%, 13%, and 22% of dasatinib-treated patients, respectively. Cytopenias can generally be managed with temporary dose interruptions or dose reductions.

During the 5-year follow-up of the DASISION trial, pleural effusions were reported in 28% of patients, most of which were grade 1/2. This occurred at a rate of approximately ≤ 8% per year, suggesting a stable incidence over time, and the effusions appear to be dose-dependent.⁹ Depending on the severity of the effusion, this may be treated with diuretics, dose interruption, and in some instances, steroids or a thoracentesis. Typically, dasatinib can be restarted at 1 dose level lower than the previous dose once the effusion has resolved.⁷ Other, less common side effects of dasatinib include pulmonary hypertension (5% of patients), as well as abdominal pain, fluid retention, headaches, fatigue, musculoskeletal pain, rash, nausea, and diarrhea. Pulmonary hypertension is typically reversible after cessation of dasatinib, and thus dasatinib should be permanently discontinued once the diagnosis is confirmed. Fluid retention is often treated with diuretics and supportive care. Nausea and diarrhea are generally manageable and occur less frequently when dasatinib is taken with food and a large glass of water. Antiemetics and antidiarrheals can be used as needed. Troublesome rash can be best managed with topical or systemic steroids as well as possible dose reduction or dose interruption.^7,9 In the DASISION trial, adverse events led to therapy discontinuation more often in  the dasatinib group than in the imatinib group (16% versus 7%).⁹ Bleeding, particularly in the setting of thrombocytopenia, has been reported in patients being treated with dasatinib as a result of the drug-induced reversible inhibition of platelet aggregation.¹⁰

Nilotinib

The structure of nilotinib is similar to that of imatinib; however, it has a markedly increased affinity for the ATP‐binding site on the BCR-ABL1 protein. It was initially given regulatory approval in the setting of imatinib failure. Nilotinib was studied at a dose of 400 mg twice daily in 321 patients who were imatinib-resistant or -intolerant. It proved to be highly effective at inducing cytogenetic remissions in the second-line setting, with 59% of patients achieving a major cytogenetic response (MCyR) and 45% achieving CCyR. With a median follow-up time of 4 years, the OS was 78%.¹¹ 

Nilotinib gained regulatory approval for use as a first-line TKI after completion of the randomized phase 3 ENESTnd (Evaluating Nilotinib Efficacy and Safety in Clinical Trials-Newly Diagnosed Patients) trial. ENESTnd was a 3-arm study comparing nilotinib 300 mg twice daily versus nilotinib 400 mg twice daily versus imatinib 400 mg daily in newly diagnosed, previously untreated patients diagnosed with CP-CML. The primary endpoint of this clinical trial was rate of MMR at 12 months.¹² Nilotinib surpassed imatinib in this regard, with 44% of patients on nilotinib 300 mg twice daily achieving MMR at 12 months versus 43% of nilotinib 400 mg twice daily patients versus 22% of the imatinib-treated patients (P < 0.001 for both comparisons). Furthermore, the rate of CCyR by 12 months was significantly higher for both nilotinib arms compared with imatinib (80% for nilotinib 300 mg, 78% for nilotinib 400 mg, and 65% for imatinib) (P < 0.001).¹² Based on this data, nilotinib 300 mg twice daily was chosen as the standard dose of nilotinib in the first-line setting. After 5 years of follow-up on the ENESTnd study, there were fewer progressions to AP/BP CML in nilotinib-treated patients compared with imatinib. MMR was achieved in 77% of nilotinib 300 mg patients compared with 60.4% of patients on the imatinib arm. MR4.5 was also more common in patients treated with nilotinib 300 mg twice daily, with a rate of 53.5% at 5 years versus 31.4% in the imatinib arm.¹³ In spite of the deeper cytogenetic and molecular responses achieved with nilotinib, this did not translate into a significant improvement in OS. The 5-year OS rate was 93.7%  in nilotinib 300 mg patients versus 91.7% in imatinib-treated patients, and this difference lacked statistical significance.¹³

Toxicity

Although some similarities exist between the toxicity profiles of nilotinib and imatinib, each drug has some distinct adverse events. On the ENESTnd trial, the rate of any grade 3/4 non-hematologic adverse event was fairly low; however, lower-grade toxicities were not uncommon. Patients treated with nilotinib 300 mg twice daily experienced rash (31%), headache (14%), pruritis (15%), and fatigue (11%) most commonly. The most frequently reported laboratory abnormalities included increased total bilirubin (53%), hypophosphatemia (32%), hyperglycemia (36%), elevated lipase (24%), increased alanine aminotransferase (ALT; 66%), and increased aspartate aminotransferase (AST; 40%). Any grade of neutropenia, thrombocytopenia, or anemia occurred at rates of 43%, 48%, and 38%, respectively.¹² Although nilotinib has a Black Box Warning from the US Food and Drug Administration for QT interval prolongation, no patients on the ENESTnd trial experienced a QT interval corrected for heart rate greater than 500 msec.¹²

More recent concerns have emerged regarding the potential for cardiovascular toxicity after long-term use of nilotinib. The 5-year update of ENESTnd reports cardiovascular events, including ischemic heart disease, ischemic cerebrovascular events, or peripheral arterial disease occurring in 7.5% of patients treated with nilotinib 300 mg twice daily compared with a rate of 2.1% in imatinib-treated patients. The frequency of these cardiovascular events increased linearly over time in both arms. Elevations in total cholesterol from baseline occurred in 27.6% of nilotinib patients compared with 3.9% of imatinib patients. Furthermore, clinically meaningful increases in low-density lipoprotein cholesterol and glycated hemoglobin occurred more frequently with nilotinib therapy.¹² 

Nilotinib should be taken on an empty stomach; therefore, patients should be made aware of the need to fast for 2 hours prior to each dose and 1 hour after each dose. Given the potential risk of QT interval prolongation, a baseline electrocardiogram (ECG) is recommended prior to initiating treatment to ensure the QT interval is within a normal range. A repeat ECG should be done approximately 7 days after nilotinib initiation to ensure no prolongation of the QT interval after starting. Close monitoring of potassium and magnesium levels is important to decrease the risk of cardiac arrhythmias, and concomitant use of drugs considered strong CYP3A4 inhibitors should be avoided.⁷

If the patient experiences any grade 3 or higher laboratory abnormalities, nilotinib should be held until resolution of the toxicity, and then restarted at a lower dose. Similarly, if patients develop significant neutropenia or thrombocytopenia, nilotinib doses should be interrupted until resolution of the cytopenias. At that point, nilotinib can be reinitiated at either the same or a lower dose. Rash can be managed by the use of topical or systemic steroids as well as potential dose reduction, interruption, or discontinuation.

Given the concerns for potential cardiovascular events with long-term use of nilotinib, caution is advised when prescribing it to any patient with a history of cardiovascular disease or peripheral arterial occlusive disease. At the first sign of new occlusive disease, nilotinib should be discontinued.⁷

Bosutinib

Bosutinib is a second-generation BCR-ABL1 TKI with activity against the Src family of kinases that was initially approved to treat patients with CP-, AP-, or BP-CML after resistance or intolerance to imatinib. Long-term data has been reported from the phase 1/2 trial of bosutinib therapy in patients with CP-CML who developed resistance or intolerance to imatinib plus dasatinib and/or nilotinib. A total of 119 patients were included in the 4-year follow-up; 38 were resistant/intolerant to imatinib and resistant to dasatinib, 50 were resistant/intolerant to imatinib and intolerant to dasatinib, 26 were resistant/intolerant to imatinib and resistant to nilotinib, and 5 were resistant/intolerant to imatinib and intolerant to nilotinib or resistant/intolerant to dasatinib and nilotinib. Bosutinib 400 mg daily was studied in this setting. Of the 38 patients with imatinib resistance/intolerance and dasatinib resistance, 39% achieved MCyR, 22% achieved CCyR, and the OS was 67%. Of the 50 patients with imatinib resistance/intolerance and dasatinib intolerance, 42% achieved MCyR, 40% achieved CCyR, and the OS was 80%. Finally, in the 26 patients with imatinib resistance/intolerance and nilotinib resistance, 38% achieved MCyR, 31% achieved CcyR, and the OS was 87%.¹⁴ 

Five-year follow-up from the phase 1/2 clinical trial which studied bosutinib 500 mg daily in CP-CML patients after imatinib failure reported data on 284 patients. By 5 years on study, 60% of patients had achieved MCyR and 50% achieved CCyR with a 71% and 69% probability, respectively, of maintaining these responses at 5 years. The 5-year OS was 84%.¹⁵ These data led to the regulatory approval of bosutinib 500 mg daily as second-line or later therapy. 

Bosutinib was initially studied in the first-line setting in the randomized phase 3 BELA (Bosutinib Efficacy and Safety in Newly Diagnosed Chronic Myeloid Leukemia) trial. This trial compared bosutinib 500 mg daily to imatinib 400 mg daily in newly diagnosed, previously untreated CP-CML patients. This trial failed to meet its primary endpoint of increased rate of CCyR at 12 months, with 70% of bosutinib patients achieving this response compared to 68% of imatinib-treated patients (P = 0.601). In spite of this, the rate of MMR at 12 months was significantly higher in the bosutinib arm (41%) compared to the imatinib arm (27%; P = 0.001).¹⁶ 

A second phase 3 trial (BFORE) was designed to study bosutinib 400 mg daily versus imatinib in newly diagnosed, previously untreated CP-CML patients. This study enrolled 536 patients who were randomly assigned 1:1 to bosutinib versus imatinib. The primary endpoint of this trial was rate of MMR at 12 months. A significantly higher number of bosutinib-treated patients achieved this response (47.2%) compared with imatinib-treated patients (36.9%, P = 0.02). Furthermore, by 12 months 77.2% of patients on the bosutinib arm had achieved CCyR compared with 66.4% on the imatinib arm, and this difference did meet statistical significance (P = 0.0075). A lower rate of progression to AP- or BP-CML was noted in bosutinib-treated patients as well (1.6% versus 2.5%). Based on this data, bosutinib gained regulatory approval for first-line therapy in CP-CML at a dose of 400 mg daily.¹⁷ 

Toxicity

On the BFORE trial, the most common treatment-emergent adverse events of any grade reported in the bosutinib-treated patients were diarrhea (70.1%), nausea (35.1%), increased ALT (30.6%), and increased AST (22.8%). Musculoskeletal pain or spasms occurred in 29.5% of patients, rash in 19.8%, fatigue in 19.4%, and headache in 18.7%. Hematologic toxicity was also reported, but most was grade 1/2. Thrombocytopenia was reported in 35.1%, anemia in 18.7%, and neutropenia in 11.2%.¹⁷

Cardiovascular events occurred in 5.2% of patients on the bosutinib arm of the BFORE trial, which was similar to the rate observed in imatinib patients. The most common cardiovascular event was QT interval prolongation, which occurred in 1.5% of patients. Pleural effusions were reported in 1.9% of patients treated with bosutinib, and none were grade 3 or higher.¹⁷

If liver enzyme elevation occurs at a value greater than 5 times the institutional upper limit of normal, bosutinib should be held until the level recovers to ≤2.5 times the upper limit of normal, at which point bosutinib can be restarted at a lower dose. If recovery takes longer than 4 weeks, bosutinib should be permanently discontinued. Liver enzymes  elevated greater than 3 times the institutional upper limit of normal and a concurrent elevation in total bilirubin to 2 times the upper limit of normal is consistent with Hy's law, and bosutinib should be discontinued. Although diarrhea is the most common toxicity associated with bosutinib, it is commonly low grade and transient. Diarrhea occurs most frequently in the first few days after initiating bosutinib. It can often be managed with over-the-counter antidiarrheal medications, but if the diarrhea is grade or higher, bosutinib should be held until recovery to grade 1 or lower. Gastrointestinal side effects may be improved by taking bosutinib with a meal and a large glass of water. Fluid retention can be managed with diuretics and supportive care. Finally, if rash occurs, this can be addressed with topical or systemic steroids as well as bosutinib dose reduction, interruption, or discontinuation.⁷

Similar to other TKIs, if bosutinib-induced cytopenias occur, treatment should be held and restarted at the same or a lower dose upon blood count recovery.⁷

Ponatinib

The most common cause of TKI resistance in CP-CML is the development of ABL kinase domain mutations. The majority of imatinib-resistant mutations can be overcome by the use of second-generation TKIs including dasatinib, nilotinib, or bosutinib. However, ponatinib is the only BCR-ABL1 TKI able to overcome a T315I mutation. The phase 2 PACE (Ponatinib Ph-positive ALL and CML Evaluation) trial enrolled patients with CP-, AP-, or BP-CML as well as patients with Ph-positive acute lymphoblastic leukemia who were resistant or intolerant to nilotinib or dasatinib, or who had evidence of a T315I mutation. The starting dose of ponatinib on this trial was 45 mg daily.¹⁸ The PACE trial enrolled 267 patients with CP-CML: 203 with resistance or intolerance to nilotinib or dasatinib, and 64 with a T315I mutation. The primary endpoint in the CP cohort was rate of MCyR at any time within 12 months of starting ponatinib. The overall rate of MCyR by 12 months in the CP-CML patients was 56%. In those with a T315I mutation, 70% achieved MCyR, which compared favorably with those with resistance or intolerance to nilotinib or dasatinib, 51% of whom achieved MCyR. CCyR was achieved in 46% of CP-CML patients (40% in the resistant/intolerant cohort and 66% in the T315I cohort). In general, patients with T315I mutations received fewer prior therapies than those in the resistant/intolerant cohort, which likely contributed to the higher response rates in the T315I patients. MR4.5 was achieved in 15% of CP-CML patients by 12 months on the PACE trial.¹⁸ The 5-year update to this study reported that 60%, 40%, and 24% of CP-CML patients achieved MCyR, MMR, and MR4.5, respectively. In the patients who achieved MCyR, the probability of maintaining this response for 5 years was 82% and the estimated 5-year OS was 73%.¹⁹

Toxicity

In 2013, after the regulatory approval of ponatinib, reports became available that the drug can cause an increase in arterial occlusive events including fatal myocardial infarctions and cerebral vascular accidents. For this reason, dose reductions were implemented in patients who were deriving clinical benefit from ponatinib. In spite of these dose reductions, ≥90% of responders maintained their response for up to 40 months.¹⁹ Although the likelihood of developing an arterial occlusive event appears higher in the first year after starting ponatinib than in later years, the cumulative incidence of events continues to increase. The 5-year follow-up to the PACE trial reports 31% of patients experiencing any grade of arterial occlusive event while on ponatinib. Aside from these events, the most common treatment-emergent adverse events in ponatinib-treated patients on the PACE trial included rash (47%), abdominal pain (46%), headache (43%), dry skin (42%), constipation (41%), and hypertension (37%). Hematologic toxicity was also common, with 46% of patients experiencing any grade of thrombocytopenia, 20% experiencing neutropenia, and 20% anemia.¹⁹

Patients receiving ponatinib therapy should be monitored closely for any evidence of arterial or venous thrombosis. In the event of an occlusive event, ponatinib should be discontinued. Similarly, in the setting of any new or worsening heart failure symptoms, ponatinib should be promptly discontinued. Management of any underlying cardiovascular risk factors including hypertension, hyperlipidemia, diabetes, or smoking history is recommended, and these patients should be referred to a cardiologist for a full evaluation. In the absence of any contraindications to aspirin, low-dose aspirin should be considered as a means of decreasing cardiovascular risks associated with ponatinib. In patients with known risk factors, a ponatinib starting dose of 30 mg daily rather than the standard 45 mg daily may be a safer option resulting in fewer arterial occlusive events, although the efficacy of this dose is still being studied in comparison to 45 mg daily.⁷

In the event of ponatinib-induced transaminitis greater than 3 times the upper limit of normal, ponatinib should be held until resolution to less than 3 times the upper limit of normal, at which point it should be resumed at a lower dose. Similarly, in the setting of elevated serum lipase or symptomatic pancreatitis, ponatinib should be held and restarted at a lower dose after resolution of symptoms.⁷ 

In the event of neutropenia or thrombocytopenia, ponatinib should be held until blood count recovery and then restarted at the same dose. If cytopenias occur for a second time, the dose of ponatinib should be lowered at the time of treatment reinitiation. If rash occurs, it can be addressed with topical or systemic steroids as well as dose reduction, interruption, or discontinuation.⁷

Case Conclusion

Given the patient's high-risk Sokal score, ideal first-line treatment is a second-generation TKI in order to increase the likelihood of achieving the desired treatment milestones and improving long-term outcomes. Her history of uncontrolled diabetes and coronary artery disease raises concerns for using nilotinib. Furthermore, her history of COPD makes dasatinib suboptimal because she would have little pulmonary reserve if she were to develop a pleural effusion. For this reason, bosutinib 400 mg daily is chosen as her first-line TKI. Shortly after starting bosutinib, she experiences diarrhea that occurs approximately 3 or 4 times daily during the first week on treatment. She is able to manage this with over-the-counter loperamide and the diarrhea resolves shortly thereafter.

After 3 months of bosutinib therapy, quantitative real-time PCR (RQ-PCR) assay on peripheral blood is done to measure BCR-ABL1 transcripts, and the result is reported at 1.2% IS. This indicates that the patient has achieved an early molecular response, which is defined as a RQ-PCR value of ≤10% IS. She undergoes RQ-PCR monitoring every 3 months, and at 12 months her results indicate a value of 0.07% IS, suggesting she has achieved a MMR. 

Conclusion

With the development of imatinib and the subsequent TKIs, dasatinib, nilotinib, bosutinib, and ponatinib, CP-CML has become a chronic disease with a life-expectancy that is similar to the general population. Given the successful treatments available for these patients, it is crucial to identify patients with this diagnosis, ensure they receive a complete, appropriate diagnostic workup including a bone marrow biopsy and aspiration with cytogenetic testing, and select the best therapy for each individual patient. Once on treatment, the importance of frequent monitoring cannot be overstated. This is the only way to be certain patients are achieving the desired treatment milestones that correlate with the favorable long-term outcomes that have been observed with TKI-based treatment of CP-CML. 

Chronic myeloid leukemia (CML) is a rare myeloproliferative neoplasm that is characterized by the presence of the Philadelphia (Ph) chromosome and uninhibited expansion of bone marrow stem cells. The Ph chromosome arises from a reciprocal translocation between the Abelson (ABL) region on chromosome 9 and the breakpoint cluster region (BCR) of chromosome 22 (t(9;22)(q34;q11.2), resulting in the BCR-ABL1 fusion gene.¹BCR-ABL1 encodes an oncoprotein with constitutive tyrosine kinase activity that promotes growth and replication through downstream pathways, which is the driving factor in the pathogenesis of CML.¹

Typical treatment for CML involves life-long use of oral BCR-ABL tyrosine kinase inhibitors (TKI). Currently, 5 TKIs have regulatory approval for treatment of this disease. With the introduction of imatinib in 2001 and the subsequent development of second- (dasatinib, nilotinib, bosutinib) and third-generation (ponatinib) TKIs, CML has become a chronic disease with a life-expectancy that is similar to that of the general population. This article reviews the diagnosis of CML and the parameters used for monitoring response to TKI therapy; the selection of initial TKI therapy is reviewed in a separate follow-up article.

Epidemiology

According to SEER data estimates, 8430 new cases of CML were diagnosed in the United States in 2018. CML is a disease of older adults, with a median age of 65 years at diagnosis, and there is a slight male predominance. Between 2011 and 2015, the number of new CML cases was 1.8 per 100,000 persons. The median overall survival (OS) in patients with newly diagnosed chronic-phase CML (CP-CML) has not been reached.² Given the effective treatments available for managing CML, it is estimated that the prevalence of CML in the United States will plateau at 180,000 patients by 2050.³

Diagnosis

Case Presentation

A 53-year-old woman presents to her primary care physician with complaints of fatigue, early satiety, left upper quadrant abdominal pain, and an 8-lb unintentional weight loss over the prior month. Her past medical history is significant for uncontrolled diabetes, coronary artery disease requiring placement of 3 cardiac stents 2 years prior, and chronic obstructive pulmonary disease (COPD) related to a 30-pack-year history of smoking. On physicial exam her spleen is palpated 8 cm below the left costal margin. A complete blood count (CBC) with differential identifies a total white blood cell (WBC) count of 124,000/μL, with a left-shifted differential including 6% basophils, 3% eosinophils, and 3% blasts; hemoglobin is 12.4 g/dL and platelet count is 801 × 10³/µL.

• How is the diagnosis of CML made?

Clinical Features

The diagnosis of CML is often suspected based on an incidental finding of leukocytosis and, in some cases, thrombocytosis. In many cases, this is an incidental finding on routine blood work, but approximately 50% of patients will present with constitutional symptoms associated with the disease. Characteristic features of the WBC differential include left-shifted maturation with neutrophilia and immature circulating myeloid cells. Basophilia and eosinophilia are often present as well. Splenomegaly is a common sign, present in 50% to 90% of patients at diagnosis. In those patients with symptoms related to CML at diagnosis, the most common presentation includes increasing fatigue, fevers, night sweats, early satiety, and weight loss. The diagnosis is confirmed by cytogenetic studies showing the Ph chromosome abnormality, t(9; 22)(q3.4;q1.1), and/or reverse transcriptase polymerase chain reaction (PCR) showing BCR-ABL1 transcripts.

• What further testing is needed when evaluating a patient for CML?

There are 3 distinct phases of CML: chronic phase (CP), accelerated phase (AP), and blast phase (BP). Bone marrow biopsy and aspiration at diagnosis are mandatory in order to determine the phase of the disease at diagnosis. This distinction is based on the percentage of blasts, promyelocytes, and basophils present as well as the platelet count and presence or absence of extramedullary disease.⁴ The vast majority of patients at diagnosis have CML that is in the chronic phase. The typical appearance in CP-CML is a hypercellular marrow with granulocytic and occasionally megakaryocytic hyperplasia. In many cases, basophilia and/or eosinophilia are noted as well. Dysplasia is not a typical finding in CML.⁵ Bone marrow fibrosis can be seen in up to one-third of patients at diagnosis, and may indicate a slightly worse prognosis.⁶ Although a diagnosis of CML can be made without a bone marrow biopsy, complete staging and prognostication are only possible with information gained from this test, including baseline karyotype and confirmation of CP versus a more advanced phase of CML.

The criteria for diagnosing AP-CML has not been agreed upon by various groups, but the modified MD Anderson Cancer Center (MDACC) criteria are used in the majority of clinical trials evaluating the efficacy of TKIs in preventing progression to advanced phases of CML. MDACC criteria define AP-CML as the presence of one of the following: 15% to 29% blasts in the peripheral blood or bone marrow, ≥ 30% peripheral blasts plus promyelocytes, ≥ 20% basophils in the blood or bone marrow, platelet count ≤ 100 × 10³/μL unrelated to therapy, and clonal cytogenetic evolution in Ph-positive metaphases (Table).⁷

BP-CML is typically defined using the criteria developed by the International Bone Marrow Transplant Registry (IBMTR): ≥ 30% blasts in the peripheral blood and/or the bone marrow or the presence of extramedullary disease.⁸ Although not typically used in clinical trials, the revised World Health Organization (WHO) criteria for BP-CML include ≥ 20% blasts in the peripheral blood or bone marrow, extramedullary blast proliferation, and large foci or clusters of blasts in the bone marrow biopsy (Table).⁹ The defining feature of CML is the presence of the Ph chromosome abnormality. In a small subset of patients, additional chromosomal abnormalities (ACA) in the Ph-positive cells may be identified at diagnosis. Some reports indicate that the presence of “major route” ACA (trisomy 8, isochromosome 17q, a second Ph chromosome, or trisomy 19) at diagnosis may negatively impact prognosis, but other reports contradict these findings.^10,11

The typical BCR breakpoint in CML is the major breakpoint cluster region (M-BCR), which results in a 210-kDa protein (p210). Alternate breakpoints that are less frequently identified are the minor BCR (mBCR or p190), which is more commonly found in Ph-positive acute lymphoblastic leukemia (ALL), and the micro BCR (µBCR or p230), which is much less common and is often characterized by chronic neutrophilia.¹² Identifying which BCR-ABL1 transcript is present in each patient using qualitative PCR is crucial in order to ensure proper monitoring during treatment.

The most sensitive method for detecting BCR-ABL1 mRNA transcripts is the quantitative real-time PCR (RQ-PCR) assay, which is typically done on peripheral blood. RQ-PCR is capable of detecting a single CML cell in the presence of ≥ 100,000 normal cells. This test should be done during the initial diagnostic workup in order to confirm the presence of BCR-ABL1 transcripts, and it is used as a standard method for monitoring response to TKI therapy.¹³ The International Scale (IS) is a standardized approach to reporting RQ-PCR results that was developed to allow comparison of results across various laboratories and has become the gold standard for reporting BCR-ABL1 transcript values.¹⁴

Determining Risk Scores

Calculating a patient’s Sokal score or EURO risk score at diagnosis remains an important component of the diagnostic workup in CP-CML, as this information has prognostic and therapeutic implications (an online calculator is available through European LeukemiaNet [ELN]). The risk for disease progression to the accelerated or blast phases is higher in patients with intermediate- or high-risk scores compared to those with a low-risk score at diagnosis. The risk of progression in intermediate- or high-risk patients is lower when a second-generation TKI (dasatinib, nilotinib, or bosutinib) is used as frontline therapy compared to imatinib, and therefore, the National Comprehensive Cancer Network (NCCN) CML Panel recommends starting with a second-generation TKI in these patients.^15-19

Case Continued

Fluorescent in-situ hybridization using a peripheral blood sample to detect the BCR-ABL gene rearrangement is performed and is positive in 87% of cells. Bone marrow biopsy and aspiration show a 95% cellular bone marrow with granulocytic hyperplasia and 1% blasts. Cytogenetics are 46,XX,t(9;22)(q34;q11.2).²⁰ RQ-PCR assay performed to measure BCR-ABL1 transcripts in the peripheral blood shows a value of 98% IS. The patient is ultimately given a diagnosis of CP-CML. Her Sokal risk score is 1.42, making her disease high risk.

• How is response to TKI therapy measured and monitored?

After confirming a diagnosis of CML and selecting the most appropriate TKI for first-line therapy, the successful management of a CML patient relies on close monitoring and follow-up to ensure patients are meeting the desired treatment milestones. Responses in CML can be assessed based on hematologic parameters, cytogenetic results, and molecular responses. A complete hematologic response (CHR) implies complete normalization of peripheral blood counts (with the exception of TKI-induced cytopenias) and resolution of any palpable splenomegaly. The majority of patients will achieve a CHR within 4 to 6 weeks after initiating CML-directed therapy.²¹

Cytogenetic Response

Cytogenetic responses are defined by the decrease in the number of Ph chromosome–positive metaphases when assessed on bone marrow cytogenetics. A partial cytogenetic response (PCyR) is defined as having 1% to 35% Ph-positive metaphases, a major cytogenetic response (MCyR) as having 0% to 35% Ph-positive metaphases, and a CCyR implies that no Ph-positive metaphases are identified on bone marrow cytogenetics. An ideal response is the achievement of PCyR after 3 months on a TKI and a CCyR after 12 months on a TKI.²²

Molecular Response

Once a patient has achieved a CCyR, monitoring their response to therapy can only be done using RQ-PCR to measure BCR-ABL1 transcripts in the peripheral blood. The NCCN and the ELN recommend monitoring RQ-PCR from the peripheral blood every 3 months in order to assess response to TKIs.^19,23 As noted, the International Scale (IS) has become the gold standard reporting system for all BCR-ABL1 transcript levels in the majority of laboratories worldwide.^14,24 Molecular responses are based on a log-reduction in BCR-ABL1 transcripts from a standardized baseline. Many molecular responses can be correlated with cytogenetic responses such that if reliable RQ-PCR testing is available, monitoring can be done using only peripheral blood RQ-PCR rather than repeat bone marrow biopsies. For example, an early molecular response (EMR) is defined as a RQ-PCR value of ≤ 10% IS, which is approximately equivalent to a PCyR.²⁵ A value of 1% IS is approximately equivalent to CCyR. A major molecular response (MMR) is a ≥ 3-log reduction in BCR-ABL1 transcripts from baseline and is a value of ≤ 0.1% IS. Deeper levels of molecular response are best described by the log-reduction in BCR-ABL1 transcripts, with a 4-log reduction denoted as MR4.0, a 4.5-log reduction as MR4.5, and so forth. Complete molecular response (CMR) is defined by the level of sensitivity of the RQ-PCR assay being used.¹⁴

The definition of relapsed disease in CML is dependent on the type of response the patient had previously achieved. Relapse could be the loss of a hematologic or cytogenetic response, but fluctuations in BCR-ABL1 transcripts on routine RQ-PCR do not necessarily indicate relapsed CML. A 1-log increase in the level of BCR-ABL1 transcripts with a concurrent loss of MMR should prompt a bone marrow biopsy in order to assess for the loss of CCyR, and thus a cytogenetic relapse; however, this loss of MMR does not define relapse in and of itself. In the setting of relapsed disease, testing should be done to look for possible ABL kinase domain mutations, and alternate therapy should be selected.¹⁹

Multiple reports have identified the prognostic relevance of achieving an EMR at 3 and 6 months after starting TKI therapy. Marin and colleagues reported that in 282 imatinib-treated patients, there was a significant improvement in 8-year OS, progression-free survival, and cumulative incidence of CCyR and CMR in patients who had BCR-ABL1 transcripts < 9.84% IS after 3 months on treatment.²⁵ This data highlights the importance of early molecular monitoring in order to ensure the best outcomes for patients with CP-CML.

The NCCN CML guidelines and ELN recommendations both agree that an ideal response after 3 months on a TKI is BCR-ABL1 transcripts < 10% IS, but treatment is not considered to be failing at this point if the patient marginally misses this milestone. After 6 months on treatment, an ideal response is considered BCR-ABL1 transcripts < 1%–10% IS. Ideally, patients will have BCR-ABL1 transcripts < 0.1%–1% IS by the time they complete 12 months of TKI therapy, suggesting that these patients have at least achieved a CCyR.^19,23 Even after patients achieve these early milestones, frequent monitoring by RQ-PCR is required to ensure that they are maintaining their response to treatment. This will help to ensure patient compliance with treatment and will also help to identify a select subset of patients who could potentially be considered for an attempt at TKI cessation (not discussed in detail here) after a minimum of 3 years on therapy.^19,26

Conclusion

Given the successful treatments available for patients with CML, it is crucial to identify patients with this disease, ensure they receive a complete, appropriate diagnostic workup including a bone marrow biopsy and aspiration with cytogenetic testing, and select the best therapy for each individual patient. Once on treatment, the importance of frequent monitoring cannot be overstated.

Chronic myeloid leukemia (CML) is a rare myeloproliferative neoplasm that is characterized by the presence of the Philadelphia (Ph) chromosome and uninhibited expansion of bone marrow stem cells. The Ph chromosome arises from a reciprocal translocation between the Abelson (ABL) region on chromosome 9 and the breakpoint cluster region (BCR) of chromosome 22 (t(9;22)(q34;q11.2), resulting in the BCR-ABL1 fusion gene.¹BCR-ABL1 encodes an oncoprotein with constitutive tyrosine kinase activity that promotes growth and replication through downstream pathways, which is the driving factor in the pathogenesis of CML.¹

Typical treatment for CML involves life-long use of oral BCR-ABL tyrosine kinase inhibitors (TKI). Currently, 5 TKIs have regulatory approval for treatment of this disease. With the introduction of imatinib in 2001 and the subsequent development of second- (dasatinib, nilotinib, bosutinib) and third-generation (ponatinib) TKIs, CML has become a chronic disease with a life-expectancy that is similar to that of the general population. This article reviews the diagnosis of CML and the parameters used for monitoring response to TKI therapy; the selection of initial TKI therapy is reviewed in a separate follow-up article.

Epidemiology

According to SEER data estimates, 8430 new cases of CML were diagnosed in the United States in 2018. CML is a disease of older adults, with a median age of 65 years at diagnosis, and there is a slight male predominance. Between 2011 and 2015, the number of new CML cases was 1.8 per 100,000 persons. The median overall survival (OS) in patients with newly diagnosed chronic-phase CML (CP-CML) has not been reached.² Given the effective treatments available for managing CML, it is estimated that the prevalence of CML in the United States will plateau at 180,000 patients by 2050.³

Diagnosis

Case Presentation

A 53-year-old woman presents to her primary care physician with complaints of fatigue, early satiety, left upper quadrant abdominal pain, and an 8-lb unintentional weight loss over the prior month. Her past medical history is significant for uncontrolled diabetes, coronary artery disease requiring placement of 3 cardiac stents 2 years prior, and chronic obstructive pulmonary disease (COPD) related to a 30-pack-year history of smoking. On physicial exam her spleen is palpated 8 cm below the left costal margin. A complete blood count (CBC) with differential identifies a total white blood cell (WBC) count of 124,000/μL, with a left-shifted differential including 6% basophils, 3% eosinophils, and 3% blasts; hemoglobin is 12.4 g/dL and platelet count is 801 × 10³/µL.

• How is the diagnosis of CML made?

Clinical Features

The diagnosis of CML is often suspected based on an incidental finding of leukocytosis and, in some cases, thrombocytosis. In many cases, this is an incidental finding on routine blood work, but approximately 50% of patients will present with constitutional symptoms associated with the disease. Characteristic features of the WBC differential include left-shifted maturation with neutrophilia and immature circulating myeloid cells. Basophilia and eosinophilia are often present as well. Splenomegaly is a common sign, present in 50% to 90% of patients at diagnosis. In those patients with symptoms related to CML at diagnosis, the most common presentation includes increasing fatigue, fevers, night sweats, early satiety, and weight loss. The diagnosis is confirmed by cytogenetic studies showing the Ph chromosome abnormality, t(9; 22)(q3.4;q1.1), and/or reverse transcriptase polymerase chain reaction (PCR) showing BCR-ABL1 transcripts.

• What further testing is needed when evaluating a patient for CML?

There are 3 distinct phases of CML: chronic phase (CP), accelerated phase (AP), and blast phase (BP). Bone marrow biopsy and aspiration at diagnosis are mandatory in order to determine the phase of the disease at diagnosis. This distinction is based on the percentage of blasts, promyelocytes, and basophils present as well as the platelet count and presence or absence of extramedullary disease.⁴ The vast majority of patients at diagnosis have CML that is in the chronic phase. The typical appearance in CP-CML is a hypercellular marrow with granulocytic and occasionally megakaryocytic hyperplasia. In many cases, basophilia and/or eosinophilia are noted as well. Dysplasia is not a typical finding in CML.⁵ Bone marrow fibrosis can be seen in up to one-third of patients at diagnosis, and may indicate a slightly worse prognosis.⁶ Although a diagnosis of CML can be made without a bone marrow biopsy, complete staging and prognostication are only possible with information gained from this test, including baseline karyotype and confirmation of CP versus a more advanced phase of CML.

The criteria for diagnosing AP-CML has not been agreed upon by various groups, but the modified MD Anderson Cancer Center (MDACC) criteria are used in the majority of clinical trials evaluating the efficacy of TKIs in preventing progression to advanced phases of CML. MDACC criteria define AP-CML as the presence of one of the following: 15% to 29% blasts in the peripheral blood or bone marrow, ≥ 30% peripheral blasts plus promyelocytes, ≥ 20% basophils in the blood or bone marrow, platelet count ≤ 100 × 10³/μL unrelated to therapy, and clonal cytogenetic evolution in Ph-positive metaphases (Table).⁷

BP-CML is typically defined using the criteria developed by the International Bone Marrow Transplant Registry (IBMTR): ≥ 30% blasts in the peripheral blood and/or the bone marrow or the presence of extramedullary disease.⁸ Although not typically used in clinical trials, the revised World Health Organization (WHO) criteria for BP-CML include ≥ 20% blasts in the peripheral blood or bone marrow, extramedullary blast proliferation, and large foci or clusters of blasts in the bone marrow biopsy (Table).⁹ The defining feature of CML is the presence of the Ph chromosome abnormality. In a small subset of patients, additional chromosomal abnormalities (ACA) in the Ph-positive cells may be identified at diagnosis. Some reports indicate that the presence of “major route” ACA (trisomy 8, isochromosome 17q, a second Ph chromosome, or trisomy 19) at diagnosis may negatively impact prognosis, but other reports contradict these findings.^10,11

The typical BCR breakpoint in CML is the major breakpoint cluster region (M-BCR), which results in a 210-kDa protein (p210). Alternate breakpoints that are less frequently identified are the minor BCR (mBCR or p190), which is more commonly found in Ph-positive acute lymphoblastic leukemia (ALL), and the micro BCR (µBCR or p230), which is much less common and is often characterized by chronic neutrophilia.¹² Identifying which BCR-ABL1 transcript is present in each patient using qualitative PCR is crucial in order to ensure proper monitoring during treatment.

The most sensitive method for detecting BCR-ABL1 mRNA transcripts is the quantitative real-time PCR (RQ-PCR) assay, which is typically done on peripheral blood. RQ-PCR is capable of detecting a single CML cell in the presence of ≥ 100,000 normal cells. This test should be done during the initial diagnostic workup in order to confirm the presence of BCR-ABL1 transcripts, and it is used as a standard method for monitoring response to TKI therapy.¹³ The International Scale (IS) is a standardized approach to reporting RQ-PCR results that was developed to allow comparison of results across various laboratories and has become the gold standard for reporting BCR-ABL1 transcript values.¹⁴

Determining Risk Scores

Calculating a patient’s Sokal score or EURO risk score at diagnosis remains an important component of the diagnostic workup in CP-CML, as this information has prognostic and therapeutic implications (an online calculator is available through European LeukemiaNet [ELN]). The risk for disease progression to the accelerated or blast phases is higher in patients with intermediate- or high-risk scores compared to those with a low-risk score at diagnosis. The risk of progression in intermediate- or high-risk patients is lower when a second-generation TKI (dasatinib, nilotinib, or bosutinib) is used as frontline therapy compared to imatinib, and therefore, the National Comprehensive Cancer Network (NCCN) CML Panel recommends starting with a second-generation TKI in these patients.^15-19

Case Continued

Fluorescent in-situ hybridization using a peripheral blood sample to detect the BCR-ABL gene rearrangement is performed and is positive in 87% of cells. Bone marrow biopsy and aspiration show a 95% cellular bone marrow with granulocytic hyperplasia and 1% blasts. Cytogenetics are 46,XX,t(9;22)(q34;q11.2).²⁰ RQ-PCR assay performed to measure BCR-ABL1 transcripts in the peripheral blood shows a value of 98% IS. The patient is ultimately given a diagnosis of CP-CML. Her Sokal risk score is 1.42, making her disease high risk.

• How is response to TKI therapy measured and monitored?

After confirming a diagnosis of CML and selecting the most appropriate TKI for first-line therapy, the successful management of a CML patient relies on close monitoring and follow-up to ensure patients are meeting the desired treatment milestones. Responses in CML can be assessed based on hematologic parameters, cytogenetic results, and molecular responses. A complete hematologic response (CHR) implies complete normalization of peripheral blood counts (with the exception of TKI-induced cytopenias) and resolution of any palpable splenomegaly. The majority of patients will achieve a CHR within 4 to 6 weeks after initiating CML-directed therapy.²¹

Cytogenetic Response

Cytogenetic responses are defined by the decrease in the number of Ph chromosome–positive metaphases when assessed on bone marrow cytogenetics. A partial cytogenetic response (PCyR) is defined as having 1% to 35% Ph-positive metaphases, a major cytogenetic response (MCyR) as having 0% to 35% Ph-positive metaphases, and a CCyR implies that no Ph-positive metaphases are identified on bone marrow cytogenetics. An ideal response is the achievement of PCyR after 3 months on a TKI and a CCyR after 12 months on a TKI.²²

Molecular Response

Once a patient has achieved a CCyR, monitoring their response to therapy can only be done using RQ-PCR to measure BCR-ABL1 transcripts in the peripheral blood. The NCCN and the ELN recommend monitoring RQ-PCR from the peripheral blood every 3 months in order to assess response to TKIs.^19,23 As noted, the International Scale (IS) has become the gold standard reporting system for all BCR-ABL1 transcript levels in the majority of laboratories worldwide.^14,24 Molecular responses are based on a log-reduction in BCR-ABL1 transcripts from a standardized baseline. Many molecular responses can be correlated with cytogenetic responses such that if reliable RQ-PCR testing is available, monitoring can be done using only peripheral blood RQ-PCR rather than repeat bone marrow biopsies. For example, an early molecular response (EMR) is defined as a RQ-PCR value of ≤ 10% IS, which is approximately equivalent to a PCyR.²⁵ A value of 1% IS is approximately equivalent to CCyR. A major molecular response (MMR) is a ≥ 3-log reduction in BCR-ABL1 transcripts from baseline and is a value of ≤ 0.1% IS. Deeper levels of molecular response are best described by the log-reduction in BCR-ABL1 transcripts, with a 4-log reduction denoted as MR4.0, a 4.5-log reduction as MR4.5, and so forth. Complete molecular response (CMR) is defined by the level of sensitivity of the RQ-PCR assay being used.¹⁴

The definition of relapsed disease in CML is dependent on the type of response the patient had previously achieved. Relapse could be the loss of a hematologic or cytogenetic response, but fluctuations in BCR-ABL1 transcripts on routine RQ-PCR do not necessarily indicate relapsed CML. A 1-log increase in the level of BCR-ABL1 transcripts with a concurrent loss of MMR should prompt a bone marrow biopsy in order to assess for the loss of CCyR, and thus a cytogenetic relapse; however, this loss of MMR does not define relapse in and of itself. In the setting of relapsed disease, testing should be done to look for possible ABL kinase domain mutations, and alternate therapy should be selected.¹⁹

Multiple reports have identified the prognostic relevance of achieving an EMR at 3 and 6 months after starting TKI therapy. Marin and colleagues reported that in 282 imatinib-treated patients, there was a significant improvement in 8-year OS, progression-free survival, and cumulative incidence of CCyR and CMR in patients who had BCR-ABL1 transcripts < 9.84% IS after 3 months on treatment.²⁵ This data highlights the importance of early molecular monitoring in order to ensure the best outcomes for patients with CP-CML.

The NCCN CML guidelines and ELN recommendations both agree that an ideal response after 3 months on a TKI is BCR-ABL1 transcripts < 10% IS, but treatment is not considered to be failing at this point if the patient marginally misses this milestone. After 6 months on treatment, an ideal response is considered BCR-ABL1 transcripts < 1%–10% IS. Ideally, patients will have BCR-ABL1 transcripts < 0.1%–1% IS by the time they complete 12 months of TKI therapy, suggesting that these patients have at least achieved a CCyR.^19,23 Even after patients achieve these early milestones, frequent monitoring by RQ-PCR is required to ensure that they are maintaining their response to treatment. This will help to ensure patient compliance with treatment and will also help to identify a select subset of patients who could potentially be considered for an attempt at TKI cessation (not discussed in detail here) after a minimum of 3 years on therapy.^19,26

Conclusion

Given the successful treatments available for patients with CML, it is crucial to identify patients with this disease, ensure they receive a complete, appropriate diagnostic workup including a bone marrow biopsy and aspiration with cytogenetic testing, and select the best therapy for each individual patient. Once on treatment, the importance of frequent monitoring cannot be overstated.

Chronic myeloid leukemia (CML) is a rare myeloproliferative neoplasm that is characterized by the presence of the Philadelphia (Ph) chromosome and uninhibited expansion of bone marrow stem cells. The Ph chromosome arises from a reciprocal translocation between the Abelson (ABL) region on chromosome 9 and the breakpoint cluster region (BCR) of chromosome 22 (t(9;22)(q34;q11.2), resulting in the BCR-ABL1 fusion gene.¹BCR-ABL1 encodes an oncoprotein with constitutive tyrosine kinase activity that promotes growth and replication through downstream pathways, which is the driving factor in the pathogenesis of CML.¹

Typical treatment for CML involves life-long use of oral BCR-ABL tyrosine kinase inhibitors (TKI). Currently, 5 TKIs have regulatory approval for treatment of this disease. With the introduction of imatinib in 2001 and the subsequent development of second- (dasatinib, nilotinib, bosutinib) and third-generation (ponatinib) TKIs, CML has become a chronic disease with a life-expectancy that is similar to that of the general population. This article reviews the diagnosis of CML and the parameters used for monitoring response to TKI therapy; the selection of initial TKI therapy is reviewed in a separate follow-up article.

Epidemiology

According to SEER data estimates, 8430 new cases of CML were diagnosed in the United States in 2018. CML is a disease of older adults, with a median age of 65 years at diagnosis, and there is a slight male predominance. Between 2011 and 2015, the number of new CML cases was 1.8 per 100,000 persons. The median overall survival (OS) in patients with newly diagnosed chronic-phase CML (CP-CML) has not been reached.² Given the effective treatments available for managing CML, it is estimated that the prevalence of CML in the United States will plateau at 180,000 patients by 2050.³

Diagnosis

Case Presentation

A 53-year-old woman presents to her primary care physician with complaints of fatigue, early satiety, left upper quadrant abdominal pain, and an 8-lb unintentional weight loss over the prior month. Her past medical history is significant for uncontrolled diabetes, coronary artery disease requiring placement of 3 cardiac stents 2 years prior, and chronic obstructive pulmonary disease (COPD) related to a 30-pack-year history of smoking. On physicial exam her spleen is palpated 8 cm below the left costal margin. A complete blood count (CBC) with differential identifies a total white blood cell (WBC) count of 124,000/μL, with a left-shifted differential including 6% basophils, 3% eosinophils, and 3% blasts; hemoglobin is 12.4 g/dL and platelet count is 801 × 10³/µL.

• How is the diagnosis of CML made?

Clinical Features

The diagnosis of CML is often suspected based on an incidental finding of leukocytosis and, in some cases, thrombocytosis. In many cases, this is an incidental finding on routine blood work, but approximately 50% of patients will present with constitutional symptoms associated with the disease. Characteristic features of the WBC differential include left-shifted maturation with neutrophilia and immature circulating myeloid cells. Basophilia and eosinophilia are often present as well. Splenomegaly is a common sign, present in 50% to 90% of patients at diagnosis. In those patients with symptoms related to CML at diagnosis, the most common presentation includes increasing fatigue, fevers, night sweats, early satiety, and weight loss. The diagnosis is confirmed by cytogenetic studies showing the Ph chromosome abnormality, t(9; 22)(q3.4;q1.1), and/or reverse transcriptase polymerase chain reaction (PCR) showing BCR-ABL1 transcripts.

• What further testing is needed when evaluating a patient for CML?

There are 3 distinct phases of CML: chronic phase (CP), accelerated phase (AP), and blast phase (BP). Bone marrow biopsy and aspiration at diagnosis are mandatory in order to determine the phase of the disease at diagnosis. This distinction is based on the percentage of blasts, promyelocytes, and basophils present as well as the platelet count and presence or absence of extramedullary disease.⁴ The vast majority of patients at diagnosis have CML that is in the chronic phase. The typical appearance in CP-CML is a hypercellular marrow with granulocytic and occasionally megakaryocytic hyperplasia. In many cases, basophilia and/or eosinophilia are noted as well. Dysplasia is not a typical finding in CML.⁵ Bone marrow fibrosis can be seen in up to one-third of patients at diagnosis, and may indicate a slightly worse prognosis.⁶ Although a diagnosis of CML can be made without a bone marrow biopsy, complete staging and prognostication are only possible with information gained from this test, including baseline karyotype and confirmation of CP versus a more advanced phase of CML.

The criteria for diagnosing AP-CML has not been agreed upon by various groups, but the modified MD Anderson Cancer Center (MDACC) criteria are used in the majority of clinical trials evaluating the efficacy of TKIs in preventing progression to advanced phases of CML. MDACC criteria define AP-CML as the presence of one of the following: 15% to 29% blasts in the peripheral blood or bone marrow, ≥ 30% peripheral blasts plus promyelocytes, ≥ 20% basophils in the blood or bone marrow, platelet count ≤ 100 × 10³/μL unrelated to therapy, and clonal cytogenetic evolution in Ph-positive metaphases (Table).⁷

BP-CML is typically defined using the criteria developed by the International Bone Marrow Transplant Registry (IBMTR): ≥ 30% blasts in the peripheral blood and/or the bone marrow or the presence of extramedullary disease.⁸ Although not typically used in clinical trials, the revised World Health Organization (WHO) criteria for BP-CML include ≥ 20% blasts in the peripheral blood or bone marrow, extramedullary blast proliferation, and large foci or clusters of blasts in the bone marrow biopsy (Table).⁹ The defining feature of CML is the presence of the Ph chromosome abnormality. In a small subset of patients, additional chromosomal abnormalities (ACA) in the Ph-positive cells may be identified at diagnosis. Some reports indicate that the presence of “major route” ACA (trisomy 8, isochromosome 17q, a second Ph chromosome, or trisomy 19) at diagnosis may negatively impact prognosis, but other reports contradict these findings.^10,11

The typical BCR breakpoint in CML is the major breakpoint cluster region (M-BCR), which results in a 210-kDa protein (p210). Alternate breakpoints that are less frequently identified are the minor BCR (mBCR or p190), which is more commonly found in Ph-positive acute lymphoblastic leukemia (ALL), and the micro BCR (µBCR or p230), which is much less common and is often characterized by chronic neutrophilia.¹² Identifying which BCR-ABL1 transcript is present in each patient using qualitative PCR is crucial in order to ensure proper monitoring during treatment.

The most sensitive method for detecting BCR-ABL1 mRNA transcripts is the quantitative real-time PCR (RQ-PCR) assay, which is typically done on peripheral blood. RQ-PCR is capable of detecting a single CML cell in the presence of ≥ 100,000 normal cells. This test should be done during the initial diagnostic workup in order to confirm the presence of BCR-ABL1 transcripts, and it is used as a standard method for monitoring response to TKI therapy.¹³ The International Scale (IS) is a standardized approach to reporting RQ-PCR results that was developed to allow comparison of results across various laboratories and has become the gold standard for reporting BCR-ABL1 transcript values.¹⁴

Determining Risk Scores

Calculating a patient’s Sokal score or EURO risk score at diagnosis remains an important component of the diagnostic workup in CP-CML, as this information has prognostic and therapeutic implications (an online calculator is available through European LeukemiaNet [ELN]). The risk for disease progression to the accelerated or blast phases is higher in patients with intermediate- or high-risk scores compared to those with a low-risk score at diagnosis. The risk of progression in intermediate- or high-risk patients is lower when a second-generation TKI (dasatinib, nilotinib, or bosutinib) is used as frontline therapy compared to imatinib, and therefore, the National Comprehensive Cancer Network (NCCN) CML Panel recommends starting with a second-generation TKI in these patients.^15-19

Case Continued

Fluorescent in-situ hybridization using a peripheral blood sample to detect the BCR-ABL gene rearrangement is performed and is positive in 87% of cells. Bone marrow biopsy and aspiration show a 95% cellular bone marrow with granulocytic hyperplasia and 1% blasts. Cytogenetics are 46,XX,t(9;22)(q34;q11.2).²⁰ RQ-PCR assay performed to measure BCR-ABL1 transcripts in the peripheral blood shows a value of 98% IS. The patient is ultimately given a diagnosis of CP-CML. Her Sokal risk score is 1.42, making her disease high risk.

• How is response to TKI therapy measured and monitored?

After confirming a diagnosis of CML and selecting the most appropriate TKI for first-line therapy, the successful management of a CML patient relies on close monitoring and follow-up to ensure patients are meeting the desired treatment milestones. Responses in CML can be assessed based on hematologic parameters, cytogenetic results, and molecular responses. A complete hematologic response (CHR) implies complete normalization of peripheral blood counts (with the exception of TKI-induced cytopenias) and resolution of any palpable splenomegaly. The majority of patients will achieve a CHR within 4 to 6 weeks after initiating CML-directed therapy.²¹

Cytogenetic Response

Cytogenetic responses are defined by the decrease in the number of Ph chromosome–positive metaphases when assessed on bone marrow cytogenetics. A partial cytogenetic response (PCyR) is defined as having 1% to 35% Ph-positive metaphases, a major cytogenetic response (MCyR) as having 0% to 35% Ph-positive metaphases, and a CCyR implies that no Ph-positive metaphases are identified on bone marrow cytogenetics. An ideal response is the achievement of PCyR after 3 months on a TKI and a CCyR after 12 months on a TKI.²²

Molecular Response

Once a patient has achieved a CCyR, monitoring their response to therapy can only be done using RQ-PCR to measure BCR-ABL1 transcripts in the peripheral blood. The NCCN and the ELN recommend monitoring RQ-PCR from the peripheral blood every 3 months in order to assess response to TKIs.^19,23 As noted, the International Scale (IS) has become the gold standard reporting system for all BCR-ABL1 transcript levels in the majority of laboratories worldwide.^14,24 Molecular responses are based on a log-reduction in BCR-ABL1 transcripts from a standardized baseline. Many molecular responses can be correlated with cytogenetic responses such that if reliable RQ-PCR testing is available, monitoring can be done using only peripheral blood RQ-PCR rather than repeat bone marrow biopsies. For example, an early molecular response (EMR) is defined as a RQ-PCR value of ≤ 10% IS, which is approximately equivalent to a PCyR.²⁵ A value of 1% IS is approximately equivalent to CCyR. A major molecular response (MMR) is a ≥ 3-log reduction in BCR-ABL1 transcripts from baseline and is a value of ≤ 0.1% IS. Deeper levels of molecular response are best described by the log-reduction in BCR-ABL1 transcripts, with a 4-log reduction denoted as MR4.0, a 4.5-log reduction as MR4.5, and so forth. Complete molecular response (CMR) is defined by the level of sensitivity of the RQ-PCR assay being used.¹⁴

The definition of relapsed disease in CML is dependent on the type of response the patient had previously achieved. Relapse could be the loss of a hematologic or cytogenetic response, but fluctuations in BCR-ABL1 transcripts on routine RQ-PCR do not necessarily indicate relapsed CML. A 1-log increase in the level of BCR-ABL1 transcripts with a concurrent loss of MMR should prompt a bone marrow biopsy in order to assess for the loss of CCyR, and thus a cytogenetic relapse; however, this loss of MMR does not define relapse in and of itself. In the setting of relapsed disease, testing should be done to look for possible ABL kinase domain mutations, and alternate therapy should be selected.¹⁹

Multiple reports have identified the prognostic relevance of achieving an EMR at 3 and 6 months after starting TKI therapy. Marin and colleagues reported that in 282 imatinib-treated patients, there was a significant improvement in 8-year OS, progression-free survival, and cumulative incidence of CCyR and CMR in patients who had BCR-ABL1 transcripts < 9.84% IS after 3 months on treatment.²⁵ This data highlights the importance of early molecular monitoring in order to ensure the best outcomes for patients with CP-CML.

The NCCN CML guidelines and ELN recommendations both agree that an ideal response after 3 months on a TKI is BCR-ABL1 transcripts < 10% IS, but treatment is not considered to be failing at this point if the patient marginally misses this milestone. After 6 months on treatment, an ideal response is considered BCR-ABL1 transcripts < 1%–10% IS. Ideally, patients will have BCR-ABL1 transcripts < 0.1%–1% IS by the time they complete 12 months of TKI therapy, suggesting that these patients have at least achieved a CCyR.^19,23 Even after patients achieve these early milestones, frequent monitoring by RQ-PCR is required to ensure that they are maintaining their response to treatment. This will help to ensure patient compliance with treatment and will also help to identify a select subset of patients who could potentially be considered for an attempt at TKI cessation (not discussed in detail here) after a minimum of 3 years on therapy.^19,26

Conclusion

Given the successful treatments available for patients with CML, it is crucial to identify patients with this disease, ensure they receive a complete, appropriate diagnostic workup including a bone marrow biopsy and aspiration with cytogenetic testing, and select the best therapy for each individual patient. Once on treatment, the importance of frequent monitoring cannot be overstated.

Aplastic anemia is a rare hematologic disorder marked by pancytopenia and a hypocellular marrow. Aplastic anemia results from either inherited or acquired causes, and the treatment approach varies significantly between the 2 causes. This article reviews the treatment of inherited and acquired forms of aplastic anemia. The approach to evaluation and diagnosis of aplastic anemia is reviewed in a separate article.

Inherited Aplastic Anemia

First-line treatment options for patients with inherited marrow failure syndromes (IMFS) are androgen therapy and hematopoietic stem cell transplant (HSCT). When evaluating patients for HSCT, it is critical to identify the presence of an IMFS, as the risk and mortality associated with the conditioning regimen, stem cell source, graft-versus-host disease (GVHD), and secondary malignancies differ between patients with IMFS and those with acquired marrow failure syndromes or hematologic malignancies.

Potential sibling donors need to be screened for donor candidacy as well as for the inherited defect.¹ Among patients with Fanconi anemia or a telomere biology disorder, the stem cell source must be considered, with bone marrow demonstrating lower rates of acute GVHD than a peripheral blood stem cell source.^2-4 In IMFS patients, the donor cell type may affect the choice of conditioning regimen.^5,6 Reduced-intensity conditioning in lieu of myeloablative conditioning without total body irradiation has proved feasible in patients with Fanconi anemia, and is associated with a reduced risk of secondary malignancies.^5,6 Incorporation of fludarabine in the conditioning regimen of patients without a matched sibling donor is associated with superior engraftment and survival^2,5,7 compared to cyclophosphamide conditioning, which was historically used in matched related donors.^6,8 The addition of fludarabine appears to be especially beneficial in older patients, in whom its use is associated with lower rates of graft failure, likely due to increased immunosuppression at the time of engraftment.^7,9 Fludarabine has also been incorporated into conditioning regimens for patients with a telomere biology disorder, but outcomes data is limited.⁵

For patients presenting with acute myeloid leukemia (AML) or a high-risk myelodysplastic syndrome (MDS) who are subsequently diagnosed with an IMFS, treatment can be more complex, as these patients are at high risk for toxicity from standard chemotherapy. Limited data suggests that induction therapy and transplantation are feasible in this group of patients, and this approach is associated with increased overall survival (OS) despite lower OS rates than those of IMFS patients who present prior to the development of MDS or AML.^10,11 Further work is needed to determine the optimal induction regimen that balances the risks of treatment-related mortality and complications associated with conditioning regimens, risk of relapse, and risk of secondary malignancies, especially in the cohort of patients diagnosed at an older age.

Acquired Aplastic Anemia

Supportive Care

While the workup and treatment plan is being established, attention should be directed at supportive care for prevention of complications. The most common complications leading to death in patients with significant pancytopenia and neutropenia are opportunistic infections and hemorrhagic complications.¹²

Transfusion support is critical to avoid symptomatic anemia and hemorrhagic complications related to thrombocytopenia, which typically occur with platelet counts lower than 10,000 cells/µL. However, transfusion carries the risk of alloimmunization (which may persist for years following transfusion) and transfusion-related graft versus host disease (trGVHD), and thus use of transfusion should be minimized when possible.^13,14 Transfusion support is often required to prevent complications associated with thrombocytopenia and anemia; all blood products given to patients with aplastic anemia should be irradiated and leukoreduced to reduce the risk of both alloimmunization and trGVHD. Guidelines from the British Society for Haematology recommend routine screening for Rh and Kell antibodies to reduce the risk of alloimmunization.¹⁵ Infectious complications remain a common cause of morbidity and mortality in patients with aplastic anemia who have prolonged neutropenia (defined as an absolute neutrophil count [ANC] < 500 cells/µL).^16-19 Therefore, patients should receive broad-spectrum antibiotics with antipseudomonal coverage. In a study by Tichelli and colleagues evaluating the role of granulocyte-colony stimulating factor (G-CSF) in patients with SAA receiving immunosuppressive therapy, 55% of all patient deaths were secondary to infection.²⁰ There was no OS benefit seen in patients who received G-CSF, though a significantly lower rate of infection was observed in the G-CSF arm compared to those not receiving G-CSF (56% versus 81%, P = 0.006).This difference was largely driven by a decrease in infectious episodes in patients with very severe aplastic anemia (VSAA) treated with G-CSF as compared to those who did not receive this therapy (22% versus 48%, P = 0.014).²⁰

Angio-invasive pulmonary aspergillosis and Zygomycetes (eg, Rhizopus, Mucor species) remain major causes of mortality related to opportunistic mycotic infections in patients with aplastic anemia.¹⁸ The infectious risk is directly related to the duration and severity of neutropenia, with one study demonstrating a significant increase in risk in AML patients with neutropenia lasting longer than 3 weeks.²¹ Invasive fungal infections carry a high mortality in patients with severe neutropenia, though due to earlier recognition and empiric antifungal therapy with extended-spectrum azoles, overall mortality secondary to invasive fungal infections is declining.^19,22

While neutropenia related to cytotoxic chemotherapy is commonly associated with gram-negative bacteria due to disruption of mucosal barriers, patients with aplastic anemia have an increased incidence of gram-positive bacteremia with staphylococcal species compared to other neutropenic populations.^18,19 This appears to be changing with time. Valdez and colleagues demonstrated a decrease in prevalence of coagulase-negative staphylococcal infections, increased prevalence of gram-positive bacilli bacteremia, and no change in prevalence of gram-negative bacteremia in patients with aplastic anemia treated between 1989 and 2008.²² Gram-negative bacteremia caused by Stenotrophomonas maltophila, Escherichia coli, Klebsiella pneumoniae, Citrobacter, and Proteus has also been reported.¹⁹ Despite a lack of clinical trials investigating the role of antifungal and antibacterial prophylaxis for patients with aplastic anemia, most centers initiate antifungal prophylaxis in patients with severe aplastic anema (SAA) or VSAA with an anti-mold agent such as voriconazole or posaconazole (which has the additional benefit compared to voriconazole of covering Mucor species).^17,23 This is especially true for patients who have received ATG or undergone HSCT. For antimicrobial prophylaxis, a fluoroquinolone antibiotic with a spectrum of activity against Pseudomonas should be considered for patients with an ANC < 500 cells/µL.¹⁷ Acyclovir or valacyclovir prophylaxis is recommended for varicella-zoster virus and herpes simplex virus. Cytomegalovirus reactivation is minimal in patients with aplastic anemia, unless multiple courses of ATG are used.

Iron overload is another complication the provider must be aware of in the setting of increased transfusions in aplastic anemia patients. Lee and colleagues demonstrated that iron chelation therapy using deferasirox is effective at reducing serum ferritin levels in patients with aplastic anemia (median ferritin level: 3254 ng/mL prior to therapy, 1854 ng/mL following), and is associated with no serious adverse events (most common adverse events included nausea, diarrhea, vomiting, and rash).²⁴ Approximately 25% of patients in this trial demonstrated an increase in creatinine, with patients taking concomitant cyclosporine more affected than those on chelation therapy alone.²⁴ For patients following HSCT or with improved hematopoiesis following immunosuppressive therapy, phlebotomy can be used to treat iron overload in lieu of chelation therapy.¹⁵

Approach to Therapy

The main treatment options for SAA and VSAA include allogeneic bone marrow transplant and immunosuppression. The deciding factors as to which treatment is best initially depends on the availability of HLA-matched related donors and age (Figure 1 and Figure 2). Survival is decreased in patients with SAA or VSAA who delay initiation of therapy, and therefore prompt referral for HLA typing and evaluation for bone marrow transplant is a very important first step in managing aplastic anemia.

Matched Sibling Donor Transplant

Current standards of care recommend HLA-matched sibling donor transplant for patients with SAA or VSAA who are younger than 50 years of age, with the caveat that integration of fludarabine and reduced cyclophosphamide dosing along with ATG shows the best overall outcomes. Locasciulli and colleagues examined outcomes in patients given either immunosuppressive therapy or sibling HSCT between 1991-1996 and 1997-2002, respectively, and found that sibling HSCT was associated with a superior 10-year OS compared to immunosuppressive therapy (73% versus 68%).²⁵ Interestingly in this study, there was no OS improvement seen with immunosuppressive therapy alone (69% versus 73%) between the 2 time periods, despite increased OS in both sibling HSCT (74% and 80%) and matched unrelated donor HSCT (38% and 65%).²⁵ Though total body irradiation has been used in the past, it is typically not included in current conditioning regimens for matched related donor transplants.²⁶

Current conditioning regimens typically use a combination of cyclophosphamide and ATG^27,28 with or without fludarabine. Fludarabine-based conditioning regimens have shown promise in patients undergoing sibling HSCT. Maury and colleagues evaluated the role of fludarabine in addition to low-dose cyclophosphamide and ATG compared to cyclophosphamide alone or in combination with ATG in patients over age 30 undergoing sibling HSCT.⁹ There was a nonsignificant improvement in 5-year OS in the fludarabine arm compared to controls (77% ± 8% versus 60% ± 3%, P = 0.14) in the pooled analysis, but when adjusted for age the fludarabine arm had a significantly lower relative risk (RR) of death (RR, 0.44; P = 0.04) compared to the control arm. Shin et al reported outcomes with fludarabine/cyclophosphamide/ATG, with excellent overall outcomes and no difference in patients older or younger than 40 years.²⁹ In addition, Kim et al evaluated their experience with patients older than 40 years of age receiving matched related donors, finding comparable outcomes in those aged 41 to 50 years compared to younger patients. Outcomes did decline in those over the age of 50 years.³⁰ Long-term data for matched related donor transplant for aplastic anemia shows excellent long-term outcomes, with minimal chronic GVHD and good performance status.³¹ Hence, these factors support the role of matched related donor transplant as the initial treatment in SAA and VSAA.

Regarding the role of transplant for patients who lack a matched related donor, a growing body of literature demonstrating identical outcomes between matched related and matched unrelated donor (MUD) transplants for pediatric patients^32,33 supports recent recommendations for upfront unrelated donor transplantation for aplastic anemia.^34,35

Immunosuppressive Therapy

For patients without an HLA-matched sibling donor or those who are older than 50 years of age, immunosuppressive therapy is the first-line therapy. ATG and cyclosporine A are the treatments of choice.³⁶ The potential effectiveness of immunosuppressive therapy in treating aplastic anemia was initially observed in patients in whom autologous transplant failed but who still experienced hematopoietic reconstitution despite the failed graft; this observation led to the hypothesis that the conditioning regimen may have an effect on hematopoiesis.^16,36,37

Anti-thymocyte globulin. Immunosuppressive therapy with ATG has been used for the treatment of aplastic anemia since the 1980s.³⁸ Historically, rabbit ATG had been used, but a 2011 study of horse ATG demonstrated superior hematological response at 6 months compared to rabbit ATG (68% versus 37%).¹⁶ Superior survival was also seen with horse ATG compared to rabbit ATG (3-year OS: 96% versus 76%). Due to these results, horse ATG is preferred over rabbit ATG. ATG should be used in combination with cyclosporine A to optimize outcomes.

Cyclosporine A. Early studies also demonstrated the efficacy of cyclosporine A in the treatment of aplastic anemia, with response rates equivalent to that of ATG monotherapy.³⁹ Recent publications still note the efficacy of cyclosporine A in the treatment of aplastic anemia. Its role as an affordable option for single-agent therapy in developing countries is intriguing.³⁹

The combination of the ATG and cyclosporine A was proven superior to either agent alone in a study by Frickhofen et al.³⁷ In this study patients were randomly assigned to a control arm that received ATG plus methylprednisolone or to an arm that received ATG plus cyclosporine A and methylprednisolone. At 6 months, 70% of patients in the cyclosporine A arm had a complete remission (CR) or partial remission compared to 46% in the control arm.⁴⁰ Further work confirmed the long-term efficacy of this regimen, reporting a 7-year OS of 55%.⁴¹ Among a pediatric population, immunosuppressive therapy was associated with an 83% 10-year OS.⁴²

It is recommended that patients remain on cyclosporine therapy for a minimum of 6 months, after which a gradual taper may be considered, although there is variation among practitioners, with some continuing immunosuppressive therapy for a minimum of 12 months due to a proportion of patients being cyclosporine dependent.^42,43 A study found that within a population of patients who responded to immunosuppressive therapy, 18% became cyclosporine dependent.⁴² The median duration of cyclosporine A treatment at full dose was 12 months, with tapering completed over a median of 19 months after patients had been in a stable CR for a minimum of 3 months. Relapse occurred more often when patients were tapered quickly (decrease ≥ 0.8 mg/kg/month) compared to slowly (0.4-0.7 mg/kg/month) or very slowly (< 0.3 mg/kg/month).

Immunosuppressive therapy plus eltrombopag. Townsley and colleagues recently investigated incorporating the use of the thrombopoietin receptor agonist eltrombopag with immunosuppressive therapy as first-line therapy in aplastic anemia.⁴⁴ When given at a dose of 150 mg daily in patients aged 12 years and older or 75 mg daily in patients younger than 12 years, in conjunction with cyclosporine A and ATG, patients demonstrated markedly improved hematological response compared to historical treatment with standard immunosuppressive therapy alone.⁴⁴ In the patient cohort administered eltrombopag starting on day 1 and continuing for 6 months, the complete response rate was 58%. Eltrombopag led to improvement in all cell lines among all treatment subgroups, and OS (censored for patients who proceeded to transplant) was 99% at 2 years.⁴⁵ Overall, toxicities associated with this therapy were low, with liver enzyme elevations most commonly observed.⁴⁴ Recently, a phase 2 trial of immunosuppressive therapy with or without eltrombopag was reported. Of the 38 patients enrolled, overall response, complete response, and time to response were not statistically different.⁴⁶ With this recent finding, the role of eltrombopag in addition to immunosuppressive therapy is not clearly defined, and further studies are warranted.

OS for patients who do not respond to immunosuppressive therapy is approximately 57% at 5 years, largely due to improved supportive measures among this patient population.^4,22 Therefore, it is important to recognize those patients who have a low chance of response so that second-line therapy can be pursued to improve outcomes.

Matched Unrelated Donor Transplant

For patients with refractory disease following immunosuppressive therapy who lack a matched sibling donor, MUD HSCT is considered standard therapy given the marked improvement in overall outcomes with modulating conditioning regimens and high-resolution HLA typing. A European Society for Blood and Marrow Transplantation analysis comparing matched sibling HSCT to MUD HSCT noted significantly higher rates of acute grade II-IV and grade III-V GVHD (grade II-IV 13% versus 25%, grade III-IV 5% versus 10%) among patients undergoing MUD transplant.⁴⁷ Chronic GVHD rates were 14% in the sibling group, as compared to 26% in the MUD group. Additional benefits seen in this analysis included improved survival when transplanted under age 20 years (84% versus 72%), when transplanted within 6 months of diagnosis (85% versus 72%), the use of ATG in the conditioning regimen (81% versus 73%), and when the donor and recipient were cytomegalovirus-negative compared to other combinations (82% versus 76%).⁴⁷ Interestingly, this study demonstrated that OS was not significantly increased when using a sibling HSCT compared to a MUD HSCT, likely as a result of improved understanding of conditioning regimens, GVHD prophylaxis, and supportive care.

Additional studies of MUD HSCT have shown outcomes similar to those seen in sibling HSCT.^4,43 A French study found a significant increase in survival in patients undergoing MUD HSCT compared to historical cohorts (2000-2005: OS 52%; 2006-2012: OS 74%).³³ The majority of patients underwent conditioning with cyclophosphamide or a combination of busulfan and cyclophosphamide, with or without fludarabine; 81% of patients included underwent in vivo T-cell depletion, and a bone marrow donor source was utilized. OS was significantly lower in patients over age 30 years undergoing MUD HSCT (57%) compared to those under age 30 years (70%). Improved OS was also seen when patients underwent transplant within 1 year of diagnosis and when a 10/10 matched donor (compared to a 9/10 mismatched donor) was utilized.⁴ A 2015 study investigated the role of MUD HSCT as frontline therapy instead of immunosuppressive therapy in patients without a matched sibling donor.³³ The 2-year OS was 96% in the MUD HSCT cohort compared to 91%, 94%, and 74% in historical cohorts of sibling HSCT, frontline immunosuppressive therapy, and second-line MUD HSCT following failed immunosuppressive therapy, respectively. Additionally, event-free survival in the MUD HSCT cohort (defined by the authors as death, lack of response, relapse, occurrence of clonal evolution/clinical paroxysmal nocturnal hemoglobinuria, malignancies developing over follow‐up, and transplant for patients receiving immunosuppressive therapy frontline) was similar compared to sibling HSCT and superior to frontline immunosuppressive therapy and second-line MUD HSCT. Furthermore, Samarasinghe et al highlighted the importance of in vivo T-cell depletion with either ATG or alemtuzumab (anti-CD52 monoclonal antibody) in the prevention of acute and chronic GVHD in both sibling HSCT and MUD HSCT.⁴⁸

With continued improvement of less toxic and more immunomodulating conditioning regimens, utilization of bone marrow as a donor cell source, in vivo T-cell depletion, and use of GVHD and antimicrobial prophylaxis, more clinical evidence supports elevating MUD HSCT in the treatment plan for patients without a matched sibling donor.⁴⁹ However, there is still a large population of patients without matched sibling or unrelated donor options. In an effort to expand the transplant pool and thus avoid clonal hematopoiesis, clinically significant paroxysmal nocturnal hemoglobinuria, and relapsed aplastic anemia, more work continues to recognize the expanding role of alternative donor transplants (cord blood and haploidentical) as another viable treatment strategy for aplastic anemia after immunosuppressive therapy failure.⁵⁰

Summary

Aplastic anemia is a rare but potentially life-threatening disorder characterized by pancytopenia and a marked reduction in the hematopoietic stem cell compartment. Treatment should be instituted as soon as the dignosis of aplastic anemia is established. Treatment outcomes are excellent with modern supportive care and the current approach to allogeneic transplantation, and therefore referral to a bone marrow transplant program to evaluate for early transplantation is the new standard of care.

Aplastic anemia is a rare hematologic disorder marked by pancytopenia and a hypocellular marrow. Aplastic anemia results from either inherited or acquired causes, and the treatment approach varies significantly between the 2 causes. This article reviews the treatment of inherited and acquired forms of aplastic anemia. The approach to evaluation and diagnosis of aplastic anemia is reviewed in a separate article.

Inherited Aplastic Anemia

First-line treatment options for patients with inherited marrow failure syndromes (IMFS) are androgen therapy and hematopoietic stem cell transplant (HSCT). When evaluating patients for HSCT, it is critical to identify the presence of an IMFS, as the risk and mortality associated with the conditioning regimen, stem cell source, graft-versus-host disease (GVHD), and secondary malignancies differ between patients with IMFS and those with acquired marrow failure syndromes or hematologic malignancies.

Potential sibling donors need to be screened for donor candidacy as well as for the inherited defect.¹ Among patients with Fanconi anemia or a telomere biology disorder, the stem cell source must be considered, with bone marrow demonstrating lower rates of acute GVHD than a peripheral blood stem cell source.^2-4 In IMFS patients, the donor cell type may affect the choice of conditioning regimen.^5,6 Reduced-intensity conditioning in lieu of myeloablative conditioning without total body irradiation has proved feasible in patients with Fanconi anemia, and is associated with a reduced risk of secondary malignancies.^5,6 Incorporation of fludarabine in the conditioning regimen of patients without a matched sibling donor is associated with superior engraftment and survival^2,5,7 compared to cyclophosphamide conditioning, which was historically used in matched related donors.^6,8 The addition of fludarabine appears to be especially beneficial in older patients, in whom its use is associated with lower rates of graft failure, likely due to increased immunosuppression at the time of engraftment.^7,9 Fludarabine has also been incorporated into conditioning regimens for patients with a telomere biology disorder, but outcomes data is limited.⁵

For patients presenting with acute myeloid leukemia (AML) or a high-risk myelodysplastic syndrome (MDS) who are subsequently diagnosed with an IMFS, treatment can be more complex, as these patients are at high risk for toxicity from standard chemotherapy. Limited data suggests that induction therapy and transplantation are feasible in this group of patients, and this approach is associated with increased overall survival (OS) despite lower OS rates than those of IMFS patients who present prior to the development of MDS or AML.^10,11 Further work is needed to determine the optimal induction regimen that balances the risks of treatment-related mortality and complications associated with conditioning regimens, risk of relapse, and risk of secondary malignancies, especially in the cohort of patients diagnosed at an older age.

Acquired Aplastic Anemia

Supportive Care

While the workup and treatment plan is being established, attention should be directed at supportive care for prevention of complications. The most common complications leading to death in patients with significant pancytopenia and neutropenia are opportunistic infections and hemorrhagic complications.¹²

Transfusion support is critical to avoid symptomatic anemia and hemorrhagic complications related to thrombocytopenia, which typically occur with platelet counts lower than 10,000 cells/µL. However, transfusion carries the risk of alloimmunization (which may persist for years following transfusion) and transfusion-related graft versus host disease (trGVHD), and thus use of transfusion should be minimized when possible.^13,14 Transfusion support is often required to prevent complications associated with thrombocytopenia and anemia; all blood products given to patients with aplastic anemia should be irradiated and leukoreduced to reduce the risk of both alloimmunization and trGVHD. Guidelines from the British Society for Haematology recommend routine screening for Rh and Kell antibodies to reduce the risk of alloimmunization.¹⁵ Infectious complications remain a common cause of morbidity and mortality in patients with aplastic anemia who have prolonged neutropenia (defined as an absolute neutrophil count [ANC] < 500 cells/µL).^16-19 Therefore, patients should receive broad-spectrum antibiotics with antipseudomonal coverage. In a study by Tichelli and colleagues evaluating the role of granulocyte-colony stimulating factor (G-CSF) in patients with SAA receiving immunosuppressive therapy, 55% of all patient deaths were secondary to infection.²⁰ There was no OS benefit seen in patients who received G-CSF, though a significantly lower rate of infection was observed in the G-CSF arm compared to those not receiving G-CSF (56% versus 81%, P = 0.006).This difference was largely driven by a decrease in infectious episodes in patients with very severe aplastic anemia (VSAA) treated with G-CSF as compared to those who did not receive this therapy (22% versus 48%, P = 0.014).²⁰

Angio-invasive pulmonary aspergillosis and Zygomycetes (eg, Rhizopus, Mucor species) remain major causes of mortality related to opportunistic mycotic infections in patients with aplastic anemia.¹⁸ The infectious risk is directly related to the duration and severity of neutropenia, with one study demonstrating a significant increase in risk in AML patients with neutropenia lasting longer than 3 weeks.²¹ Invasive fungal infections carry a high mortality in patients with severe neutropenia, though due to earlier recognition and empiric antifungal therapy with extended-spectrum azoles, overall mortality secondary to invasive fungal infections is declining.^19,22

While neutropenia related to cytotoxic chemotherapy is commonly associated with gram-negative bacteria due to disruption of mucosal barriers, patients with aplastic anemia have an increased incidence of gram-positive bacteremia with staphylococcal species compared to other neutropenic populations.^18,19 This appears to be changing with time. Valdez and colleagues demonstrated a decrease in prevalence of coagulase-negative staphylococcal infections, increased prevalence of gram-positive bacilli bacteremia, and no change in prevalence of gram-negative bacteremia in patients with aplastic anemia treated between 1989 and 2008.²² Gram-negative bacteremia caused by Stenotrophomonas maltophila, Escherichia coli, Klebsiella pneumoniae, Citrobacter, and Proteus has also been reported.¹⁹ Despite a lack of clinical trials investigating the role of antifungal and antibacterial prophylaxis for patients with aplastic anemia, most centers initiate antifungal prophylaxis in patients with severe aplastic anema (SAA) or VSAA with an anti-mold agent such as voriconazole or posaconazole (which has the additional benefit compared to voriconazole of covering Mucor species).^17,23 This is especially true for patients who have received ATG or undergone HSCT. For antimicrobial prophylaxis, a fluoroquinolone antibiotic with a spectrum of activity against Pseudomonas should be considered for patients with an ANC < 500 cells/µL.¹⁷ Acyclovir or valacyclovir prophylaxis is recommended for varicella-zoster virus and herpes simplex virus. Cytomegalovirus reactivation is minimal in patients with aplastic anemia, unless multiple courses of ATG are used.

Iron overload is another complication the provider must be aware of in the setting of increased transfusions in aplastic anemia patients. Lee and colleagues demonstrated that iron chelation therapy using deferasirox is effective at reducing serum ferritin levels in patients with aplastic anemia (median ferritin level: 3254 ng/mL prior to therapy, 1854 ng/mL following), and is associated with no serious adverse events (most common adverse events included nausea, diarrhea, vomiting, and rash).²⁴ Approximately 25% of patients in this trial demonstrated an increase in creatinine, with patients taking concomitant cyclosporine more affected than those on chelation therapy alone.²⁴ For patients following HSCT or with improved hematopoiesis following immunosuppressive therapy, phlebotomy can be used to treat iron overload in lieu of chelation therapy.¹⁵

Approach to Therapy

The main treatment options for SAA and VSAA include allogeneic bone marrow transplant and immunosuppression. The deciding factors as to which treatment is best initially depends on the availability of HLA-matched related donors and age (Figure 1 and Figure 2). Survival is decreased in patients with SAA or VSAA who delay initiation of therapy, and therefore prompt referral for HLA typing and evaluation for bone marrow transplant is a very important first step in managing aplastic anemia.

Matched Sibling Donor Transplant

Current standards of care recommend HLA-matched sibling donor transplant for patients with SAA or VSAA who are younger than 50 years of age, with the caveat that integration of fludarabine and reduced cyclophosphamide dosing along with ATG shows the best overall outcomes. Locasciulli and colleagues examined outcomes in patients given either immunosuppressive therapy or sibling HSCT between 1991-1996 and 1997-2002, respectively, and found that sibling HSCT was associated with a superior 10-year OS compared to immunosuppressive therapy (73% versus 68%).²⁵ Interestingly in this study, there was no OS improvement seen with immunosuppressive therapy alone (69% versus 73%) between the 2 time periods, despite increased OS in both sibling HSCT (74% and 80%) and matched unrelated donor HSCT (38% and 65%).²⁵ Though total body irradiation has been used in the past, it is typically not included in current conditioning regimens for matched related donor transplants.²⁶

Current conditioning regimens typically use a combination of cyclophosphamide and ATG^27,28 with or without fludarabine. Fludarabine-based conditioning regimens have shown promise in patients undergoing sibling HSCT. Maury and colleagues evaluated the role of fludarabine in addition to low-dose cyclophosphamide and ATG compared to cyclophosphamide alone or in combination with ATG in patients over age 30 undergoing sibling HSCT.⁹ There was a nonsignificant improvement in 5-year OS in the fludarabine arm compared to controls (77% ± 8% versus 60% ± 3%, P = 0.14) in the pooled analysis, but when adjusted for age the fludarabine arm had a significantly lower relative risk (RR) of death (RR, 0.44; P = 0.04) compared to the control arm. Shin et al reported outcomes with fludarabine/cyclophosphamide/ATG, with excellent overall outcomes and no difference in patients older or younger than 40 years.²⁹ In addition, Kim et al evaluated their experience with patients older than 40 years of age receiving matched related donors, finding comparable outcomes in those aged 41 to 50 years compared to younger patients. Outcomes did decline in those over the age of 50 years.³⁰ Long-term data for matched related donor transplant for aplastic anemia shows excellent long-term outcomes, with minimal chronic GVHD and good performance status.³¹ Hence, these factors support the role of matched related donor transplant as the initial treatment in SAA and VSAA.

Regarding the role of transplant for patients who lack a matched related donor, a growing body of literature demonstrating identical outcomes between matched related and matched unrelated donor (MUD) transplants for pediatric patients^32,33 supports recent recommendations for upfront unrelated donor transplantation for aplastic anemia.^34,35

Immunosuppressive Therapy

For patients without an HLA-matched sibling donor or those who are older than 50 years of age, immunosuppressive therapy is the first-line therapy. ATG and cyclosporine A are the treatments of choice.³⁶ The potential effectiveness of immunosuppressive therapy in treating aplastic anemia was initially observed in patients in whom autologous transplant failed but who still experienced hematopoietic reconstitution despite the failed graft; this observation led to the hypothesis that the conditioning regimen may have an effect on hematopoiesis.^16,36,37

Anti-thymocyte globulin. Immunosuppressive therapy with ATG has been used for the treatment of aplastic anemia since the 1980s.³⁸ Historically, rabbit ATG had been used, but a 2011 study of horse ATG demonstrated superior hematological response at 6 months compared to rabbit ATG (68% versus 37%).¹⁶ Superior survival was also seen with horse ATG compared to rabbit ATG (3-year OS: 96% versus 76%). Due to these results, horse ATG is preferred over rabbit ATG. ATG should be used in combination with cyclosporine A to optimize outcomes.

Cyclosporine A. Early studies also demonstrated the efficacy of cyclosporine A in the treatment of aplastic anemia, with response rates equivalent to that of ATG monotherapy.³⁹ Recent publications still note the efficacy of cyclosporine A in the treatment of aplastic anemia. Its role as an affordable option for single-agent therapy in developing countries is intriguing.³⁹

The combination of the ATG and cyclosporine A was proven superior to either agent alone in a study by Frickhofen et al.³⁷ In this study patients were randomly assigned to a control arm that received ATG plus methylprednisolone or to an arm that received ATG plus cyclosporine A and methylprednisolone. At 6 months, 70% of patients in the cyclosporine A arm had a complete remission (CR) or partial remission compared to 46% in the control arm.⁴⁰ Further work confirmed the long-term efficacy of this regimen, reporting a 7-year OS of 55%.⁴¹ Among a pediatric population, immunosuppressive therapy was associated with an 83% 10-year OS.⁴²

It is recommended that patients remain on cyclosporine therapy for a minimum of 6 months, after which a gradual taper may be considered, although there is variation among practitioners, with some continuing immunosuppressive therapy for a minimum of 12 months due to a proportion of patients being cyclosporine dependent.^42,43 A study found that within a population of patients who responded to immunosuppressive therapy, 18% became cyclosporine dependent.⁴² The median duration of cyclosporine A treatment at full dose was 12 months, with tapering completed over a median of 19 months after patients had been in a stable CR for a minimum of 3 months. Relapse occurred more often when patients were tapered quickly (decrease ≥ 0.8 mg/kg/month) compared to slowly (0.4-0.7 mg/kg/month) or very slowly (< 0.3 mg/kg/month).

Immunosuppressive therapy plus eltrombopag. Townsley and colleagues recently investigated incorporating the use of the thrombopoietin receptor agonist eltrombopag with immunosuppressive therapy as first-line therapy in aplastic anemia.⁴⁴ When given at a dose of 150 mg daily in patients aged 12 years and older or 75 mg daily in patients younger than 12 years, in conjunction with cyclosporine A and ATG, patients demonstrated markedly improved hematological response compared to historical treatment with standard immunosuppressive therapy alone.⁴⁴ In the patient cohort administered eltrombopag starting on day 1 and continuing for 6 months, the complete response rate was 58%. Eltrombopag led to improvement in all cell lines among all treatment subgroups, and OS (censored for patients who proceeded to transplant) was 99% at 2 years.⁴⁵ Overall, toxicities associated with this therapy were low, with liver enzyme elevations most commonly observed.⁴⁴ Recently, a phase 2 trial of immunosuppressive therapy with or without eltrombopag was reported. Of the 38 patients enrolled, overall response, complete response, and time to response were not statistically different.⁴⁶ With this recent finding, the role of eltrombopag in addition to immunosuppressive therapy is not clearly defined, and further studies are warranted.

OS for patients who do not respond to immunosuppressive therapy is approximately 57% at 5 years, largely due to improved supportive measures among this patient population.^4,22 Therefore, it is important to recognize those patients who have a low chance of response so that second-line therapy can be pursued to improve outcomes.

Matched Unrelated Donor Transplant

For patients with refractory disease following immunosuppressive therapy who lack a matched sibling donor, MUD HSCT is considered standard therapy given the marked improvement in overall outcomes with modulating conditioning regimens and high-resolution HLA typing. A European Society for Blood and Marrow Transplantation analysis comparing matched sibling HSCT to MUD HSCT noted significantly higher rates of acute grade II-IV and grade III-V GVHD (grade II-IV 13% versus 25%, grade III-IV 5% versus 10%) among patients undergoing MUD transplant.⁴⁷ Chronic GVHD rates were 14% in the sibling group, as compared to 26% in the MUD group. Additional benefits seen in this analysis included improved survival when transplanted under age 20 years (84% versus 72%), when transplanted within 6 months of diagnosis (85% versus 72%), the use of ATG in the conditioning regimen (81% versus 73%), and when the donor and recipient were cytomegalovirus-negative compared to other combinations (82% versus 76%).⁴⁷ Interestingly, this study demonstrated that OS was not significantly increased when using a sibling HSCT compared to a MUD HSCT, likely as a result of improved understanding of conditioning regimens, GVHD prophylaxis, and supportive care.

Additional studies of MUD HSCT have shown outcomes similar to those seen in sibling HSCT.^4,43 A French study found a significant increase in survival in patients undergoing MUD HSCT compared to historical cohorts (2000-2005: OS 52%; 2006-2012: OS 74%).³³ The majority of patients underwent conditioning with cyclophosphamide or a combination of busulfan and cyclophosphamide, with or without fludarabine; 81% of patients included underwent in vivo T-cell depletion, and a bone marrow donor source was utilized. OS was significantly lower in patients over age 30 years undergoing MUD HSCT (57%) compared to those under age 30 years (70%). Improved OS was also seen when patients underwent transplant within 1 year of diagnosis and when a 10/10 matched donor (compared to a 9/10 mismatched donor) was utilized.⁴ A 2015 study investigated the role of MUD HSCT as frontline therapy instead of immunosuppressive therapy in patients without a matched sibling donor.³³ The 2-year OS was 96% in the MUD HSCT cohort compared to 91%, 94%, and 74% in historical cohorts of sibling HSCT, frontline immunosuppressive therapy, and second-line MUD HSCT following failed immunosuppressive therapy, respectively. Additionally, event-free survival in the MUD HSCT cohort (defined by the authors as death, lack of response, relapse, occurrence of clonal evolution/clinical paroxysmal nocturnal hemoglobinuria, malignancies developing over follow‐up, and transplant for patients receiving immunosuppressive therapy frontline) was similar compared to sibling HSCT and superior to frontline immunosuppressive therapy and second-line MUD HSCT. Furthermore, Samarasinghe et al highlighted the importance of in vivo T-cell depletion with either ATG or alemtuzumab (anti-CD52 monoclonal antibody) in the prevention of acute and chronic GVHD in both sibling HSCT and MUD HSCT.⁴⁸

With continued improvement of less toxic and more immunomodulating conditioning regimens, utilization of bone marrow as a donor cell source, in vivo T-cell depletion, and use of GVHD and antimicrobial prophylaxis, more clinical evidence supports elevating MUD HSCT in the treatment plan for patients without a matched sibling donor.⁴⁹ However, there is still a large population of patients without matched sibling or unrelated donor options. In an effort to expand the transplant pool and thus avoid clonal hematopoiesis, clinically significant paroxysmal nocturnal hemoglobinuria, and relapsed aplastic anemia, more work continues to recognize the expanding role of alternative donor transplants (cord blood and haploidentical) as another viable treatment strategy for aplastic anemia after immunosuppressive therapy failure.⁵⁰

Summary

Aplastic anemia is a rare but potentially life-threatening disorder characterized by pancytopenia and a marked reduction in the hematopoietic stem cell compartment. Treatment should be instituted as soon as the dignosis of aplastic anemia is established. Treatment outcomes are excellent with modern supportive care and the current approach to allogeneic transplantation, and therefore referral to a bone marrow transplant program to evaluate for early transplantation is the new standard of care.

Aplastic anemia is a rare hematologic disorder marked by pancytopenia and a hypocellular marrow. Aplastic anemia results from either inherited or acquired causes, and the treatment approach varies significantly between the 2 causes. This article reviews the treatment of inherited and acquired forms of aplastic anemia. The approach to evaluation and diagnosis of aplastic anemia is reviewed in a separate article.

Inherited Aplastic Anemia

First-line treatment options for patients with inherited marrow failure syndromes (IMFS) are androgen therapy and hematopoietic stem cell transplant (HSCT). When evaluating patients for HSCT, it is critical to identify the presence of an IMFS, as the risk and mortality associated with the conditioning regimen, stem cell source, graft-versus-host disease (GVHD), and secondary malignancies differ between patients with IMFS and those with acquired marrow failure syndromes or hematologic malignancies.

Potential sibling donors need to be screened for donor candidacy as well as for the inherited defect.¹ Among patients with Fanconi anemia or a telomere biology disorder, the stem cell source must be considered, with bone marrow demonstrating lower rates of acute GVHD than a peripheral blood stem cell source.^2-4 In IMFS patients, the donor cell type may affect the choice of conditioning regimen.^5,6 Reduced-intensity conditioning in lieu of myeloablative conditioning without total body irradiation has proved feasible in patients with Fanconi anemia, and is associated with a reduced risk of secondary malignancies.^5,6 Incorporation of fludarabine in the conditioning regimen of patients without a matched sibling donor is associated with superior engraftment and survival^2,5,7 compared to cyclophosphamide conditioning, which was historically used in matched related donors.^6,8 The addition of fludarabine appears to be especially beneficial in older patients, in whom its use is associated with lower rates of graft failure, likely due to increased immunosuppression at the time of engraftment.^7,9 Fludarabine has also been incorporated into conditioning regimens for patients with a telomere biology disorder, but outcomes data is limited.⁵

For patients presenting with acute myeloid leukemia (AML) or a high-risk myelodysplastic syndrome (MDS) who are subsequently diagnosed with an IMFS, treatment can be more complex, as these patients are at high risk for toxicity from standard chemotherapy. Limited data suggests that induction therapy and transplantation are feasible in this group of patients, and this approach is associated with increased overall survival (OS) despite lower OS rates than those of IMFS patients who present prior to the development of MDS or AML.^10,11 Further work is needed to determine the optimal induction regimen that balances the risks of treatment-related mortality and complications associated with conditioning regimens, risk of relapse, and risk of secondary malignancies, especially in the cohort of patients diagnosed at an older age.

Acquired Aplastic Anemia

Supportive Care

While the workup and treatment plan is being established, attention should be directed at supportive care for prevention of complications. The most common complications leading to death in patients with significant pancytopenia and neutropenia are opportunistic infections and hemorrhagic complications.¹²

Transfusion support is critical to avoid symptomatic anemia and hemorrhagic complications related to thrombocytopenia, which typically occur with platelet counts lower than 10,000 cells/µL. However, transfusion carries the risk of alloimmunization (which may persist for years following transfusion) and transfusion-related graft versus host disease (trGVHD), and thus use of transfusion should be minimized when possible.^13,14 Transfusion support is often required to prevent complications associated with thrombocytopenia and anemia; all blood products given to patients with aplastic anemia should be irradiated and leukoreduced to reduce the risk of both alloimmunization and trGVHD. Guidelines from the British Society for Haematology recommend routine screening for Rh and Kell antibodies to reduce the risk of alloimmunization.¹⁵ Infectious complications remain a common cause of morbidity and mortality in patients with aplastic anemia who have prolonged neutropenia (defined as an absolute neutrophil count [ANC] < 500 cells/µL).^16-19 Therefore, patients should receive broad-spectrum antibiotics with antipseudomonal coverage. In a study by Tichelli and colleagues evaluating the role of granulocyte-colony stimulating factor (G-CSF) in patients with SAA receiving immunosuppressive therapy, 55% of all patient deaths were secondary to infection.²⁰ There was no OS benefit seen in patients who received G-CSF, though a significantly lower rate of infection was observed in the G-CSF arm compared to those not receiving G-CSF (56% versus 81%, P = 0.006).This difference was largely driven by a decrease in infectious episodes in patients with very severe aplastic anemia (VSAA) treated with G-CSF as compared to those who did not receive this therapy (22% versus 48%, P = 0.014).²⁰

Angio-invasive pulmonary aspergillosis and Zygomycetes (eg, Rhizopus, Mucor species) remain major causes of mortality related to opportunistic mycotic infections in patients with aplastic anemia.¹⁸ The infectious risk is directly related to the duration and severity of neutropenia, with one study demonstrating a significant increase in risk in AML patients with neutropenia lasting longer than 3 weeks.²¹ Invasive fungal infections carry a high mortality in patients with severe neutropenia, though due to earlier recognition and empiric antifungal therapy with extended-spectrum azoles, overall mortality secondary to invasive fungal infections is declining.^19,22

While neutropenia related to cytotoxic chemotherapy is commonly associated with gram-negative bacteria due to disruption of mucosal barriers, patients with aplastic anemia have an increased incidence of gram-positive bacteremia with staphylococcal species compared to other neutropenic populations.^18,19 This appears to be changing with time. Valdez and colleagues demonstrated a decrease in prevalence of coagulase-negative staphylococcal infections, increased prevalence of gram-positive bacilli bacteremia, and no change in prevalence of gram-negative bacteremia in patients with aplastic anemia treated between 1989 and 2008.²² Gram-negative bacteremia caused by Stenotrophomonas maltophila, Escherichia coli, Klebsiella pneumoniae, Citrobacter, and Proteus has also been reported.¹⁹ Despite a lack of clinical trials investigating the role of antifungal and antibacterial prophylaxis for patients with aplastic anemia, most centers initiate antifungal prophylaxis in patients with severe aplastic anema (SAA) or VSAA with an anti-mold agent such as voriconazole or posaconazole (which has the additional benefit compared to voriconazole of covering Mucor species).^17,23 This is especially true for patients who have received ATG or undergone HSCT. For antimicrobial prophylaxis, a fluoroquinolone antibiotic with a spectrum of activity against Pseudomonas should be considered for patients with an ANC < 500 cells/µL.¹⁷ Acyclovir or valacyclovir prophylaxis is recommended for varicella-zoster virus and herpes simplex virus. Cytomegalovirus reactivation is minimal in patients with aplastic anemia, unless multiple courses of ATG are used.

Iron overload is another complication the provider must be aware of in the setting of increased transfusions in aplastic anemia patients. Lee and colleagues demonstrated that iron chelation therapy using deferasirox is effective at reducing serum ferritin levels in patients with aplastic anemia (median ferritin level: 3254 ng/mL prior to therapy, 1854 ng/mL following), and is associated with no serious adverse events (most common adverse events included nausea, diarrhea, vomiting, and rash).²⁴ Approximately 25% of patients in this trial demonstrated an increase in creatinine, with patients taking concomitant cyclosporine more affected than those on chelation therapy alone.²⁴ For patients following HSCT or with improved hematopoiesis following immunosuppressive therapy, phlebotomy can be used to treat iron overload in lieu of chelation therapy.¹⁵

Approach to Therapy

The main treatment options for SAA and VSAA include allogeneic bone marrow transplant and immunosuppression. The deciding factors as to which treatment is best initially depends on the availability of HLA-matched related donors and age (Figure 1 and Figure 2). Survival is decreased in patients with SAA or VSAA who delay initiation of therapy, and therefore prompt referral for HLA typing and evaluation for bone marrow transplant is a very important first step in managing aplastic anemia.

Matched Sibling Donor Transplant

Current standards of care recommend HLA-matched sibling donor transplant for patients with SAA or VSAA who are younger than 50 years of age, with the caveat that integration of fludarabine and reduced cyclophosphamide dosing along with ATG shows the best overall outcomes. Locasciulli and colleagues examined outcomes in patients given either immunosuppressive therapy or sibling HSCT between 1991-1996 and 1997-2002, respectively, and found that sibling HSCT was associated with a superior 10-year OS compared to immunosuppressive therapy (73% versus 68%).²⁵ Interestingly in this study, there was no OS improvement seen with immunosuppressive therapy alone (69% versus 73%) between the 2 time periods, despite increased OS in both sibling HSCT (74% and 80%) and matched unrelated donor HSCT (38% and 65%).²⁵ Though total body irradiation has been used in the past, it is typically not included in current conditioning regimens for matched related donor transplants.²⁶

Current conditioning regimens typically use a combination of cyclophosphamide and ATG^27,28 with or without fludarabine. Fludarabine-based conditioning regimens have shown promise in patients undergoing sibling HSCT. Maury and colleagues evaluated the role of fludarabine in addition to low-dose cyclophosphamide and ATG compared to cyclophosphamide alone or in combination with ATG in patients over age 30 undergoing sibling HSCT.⁹ There was a nonsignificant improvement in 5-year OS in the fludarabine arm compared to controls (77% ± 8% versus 60% ± 3%, P = 0.14) in the pooled analysis, but when adjusted for age the fludarabine arm had a significantly lower relative risk (RR) of death (RR, 0.44; P = 0.04) compared to the control arm. Shin et al reported outcomes with fludarabine/cyclophosphamide/ATG, with excellent overall outcomes and no difference in patients older or younger than 40 years.²⁹ In addition, Kim et al evaluated their experience with patients older than 40 years of age receiving matched related donors, finding comparable outcomes in those aged 41 to 50 years compared to younger patients. Outcomes did decline in those over the age of 50 years.³⁰ Long-term data for matched related donor transplant for aplastic anemia shows excellent long-term outcomes, with minimal chronic GVHD and good performance status.³¹ Hence, these factors support the role of matched related donor transplant as the initial treatment in SAA and VSAA.

Regarding the role of transplant for patients who lack a matched related donor, a growing body of literature demonstrating identical outcomes between matched related and matched unrelated donor (MUD) transplants for pediatric patients^32,33 supports recent recommendations for upfront unrelated donor transplantation for aplastic anemia.^34,35

Immunosuppressive Therapy

For patients without an HLA-matched sibling donor or those who are older than 50 years of age, immunosuppressive therapy is the first-line therapy. ATG and cyclosporine A are the treatments of choice.³⁶ The potential effectiveness of immunosuppressive therapy in treating aplastic anemia was initially observed in patients in whom autologous transplant failed but who still experienced hematopoietic reconstitution despite the failed graft; this observation led to the hypothesis that the conditioning regimen may have an effect on hematopoiesis.^16,36,37

Anti-thymocyte globulin. Immunosuppressive therapy with ATG has been used for the treatment of aplastic anemia since the 1980s.³⁸ Historically, rabbit ATG had been used, but a 2011 study of horse ATG demonstrated superior hematological response at 6 months compared to rabbit ATG (68% versus 37%).¹⁶ Superior survival was also seen with horse ATG compared to rabbit ATG (3-year OS: 96% versus 76%). Due to these results, horse ATG is preferred over rabbit ATG. ATG should be used in combination with cyclosporine A to optimize outcomes.

Cyclosporine A. Early studies also demonstrated the efficacy of cyclosporine A in the treatment of aplastic anemia, with response rates equivalent to that of ATG monotherapy.³⁹ Recent publications still note the efficacy of cyclosporine A in the treatment of aplastic anemia. Its role as an affordable option for single-agent therapy in developing countries is intriguing.³⁹

The combination of the ATG and cyclosporine A was proven superior to either agent alone in a study by Frickhofen et al.³⁷ In this study patients were randomly assigned to a control arm that received ATG plus methylprednisolone or to an arm that received ATG plus cyclosporine A and methylprednisolone. At 6 months, 70% of patients in the cyclosporine A arm had a complete remission (CR) or partial remission compared to 46% in the control arm.⁴⁰ Further work confirmed the long-term efficacy of this regimen, reporting a 7-year OS of 55%.⁴¹ Among a pediatric population, immunosuppressive therapy was associated with an 83% 10-year OS.⁴²

It is recommended that patients remain on cyclosporine therapy for a minimum of 6 months, after which a gradual taper may be considered, although there is variation among practitioners, with some continuing immunosuppressive therapy for a minimum of 12 months due to a proportion of patients being cyclosporine dependent.^42,43 A study found that within a population of patients who responded to immunosuppressive therapy, 18% became cyclosporine dependent.⁴² The median duration of cyclosporine A treatment at full dose was 12 months, with tapering completed over a median of 19 months after patients had been in a stable CR for a minimum of 3 months. Relapse occurred more often when patients were tapered quickly (decrease ≥ 0.8 mg/kg/month) compared to slowly (0.4-0.7 mg/kg/month) or very slowly (< 0.3 mg/kg/month).

Immunosuppressive therapy plus eltrombopag. Townsley and colleagues recently investigated incorporating the use of the thrombopoietin receptor agonist eltrombopag with immunosuppressive therapy as first-line therapy in aplastic anemia.⁴⁴ When given at a dose of 150 mg daily in patients aged 12 years and older or 75 mg daily in patients younger than 12 years, in conjunction with cyclosporine A and ATG, patients demonstrated markedly improved hematological response compared to historical treatment with standard immunosuppressive therapy alone.⁴⁴ In the patient cohort administered eltrombopag starting on day 1 and continuing for 6 months, the complete response rate was 58%. Eltrombopag led to improvement in all cell lines among all treatment subgroups, and OS (censored for patients who proceeded to transplant) was 99% at 2 years.⁴⁵ Overall, toxicities associated with this therapy were low, with liver enzyme elevations most commonly observed.⁴⁴ Recently, a phase 2 trial of immunosuppressive therapy with or without eltrombopag was reported. Of the 38 patients enrolled, overall response, complete response, and time to response were not statistically different.⁴⁶ With this recent finding, the role of eltrombopag in addition to immunosuppressive therapy is not clearly defined, and further studies are warranted.

OS for patients who do not respond to immunosuppressive therapy is approximately 57% at 5 years, largely due to improved supportive measures among this patient population.^4,22 Therefore, it is important to recognize those patients who have a low chance of response so that second-line therapy can be pursued to improve outcomes.

Matched Unrelated Donor Transplant

For patients with refractory disease following immunosuppressive therapy who lack a matched sibling donor, MUD HSCT is considered standard therapy given the marked improvement in overall outcomes with modulating conditioning regimens and high-resolution HLA typing. A European Society for Blood and Marrow Transplantation analysis comparing matched sibling HSCT to MUD HSCT noted significantly higher rates of acute grade II-IV and grade III-V GVHD (grade II-IV 13% versus 25%, grade III-IV 5% versus 10%) among patients undergoing MUD transplant.⁴⁷ Chronic GVHD rates were 14% in the sibling group, as compared to 26% in the MUD group. Additional benefits seen in this analysis included improved survival when transplanted under age 20 years (84% versus 72%), when transplanted within 6 months of diagnosis (85% versus 72%), the use of ATG in the conditioning regimen (81% versus 73%), and when the donor and recipient were cytomegalovirus-negative compared to other combinations (82% versus 76%).⁴⁷ Interestingly, this study demonstrated that OS was not significantly increased when using a sibling HSCT compared to a MUD HSCT, likely as a result of improved understanding of conditioning regimens, GVHD prophylaxis, and supportive care.

Additional studies of MUD HSCT have shown outcomes similar to those seen in sibling HSCT.^4,43 A French study found a significant increase in survival in patients undergoing MUD HSCT compared to historical cohorts (2000-2005: OS 52%; 2006-2012: OS 74%).³³ The majority of patients underwent conditioning with cyclophosphamide or a combination of busulfan and cyclophosphamide, with or without fludarabine; 81% of patients included underwent in vivo T-cell depletion, and a bone marrow donor source was utilized. OS was significantly lower in patients over age 30 years undergoing MUD HSCT (57%) compared to those under age 30 years (70%). Improved OS was also seen when patients underwent transplant within 1 year of diagnosis and when a 10/10 matched donor (compared to a 9/10 mismatched donor) was utilized.⁴ A 2015 study investigated the role of MUD HSCT as frontline therapy instead of immunosuppressive therapy in patients without a matched sibling donor.³³ The 2-year OS was 96% in the MUD HSCT cohort compared to 91%, 94%, and 74% in historical cohorts of sibling HSCT, frontline immunosuppressive therapy, and second-line MUD HSCT following failed immunosuppressive therapy, respectively. Additionally, event-free survival in the MUD HSCT cohort (defined by the authors as death, lack of response, relapse, occurrence of clonal evolution/clinical paroxysmal nocturnal hemoglobinuria, malignancies developing over follow‐up, and transplant for patients receiving immunosuppressive therapy frontline) was similar compared to sibling HSCT and superior to frontline immunosuppressive therapy and second-line MUD HSCT. Furthermore, Samarasinghe et al highlighted the importance of in vivo T-cell depletion with either ATG or alemtuzumab (anti-CD52 monoclonal antibody) in the prevention of acute and chronic GVHD in both sibling HSCT and MUD HSCT.⁴⁸

With continued improvement of less toxic and more immunomodulating conditioning regimens, utilization of bone marrow as a donor cell source, in vivo T-cell depletion, and use of GVHD and antimicrobial prophylaxis, more clinical evidence supports elevating MUD HSCT in the treatment plan for patients without a matched sibling donor.⁴⁹ However, there is still a large population of patients without matched sibling or unrelated donor options. In an effort to expand the transplant pool and thus avoid clonal hematopoiesis, clinically significant paroxysmal nocturnal hemoglobinuria, and relapsed aplastic anemia, more work continues to recognize the expanding role of alternative donor transplants (cord blood and haploidentical) as another viable treatment strategy for aplastic anemia after immunosuppressive therapy failure.⁵⁰

Summary

Aplastic anemia is a rare but potentially life-threatening disorder characterized by pancytopenia and a marked reduction in the hematopoietic stem cell compartment. Treatment should be instituted as soon as the dignosis of aplastic anemia is established. Treatment outcomes are excellent with modern supportive care and the current approach to allogeneic transplantation, and therefore referral to a bone marrow transplant program to evaluate for early transplantation is the new standard of care.

Aplastic anemia is a clinical and pathological entity of bone marrow failure that causes progressive loss of hematopoietic progenitor stem cells (HPSC), resulting in pancytopenia.¹ Patients may present along a spectrum, ranging from being asymptomatic with incidental findings on peripheral blood testing to having life-threatening neutropenic infections or bleeding. Aplastic anemia results from either inherited or acquired causes, and the pathophysiology and treatment approach vary significantly between these 2 causes. Therefore, recognition of inherited marrow failure diseases, such as Fanconi anemia and telomere biology disorders, is critical to establishing the management plan. This article reviews the epidemiology, pathophysiology, clinical presentation, and diagnosis of aplastic anemia. Treatment of aplastic anemia is reviewed in a separate article.

Epidemiology

Aplastic anemia is a rare disorder, with an incidence of approximately 1.5 to 7 cases per million individuals per year.^2,3 A recent Scandinavian study reported that the incidence of aplastic anemia among the Swedish population is 2.3 cases per million individuals per year, with a median age at diagnosis of 60 years and a slight female predominance (52% versus 48%, respectively).² This data is congruent with prior observations made in Barcelona, where the incidence was 2.34 cases per million individuals per year, albeit with a slightly higher incidence in males compared to females (2.54 versus 2.16, respectively).⁴ The incidence of aplastic anemia varies globally, with a disproportionate increase in incidence seen among Asian populations, with rates as high as 8.8 per million individuals per year.^3-5 This variation in incidence in Asia versus other countries has not been well explained. There appears to be a bimodal distribution, with incidence peaks seen in young adults and in older adults.^2,3,6

Pathophysiology

Acquired Aplastic Anemia

The leading hypothesis as to the cause of most cases of acquired aplastic anemia is that a dysregulated immune system destroys hematopoietic progenitor cells. Inciting etiologies implicated in the development of acquired aplastic anemia include pregnancy, infection, medications, and exposure to certain chemicals, such as benzene.^1,7 The historical understanding of acquired aplastic anemia implicates cytotoxic T-lymphocyte–mediated destruction of CD34+ hematopoietic stem cells.^1,8,9 This hypothesis served as the basis for treatment of acquired aplastic anemia with immunosuppressive therapy, predominantly anti-thymocyte globulin (ATG) combined with cyclosporine A.^1,8 More recent work has focused on cytokine interactions, particularly the suppressive role of interferon (IFN)-γ on hematopoietic stem cells independent of T-lymphocyte–mediated hematopoietic destruction, which has been demonstrated in a murine model.⁸ The interaction of IFN-γ with the hematopoietic stem cells pool is dynamic. IFN-γ levels are elevated during an acute inflammatory response such as a viral infection, providing further basis for the immune-mediated nature of the acquired disease.¹⁰ Specifically, in vitro studies suggest the effects of IFN-γ on HPSC may be secondary to interruption of thrombopoietin and its respective signaling pathways, which play a key role in hematopoietic stem cell renewal.¹¹ Eltrombopag, a thrombopoietin receptor antagonist, has shown promise in the treatment of refractory aplastic anemia, with studies indicating that its effectiveness is independent of IFN-γ levels.^11,12

Inherited Aplastic Anemia

The inherited marrow failure syndromes (IMFSs) are a group of disorders characterized by cellular maintenance and repair defects, leading to cytopenias, increased cancer risk, structural defects, and risk of end organ damage, such as liver cirrhosis and pulmonary fibrosis.^13-15 The most common diseases include Fanconi anemia, dyskeratosis congenita/telomere biology disorders, Diamond-Blackfan anemia, and Shwachman-Diamond syndrome, but with the advent of whole exome sequencing new syndromes continue to be discovered. While classically these disorders present in children, adult presentations of these syndromes are now commonplace. Broadly, the pathophysiology of inherited aplastic anemia relates to the defective hematopoietic progenitor cells and an accelerated decline of the hematopoietic stem cell compartment.

The most common IMFS, Fanconi anemia and telomere biology disorders, are associated with numerous mutations in DNA damage repair pathways and telomere maintenance pathways. TERT, DKC, and TERC mutations are most commonly associated with dyskeratosis congenita, but may also be found infrequently in patients with aplastic anemia presenting at an older age in the absence of the classic phenotypical features.^1,16,17 The recognition of an underlying genetic disorder or telomere biology disorder leading to constitutional aplastic anemia is significant, as these conditions are associated not only with marrow failure, but also endocrinopathies, organ fibrosis, and solid organ malignancies.^13-15 In particular, mutations in the TERT and TERC genes have been associated with dyskeratosis congenita as well as pulmonary fibrosis and cirrhosis.^18,19 The implications of early diagnosis of an IMFS lie in the approach to treatment and prognosis.

Clonal Disorders and Secondary Malignancies

Myelodysplastic syndrome (MDS) and secondary acute myeloid leukemia (AML) are 2 clonal disorders that may arise from a background of aplastic anemia.^9,20,21 Hypoplastic MDS can be difficult to differentiate from aplastic anemia at diagnosis based on morphology alone, although recent work has demonstrated that molecular testing for somatic mutations in ASXL1, DNMT3A, and BCOR can aid in differentiating a subset of aplastic anemia patients who are more likely to progress to MDS.²¹ Clonal populations of cells harboring 6p uniparental disomy are seen in more than 10% of patients with aplastic anemia on cytogenetic analysis, which can help differentiate the diseases.⁹ Yoshizato and colleagues found lower rates of ASXL1 and DNMT3A mutations in patients with aplastic anemia as compared with patients with MDS or AML. In this study, patients with aplastic anemia had higher rates of mutations in PIGA (reflecting the increased paroxysmal nocturnal hemoglobinuria [PNH] clonality seen in aplastic anemia) and BCOR.⁹ Mutations were also found in genes commonly mutated in MDS and AML, including TET2, RUNX1, TP53, and JAK2, albeit at lower frequencies.⁹ These mutations as a whole have not predicted response to therapy or prognosis. However, when performing survival analysis in patients with specific mutations, those commonly encountered in MDS/AML (ASXL1, DNMT3A, TP53, RUNX1, CSMD1) are associated with faster progression to overt MDS/AML and decreased overall survival (OS),^20,21 suggesting these mutations may represent early clonality that can lead to clonal evolution and the development of secondary malignancies. Conversely, mutations in BCOR and BCORL appear to identify patients who may have a favorable outcome in response to immunosuppressive therapy and, similar to patients with PIGA mutations, improved OS.⁹

Paroxysmal Nocturnal Hemoglobinuria

In addition to having an increased risk of myelodysplasia and malignancy due to the development of a dominant pre-malignant clone, patients with aplastic anemia often harbor progenitor cell clones associated with PNH.^1,17 PNH clones have been identified in more than 50% of patients with aplastic anemia.^22,23 PNH represents a clonal disorder of hematopoiesis in which cells harbor X-linked somatic mutations in the PIGA gene; this gene encodes a protein responsible for the synthesis of glycosylphosphatidylinositol (GPI) anchors on the cell surface.^22,24 The lack of these cell surface proteins, specifically CD55 (also known as decay accelerating factor) and CD59 (also known as membrane inhibitor of reactive lysis), predisposes red cells to increased complement-mediated lysis.²⁵ The exact mechanism for the development of these clones in patients with aplastic anemia is not fully understood. Current theories hypothesize that these clones are protected from the immune-mediated destruction of normal hematopoietic stem cells due to the absence of the cell surface proteins.^1,20 The role of these clones over time in patients with aplastic anemia is less clear, though recent work demonstrated that despite differences in clonality over the disease course, aplastic anemia patients with small PNH clones are less likely to develop overt hemolysis and larger PNH clones compared to patients harboring larger (≥ 50%) PNH clones at diagnosis.^23,26,27 Additionally, PNH clones in patients with aplastic anemia infrequently become clinically significant.²⁷ It should be noted that these conditions exist along a continuum; that is, patients with aplastic anemia may develop PNH clones, while conversely patients with PNH may develop aplastic anemia.²⁰ Patients with PNH clones should be followed via peripheral blood flow cytometry in addition to complete blood count to track clonal stability and identify clinically significant PNH among aplastic anemia patients.²⁸

Patients with aplastic anemia typically are diagnosed either due to asymptomatic cytopenias found on peripheral blood sampling, symptomatic anemia, bleeding secondary to thrombocytopenia, or wound healing and infectious complications related to neutropenia.²⁹ A thorough history to understand the timing of symptoms, recent infectious symptoms/exposure, habits, and chemical or toxin exposures (including medications, travel, and supplements) helps guide diagnostic testing. Family history is also critical, with attention given to premature graying, pulmonary, renal, and liver disease, and blood disorders.

Patients with an IMFS, (eg, Fanconi anemia or dyskeratosis congenita) may have associated phenotypical findings such as urogenital abnormalities or short stature; in addition, those with dyskeratosis congenita may present with the classic triad of oral leukoplakia, lacy skin pigmentation, and dystrophic nails.⁷ However, in patients with IMFS, classic phenotypical findings may be lacking in up to 30% to 40% of patients.⁷ As described previously, while congenital malformations are common in Fanconi anemia and dyskeratosis congenita, a third of patients may have no or only subtle phenotypical abnormalities, including alterations in skin or hair pigmentation, skeletal and growth abnormalities, and endocrine disorders.³⁰ The International Fanconi Anemia Registry identified central nervous system, genitourinary, skin and musculoskeletal, ophthalmic, and gastrointestinal system malformations among children with Fanconi anemia.^31,32 Patients with dyskeratosis congenita may present with pulmonary fibrosis, hepatic cirrhosis, or premature graying, as highlighted in a recent study by DiNardo and colleagues.³³ Therefore, physicians must have a heightened index of suspicion in patients with subtle phenotypical findings and associated cytopenias.

Diagnosis

Differential Diagnosis

The diagnosis of aplastic anemia should be suspected in any patient presenting with pancytopenia. Aplastic anemia is a diagnosis of exclusion.³⁴ Other conditions associated with peripheral blood pancytopenia should be considered including infections (HIV, hepatitis, parvovirus B19, cytomegalovirus, Epstein-Barr virus, varicella-zoster virus), nutritional deficiencies (vitamin B12, folate, copper, zinc), autoimmune disease (systemic lupus erythematosus, rheumatoid arthritis, hemophagocytic lymphohistiocytosis), hypersplenism, marrow-occupying diseases (eg, leukemia, lymphoma, MDS), solid malignancies, and fibrosis (Table).⁷

Diagnostic Evaluation

The workup for aplastic anemia should include a thorough history and physical exam to search simultaneously for alternative diagnoses and clues pointing to potential etiologic agents.⁷ Diagnostic tests to be performed include a complete blood count with differential, reticulocyte count, immature platelet fraction, flow cytometry (to rule out lymphoproliferative disorders and atypical myeloid cells and to evaluate for PNH), and bone marrow biopsy with subsequent cytogenetic, immunohistochemical, and molecular testing.³⁵ The typical findings in aplastic anemia include peripheral blood pancytopenia without dysplastic features and bone marrow biopsy demonstrating a hypocellular marrow.⁷ A relative lymphocytosis in the peripheral blood is common.⁷ In patients with a significant PNH clone, a macrocytosis along with elevated lactate dehydrogenase and elevated reticulocyte and granulocyte counts may be present.³⁶

The diagnosis (based on the Camitta criteria³⁷ and modified Camitta criteria³⁸ for severe aplastic anemia) requires 2 of the following findings on peripheral blood samples:

• Absolute neutrophil count (ANC) < 500 cells/µL
• Platelet count < 20,000 cells/µL
• Reticulocyte count < 1% corrected or < 20,000 cells/µL.³⁵

In addition to peripheral blood findings, bone marrow biopsy is essential for the diagnosis, and should demonstrate a markedly hypocellular marrow (cellularity < 25%), occasionally with an increase in T lymphocytes.^7,39 Because marrow cellularity varies with age and can be challenging to assess, additional biopsies may be needed to confirm the diagnosis.²⁹ A 1- to 2-cm core biopsy is necessary to confirm hypocellularity, as small areas of residual hematopoiesis may be present and obscure the diagnosis.³⁵

Excluding Hypocellular MDS and IMFS

A diagnostic challenge is the exclusion of hypocellular MDS, especially in the older adult presenting with aplastic anemia, as patients with aplastic anemia may have some degree of erythroid dysplasia on bone marrow morphology.³⁶ The presence of a PNH clone on flow cytometry can aid in diagnosing aplastic anemia and excluding MDS,³⁴ although PNH clones can be present in refractory anemia MDS. Patients with aplastic anemia have a lower ratio of CD34+ cells compared to those with hypoplastic MDS, with one study demonstrating a mean CD34+ percentage of < 0.5% in aplastic anemia versus 3.7% in hypoplastic MDS.⁴⁰ Cytogenetic and molecular testing can also aid in making this distinction by identifying mutations commonly implicated in MDS.⁷ The presence of monosomy 7 (-7) in aplastic anemia patients is associated with a poor overall prognosis.^34,41

Peripheral blood screening using chromosome breakage analysis (done using either mitomycin C or diepoxybutane as in vitro DNA-crosslinking agents) and telomere length testing (of peripheral blood leukocytes) is necessary to exclude the main IMFS, Fanconi anemia and telomere biology disorders, respectively. Ruling out these conditions is imperative, as the approach to treatment varies significantly between IMFS and aplastic anemia. Patients with shortened telomeres should undergo genetic screening for mutations in the telomere maintenance genes to evaluate the underlying defect leading to shortened telomeres. Patients with increased peripheral blood breakage should have genetic testing to detect mutations associated with Fanconi anemia.

Classification

Once the diagnosis of aplastic anemia has been made, the patient should be classified according to the severity of their disease. Disease severity is determined based on peripheral blood ANC:³⁴ non-severe aplastic anemia (NSAA), ANC > 500 polymorphonuclear neutrophils (PMNs)/µL; severe aplastic anemia (SAA), 200–500 PMNs/µL; and very severe (VSAA), 0–200 PMNs/µL.^4,34 Disease classification is important, as VSAA is associated with a decreased OS compared to SAA.² Disease classification may affect treatment decisions, as patients with NSAA may be observed for a short period of time, while conversely patients with SAA have a worse prognosis with delays in therapy.^42-44

Summary

Aplastic anemia is a rare but potentially life-threatening disorder characterized by pancytopenia and a marked reduction in the hematopoietic stem cell compartment. It can be acquired or associated with an IMFS, and the treatment and prognosis vary dramatically between these 2 etiologies. Work-up and diagnosis involves investigating IMFSs and ruling out malignant or infectious etiologies for pancytopenia. After aplastic anemia has been diagnosed, the patient should be classified according to the severity of their disease based on peripheral blood ANC.

Aplastic anemia is a clinical and pathological entity of bone marrow failure that causes progressive loss of hematopoietic progenitor stem cells (HPSC), resulting in pancytopenia.¹ Patients may present along a spectrum, ranging from being asymptomatic with incidental findings on peripheral blood testing to having life-threatening neutropenic infections or bleeding. Aplastic anemia results from either inherited or acquired causes, and the pathophysiology and treatment approach vary significantly between these 2 causes. Therefore, recognition of inherited marrow failure diseases, such as Fanconi anemia and telomere biology disorders, is critical to establishing the management plan. This article reviews the epidemiology, pathophysiology, clinical presentation, and diagnosis of aplastic anemia. Treatment of aplastic anemia is reviewed in a separate article.

Epidemiology

Aplastic anemia is a rare disorder, with an incidence of approximately 1.5 to 7 cases per million individuals per year.^2,3 A recent Scandinavian study reported that the incidence of aplastic anemia among the Swedish population is 2.3 cases per million individuals per year, with a median age at diagnosis of 60 years and a slight female predominance (52% versus 48%, respectively).² This data is congruent with prior observations made in Barcelona, where the incidence was 2.34 cases per million individuals per year, albeit with a slightly higher incidence in males compared to females (2.54 versus 2.16, respectively).⁴ The incidence of aplastic anemia varies globally, with a disproportionate increase in incidence seen among Asian populations, with rates as high as 8.8 per million individuals per year.^3-5 This variation in incidence in Asia versus other countries has not been well explained. There appears to be a bimodal distribution, with incidence peaks seen in young adults and in older adults.^2,3,6

Pathophysiology

Acquired Aplastic Anemia

The leading hypothesis as to the cause of most cases of acquired aplastic anemia is that a dysregulated immune system destroys hematopoietic progenitor cells. Inciting etiologies implicated in the development of acquired aplastic anemia include pregnancy, infection, medications, and exposure to certain chemicals, such as benzene.^1,7 The historical understanding of acquired aplastic anemia implicates cytotoxic T-lymphocyte–mediated destruction of CD34+ hematopoietic stem cells.^1,8,9 This hypothesis served as the basis for treatment of acquired aplastic anemia with immunosuppressive therapy, predominantly anti-thymocyte globulin (ATG) combined with cyclosporine A.^1,8 More recent work has focused on cytokine interactions, particularly the suppressive role of interferon (IFN)-γ on hematopoietic stem cells independent of T-lymphocyte–mediated hematopoietic destruction, which has been demonstrated in a murine model.⁸ The interaction of IFN-γ with the hematopoietic stem cells pool is dynamic. IFN-γ levels are elevated during an acute inflammatory response such as a viral infection, providing further basis for the immune-mediated nature of the acquired disease.¹⁰ Specifically, in vitro studies suggest the effects of IFN-γ on HPSC may be secondary to interruption of thrombopoietin and its respective signaling pathways, which play a key role in hematopoietic stem cell renewal.¹¹ Eltrombopag, a thrombopoietin receptor antagonist, has shown promise in the treatment of refractory aplastic anemia, with studies indicating that its effectiveness is independent of IFN-γ levels.^11,12

Inherited Aplastic Anemia

The inherited marrow failure syndromes (IMFSs) are a group of disorders characterized by cellular maintenance and repair defects, leading to cytopenias, increased cancer risk, structural defects, and risk of end organ damage, such as liver cirrhosis and pulmonary fibrosis.^13-15 The most common diseases include Fanconi anemia, dyskeratosis congenita/telomere biology disorders, Diamond-Blackfan anemia, and Shwachman-Diamond syndrome, but with the advent of whole exome sequencing new syndromes continue to be discovered. While classically these disorders present in children, adult presentations of these syndromes are now commonplace. Broadly, the pathophysiology of inherited aplastic anemia relates to the defective hematopoietic progenitor cells and an accelerated decline of the hematopoietic stem cell compartment.

The most common IMFS, Fanconi anemia and telomere biology disorders, are associated with numerous mutations in DNA damage repair pathways and telomere maintenance pathways. TERT, DKC, and TERC mutations are most commonly associated with dyskeratosis congenita, but may also be found infrequently in patients with aplastic anemia presenting at an older age in the absence of the classic phenotypical features.^1,16,17 The recognition of an underlying genetic disorder or telomere biology disorder leading to constitutional aplastic anemia is significant, as these conditions are associated not only with marrow failure, but also endocrinopathies, organ fibrosis, and solid organ malignancies.^13-15 In particular, mutations in the TERT and TERC genes have been associated with dyskeratosis congenita as well as pulmonary fibrosis and cirrhosis.^18,19 The implications of early diagnosis of an IMFS lie in the approach to treatment and prognosis.

Clonal Disorders and Secondary Malignancies

Myelodysplastic syndrome (MDS) and secondary acute myeloid leukemia (AML) are 2 clonal disorders that may arise from a background of aplastic anemia.^9,20,21 Hypoplastic MDS can be difficult to differentiate from aplastic anemia at diagnosis based on morphology alone, although recent work has demonstrated that molecular testing for somatic mutations in ASXL1, DNMT3A, and BCOR can aid in differentiating a subset of aplastic anemia patients who are more likely to progress to MDS.²¹ Clonal populations of cells harboring 6p uniparental disomy are seen in more than 10% of patients with aplastic anemia on cytogenetic analysis, which can help differentiate the diseases.⁹ Yoshizato and colleagues found lower rates of ASXL1 and DNMT3A mutations in patients with aplastic anemia as compared with patients with MDS or AML. In this study, patients with aplastic anemia had higher rates of mutations in PIGA (reflecting the increased paroxysmal nocturnal hemoglobinuria [PNH] clonality seen in aplastic anemia) and BCOR.⁹ Mutations were also found in genes commonly mutated in MDS and AML, including TET2, RUNX1, TP53, and JAK2, albeit at lower frequencies.⁹ These mutations as a whole have not predicted response to therapy or prognosis. However, when performing survival analysis in patients with specific mutations, those commonly encountered in MDS/AML (ASXL1, DNMT3A, TP53, RUNX1, CSMD1) are associated with faster progression to overt MDS/AML and decreased overall survival (OS),^20,21 suggesting these mutations may represent early clonality that can lead to clonal evolution and the development of secondary malignancies. Conversely, mutations in BCOR and BCORL appear to identify patients who may have a favorable outcome in response to immunosuppressive therapy and, similar to patients with PIGA mutations, improved OS.⁹

Paroxysmal Nocturnal Hemoglobinuria

In addition to having an increased risk of myelodysplasia and malignancy due to the development of a dominant pre-malignant clone, patients with aplastic anemia often harbor progenitor cell clones associated with PNH.^1,17 PNH clones have been identified in more than 50% of patients with aplastic anemia.^22,23 PNH represents a clonal disorder of hematopoiesis in which cells harbor X-linked somatic mutations in the PIGA gene; this gene encodes a protein responsible for the synthesis of glycosylphosphatidylinositol (GPI) anchors on the cell surface.^22,24 The lack of these cell surface proteins, specifically CD55 (also known as decay accelerating factor) and CD59 (also known as membrane inhibitor of reactive lysis), predisposes red cells to increased complement-mediated lysis.²⁵ The exact mechanism for the development of these clones in patients with aplastic anemia is not fully understood. Current theories hypothesize that these clones are protected from the immune-mediated destruction of normal hematopoietic stem cells due to the absence of the cell surface proteins.^1,20 The role of these clones over time in patients with aplastic anemia is less clear, though recent work demonstrated that despite differences in clonality over the disease course, aplastic anemia patients with small PNH clones are less likely to develop overt hemolysis and larger PNH clones compared to patients harboring larger (≥ 50%) PNH clones at diagnosis.^23,26,27 Additionally, PNH clones in patients with aplastic anemia infrequently become clinically significant.²⁷ It should be noted that these conditions exist along a continuum; that is, patients with aplastic anemia may develop PNH clones, while conversely patients with PNH may develop aplastic anemia.²⁰ Patients with PNH clones should be followed via peripheral blood flow cytometry in addition to complete blood count to track clonal stability and identify clinically significant PNH among aplastic anemia patients.²⁸

Patients with aplastic anemia typically are diagnosed either due to asymptomatic cytopenias found on peripheral blood sampling, symptomatic anemia, bleeding secondary to thrombocytopenia, or wound healing and infectious complications related to neutropenia.²⁹ A thorough history to understand the timing of symptoms, recent infectious symptoms/exposure, habits, and chemical or toxin exposures (including medications, travel, and supplements) helps guide diagnostic testing. Family history is also critical, with attention given to premature graying, pulmonary, renal, and liver disease, and blood disorders.

Patients with an IMFS, (eg, Fanconi anemia or dyskeratosis congenita) may have associated phenotypical findings such as urogenital abnormalities or short stature; in addition, those with dyskeratosis congenita may present with the classic triad of oral leukoplakia, lacy skin pigmentation, and dystrophic nails.⁷ However, in patients with IMFS, classic phenotypical findings may be lacking in up to 30% to 40% of patients.⁷ As described previously, while congenital malformations are common in Fanconi anemia and dyskeratosis congenita, a third of patients may have no or only subtle phenotypical abnormalities, including alterations in skin or hair pigmentation, skeletal and growth abnormalities, and endocrine disorders.³⁰ The International Fanconi Anemia Registry identified central nervous system, genitourinary, skin and musculoskeletal, ophthalmic, and gastrointestinal system malformations among children with Fanconi anemia.^31,32 Patients with dyskeratosis congenita may present with pulmonary fibrosis, hepatic cirrhosis, or premature graying, as highlighted in a recent study by DiNardo and colleagues.³³ Therefore, physicians must have a heightened index of suspicion in patients with subtle phenotypical findings and associated cytopenias.

Diagnosis

Differential Diagnosis

The diagnosis of aplastic anemia should be suspected in any patient presenting with pancytopenia. Aplastic anemia is a diagnosis of exclusion.³⁴ Other conditions associated with peripheral blood pancytopenia should be considered including infections (HIV, hepatitis, parvovirus B19, cytomegalovirus, Epstein-Barr virus, varicella-zoster virus), nutritional deficiencies (vitamin B12, folate, copper, zinc), autoimmune disease (systemic lupus erythematosus, rheumatoid arthritis, hemophagocytic lymphohistiocytosis), hypersplenism, marrow-occupying diseases (eg, leukemia, lymphoma, MDS), solid malignancies, and fibrosis (Table).⁷

Diagnostic Evaluation

The workup for aplastic anemia should include a thorough history and physical exam to search simultaneously for alternative diagnoses and clues pointing to potential etiologic agents.⁷ Diagnostic tests to be performed include a complete blood count with differential, reticulocyte count, immature platelet fraction, flow cytometry (to rule out lymphoproliferative disorders and atypical myeloid cells and to evaluate for PNH), and bone marrow biopsy with subsequent cytogenetic, immunohistochemical, and molecular testing.³⁵ The typical findings in aplastic anemia include peripheral blood pancytopenia without dysplastic features and bone marrow biopsy demonstrating a hypocellular marrow.⁷ A relative lymphocytosis in the peripheral blood is common.⁷ In patients with a significant PNH clone, a macrocytosis along with elevated lactate dehydrogenase and elevated reticulocyte and granulocyte counts may be present.³⁶

The diagnosis (based on the Camitta criteria³⁷ and modified Camitta criteria³⁸ for severe aplastic anemia) requires 2 of the following findings on peripheral blood samples:

• Absolute neutrophil count (ANC) < 500 cells/µL
• Platelet count < 20,000 cells/µL
• Reticulocyte count < 1% corrected or < 20,000 cells/µL.³⁵

In addition to peripheral blood findings, bone marrow biopsy is essential for the diagnosis, and should demonstrate a markedly hypocellular marrow (cellularity < 25%), occasionally with an increase in T lymphocytes.^7,39 Because marrow cellularity varies with age and can be challenging to assess, additional biopsies may be needed to confirm the diagnosis.²⁹ A 1- to 2-cm core biopsy is necessary to confirm hypocellularity, as small areas of residual hematopoiesis may be present and obscure the diagnosis.³⁵

Excluding Hypocellular MDS and IMFS

A diagnostic challenge is the exclusion of hypocellular MDS, especially in the older adult presenting with aplastic anemia, as patients with aplastic anemia may have some degree of erythroid dysplasia on bone marrow morphology.³⁶ The presence of a PNH clone on flow cytometry can aid in diagnosing aplastic anemia and excluding MDS,³⁴ although PNH clones can be present in refractory anemia MDS. Patients with aplastic anemia have a lower ratio of CD34+ cells compared to those with hypoplastic MDS, with one study demonstrating a mean CD34+ percentage of < 0.5% in aplastic anemia versus 3.7% in hypoplastic MDS.⁴⁰ Cytogenetic and molecular testing can also aid in making this distinction by identifying mutations commonly implicated in MDS.⁷ The presence of monosomy 7 (-7) in aplastic anemia patients is associated with a poor overall prognosis.^34,41

Peripheral blood screening using chromosome breakage analysis (done using either mitomycin C or diepoxybutane as in vitro DNA-crosslinking agents) and telomere length testing (of peripheral blood leukocytes) is necessary to exclude the main IMFS, Fanconi anemia and telomere biology disorders, respectively. Ruling out these conditions is imperative, as the approach to treatment varies significantly between IMFS and aplastic anemia. Patients with shortened telomeres should undergo genetic screening for mutations in the telomere maintenance genes to evaluate the underlying defect leading to shortened telomeres. Patients with increased peripheral blood breakage should have genetic testing to detect mutations associated with Fanconi anemia.

Classification

Once the diagnosis of aplastic anemia has been made, the patient should be classified according to the severity of their disease. Disease severity is determined based on peripheral blood ANC:³⁴ non-severe aplastic anemia (NSAA), ANC > 500 polymorphonuclear neutrophils (PMNs)/µL; severe aplastic anemia (SAA), 200–500 PMNs/µL; and very severe (VSAA), 0–200 PMNs/µL.^4,34 Disease classification is important, as VSAA is associated with a decreased OS compared to SAA.² Disease classification may affect treatment decisions, as patients with NSAA may be observed for a short period of time, while conversely patients with SAA have a worse prognosis with delays in therapy.^42-44

Summary

Aplastic anemia is a rare but potentially life-threatening disorder characterized by pancytopenia and a marked reduction in the hematopoietic stem cell compartment. It can be acquired or associated with an IMFS, and the treatment and prognosis vary dramatically between these 2 etiologies. Work-up and diagnosis involves investigating IMFSs and ruling out malignant or infectious etiologies for pancytopenia. After aplastic anemia has been diagnosed, the patient should be classified according to the severity of their disease based on peripheral blood ANC.

Aplastic anemia is a clinical and pathological entity of bone marrow failure that causes progressive loss of hematopoietic progenitor stem cells (HPSC), resulting in pancytopenia.¹ Patients may present along a spectrum, ranging from being asymptomatic with incidental findings on peripheral blood testing to having life-threatening neutropenic infections or bleeding. Aplastic anemia results from either inherited or acquired causes, and the pathophysiology and treatment approach vary significantly between these 2 causes. Therefore, recognition of inherited marrow failure diseases, such as Fanconi anemia and telomere biology disorders, is critical to establishing the management plan. This article reviews the epidemiology, pathophysiology, clinical presentation, and diagnosis of aplastic anemia. Treatment of aplastic anemia is reviewed in a separate article.

Epidemiology

Aplastic anemia is a rare disorder, with an incidence of approximately 1.5 to 7 cases per million individuals per year.^2,3 A recent Scandinavian study reported that the incidence of aplastic anemia among the Swedish population is 2.3 cases per million individuals per year, with a median age at diagnosis of 60 years and a slight female predominance (52% versus 48%, respectively).² This data is congruent with prior observations made in Barcelona, where the incidence was 2.34 cases per million individuals per year, albeit with a slightly higher incidence in males compared to females (2.54 versus 2.16, respectively).⁴ The incidence of aplastic anemia varies globally, with a disproportionate increase in incidence seen among Asian populations, with rates as high as 8.8 per million individuals per year.^3-5 This variation in incidence in Asia versus other countries has not been well explained. There appears to be a bimodal distribution, with incidence peaks seen in young adults and in older adults.^2,3,6

Pathophysiology

Acquired Aplastic Anemia

The leading hypothesis as to the cause of most cases of acquired aplastic anemia is that a dysregulated immune system destroys hematopoietic progenitor cells. Inciting etiologies implicated in the development of acquired aplastic anemia include pregnancy, infection, medications, and exposure to certain chemicals, such as benzene.^1,7 The historical understanding of acquired aplastic anemia implicates cytotoxic T-lymphocyte–mediated destruction of CD34+ hematopoietic stem cells.^1,8,9 This hypothesis served as the basis for treatment of acquired aplastic anemia with immunosuppressive therapy, predominantly anti-thymocyte globulin (ATG) combined with cyclosporine A.^1,8 More recent work has focused on cytokine interactions, particularly the suppressive role of interferon (IFN)-γ on hematopoietic stem cells independent of T-lymphocyte–mediated hematopoietic destruction, which has been demonstrated in a murine model.⁸ The interaction of IFN-γ with the hematopoietic stem cells pool is dynamic. IFN-γ levels are elevated during an acute inflammatory response such as a viral infection, providing further basis for the immune-mediated nature of the acquired disease.¹⁰ Specifically, in vitro studies suggest the effects of IFN-γ on HPSC may be secondary to interruption of thrombopoietin and its respective signaling pathways, which play a key role in hematopoietic stem cell renewal.¹¹ Eltrombopag, a thrombopoietin receptor antagonist, has shown promise in the treatment of refractory aplastic anemia, with studies indicating that its effectiveness is independent of IFN-γ levels.^11,12

Inherited Aplastic Anemia

The inherited marrow failure syndromes (IMFSs) are a group of disorders characterized by cellular maintenance and repair defects, leading to cytopenias, increased cancer risk, structural defects, and risk of end organ damage, such as liver cirrhosis and pulmonary fibrosis.^13-15 The most common diseases include Fanconi anemia, dyskeratosis congenita/telomere biology disorders, Diamond-Blackfan anemia, and Shwachman-Diamond syndrome, but with the advent of whole exome sequencing new syndromes continue to be discovered. While classically these disorders present in children, adult presentations of these syndromes are now commonplace. Broadly, the pathophysiology of inherited aplastic anemia relates to the defective hematopoietic progenitor cells and an accelerated decline of the hematopoietic stem cell compartment.

The most common IMFS, Fanconi anemia and telomere biology disorders, are associated with numerous mutations in DNA damage repair pathways and telomere maintenance pathways. TERT, DKC, and TERC mutations are most commonly associated with dyskeratosis congenita, but may also be found infrequently in patients with aplastic anemia presenting at an older age in the absence of the classic phenotypical features.^1,16,17 The recognition of an underlying genetic disorder or telomere biology disorder leading to constitutional aplastic anemia is significant, as these conditions are associated not only with marrow failure, but also endocrinopathies, organ fibrosis, and solid organ malignancies.^13-15 In particular, mutations in the TERT and TERC genes have been associated with dyskeratosis congenita as well as pulmonary fibrosis and cirrhosis.^18,19 The implications of early diagnosis of an IMFS lie in the approach to treatment and prognosis.

Clonal Disorders and Secondary Malignancies

Myelodysplastic syndrome (MDS) and secondary acute myeloid leukemia (AML) are 2 clonal disorders that may arise from a background of aplastic anemia.^9,20,21 Hypoplastic MDS can be difficult to differentiate from aplastic anemia at diagnosis based on morphology alone, although recent work has demonstrated that molecular testing for somatic mutations in ASXL1, DNMT3A, and BCOR can aid in differentiating a subset of aplastic anemia patients who are more likely to progress to MDS.²¹ Clonal populations of cells harboring 6p uniparental disomy are seen in more than 10% of patients with aplastic anemia on cytogenetic analysis, which can help differentiate the diseases.⁹ Yoshizato and colleagues found lower rates of ASXL1 and DNMT3A mutations in patients with aplastic anemia as compared with patients with MDS or AML. In this study, patients with aplastic anemia had higher rates of mutations in PIGA (reflecting the increased paroxysmal nocturnal hemoglobinuria [PNH] clonality seen in aplastic anemia) and BCOR.⁹ Mutations were also found in genes commonly mutated in MDS and AML, including TET2, RUNX1, TP53, and JAK2, albeit at lower frequencies.⁹ These mutations as a whole have not predicted response to therapy or prognosis. However, when performing survival analysis in patients with specific mutations, those commonly encountered in MDS/AML (ASXL1, DNMT3A, TP53, RUNX1, CSMD1) are associated with faster progression to overt MDS/AML and decreased overall survival (OS),^20,21 suggesting these mutations may represent early clonality that can lead to clonal evolution and the development of secondary malignancies. Conversely, mutations in BCOR and BCORL appear to identify patients who may have a favorable outcome in response to immunosuppressive therapy and, similar to patients with PIGA mutations, improved OS.⁹

Paroxysmal Nocturnal Hemoglobinuria

In addition to having an increased risk of myelodysplasia and malignancy due to the development of a dominant pre-malignant clone, patients with aplastic anemia often harbor progenitor cell clones associated with PNH.^1,17 PNH clones have been identified in more than 50% of patients with aplastic anemia.^22,23 PNH represents a clonal disorder of hematopoiesis in which cells harbor X-linked somatic mutations in the PIGA gene; this gene encodes a protein responsible for the synthesis of glycosylphosphatidylinositol (GPI) anchors on the cell surface.^22,24 The lack of these cell surface proteins, specifically CD55 (also known as decay accelerating factor) and CD59 (also known as membrane inhibitor of reactive lysis), predisposes red cells to increased complement-mediated lysis.²⁵ The exact mechanism for the development of these clones in patients with aplastic anemia is not fully understood. Current theories hypothesize that these clones are protected from the immune-mediated destruction of normal hematopoietic stem cells due to the absence of the cell surface proteins.^1,20 The role of these clones over time in patients with aplastic anemia is less clear, though recent work demonstrated that despite differences in clonality over the disease course, aplastic anemia patients with small PNH clones are less likely to develop overt hemolysis and larger PNH clones compared to patients harboring larger (≥ 50%) PNH clones at diagnosis.^23,26,27 Additionally, PNH clones in patients with aplastic anemia infrequently become clinically significant.²⁷ It should be noted that these conditions exist along a continuum; that is, patients with aplastic anemia may develop PNH clones, while conversely patients with PNH may develop aplastic anemia.²⁰ Patients with PNH clones should be followed via peripheral blood flow cytometry in addition to complete blood count to track clonal stability and identify clinically significant PNH among aplastic anemia patients.²⁸

Patients with aplastic anemia typically are diagnosed either due to asymptomatic cytopenias found on peripheral blood sampling, symptomatic anemia, bleeding secondary to thrombocytopenia, or wound healing and infectious complications related to neutropenia.²⁹ A thorough history to understand the timing of symptoms, recent infectious symptoms/exposure, habits, and chemical or toxin exposures (including medications, travel, and supplements) helps guide diagnostic testing. Family history is also critical, with attention given to premature graying, pulmonary, renal, and liver disease, and blood disorders.

Patients with an IMFS, (eg, Fanconi anemia or dyskeratosis congenita) may have associated phenotypical findings such as urogenital abnormalities or short stature; in addition, those with dyskeratosis congenita may present with the classic triad of oral leukoplakia, lacy skin pigmentation, and dystrophic nails.⁷ However, in patients with IMFS, classic phenotypical findings may be lacking in up to 30% to 40% of patients.⁷ As described previously, while congenital malformations are common in Fanconi anemia and dyskeratosis congenita, a third of patients may have no or only subtle phenotypical abnormalities, including alterations in skin or hair pigmentation, skeletal and growth abnormalities, and endocrine disorders.³⁰ The International Fanconi Anemia Registry identified central nervous system, genitourinary, skin and musculoskeletal, ophthalmic, and gastrointestinal system malformations among children with Fanconi anemia.^31,32 Patients with dyskeratosis congenita may present with pulmonary fibrosis, hepatic cirrhosis, or premature graying, as highlighted in a recent study by DiNardo and colleagues.³³ Therefore, physicians must have a heightened index of suspicion in patients with subtle phenotypical findings and associated cytopenias.

Diagnosis

Differential Diagnosis

The diagnosis of aplastic anemia should be suspected in any patient presenting with pancytopenia. Aplastic anemia is a diagnosis of exclusion.³⁴ Other conditions associated with peripheral blood pancytopenia should be considered including infections (HIV, hepatitis, parvovirus B19, cytomegalovirus, Epstein-Barr virus, varicella-zoster virus), nutritional deficiencies (vitamin B12, folate, copper, zinc), autoimmune disease (systemic lupus erythematosus, rheumatoid arthritis, hemophagocytic lymphohistiocytosis), hypersplenism, marrow-occupying diseases (eg, leukemia, lymphoma, MDS), solid malignancies, and fibrosis (Table).⁷

Diagnostic Evaluation

The workup for aplastic anemia should include a thorough history and physical exam to search simultaneously for alternative diagnoses and clues pointing to potential etiologic agents.⁷ Diagnostic tests to be performed include a complete blood count with differential, reticulocyte count, immature platelet fraction, flow cytometry (to rule out lymphoproliferative disorders and atypical myeloid cells and to evaluate for PNH), and bone marrow biopsy with subsequent cytogenetic, immunohistochemical, and molecular testing.³⁵ The typical findings in aplastic anemia include peripheral blood pancytopenia without dysplastic features and bone marrow biopsy demonstrating a hypocellular marrow.⁷ A relative lymphocytosis in the peripheral blood is common.⁷ In patients with a significant PNH clone, a macrocytosis along with elevated lactate dehydrogenase and elevated reticulocyte and granulocyte counts may be present.³⁶

The diagnosis (based on the Camitta criteria³⁷ and modified Camitta criteria³⁸ for severe aplastic anemia) requires 2 of the following findings on peripheral blood samples:

• Absolute neutrophil count (ANC) < 500 cells/µL
• Platelet count < 20,000 cells/µL
• Reticulocyte count < 1% corrected or < 20,000 cells/µL.³⁵

In addition to peripheral blood findings, bone marrow biopsy is essential for the diagnosis, and should demonstrate a markedly hypocellular marrow (cellularity < 25%), occasionally with an increase in T lymphocytes.^7,39 Because marrow cellularity varies with age and can be challenging to assess, additional biopsies may be needed to confirm the diagnosis.²⁹ A 1- to 2-cm core biopsy is necessary to confirm hypocellularity, as small areas of residual hematopoiesis may be present and obscure the diagnosis.³⁵

Excluding Hypocellular MDS and IMFS

A diagnostic challenge is the exclusion of hypocellular MDS, especially in the older adult presenting with aplastic anemia, as patients with aplastic anemia may have some degree of erythroid dysplasia on bone marrow morphology.³⁶ The presence of a PNH clone on flow cytometry can aid in diagnosing aplastic anemia and excluding MDS,³⁴ although PNH clones can be present in refractory anemia MDS. Patients with aplastic anemia have a lower ratio of CD34+ cells compared to those with hypoplastic MDS, with one study demonstrating a mean CD34+ percentage of < 0.5% in aplastic anemia versus 3.7% in hypoplastic MDS.⁴⁰ Cytogenetic and molecular testing can also aid in making this distinction by identifying mutations commonly implicated in MDS.⁷ The presence of monosomy 7 (-7) in aplastic anemia patients is associated with a poor overall prognosis.^34,41

Peripheral blood screening using chromosome breakage analysis (done using either mitomycin C or diepoxybutane as in vitro DNA-crosslinking agents) and telomere length testing (of peripheral blood leukocytes) is necessary to exclude the main IMFS, Fanconi anemia and telomere biology disorders, respectively. Ruling out these conditions is imperative, as the approach to treatment varies significantly between IMFS and aplastic anemia. Patients with shortened telomeres should undergo genetic screening for mutations in the telomere maintenance genes to evaluate the underlying defect leading to shortened telomeres. Patients with increased peripheral blood breakage should have genetic testing to detect mutations associated with Fanconi anemia.

Classification

Once the diagnosis of aplastic anemia has been made, the patient should be classified according to the severity of their disease. Disease severity is determined based on peripheral blood ANC:³⁴ non-severe aplastic anemia (NSAA), ANC > 500 polymorphonuclear neutrophils (PMNs)/µL; severe aplastic anemia (SAA), 200–500 PMNs/µL; and very severe (VSAA), 0–200 PMNs/µL.^4,34 Disease classification is important, as VSAA is associated with a decreased OS compared to SAA.² Disease classification may affect treatment decisions, as patients with NSAA may be observed for a short period of time, while conversely patients with SAA have a worse prognosis with delays in therapy.^42-44

Summary

Aplastic anemia is a rare but potentially life-threatening disorder characterized by pancytopenia and a marked reduction in the hematopoietic stem cell compartment. It can be acquired or associated with an IMFS, and the treatment and prognosis vary dramatically between these 2 etiologies. Work-up and diagnosis involves investigating IMFSs and ruling out malignant or infectious etiologies for pancytopenia. After aplastic anemia has been diagnosed, the patient should be classified according to the severity of their disease based on peripheral blood ANC.

INTRODUCTION

Small cell lung cancer (SCLC) is an aggressive cancer of neuroendocrine origin that accounts for approximately 15% of all lung cancer cases, with approximately 33,000 patients diagnosed annually.¹ The incidence of SCLC in the United States has steadily declined over the past 30 years, presumably because of a decrease in the number of smokers and a change to low-tar filter cigarettes.² Although the overall incidence of SCLC has been decreasing, the incidence in women is increasing and the male-to-female incidence ratio is now 1:1.³ Nearly all cases of SCLC are associated with heavy tobacco exposure, making it a heterogeneous disease with a complex genomic landscape consisting of thousands of mutations.^4,5 Despite recent advances in the treatment of non-small cell lung cancer, the therapeutic options for SCLC remain limited, with a median overall survival (OS) of 9 months in patients with advanced disease.

DIAGNOSIS AND STAGING

CASE PRESENTATION

A 61-year-old man presents to the emergency department with progressive shortness of breath and cough over the past 6 weeks. He also reports a 20-lb weight loss over the same period. He is a current smoker and has been smoking 1 pack of cigarettes per day since the age of 18 years. A chest radiograph obtained in the emergency department shows a right hilar mass. Computed tomography (CT) scan confirms the presence of a 4.5-cm right hilar mass and enlarged mediastinal lymph nodes bilaterally.

• What are the next steps in diagnosis?

SCLC is characterized by rapid growth and early hematogenous metastasis. Consequently, only 25% of patients have limited-stage disease at the time of diagnosis. According to the Veterans Administration Lung Study Group (VALSG) staging system, limited-stage disease is defined as tumor that is confined to 1 hemithorax and can be encompassed within 1 radiation field. This typically includes mediastinal lymph nodes and ipsilateral supraclavicular lymph nodes. Approximately 75% of patients present with extensive-stage disease, which is defined as disease that cannot be classified as limited, including disease that extends beyond 1 hemithorax. Extensive-stage disease includes the presence of malignant pleural effusion and/or distant metastasis.⁶ The VALSG classification and staging system is more commonly used in clinical practice than the American Joint Committee on Cancer TNM staging system because it is less complex and directs treatment decisions, as most of the literature on SCLC classifies patients based on the VALSG system.⁷

Given SCLC’s propensity to metastasize quickly, none of the currently available screening methods have proven successful in early detection of SCLC. In the National Lung Cancer Screening Trial, 86% of the 125 patients who were diagnosed with SCLC while undergoing annual low-dose chest CT scans had advanced disease at diagnosis.^8,9 These results highlight the fact that most cases of SCLC develop in the interval between annual screening imaging.

SCLC frequently presents with a large hilar mass that is symptomatic. Common symptoms include shortness of breath and cough. In addition, patients with SCLC usually have bulky mediastinal adenopathy at presentation. SCLC is commonly located submucosally in the bronchus, and therefore hemoptysis is not a very common symptom at the time of presentation. Patients may present with superior vena cava syndrome from local compression by the tumor. Not infrequently, SCLC is associated with paraneoplastic syndromes that arise due to ectopic secretion of hormones or antibodies by the tumor cells. The paraneoplastic syndromes can be broadly categorized as endocrine or neurologic (Table 1). The presence of a paraneoplastic syndrome is often a clue to the potential diagnosis of SCLC in the presence of a hilar mass. Additionally, some paraneoplastic syndromes, more specifically endocrine paraneoplastic syndromes, follow the pattern of disease response and relapse, and therefore can sometimes serve as an early marker of disease relapse or progression.

The common sites of metastases include brain, liver, and bone. Therefore, the staging workup should include fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT scan. Contrast-enhanced CT scan of the chest and abdomen and bone scan can be obtained for staging in lieu of PET scan. Due to the physiologic FDG uptake, cerebral metastases cannot be assessed with sufficient certainty using PET-CT.¹⁰ Therefore, brain imaging with contrast-enhanced CT or magnetic resonance imaging (MRI) is also necessary. Although the incidence of metastasis to bone marrow is less than 10%, bone marrow aspiration and biopsy are warranted in patients with unexplained cytopenias, especially when the cytopenia is associated with teardrop-shaped red cells or nucleated red cells on peripheral blood smear, findings indicative of a marrow infiltrative process.⁷ The tissue diagnosis is established by obtaining a biopsy of the primary tumor or 1 of the metastatic sites. In localized disease, bronchoscopy (with endobronchial ultrasound, if necessary) with biopsy of the centrally located tumor and/or lymph node is required. Histologically, SCLC consists of monomorphic cells, a high nuclear-cytoplasmic ratio, and confluent necrosis. The tumor cells are positive for chromogranin, synaptophysin, and CD56 by immunohistochemistry, and very frequently are also positive for thyroid transcription factor 1.¹¹ Although serum tumor markers, including neuron-specific enolase and progastrin-releasing peptide, are frequently elevated in patients with SCLC, these markers are of limited value in clinical practice because they lack sensitivity and specificity.¹²

MANAGEMENT OF LIMITED-STAGE DISEASE

CASE CONTINUED

The patient undergoes FDG PET scan, which shows the presence of a hypermetabolic right hilar mass in addition to enlarged and hypermetabolic bilateral mediastinal lymph nodes. There are no other areas of FDG avidity. Brain MRI does not show any evidence of brain metastasis. Thus, the patient is confirmed to have limited-stage SCLC.

• What is the standard of care treatment for limited-stage SCLC?

SCLC is exquisitely sensitive to both chemotherapy and radiation, especially at the time of initial presentation. The standard of care treatment of limited-stage SCLC is 4 cycles of platinum-based chemotherapy in combination with thoracic radiation started within the first 2 cycles of chemotherapy (Figure 1).

CHOICE OF CHEMOTHERAPY

Etoposide and cisplatin is the most commonly used initial combination chemotherapy regimen in limited-stage SCLC.¹⁴ This combination has largely replaced anthracycline-based regimens given its favorable efficacy and toxicity profile.^15–17 Several small randomized trials have shown comparable efficacy of carboplatin and etoposide in extensive-stage SCLC.^18–20 A meta-analysis of 4 randomized trials comparing cisplatin-based versus carboplatin-based regimens in 663 patients with SCLC (32% had limited-stage disease and 68% had extensive-stage disease) showed no statistically significant difference in response rate, progression-free survival (PFS), or OS between the 2 regimens.²¹ Therefore, in clinical practice carboplatin is frequently used instead of cisplatin in patients with extensive-stage disease. In patients with limited-stage disease, cisplatin is still the drug of choice. However, the toxicity profile of the 2 regimens is different. Cisplatin-based regimens are more commonly associated with neuropathy, nephrotoxicity, and chemotherapy-induced nausea/vomiting,¹⁸ while carboplatin-based regimens are more myelosuppressive.²² In addition, the combination of thoracic radiation with either of these regimens is associated with a higher risk of esophagitis, pneumonitis, and myelosuppression.²³ The use of myeloid growth factors is not recommended in patients undergoing concurrent chemoradiation.²⁴ Of note, intravenous etoposide is always preferred over oral etoposide, especially in the curative setting given the unreliable absorption and bioavailability of oral formulations.

THORACIC RADIOTHERAPY

Adding thoracic radiotherapy to platinum-etoposide chemotherapy improves local control and OS. Two meta-analyses of 13 trials including more than 2000 patients have shown a 25% to 30% decrease in local failure and a 5% to 7% increase in 2-year OS with chemoradiation compared to chemotherapy alone in limited-stage SCLC.^25,26 Early (within the first 2 cycles) concurrent thoracic radiation is superior to delayed and/or sequential radiation in terms of local control and OS.^23,27,28 The dose and fractionation of thoracic radiation in limited-stage SCLC has remained a controversial issue. The Eastern Cooperative Oncology Group/Radiation Therapy Oncology Group randomized trial compared 45 Gy of radiotherapy delivered twice daily over a period of 3 weeks to 45 Gy once daily over 5 weeks concurrently with chemotherapy. The twice daily regimen led to a 10% improvement in 5-year OS (26% versus 16%), but a higher incidence of grade 3 and 4 adverse events.¹³ Despite the survival advantage demonstrated by hyperfractionated radiotherapy, the results need to be interpreted with caution because the radiation doses are not biologically equivalent. In addition, the difficult logistics of patients receiving radiation twice a day has limited the routine implementation of this strategy. Subsequently, another randomized phase 3 trial (CONVERT) compared 45 Gy radiotherapy twice daily with 66 Gy radiotherapy once daily in limited-stage SCLC.²⁹ This trial did not show any difference in OS. The patients in the twice daily arm had a higher incidence of grade 4 neutropenia. Considering the results of these trials, both strategies—45 Gy fractionated twice daily or 60 Gy fractionated once daily, delivered concurrently with chemotherapy—are acceptable in the setting of limited-stage SCLC. However, quite often a hyperfractionated regimen is not feasible for patients and many radiation oncology centers. Hopefully, the ongoing CALGB 30610 study will clarify the optimal radiation schedule for limited-stage disease.

PROPHYLACTIC CRANIAL IRRADIATION

Approximately 75% of patients with limited-stage disease experience disease recurrence, and brain is the site of recurrence in approximately half of these patients.³⁰ Prophylactic cranial irradiation (PCI) consisting of 25 Gy radiotherapy delivered in 10 fractions has been shown to be effective in decreasing the incidence of cerebral metastases.^30–32 Although individual small studies have not shown a survival benefit of PCI because of small sample size and limited power, a meta-analysis of these studies has shown a 25% decrease in the 3-year incidence of brain metastasis and 5.4% increase in 3-year OS.³⁰ Most patients included in these studies had limited-stage disease. Therefore, PCI is the standard of care for patients with limited-stage disease who attain a partial or complete response to chemoradiation.

ROLE OF SURGERY

Surgical resection may be an acceptable choice in a very limited subset of patients with peripherally located small (< 5 cm) tumors where mediastinal lymph nodes have been confirmed to be uninvolved with complete mediastinal staging.^33,34 Most of the data in this setting are derived from retrospective studies.^35,36 A 5-year OS between 40% and 60% has been reported with this strategy in patients with clinical stage I disease. In general, when surgery is considered, lobectomy with mediastinal lymph node dissection followed by chemotherapy (if there is no nodal involvement) or chemoradiation (if nodal involvement) is recommended.^37,38 Wedge or segmental resections are not considered to be optimal surgical options.

MANAGEMENT OF EXTENSIVE-STAGE DISEASE

CASE CONTINUED

The patient receives 4 cycles of cisplatin and etoposide along with 70 Gy radiotherapy concurrently with the first 2 cycles of chemotherapy. His post-treatment CT scans show a partial response. He undergoes PCI 6 weeks after completion of treatment. At routine follow-up 18 months later, he is doing generally well except for mildly decreased appetite and an unintentional weight loss of 5 lb. CT scans demonstrate multiple hypodense liver lesions ranging from 7 mm to 2 cm in size and a 2-cm left adrenal gland lesion highly concerning for metastasis. FDG PET scan confirms that the adrenal and liver lesions are hypermetabolic. In addition, the PET scan shows multiple FDG-avid bone lesions throughout the spine. Brain MRI is negative for brain metastasis.

• What is the standard of care for treatment of extensive-stage disease?

Chemotherapy is the mainstay of treatment for extensive-stage SCLC; the goals of treatment are prolongation of survival, prevention or alleviation of cancer-related symptoms, and improvement in quality of life. The combination of etoposide with a platinum agent (carboplatin or cisplatin) is the preferred first-line treatment option. Carboplatin is more commonly used in clinical practice in this setting because of its comparable efficacy and better tolerability compared to cisplatin (Figure 2).²¹ A Japanese phase 3 trial comparing cisplatin plus irinotecan with cisplatin plus etoposide in the first-line setting in extensive-stage SCLC showed improvement in median and 2-year OS with the cisplatin/irinotecan regimen; however, 2 subsequent phase 3 trials conducted in the United States comparing these 2 regimens did not show any difference in OS. In addition, the cisplatin/irinotecan regimen was more toxic than the etoposide-based regimen.^39,40 Therefore, 4 to 6 cycles of platinum/etoposide remains the standard of care first-line treatment for extensive-stage SCLC in the United States. The combination yields a 60% to 70% response rate, but the majority of patients invariably experience disease progression, with a median OS of 9 to 11 months.⁴¹ Maintenance chemotherapy beyond the initial 4 to 6 cycles does not improve survival and is associated with higher cumulative toxicity.⁴²

Multiple attempts at improving first-line chemotherapy in extensive-stage disease have failed to show any meaningful difference in OS. For example, the addition of ifosfamide, palifosfamide, cyclophosphamide, taxane, or anthracycline to platinum doublet failed to show improvement in OS and led to more toxicity.^43–46 Additionally, the use of alternating or cyclic chemotherapies in an attempt to curb drug resistance has also failed to show survival benefit.^47–49 The addition of the antiangiogenic agent bevacizumab to standard platinum-based doublet has not prolonged OS in SCLC and has led to an unacceptably higher rate of tracheoesophageal fistula when used in conjunction with chemoradiation in limited-stage disease.^50–55 Finally, the immune checkpoint inhibitor ipilimumab in combination with platinum plus etoposide failed to improve PFS or OS compared to platinum plus etoposide alone in a recent phase 3 trial, and maintenance pembrolizumab after completion of platinum-based chemotherapy did not improve PFS.^56,57

More recently, a phase 2 study of pembrolizumab in extensive-stage SCLC (KEYNOTE 158) reported an overall response rate of 35.7%, median PFS of 2.1 months, and median OS of 14.6 months in patients who tested positive for programmed death ligand-1 (PD-L1) expression (which was defined as a PD-L1 Combined Positive Score ≥ 1).⁵⁸ The median duration of response has not been reached in this study, indicating that pembrolizumab may be a promising approach in patients with extensive-stage SCLC, especially for those with PD-L1–positive tumors.

Patients with extensive-stage disease who have brain metastasis at the time of diagnosis can be treated with systemic chemotherapy first if the brain metastases are asymptomatic and there is significant extracranial disease burden. In that case, whole brain radiotherapy should be given after completion of systemic therapy.

SECOND-LINE CHEMOTHERAPY

Despite being exquisitely chemosensitive, SCLC is associated with a very poor prognosis largely because of invariable disease progression following first-line therapy and lack of effective second-line treatment options that can lead to appreciable disease control. The choice of second-line treatment is predominantly determined by the time of disease relapse after first-line platinum-based therapy. If this interval is 6 months or longer, re-treatment utilizing the same platinum doublet is appropriate. However, if the interval is 6 months or less, second-line systemic therapy options should be explored. Unfortunately, the response rate tends to be less than 10% with most of the second-line therapies in platinum-resistant disease (defined as disease progression within 3 months of receiving platinum-based therapy). If disease progression occurs between 3 and 6 months after completion of platinum-based therapy, the response rate with second-line chemotherapy is in the range of 25%.^59,60

A number of second-line chemotherapy options have been explored in small studies, including topotecan, irinotecan, paclitaxel, docetaxel, temozolomide, vinorelbine, oral etoposide, gemcitabine, bendamustine, and CAV (cyclophosphamide, doxorubicin, vincristine) (Table 2).^61–77 Of these, topotecan (oral or intravenous) is the only second-line agent approved by the US Food and Drug Administration (FDA) for extensive-stage SCLC; this approval is based on a phase 3 trial showing a survival benefit with topotecan compared to best supportive care. The duration of second-line chemotherapy is not defined and is largely driven by duration of response and occurrence of side effects. Given the lack of effective treatment options, patients with platinum-resistant disease should be considered for clinical trials whenever possible. In patients with disease progression after second-line therapy, a third-line treatment can be considered if their Eastern Cooperative Oncology Group (ECOG) performance status is 0 to 2, but it is unclear whether further treatments would improve survival.

IMMUNOTHERAPY

The role of immune checkpoint inhibitors in the treatment of SCLC is evolving, and currently there are no FDA-approved immunotherapy agents for treating SCLC. A recently conducted phase 1/2 trial (CheckMate 032) studied the anti-programmed death(PD)-1 antibody nivolumab with or without the anti-cytotoxic T-lymphocyte–associated antigen (CTLA) -4 antibody ipilimumab in patients with relapsed SCLC. The authors reported response rates of 10% with nivolumab 3 mg/kg and 21% with nivolumab 1 mg/kg plus ipilimumab 3 mg/kg.^78,79 The 2-year OS was 26% with the combination and 14% with single-agent nivolumab. Only 18% of patients had PD-L1 expression of ≥ 1%, and the response rate did not correlate with PD-L1 status. The rate of grade 3 or 4 adverse events was approximately 20%, and only 10% of patients discontinued treatment because of toxicity. Based on these data, nivolumab plus ipilimumab is now included in the National Comprehensive Cancer Network guidelines as an option for patients with SCLC who experience disease relapse within 6 months of receiving platinum-based therapy;⁷ however, it is questionable whether routine use of this combination is justified based on currently available data. The evidence for the combination of nivolumab and ipilimumab remains limited. The efficacy and toxicity data from both randomized and nonrandomized cohorts were presented together, making it hard to interpret the results.

Another phase 1b study (KEYNOTE-028) evaluated the anti-PD-1 antibody pembrolizumab (10 mg/kg intravenously every 2 weeks) in patients with relapsed SCLC who had received 1 or more prior lines of therapy and had PD-L1 expression of ≥ 1%. This study showed a response rate of 33%, with a median duration of response of 19 months and 1-year OS of 38%.⁸⁰ Although only 28% of screened patients had PD-L1 expression of ≥ 1%, these results indicated that at least a subset of SCLC patients are able to achieve durable responses with immune checkpoint inhibition. A number of clinical trials utilizing immune checkpoint inhibitors in various combinations and settings are currently underway.

ROLE OF PROPHYLACTIC CRANIAL IRRADIATION

The role of PCI in extensive-stage SCLC is not clearly defined. A randomized phase 3 trial conducted by the European Organization for Research and Treatment of Cancer (EORTC) comparing PCI with no PCI in patients with extensive-stage SCLC who had a partial or complete response to initial platinum-based chemotherapy showed a decrease in the incidence of symptomatic brain metastasis and improvement in 1-year OS with PCI.⁸¹ However, this trial did not require mandatory brain imaging prior to PCI, and thus it is unclear if some patients in the PCI group had asymptomatic brain metastasis prior to enrollment and therefore received therapeutic benefit from brain radiation. Additionally, the dose and fractionation of PCI was not standardized across patient groups.

A more recent phase 3 study conducted in Japan that compared PCI (25 Gy in 10 fractions) with no PCI reported no difference in survival between the 2 groups.⁸² As opposed to the EORTC study, the Japanese study did require baseline brain imaging to confirm the absence of brain metastasis prior to enrollment. In addition, the control patients underwent periodic brain MRI to allow early detection of brain metastasis. Given the emergence of the new data, the impact of PCI on survival in patients with extensive-stage SCLC is unproven, and PCI likely has a role in a highly selected small group of patients with extensive-stage SCLC. PCI is not recommended for patients with poor performance status (ECOG performance score of 3 or 4) or underlying neurocognitive disorders.^34,83

The NMDA-receptor antagonist memantine can be used in patients undergoing PCI to delay the occurrence of cognitive dysfunction.⁶¹ Memantine 20 mg daily delayed time to cognitive decline and reduced the rate of decline in memory, executive function, and processing speed compared to placebo in patients receiving whole brain radiotherapy.⁸⁴

ROLE OF RADIOTHERAPY

A subset of patients with extensive-stage SCLC may benefit from consolidative thoracic radiotherapy after completion of platinum-based chemotherapy. A randomized trial that enrolled patients who achieved complete or near complete response after 3 cycles of cisplatin plus etoposide compared thoracic radiotherapy in combination with continued chemotherapy versus chemotherapy alone.⁸⁵ The median OS was longer with the addition of thoracic radiotherapy compared to chemotherapy alone. Another phase 3 trial did not show improvement in 1-year OS with consolidative thoracic radiotherapy, but 2-year OS and 6-month PFS were longer.⁸⁶ In general, consolidative thoracic radiotherapy benefits patients who have residual thoracic disease and low-bulk extrathoracic disease that has responded to systemic therapy.⁸⁷ In addition, patients who initially presented with bulky symptomatic thoracic disease should also be considered for consolidative radiation.

Similar to other solid tumors, radiotherapy should be utilized for palliative purposes in patients with painful bone metastasis, spinal cord compression, or brain metastasis. Surgery is generally not recommended for spinal cord compression given the short life expectancy of patients with extensive-stage disease. Whole brain radiotherapy is preferred over stereotactic radiosurgery because micrometastasis is frequently present even in the setting of 1 or 2 radiographically evident brain metastasis.

NOVEL THERAPIES

The very complex genetic landscape of SCLC accounts for its resistance to conventional therapy and high recurrence rate; however, at the same time this complexity can form the basis for effective targeted therapy for the disease. One of the major factors hindering the development of targeted therapies in SCLC is limited availability of tissue due to small tissue samples and the frequent presence of significant necrosis in the samples. In recent years, several different therapeutic strategies and targeted agents have been investigated for their potential role in SCLC. Several of them, including EGFR tyrosine kinase inhibitors (TKIs), BCR-ABL TKIs, mTOR inhibitors, and VEGF inhibitors, have not been shown to provide a survival advantage in this disease. Several others, including PARP inhibitors, cellular developmental pathway inhibitors, and antibody-drug conjugates, are being tested. A phase 1 study of veliparib combined with cisplatin and etoposide in patients with previously untreated extensive-stage SCLC demonstrated a complete response in 14.3%, a partial response in 57.1%, and stable disease in 28.6% of patients with an acceptable safety profile.⁸⁸ So far, none of these agents are approved for use in SCLC, and the majority are in early- phase clinical trials.⁸⁹

One of the emerging targets in the treatment of SCLC is delta-like protein 3 (DLL3). DLL3 is expressed on more than 80% of SCLC tumor cells and cancer stem cells. Rovalpituzumab tesirine is an antibody-drug conjugate consisting of humanized anti-DLL3 monoclonal antibody linked to SC-DR002, a DNA-crosslinking agent. A phase 1 trial of rovalpituzumab in patients with relapsed SCLC after 1 or 2 prior lines of therapy reported a response rate of 31% in patients with DLL3 expression of ≥ 50%. The median duration of response and median PFS were both 4.6 months.⁹⁰ Rovalpituzumab is currently in later phases of clinical trials and has a potential to serve as an option for patients with extensive-stage disease after disease progression on platinum-based therapy.

SUMMARY

Four to 6 cycles of carboplatin and etoposide remain the standard of care first-line treatment for patients with extensive stage SCLC. The only FDA-approved second-line treatment option is topotecan. Re-treatment with the original platinum doublet is a reasonable option for patients who have disease progression 6 months or longer after completion of platinum-based therapy. The immune checkpoint inhibitors pembrolizumab and combination nivolumab and ipilimumab have shown promising results in the second-line setting and beyond. The role of PCI has become more controversial in recent years, and periodic brain MRI in lieu of PCI is now an acceptable approach.

For patients undergoing treatment for limited-stage SCLC, response assessment with contrast-enhanced CT of the chest/abdomen should be performed after completion of 4 cycles of chemotherapy and thoracic radiation.⁷ The surveillance guidelines consist of history, physical exam, and imaging every 3 months during the first 2 years, every 6 months during the third year, and annually thereafter. If PCI is not performed, brain MRI or contrast-enhanced CT scan should be performed every 3 or 4 months during the first 2 years of follow up. For extensive-stage disease, response assessment should be performed after every 2 cycles of therapy. After completion of therapy, history, physical exam, and imaging should be done every 2 months during the first year, every 3 or 4 months during years 2 and 3, every 6 months during years 4 and 5, and annually thereafter. Routine use of PET scan for surveillance is not recommended. Any new pulmonary nodule should prompt evaluation for a second primary lung malignancy. Finally, smoking cessation counseling is an integral part of management of any patient with SCLC and should be included with every clinic visit.

CONCLUSION

SCLC is a heterogeneous and genetically complex disease with a very high mortality rate. The current standard of care includes concurrent chemoradiation with cisplatin and etoposide for limited-stage SCLC and the combination of platinum and etoposide for extensive SCLC. A number of novel treatment approaches, including immune checkpoint inhibitors and antibody-drug conjugates, have had promising results in early clinical trials. Given the limited treatment options and large unmet need for new treatment options, enrollment in clinical trials is strongly recommended for patients with SCLC.

INTRODUCTION

Small cell lung cancer (SCLC) is an aggressive cancer of neuroendocrine origin that accounts for approximately 15% of all lung cancer cases, with approximately 33,000 patients diagnosed annually.¹ The incidence of SCLC in the United States has steadily declined over the past 30 years, presumably because of a decrease in the number of smokers and a change to low-tar filter cigarettes.² Although the overall incidence of SCLC has been decreasing, the incidence in women is increasing and the male-to-female incidence ratio is now 1:1.³ Nearly all cases of SCLC are associated with heavy tobacco exposure, making it a heterogeneous disease with a complex genomic landscape consisting of thousands of mutations.^4,5 Despite recent advances in the treatment of non-small cell lung cancer, the therapeutic options for SCLC remain limited, with a median overall survival (OS) of 9 months in patients with advanced disease.

DIAGNOSIS AND STAGING

CASE PRESENTATION

A 61-year-old man presents to the emergency department with progressive shortness of breath and cough over the past 6 weeks. He also reports a 20-lb weight loss over the same period. He is a current smoker and has been smoking 1 pack of cigarettes per day since the age of 18 years. A chest radiograph obtained in the emergency department shows a right hilar mass. Computed tomography (CT) scan confirms the presence of a 4.5-cm right hilar mass and enlarged mediastinal lymph nodes bilaterally.

• What are the next steps in diagnosis?

SCLC is characterized by rapid growth and early hematogenous metastasis. Consequently, only 25% of patients have limited-stage disease at the time of diagnosis. According to the Veterans Administration Lung Study Group (VALSG) staging system, limited-stage disease is defined as tumor that is confined to 1 hemithorax and can be encompassed within 1 radiation field. This typically includes mediastinal lymph nodes and ipsilateral supraclavicular lymph nodes. Approximately 75% of patients present with extensive-stage disease, which is defined as disease that cannot be classified as limited, including disease that extends beyond 1 hemithorax. Extensive-stage disease includes the presence of malignant pleural effusion and/or distant metastasis.⁶ The VALSG classification and staging system is more commonly used in clinical practice than the American Joint Committee on Cancer TNM staging system because it is less complex and directs treatment decisions, as most of the literature on SCLC classifies patients based on the VALSG system.⁷

Given SCLC’s propensity to metastasize quickly, none of the currently available screening methods have proven successful in early detection of SCLC. In the National Lung Cancer Screening Trial, 86% of the 125 patients who were diagnosed with SCLC while undergoing annual low-dose chest CT scans had advanced disease at diagnosis.^8,9 These results highlight the fact that most cases of SCLC develop in the interval between annual screening imaging.

SCLC frequently presents with a large hilar mass that is symptomatic. Common symptoms include shortness of breath and cough. In addition, patients with SCLC usually have bulky mediastinal adenopathy at presentation. SCLC is commonly located submucosally in the bronchus, and therefore hemoptysis is not a very common symptom at the time of presentation. Patients may present with superior vena cava syndrome from local compression by the tumor. Not infrequently, SCLC is associated with paraneoplastic syndromes that arise due to ectopic secretion of hormones or antibodies by the tumor cells. The paraneoplastic syndromes can be broadly categorized as endocrine or neurologic (Table 1). The presence of a paraneoplastic syndrome is often a clue to the potential diagnosis of SCLC in the presence of a hilar mass. Additionally, some paraneoplastic syndromes, more specifically endocrine paraneoplastic syndromes, follow the pattern of disease response and relapse, and therefore can sometimes serve as an early marker of disease relapse or progression.

The common sites of metastases include brain, liver, and bone. Therefore, the staging workup should include fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT scan. Contrast-enhanced CT scan of the chest and abdomen and bone scan can be obtained for staging in lieu of PET scan. Due to the physiologic FDG uptake, cerebral metastases cannot be assessed with sufficient certainty using PET-CT.¹⁰ Therefore, brain imaging with contrast-enhanced CT or magnetic resonance imaging (MRI) is also necessary. Although the incidence of metastasis to bone marrow is less than 10%, bone marrow aspiration and biopsy are warranted in patients with unexplained cytopenias, especially when the cytopenia is associated with teardrop-shaped red cells or nucleated red cells on peripheral blood smear, findings indicative of a marrow infiltrative process.⁷ The tissue diagnosis is established by obtaining a biopsy of the primary tumor or 1 of the metastatic sites. In localized disease, bronchoscopy (with endobronchial ultrasound, if necessary) with biopsy of the centrally located tumor and/or lymph node is required. Histologically, SCLC consists of monomorphic cells, a high nuclear-cytoplasmic ratio, and confluent necrosis. The tumor cells are positive for chromogranin, synaptophysin, and CD56 by immunohistochemistry, and very frequently are also positive for thyroid transcription factor 1.¹¹ Although serum tumor markers, including neuron-specific enolase and progastrin-releasing peptide, are frequently elevated in patients with SCLC, these markers are of limited value in clinical practice because they lack sensitivity and specificity.¹²

MANAGEMENT OF LIMITED-STAGE DISEASE

CASE CONTINUED

The patient undergoes FDG PET scan, which shows the presence of a hypermetabolic right hilar mass in addition to enlarged and hypermetabolic bilateral mediastinal lymph nodes. There are no other areas of FDG avidity. Brain MRI does not show any evidence of brain metastasis. Thus, the patient is confirmed to have limited-stage SCLC.

• What is the standard of care treatment for limited-stage SCLC?

SCLC is exquisitely sensitive to both chemotherapy and radiation, especially at the time of initial presentation. The standard of care treatment of limited-stage SCLC is 4 cycles of platinum-based chemotherapy in combination with thoracic radiation started within the first 2 cycles of chemotherapy (Figure 1).

CHOICE OF CHEMOTHERAPY

Etoposide and cisplatin is the most commonly used initial combination chemotherapy regimen in limited-stage SCLC.¹⁴ This combination has largely replaced anthracycline-based regimens given its favorable efficacy and toxicity profile.^15–17 Several small randomized trials have shown comparable efficacy of carboplatin and etoposide in extensive-stage SCLC.^18–20 A meta-analysis of 4 randomized trials comparing cisplatin-based versus carboplatin-based regimens in 663 patients with SCLC (32% had limited-stage disease and 68% had extensive-stage disease) showed no statistically significant difference in response rate, progression-free survival (PFS), or OS between the 2 regimens.²¹ Therefore, in clinical practice carboplatin is frequently used instead of cisplatin in patients with extensive-stage disease. In patients with limited-stage disease, cisplatin is still the drug of choice. However, the toxicity profile of the 2 regimens is different. Cisplatin-based regimens are more commonly associated with neuropathy, nephrotoxicity, and chemotherapy-induced nausea/vomiting,¹⁸ while carboplatin-based regimens are more myelosuppressive.²² In addition, the combination of thoracic radiation with either of these regimens is associated with a higher risk of esophagitis, pneumonitis, and myelosuppression.²³ The use of myeloid growth factors is not recommended in patients undergoing concurrent chemoradiation.²⁴ Of note, intravenous etoposide is always preferred over oral etoposide, especially in the curative setting given the unreliable absorption and bioavailability of oral formulations.

THORACIC RADIOTHERAPY

Adding thoracic radiotherapy to platinum-etoposide chemotherapy improves local control and OS. Two meta-analyses of 13 trials including more than 2000 patients have shown a 25% to 30% decrease in local failure and a 5% to 7% increase in 2-year OS with chemoradiation compared to chemotherapy alone in limited-stage SCLC.^25,26 Early (within the first 2 cycles) concurrent thoracic radiation is superior to delayed and/or sequential radiation in terms of local control and OS.^23,27,28 The dose and fractionation of thoracic radiation in limited-stage SCLC has remained a controversial issue. The Eastern Cooperative Oncology Group/Radiation Therapy Oncology Group randomized trial compared 45 Gy of radiotherapy delivered twice daily over a period of 3 weeks to 45 Gy once daily over 5 weeks concurrently with chemotherapy. The twice daily regimen led to a 10% improvement in 5-year OS (26% versus 16%), but a higher incidence of grade 3 and 4 adverse events.¹³ Despite the survival advantage demonstrated by hyperfractionated radiotherapy, the results need to be interpreted with caution because the radiation doses are not biologically equivalent. In addition, the difficult logistics of patients receiving radiation twice a day has limited the routine implementation of this strategy. Subsequently, another randomized phase 3 trial (CONVERT) compared 45 Gy radiotherapy twice daily with 66 Gy radiotherapy once daily in limited-stage SCLC.²⁹ This trial did not show any difference in OS. The patients in the twice daily arm had a higher incidence of grade 4 neutropenia. Considering the results of these trials, both strategies—45 Gy fractionated twice daily or 60 Gy fractionated once daily, delivered concurrently with chemotherapy—are acceptable in the setting of limited-stage SCLC. However, quite often a hyperfractionated regimen is not feasible for patients and many radiation oncology centers. Hopefully, the ongoing CALGB 30610 study will clarify the optimal radiation schedule for limited-stage disease.

PROPHYLACTIC CRANIAL IRRADIATION

Approximately 75% of patients with limited-stage disease experience disease recurrence, and brain is the site of recurrence in approximately half of these patients.³⁰ Prophylactic cranial irradiation (PCI) consisting of 25 Gy radiotherapy delivered in 10 fractions has been shown to be effective in decreasing the incidence of cerebral metastases.^30–32 Although individual small studies have not shown a survival benefit of PCI because of small sample size and limited power, a meta-analysis of these studies has shown a 25% decrease in the 3-year incidence of brain metastasis and 5.4% increase in 3-year OS.³⁰ Most patients included in these studies had limited-stage disease. Therefore, PCI is the standard of care for patients with limited-stage disease who attain a partial or complete response to chemoradiation.

ROLE OF SURGERY

Surgical resection may be an acceptable choice in a very limited subset of patients with peripherally located small (< 5 cm) tumors where mediastinal lymph nodes have been confirmed to be uninvolved with complete mediastinal staging.^33,34 Most of the data in this setting are derived from retrospective studies.^35,36 A 5-year OS between 40% and 60% has been reported with this strategy in patients with clinical stage I disease. In general, when surgery is considered, lobectomy with mediastinal lymph node dissection followed by chemotherapy (if there is no nodal involvement) or chemoradiation (if nodal involvement) is recommended.^37,38 Wedge or segmental resections are not considered to be optimal surgical options.

MANAGEMENT OF EXTENSIVE-STAGE DISEASE

CASE CONTINUED

The patient receives 4 cycles of cisplatin and etoposide along with 70 Gy radiotherapy concurrently with the first 2 cycles of chemotherapy. His post-treatment CT scans show a partial response. He undergoes PCI 6 weeks after completion of treatment. At routine follow-up 18 months later, he is doing generally well except for mildly decreased appetite and an unintentional weight loss of 5 lb. CT scans demonstrate multiple hypodense liver lesions ranging from 7 mm to 2 cm in size and a 2-cm left adrenal gland lesion highly concerning for metastasis. FDG PET scan confirms that the adrenal and liver lesions are hypermetabolic. In addition, the PET scan shows multiple FDG-avid bone lesions throughout the spine. Brain MRI is negative for brain metastasis.

• What is the standard of care for treatment of extensive-stage disease?

Chemotherapy is the mainstay of treatment for extensive-stage SCLC; the goals of treatment are prolongation of survival, prevention or alleviation of cancer-related symptoms, and improvement in quality of life. The combination of etoposide with a platinum agent (carboplatin or cisplatin) is the preferred first-line treatment option. Carboplatin is more commonly used in clinical practice in this setting because of its comparable efficacy and better tolerability compared to cisplatin (Figure 2).²¹ A Japanese phase 3 trial comparing cisplatin plus irinotecan with cisplatin plus etoposide in the first-line setting in extensive-stage SCLC showed improvement in median and 2-year OS with the cisplatin/irinotecan regimen; however, 2 subsequent phase 3 trials conducted in the United States comparing these 2 regimens did not show any difference in OS. In addition, the cisplatin/irinotecan regimen was more toxic than the etoposide-based regimen.^39,40 Therefore, 4 to 6 cycles of platinum/etoposide remains the standard of care first-line treatment for extensive-stage SCLC in the United States. The combination yields a 60% to 70% response rate, but the majority of patients invariably experience disease progression, with a median OS of 9 to 11 months.⁴¹ Maintenance chemotherapy beyond the initial 4 to 6 cycles does not improve survival and is associated with higher cumulative toxicity.⁴²

Multiple attempts at improving first-line chemotherapy in extensive-stage disease have failed to show any meaningful difference in OS. For example, the addition of ifosfamide, palifosfamide, cyclophosphamide, taxane, or anthracycline to platinum doublet failed to show improvement in OS and led to more toxicity.^43–46 Additionally, the use of alternating or cyclic chemotherapies in an attempt to curb drug resistance has also failed to show survival benefit.^47–49 The addition of the antiangiogenic agent bevacizumab to standard platinum-based doublet has not prolonged OS in SCLC and has led to an unacceptably higher rate of tracheoesophageal fistula when used in conjunction with chemoradiation in limited-stage disease.^50–55 Finally, the immune checkpoint inhibitor ipilimumab in combination with platinum plus etoposide failed to improve PFS or OS compared to platinum plus etoposide alone in a recent phase 3 trial, and maintenance pembrolizumab after completion of platinum-based chemotherapy did not improve PFS.^56,57

More recently, a phase 2 study of pembrolizumab in extensive-stage SCLC (KEYNOTE 158) reported an overall response rate of 35.7%, median PFS of 2.1 months, and median OS of 14.6 months in patients who tested positive for programmed death ligand-1 (PD-L1) expression (which was defined as a PD-L1 Combined Positive Score ≥ 1).⁵⁸ The median duration of response has not been reached in this study, indicating that pembrolizumab may be a promising approach in patients with extensive-stage SCLC, especially for those with PD-L1–positive tumors.

Patients with extensive-stage disease who have brain metastasis at the time of diagnosis can be treated with systemic chemotherapy first if the brain metastases are asymptomatic and there is significant extracranial disease burden. In that case, whole brain radiotherapy should be given after completion of systemic therapy.

SECOND-LINE CHEMOTHERAPY

Despite being exquisitely chemosensitive, SCLC is associated with a very poor prognosis largely because of invariable disease progression following first-line therapy and lack of effective second-line treatment options that can lead to appreciable disease control. The choice of second-line treatment is predominantly determined by the time of disease relapse after first-line platinum-based therapy. If this interval is 6 months or longer, re-treatment utilizing the same platinum doublet is appropriate. However, if the interval is 6 months or less, second-line systemic therapy options should be explored. Unfortunately, the response rate tends to be less than 10% with most of the second-line therapies in platinum-resistant disease (defined as disease progression within 3 months of receiving platinum-based therapy). If disease progression occurs between 3 and 6 months after completion of platinum-based therapy, the response rate with second-line chemotherapy is in the range of 25%.^59,60

A number of second-line chemotherapy options have been explored in small studies, including topotecan, irinotecan, paclitaxel, docetaxel, temozolomide, vinorelbine, oral etoposide, gemcitabine, bendamustine, and CAV (cyclophosphamide, doxorubicin, vincristine) (Table 2).^61–77 Of these, topotecan (oral or intravenous) is the only second-line agent approved by the US Food and Drug Administration (FDA) for extensive-stage SCLC; this approval is based on a phase 3 trial showing a survival benefit with topotecan compared to best supportive care. The duration of second-line chemotherapy is not defined and is largely driven by duration of response and occurrence of side effects. Given the lack of effective treatment options, patients with platinum-resistant disease should be considered for clinical trials whenever possible. In patients with disease progression after second-line therapy, a third-line treatment can be considered if their Eastern Cooperative Oncology Group (ECOG) performance status is 0 to 2, but it is unclear whether further treatments would improve survival.

IMMUNOTHERAPY

The role of immune checkpoint inhibitors in the treatment of SCLC is evolving, and currently there are no FDA-approved immunotherapy agents for treating SCLC. A recently conducted phase 1/2 trial (CheckMate 032) studied the anti-programmed death(PD)-1 antibody nivolumab with or without the anti-cytotoxic T-lymphocyte–associated antigen (CTLA) -4 antibody ipilimumab in patients with relapsed SCLC. The authors reported response rates of 10% with nivolumab 3 mg/kg and 21% with nivolumab 1 mg/kg plus ipilimumab 3 mg/kg.^78,79 The 2-year OS was 26% with the combination and 14% with single-agent nivolumab. Only 18% of patients had PD-L1 expression of ≥ 1%, and the response rate did not correlate with PD-L1 status. The rate of grade 3 or 4 adverse events was approximately 20%, and only 10% of patients discontinued treatment because of toxicity. Based on these data, nivolumab plus ipilimumab is now included in the National Comprehensive Cancer Network guidelines as an option for patients with SCLC who experience disease relapse within 6 months of receiving platinum-based therapy;⁷ however, it is questionable whether routine use of this combination is justified based on currently available data. The evidence for the combination of nivolumab and ipilimumab remains limited. The efficacy and toxicity data from both randomized and nonrandomized cohorts were presented together, making it hard to interpret the results.

Another phase 1b study (KEYNOTE-028) evaluated the anti-PD-1 antibody pembrolizumab (10 mg/kg intravenously every 2 weeks) in patients with relapsed SCLC who had received 1 or more prior lines of therapy and had PD-L1 expression of ≥ 1%. This study showed a response rate of 33%, with a median duration of response of 19 months and 1-year OS of 38%.⁸⁰ Although only 28% of screened patients had PD-L1 expression of ≥ 1%, these results indicated that at least a subset of SCLC patients are able to achieve durable responses with immune checkpoint inhibition. A number of clinical trials utilizing immune checkpoint inhibitors in various combinations and settings are currently underway.

ROLE OF PROPHYLACTIC CRANIAL IRRADIATION

The role of PCI in extensive-stage SCLC is not clearly defined. A randomized phase 3 trial conducted by the European Organization for Research and Treatment of Cancer (EORTC) comparing PCI with no PCI in patients with extensive-stage SCLC who had a partial or complete response to initial platinum-based chemotherapy showed a decrease in the incidence of symptomatic brain metastasis and improvement in 1-year OS with PCI.⁸¹ However, this trial did not require mandatory brain imaging prior to PCI, and thus it is unclear if some patients in the PCI group had asymptomatic brain metastasis prior to enrollment and therefore received therapeutic benefit from brain radiation. Additionally, the dose and fractionation of PCI was not standardized across patient groups.

A more recent phase 3 study conducted in Japan that compared PCI (25 Gy in 10 fractions) with no PCI reported no difference in survival between the 2 groups.⁸² As opposed to the EORTC study, the Japanese study did require baseline brain imaging to confirm the absence of brain metastasis prior to enrollment. In addition, the control patients underwent periodic brain MRI to allow early detection of brain metastasis. Given the emergence of the new data, the impact of PCI on survival in patients with extensive-stage SCLC is unproven, and PCI likely has a role in a highly selected small group of patients with extensive-stage SCLC. PCI is not recommended for patients with poor performance status (ECOG performance score of 3 or 4) or underlying neurocognitive disorders.^34,83

The NMDA-receptor antagonist memantine can be used in patients undergoing PCI to delay the occurrence of cognitive dysfunction.⁶¹ Memantine 20 mg daily delayed time to cognitive decline and reduced the rate of decline in memory, executive function, and processing speed compared to placebo in patients receiving whole brain radiotherapy.⁸⁴

ROLE OF RADIOTHERAPY

A subset of patients with extensive-stage SCLC may benefit from consolidative thoracic radiotherapy after completion of platinum-based chemotherapy. A randomized trial that enrolled patients who achieved complete or near complete response after 3 cycles of cisplatin plus etoposide compared thoracic radiotherapy in combination with continued chemotherapy versus chemotherapy alone.⁸⁵ The median OS was longer with the addition of thoracic radiotherapy compared to chemotherapy alone. Another phase 3 trial did not show improvement in 1-year OS with consolidative thoracic radiotherapy, but 2-year OS and 6-month PFS were longer.⁸⁶ In general, consolidative thoracic radiotherapy benefits patients who have residual thoracic disease and low-bulk extrathoracic disease that has responded to systemic therapy.⁸⁷ In addition, patients who initially presented with bulky symptomatic thoracic disease should also be considered for consolidative radiation.

Similar to other solid tumors, radiotherapy should be utilized for palliative purposes in patients with painful bone metastasis, spinal cord compression, or brain metastasis. Surgery is generally not recommended for spinal cord compression given the short life expectancy of patients with extensive-stage disease. Whole brain radiotherapy is preferred over stereotactic radiosurgery because micrometastasis is frequently present even in the setting of 1 or 2 radiographically evident brain metastasis.

NOVEL THERAPIES

The very complex genetic landscape of SCLC accounts for its resistance to conventional therapy and high recurrence rate; however, at the same time this complexity can form the basis for effective targeted therapy for the disease. One of the major factors hindering the development of targeted therapies in SCLC is limited availability of tissue due to small tissue samples and the frequent presence of significant necrosis in the samples. In recent years, several different therapeutic strategies and targeted agents have been investigated for their potential role in SCLC. Several of them, including EGFR tyrosine kinase inhibitors (TKIs), BCR-ABL TKIs, mTOR inhibitors, and VEGF inhibitors, have not been shown to provide a survival advantage in this disease. Several others, including PARP inhibitors, cellular developmental pathway inhibitors, and antibody-drug conjugates, are being tested. A phase 1 study of veliparib combined with cisplatin and etoposide in patients with previously untreated extensive-stage SCLC demonstrated a complete response in 14.3%, a partial response in 57.1%, and stable disease in 28.6% of patients with an acceptable safety profile.⁸⁸ So far, none of these agents are approved for use in SCLC, and the majority are in early- phase clinical trials.⁸⁹

One of the emerging targets in the treatment of SCLC is delta-like protein 3 (DLL3). DLL3 is expressed on more than 80% of SCLC tumor cells and cancer stem cells. Rovalpituzumab tesirine is an antibody-drug conjugate consisting of humanized anti-DLL3 monoclonal antibody linked to SC-DR002, a DNA-crosslinking agent. A phase 1 trial of rovalpituzumab in patients with relapsed SCLC after 1 or 2 prior lines of therapy reported a response rate of 31% in patients with DLL3 expression of ≥ 50%. The median duration of response and median PFS were both 4.6 months.⁹⁰ Rovalpituzumab is currently in later phases of clinical trials and has a potential to serve as an option for patients with extensive-stage disease after disease progression on platinum-based therapy.

SUMMARY

Four to 6 cycles of carboplatin and etoposide remain the standard of care first-line treatment for patients with extensive stage SCLC. The only FDA-approved second-line treatment option is topotecan. Re-treatment with the original platinum doublet is a reasonable option for patients who have disease progression 6 months or longer after completion of platinum-based therapy. The immune checkpoint inhibitors pembrolizumab and combination nivolumab and ipilimumab have shown promising results in the second-line setting and beyond. The role of PCI has become more controversial in recent years, and periodic brain MRI in lieu of PCI is now an acceptable approach.

For patients undergoing treatment for limited-stage SCLC, response assessment with contrast-enhanced CT of the chest/abdomen should be performed after completion of 4 cycles of chemotherapy and thoracic radiation.⁷ The surveillance guidelines consist of history, physical exam, and imaging every 3 months during the first 2 years, every 6 months during the third year, and annually thereafter. If PCI is not performed, brain MRI or contrast-enhanced CT scan should be performed every 3 or 4 months during the first 2 years of follow up. For extensive-stage disease, response assessment should be performed after every 2 cycles of therapy. After completion of therapy, history, physical exam, and imaging should be done every 2 months during the first year, every 3 or 4 months during years 2 and 3, every 6 months during years 4 and 5, and annually thereafter. Routine use of PET scan for surveillance is not recommended. Any new pulmonary nodule should prompt evaluation for a second primary lung malignancy. Finally, smoking cessation counseling is an integral part of management of any patient with SCLC and should be included with every clinic visit.

CONCLUSION

SCLC is a heterogeneous and genetically complex disease with a very high mortality rate. The current standard of care includes concurrent chemoradiation with cisplatin and etoposide for limited-stage SCLC and the combination of platinum and etoposide for extensive SCLC. A number of novel treatment approaches, including immune checkpoint inhibitors and antibody-drug conjugates, have had promising results in early clinical trials. Given the limited treatment options and large unmet need for new treatment options, enrollment in clinical trials is strongly recommended for patients with SCLC.

INTRODUCTION

Small cell lung cancer (SCLC) is an aggressive cancer of neuroendocrine origin that accounts for approximately 15% of all lung cancer cases, with approximately 33,000 patients diagnosed annually.¹ The incidence of SCLC in the United States has steadily declined over the past 30 years, presumably because of a decrease in the number of smokers and a change to low-tar filter cigarettes.² Although the overall incidence of SCLC has been decreasing, the incidence in women is increasing and the male-to-female incidence ratio is now 1:1.³ Nearly all cases of SCLC are associated with heavy tobacco exposure, making it a heterogeneous disease with a complex genomic landscape consisting of thousands of mutations.^4,5 Despite recent advances in the treatment of non-small cell lung cancer, the therapeutic options for SCLC remain limited, with a median overall survival (OS) of 9 months in patients with advanced disease.

DIAGNOSIS AND STAGING

CASE PRESENTATION

A 61-year-old man presents to the emergency department with progressive shortness of breath and cough over the past 6 weeks. He also reports a 20-lb weight loss over the same period. He is a current smoker and has been smoking 1 pack of cigarettes per day since the age of 18 years. A chest radiograph obtained in the emergency department shows a right hilar mass. Computed tomography (CT) scan confirms the presence of a 4.5-cm right hilar mass and enlarged mediastinal lymph nodes bilaterally.

• What are the next steps in diagnosis?

SCLC is characterized by rapid growth and early hematogenous metastasis. Consequently, only 25% of patients have limited-stage disease at the time of diagnosis. According to the Veterans Administration Lung Study Group (VALSG) staging system, limited-stage disease is defined as tumor that is confined to 1 hemithorax and can be encompassed within 1 radiation field. This typically includes mediastinal lymph nodes and ipsilateral supraclavicular lymph nodes. Approximately 75% of patients present with extensive-stage disease, which is defined as disease that cannot be classified as limited, including disease that extends beyond 1 hemithorax. Extensive-stage disease includes the presence of malignant pleural effusion and/or distant metastasis.⁶ The VALSG classification and staging system is more commonly used in clinical practice than the American Joint Committee on Cancer TNM staging system because it is less complex and directs treatment decisions, as most of the literature on SCLC classifies patients based on the VALSG system.⁷

Given SCLC’s propensity to metastasize quickly, none of the currently available screening methods have proven successful in early detection of SCLC. In the National Lung Cancer Screening Trial, 86% of the 125 patients who were diagnosed with SCLC while undergoing annual low-dose chest CT scans had advanced disease at diagnosis.^8,9 These results highlight the fact that most cases of SCLC develop in the interval between annual screening imaging.

SCLC frequently presents with a large hilar mass that is symptomatic. Common symptoms include shortness of breath and cough. In addition, patients with SCLC usually have bulky mediastinal adenopathy at presentation. SCLC is commonly located submucosally in the bronchus, and therefore hemoptysis is not a very common symptom at the time of presentation. Patients may present with superior vena cava syndrome from local compression by the tumor. Not infrequently, SCLC is associated with paraneoplastic syndromes that arise due to ectopic secretion of hormones or antibodies by the tumor cells. The paraneoplastic syndromes can be broadly categorized as endocrine or neurologic (Table 1). The presence of a paraneoplastic syndrome is often a clue to the potential diagnosis of SCLC in the presence of a hilar mass. Additionally, some paraneoplastic syndromes, more specifically endocrine paraneoplastic syndromes, follow the pattern of disease response and relapse, and therefore can sometimes serve as an early marker of disease relapse or progression.

The common sites of metastases include brain, liver, and bone. Therefore, the staging workup should include fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT scan. Contrast-enhanced CT scan of the chest and abdomen and bone scan can be obtained for staging in lieu of PET scan. Due to the physiologic FDG uptake, cerebral metastases cannot be assessed with sufficient certainty using PET-CT.¹⁰ Therefore, brain imaging with contrast-enhanced CT or magnetic resonance imaging (MRI) is also necessary. Although the incidence of metastasis to bone marrow is less than 10%, bone marrow aspiration and biopsy are warranted in patients with unexplained cytopenias, especially when the cytopenia is associated with teardrop-shaped red cells or nucleated red cells on peripheral blood smear, findings indicative of a marrow infiltrative process.⁷ The tissue diagnosis is established by obtaining a biopsy of the primary tumor or 1 of the metastatic sites. In localized disease, bronchoscopy (with endobronchial ultrasound, if necessary) with biopsy of the centrally located tumor and/or lymph node is required. Histologically, SCLC consists of monomorphic cells, a high nuclear-cytoplasmic ratio, and confluent necrosis. The tumor cells are positive for chromogranin, synaptophysin, and CD56 by immunohistochemistry, and very frequently are also positive for thyroid transcription factor 1.¹¹ Although serum tumor markers, including neuron-specific enolase and progastrin-releasing peptide, are frequently elevated in patients with SCLC, these markers are of limited value in clinical practice because they lack sensitivity and specificity.¹²

MANAGEMENT OF LIMITED-STAGE DISEASE

CASE CONTINUED

The patient undergoes FDG PET scan, which shows the presence of a hypermetabolic right hilar mass in addition to enlarged and hypermetabolic bilateral mediastinal lymph nodes. There are no other areas of FDG avidity. Brain MRI does not show any evidence of brain metastasis. Thus, the patient is confirmed to have limited-stage SCLC.

• What is the standard of care treatment for limited-stage SCLC?

SCLC is exquisitely sensitive to both chemotherapy and radiation, especially at the time of initial presentation. The standard of care treatment of limited-stage SCLC is 4 cycles of platinum-based chemotherapy in combination with thoracic radiation started within the first 2 cycles of chemotherapy (Figure 1).

CHOICE OF CHEMOTHERAPY

Etoposide and cisplatin is the most commonly used initial combination chemotherapy regimen in limited-stage SCLC.¹⁴ This combination has largely replaced anthracycline-based regimens given its favorable efficacy and toxicity profile.^15–17 Several small randomized trials have shown comparable efficacy of carboplatin and etoposide in extensive-stage SCLC.^18–20 A meta-analysis of 4 randomized trials comparing cisplatin-based versus carboplatin-based regimens in 663 patients with SCLC (32% had limited-stage disease and 68% had extensive-stage disease) showed no statistically significant difference in response rate, progression-free survival (PFS), or OS between the 2 regimens.²¹ Therefore, in clinical practice carboplatin is frequently used instead of cisplatin in patients with extensive-stage disease. In patients with limited-stage disease, cisplatin is still the drug of choice. However, the toxicity profile of the 2 regimens is different. Cisplatin-based regimens are more commonly associated with neuropathy, nephrotoxicity, and chemotherapy-induced nausea/vomiting,¹⁸ while carboplatin-based regimens are more myelosuppressive.²² In addition, the combination of thoracic radiation with either of these regimens is associated with a higher risk of esophagitis, pneumonitis, and myelosuppression.²³ The use of myeloid growth factors is not recommended in patients undergoing concurrent chemoradiation.²⁴ Of note, intravenous etoposide is always preferred over oral etoposide, especially in the curative setting given the unreliable absorption and bioavailability of oral formulations.

THORACIC RADIOTHERAPY

Adding thoracic radiotherapy to platinum-etoposide chemotherapy improves local control and OS. Two meta-analyses of 13 trials including more than 2000 patients have shown a 25% to 30% decrease in local failure and a 5% to 7% increase in 2-year OS with chemoradiation compared to chemotherapy alone in limited-stage SCLC.^25,26 Early (within the first 2 cycles) concurrent thoracic radiation is superior to delayed and/or sequential radiation in terms of local control and OS.^23,27,28 The dose and fractionation of thoracic radiation in limited-stage SCLC has remained a controversial issue. The Eastern Cooperative Oncology Group/Radiation Therapy Oncology Group randomized trial compared 45 Gy of radiotherapy delivered twice daily over a period of 3 weeks to 45 Gy once daily over 5 weeks concurrently with chemotherapy. The twice daily regimen led to a 10% improvement in 5-year OS (26% versus 16%), but a higher incidence of grade 3 and 4 adverse events.¹³ Despite the survival advantage demonstrated by hyperfractionated radiotherapy, the results need to be interpreted with caution because the radiation doses are not biologically equivalent. In addition, the difficult logistics of patients receiving radiation twice a day has limited the routine implementation of this strategy. Subsequently, another randomized phase 3 trial (CONVERT) compared 45 Gy radiotherapy twice daily with 66 Gy radiotherapy once daily in limited-stage SCLC.²⁹ This trial did not show any difference in OS. The patients in the twice daily arm had a higher incidence of grade 4 neutropenia. Considering the results of these trials, both strategies—45 Gy fractionated twice daily or 60 Gy fractionated once daily, delivered concurrently with chemotherapy—are acceptable in the setting of limited-stage SCLC. However, quite often a hyperfractionated regimen is not feasible for patients and many radiation oncology centers. Hopefully, the ongoing CALGB 30610 study will clarify the optimal radiation schedule for limited-stage disease.

PROPHYLACTIC CRANIAL IRRADIATION

Approximately 75% of patients with limited-stage disease experience disease recurrence, and brain is the site of recurrence in approximately half of these patients.³⁰ Prophylactic cranial irradiation (PCI) consisting of 25 Gy radiotherapy delivered in 10 fractions has been shown to be effective in decreasing the incidence of cerebral metastases.^30–32 Although individual small studies have not shown a survival benefit of PCI because of small sample size and limited power, a meta-analysis of these studies has shown a 25% decrease in the 3-year incidence of brain metastasis and 5.4% increase in 3-year OS.³⁰ Most patients included in these studies had limited-stage disease. Therefore, PCI is the standard of care for patients with limited-stage disease who attain a partial or complete response to chemoradiation.

ROLE OF SURGERY

Surgical resection may be an acceptable choice in a very limited subset of patients with peripherally located small (< 5 cm) tumors where mediastinal lymph nodes have been confirmed to be uninvolved with complete mediastinal staging.^33,34 Most of the data in this setting are derived from retrospective studies.^35,36 A 5-year OS between 40% and 60% has been reported with this strategy in patients with clinical stage I disease. In general, when surgery is considered, lobectomy with mediastinal lymph node dissection followed by chemotherapy (if there is no nodal involvement) or chemoradiation (if nodal involvement) is recommended.^37,38 Wedge or segmental resections are not considered to be optimal surgical options.

MANAGEMENT OF EXTENSIVE-STAGE DISEASE

CASE CONTINUED

The patient receives 4 cycles of cisplatin and etoposide along with 70 Gy radiotherapy concurrently with the first 2 cycles of chemotherapy. His post-treatment CT scans show a partial response. He undergoes PCI 6 weeks after completion of treatment. At routine follow-up 18 months later, he is doing generally well except for mildly decreased appetite and an unintentional weight loss of 5 lb. CT scans demonstrate multiple hypodense liver lesions ranging from 7 mm to 2 cm in size and a 2-cm left adrenal gland lesion highly concerning for metastasis. FDG PET scan confirms that the adrenal and liver lesions are hypermetabolic. In addition, the PET scan shows multiple FDG-avid bone lesions throughout the spine. Brain MRI is negative for brain metastasis.

• What is the standard of care for treatment of extensive-stage disease?

Chemotherapy is the mainstay of treatment for extensive-stage SCLC; the goals of treatment are prolongation of survival, prevention or alleviation of cancer-related symptoms, and improvement in quality of life. The combination of etoposide with a platinum agent (carboplatin or cisplatin) is the preferred first-line treatment option. Carboplatin is more commonly used in clinical practice in this setting because of its comparable efficacy and better tolerability compared to cisplatin (Figure 2).²¹ A Japanese phase 3 trial comparing cisplatin plus irinotecan with cisplatin plus etoposide in the first-line setting in extensive-stage SCLC showed improvement in median and 2-year OS with the cisplatin/irinotecan regimen; however, 2 subsequent phase 3 trials conducted in the United States comparing these 2 regimens did not show any difference in OS. In addition, the cisplatin/irinotecan regimen was more toxic than the etoposide-based regimen.^39,40 Therefore, 4 to 6 cycles of platinum/etoposide remains the standard of care first-line treatment for extensive-stage SCLC in the United States. The combination yields a 60% to 70% response rate, but the majority of patients invariably experience disease progression, with a median OS of 9 to 11 months.⁴¹ Maintenance chemotherapy beyond the initial 4 to 6 cycles does not improve survival and is associated with higher cumulative toxicity.⁴²

Multiple attempts at improving first-line chemotherapy in extensive-stage disease have failed to show any meaningful difference in OS. For example, the addition of ifosfamide, palifosfamide, cyclophosphamide, taxane, or anthracycline to platinum doublet failed to show improvement in OS and led to more toxicity.^43–46 Additionally, the use of alternating or cyclic chemotherapies in an attempt to curb drug resistance has also failed to show survival benefit.^47–49 The addition of the antiangiogenic agent bevacizumab to standard platinum-based doublet has not prolonged OS in SCLC and has led to an unacceptably higher rate of tracheoesophageal fistula when used in conjunction with chemoradiation in limited-stage disease.^50–55 Finally, the immune checkpoint inhibitor ipilimumab in combination with platinum plus etoposide failed to improve PFS or OS compared to platinum plus etoposide alone in a recent phase 3 trial, and maintenance pembrolizumab after completion of platinum-based chemotherapy did not improve PFS.^56,57

More recently, a phase 2 study of pembrolizumab in extensive-stage SCLC (KEYNOTE 158) reported an overall response rate of 35.7%, median PFS of 2.1 months, and median OS of 14.6 months in patients who tested positive for programmed death ligand-1 (PD-L1) expression (which was defined as a PD-L1 Combined Positive Score ≥ 1).⁵⁸ The median duration of response has not been reached in this study, indicating that pembrolizumab may be a promising approach in patients with extensive-stage SCLC, especially for those with PD-L1–positive tumors.

Patients with extensive-stage disease who have brain metastasis at the time of diagnosis can be treated with systemic chemotherapy first if the brain metastases are asymptomatic and there is significant extracranial disease burden. In that case, whole brain radiotherapy should be given after completion of systemic therapy.

SECOND-LINE CHEMOTHERAPY

Despite being exquisitely chemosensitive, SCLC is associated with a very poor prognosis largely because of invariable disease progression following first-line therapy and lack of effective second-line treatment options that can lead to appreciable disease control. The choice of second-line treatment is predominantly determined by the time of disease relapse after first-line platinum-based therapy. If this interval is 6 months or longer, re-treatment utilizing the same platinum doublet is appropriate. However, if the interval is 6 months or less, second-line systemic therapy options should be explored. Unfortunately, the response rate tends to be less than 10% with most of the second-line therapies in platinum-resistant disease (defined as disease progression within 3 months of receiving platinum-based therapy). If disease progression occurs between 3 and 6 months after completion of platinum-based therapy, the response rate with second-line chemotherapy is in the range of 25%.^59,60

A number of second-line chemotherapy options have been explored in small studies, including topotecan, irinotecan, paclitaxel, docetaxel, temozolomide, vinorelbine, oral etoposide, gemcitabine, bendamustine, and CAV (cyclophosphamide, doxorubicin, vincristine) (Table 2).^61–77 Of these, topotecan (oral or intravenous) is the only second-line agent approved by the US Food and Drug Administration (FDA) for extensive-stage SCLC; this approval is based on a phase 3 trial showing a survival benefit with topotecan compared to best supportive care. The duration of second-line chemotherapy is not defined and is largely driven by duration of response and occurrence of side effects. Given the lack of effective treatment options, patients with platinum-resistant disease should be considered for clinical trials whenever possible. In patients with disease progression after second-line therapy, a third-line treatment can be considered if their Eastern Cooperative Oncology Group (ECOG) performance status is 0 to 2, but it is unclear whether further treatments would improve survival.

IMMUNOTHERAPY

The role of immune checkpoint inhibitors in the treatment of SCLC is evolving, and currently there are no FDA-approved immunotherapy agents for treating SCLC. A recently conducted phase 1/2 trial (CheckMate 032) studied the anti-programmed death(PD)-1 antibody nivolumab with or without the anti-cytotoxic T-lymphocyte–associated antigen (CTLA) -4 antibody ipilimumab in patients with relapsed SCLC. The authors reported response rates of 10% with nivolumab 3 mg/kg and 21% with nivolumab 1 mg/kg plus ipilimumab 3 mg/kg.^78,79 The 2-year OS was 26% with the combination and 14% with single-agent nivolumab. Only 18% of patients had PD-L1 expression of ≥ 1%, and the response rate did not correlate with PD-L1 status. The rate of grade 3 or 4 adverse events was approximately 20%, and only 10% of patients discontinued treatment because of toxicity. Based on these data, nivolumab plus ipilimumab is now included in the National Comprehensive Cancer Network guidelines as an option for patients with SCLC who experience disease relapse within 6 months of receiving platinum-based therapy;⁷ however, it is questionable whether routine use of this combination is justified based on currently available data. The evidence for the combination of nivolumab and ipilimumab remains limited. The efficacy and toxicity data from both randomized and nonrandomized cohorts were presented together, making it hard to interpret the results.

Another phase 1b study (KEYNOTE-028) evaluated the anti-PD-1 antibody pembrolizumab (10 mg/kg intravenously every 2 weeks) in patients with relapsed SCLC who had received 1 or more prior lines of therapy and had PD-L1 expression of ≥ 1%. This study showed a response rate of 33%, with a median duration of response of 19 months and 1-year OS of 38%.⁸⁰ Although only 28% of screened patients had PD-L1 expression of ≥ 1%, these results indicated that at least a subset of SCLC patients are able to achieve durable responses with immune checkpoint inhibition. A number of clinical trials utilizing immune checkpoint inhibitors in various combinations and settings are currently underway.

ROLE OF PROPHYLACTIC CRANIAL IRRADIATION

The role of PCI in extensive-stage SCLC is not clearly defined. A randomized phase 3 trial conducted by the European Organization for Research and Treatment of Cancer (EORTC) comparing PCI with no PCI in patients with extensive-stage SCLC who had a partial or complete response to initial platinum-based chemotherapy showed a decrease in the incidence of symptomatic brain metastasis and improvement in 1-year OS with PCI.⁸¹ However, this trial did not require mandatory brain imaging prior to PCI, and thus it is unclear if some patients in the PCI group had asymptomatic brain metastasis prior to enrollment and therefore received therapeutic benefit from brain radiation. Additionally, the dose and fractionation of PCI was not standardized across patient groups.

A more recent phase 3 study conducted in Japan that compared PCI (25 Gy in 10 fractions) with no PCI reported no difference in survival between the 2 groups.⁸² As opposed to the EORTC study, the Japanese study did require baseline brain imaging to confirm the absence of brain metastasis prior to enrollment. In addition, the control patients underwent periodic brain MRI to allow early detection of brain metastasis. Given the emergence of the new data, the impact of PCI on survival in patients with extensive-stage SCLC is unproven, and PCI likely has a role in a highly selected small group of patients with extensive-stage SCLC. PCI is not recommended for patients with poor performance status (ECOG performance score of 3 or 4) or underlying neurocognitive disorders.^34,83

The NMDA-receptor antagonist memantine can be used in patients undergoing PCI to delay the occurrence of cognitive dysfunction.⁶¹ Memantine 20 mg daily delayed time to cognitive decline and reduced the rate of decline in memory, executive function, and processing speed compared to placebo in patients receiving whole brain radiotherapy.⁸⁴

ROLE OF RADIOTHERAPY

A subset of patients with extensive-stage SCLC may benefit from consolidative thoracic radiotherapy after completion of platinum-based chemotherapy. A randomized trial that enrolled patients who achieved complete or near complete response after 3 cycles of cisplatin plus etoposide compared thoracic radiotherapy in combination with continued chemotherapy versus chemotherapy alone.⁸⁵ The median OS was longer with the addition of thoracic radiotherapy compared to chemotherapy alone. Another phase 3 trial did not show improvement in 1-year OS with consolidative thoracic radiotherapy, but 2-year OS and 6-month PFS were longer.⁸⁶ In general, consolidative thoracic radiotherapy benefits patients who have residual thoracic disease and low-bulk extrathoracic disease that has responded to systemic therapy.⁸⁷ In addition, patients who initially presented with bulky symptomatic thoracic disease should also be considered for consolidative radiation.

Similar to other solid tumors, radiotherapy should be utilized for palliative purposes in patients with painful bone metastasis, spinal cord compression, or brain metastasis. Surgery is generally not recommended for spinal cord compression given the short life expectancy of patients with extensive-stage disease. Whole brain radiotherapy is preferred over stereotactic radiosurgery because micrometastasis is frequently present even in the setting of 1 or 2 radiographically evident brain metastasis.

NOVEL THERAPIES

The very complex genetic landscape of SCLC accounts for its resistance to conventional therapy and high recurrence rate; however, at the same time this complexity can form the basis for effective targeted therapy for the disease. One of the major factors hindering the development of targeted therapies in SCLC is limited availability of tissue due to small tissue samples and the frequent presence of significant necrosis in the samples. In recent years, several different therapeutic strategies and targeted agents have been investigated for their potential role in SCLC. Several of them, including EGFR tyrosine kinase inhibitors (TKIs), BCR-ABL TKIs, mTOR inhibitors, and VEGF inhibitors, have not been shown to provide a survival advantage in this disease. Several others, including PARP inhibitors, cellular developmental pathway inhibitors, and antibody-drug conjugates, are being tested. A phase 1 study of veliparib combined with cisplatin and etoposide in patients with previously untreated extensive-stage SCLC demonstrated a complete response in 14.3%, a partial response in 57.1%, and stable disease in 28.6% of patients with an acceptable safety profile.⁸⁸ So far, none of these agents are approved for use in SCLC, and the majority are in early- phase clinical trials.⁸⁹

One of the emerging targets in the treatment of SCLC is delta-like protein 3 (DLL3). DLL3 is expressed on more than 80% of SCLC tumor cells and cancer stem cells. Rovalpituzumab tesirine is an antibody-drug conjugate consisting of humanized anti-DLL3 monoclonal antibody linked to SC-DR002, a DNA-crosslinking agent. A phase 1 trial of rovalpituzumab in patients with relapsed SCLC after 1 or 2 prior lines of therapy reported a response rate of 31% in patients with DLL3 expression of ≥ 50%. The median duration of response and median PFS were both 4.6 months.⁹⁰ Rovalpituzumab is currently in later phases of clinical trials and has a potential to serve as an option for patients with extensive-stage disease after disease progression on platinum-based therapy.

SUMMARY

Four to 6 cycles of carboplatin and etoposide remain the standard of care first-line treatment for patients with extensive stage SCLC. The only FDA-approved second-line treatment option is topotecan. Re-treatment with the original platinum doublet is a reasonable option for patients who have disease progression 6 months or longer after completion of platinum-based therapy. The immune checkpoint inhibitors pembrolizumab and combination nivolumab and ipilimumab have shown promising results in the second-line setting and beyond. The role of PCI has become more controversial in recent years, and periodic brain MRI in lieu of PCI is now an acceptable approach.

For patients undergoing treatment for limited-stage SCLC, response assessment with contrast-enhanced CT of the chest/abdomen should be performed after completion of 4 cycles of chemotherapy and thoracic radiation.⁷ The surveillance guidelines consist of history, physical exam, and imaging every 3 months during the first 2 years, every 6 months during the third year, and annually thereafter. If PCI is not performed, brain MRI or contrast-enhanced CT scan should be performed every 3 or 4 months during the first 2 years of follow up. For extensive-stage disease, response assessment should be performed after every 2 cycles of therapy. After completion of therapy, history, physical exam, and imaging should be done every 2 months during the first year, every 3 or 4 months during years 2 and 3, every 6 months during years 4 and 5, and annually thereafter. Routine use of PET scan for surveillance is not recommended. Any new pulmonary nodule should prompt evaluation for a second primary lung malignancy. Finally, smoking cessation counseling is an integral part of management of any patient with SCLC and should be included with every clinic visit.

CONCLUSION

SCLC is a heterogeneous and genetically complex disease with a very high mortality rate. The current standard of care includes concurrent chemoradiation with cisplatin and etoposide for limited-stage SCLC and the combination of platinum and etoposide for extensive SCLC. A number of novel treatment approaches, including immune checkpoint inhibitors and antibody-drug conjugates, have had promising results in early clinical trials. Given the limited treatment options and large unmet need for new treatment options, enrollment in clinical trials is strongly recommended for patients with SCLC.