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Factor VIII replacement therapies and gene therapy may soon reduce the need for factor VIII concentrate in hemophilia A, but concentrate, a staple of therapy for hemophilia A since the 1950s, will still likely have a role in certain circumstances, a hematology expert said.
“Factor VIII concentrate therapy should still be available for hemophilia A therapy in the future, for the treatment of breakthrough bleeds in non–factor substitution therapy cases, to obtain retain reliable levels of laboratory-measurable hemostatic activity, for enhanced global access to hemophilia A therapy, and finally – and somewhat speculatively – to treat nonhemostatic functions if these are better defined in future preclinical investigations,” said David Lillicrap, MD, from Queen’s University in Kingston, Ont.
He discussed factor VIII biology and the pros and cons of alternatives to factor VIII concentrate at the annual congress of the European Association for Haemophilia and Allied Disorders.
One factor, multiple sources
It has been known since at least the late 1960s and early ‘70s that the liver is a significant source of factor VIII, primarily through liver sinusoidal endothelial cells (LSECs), but more recent studies have revealed other, nonhepatic sites of factor VIII expression, including the kidneys, lungs, spleen, lymph nodes, heat, intestines, skin an pulmonary artery, he said.
Endothelial cells proven to express factor VIII included LSECs, lymphatic endothelium, glomerular endothelium, and high endothelial venules.
“This information suggests that maybe a site of factor VIII synthesis could be important for a function that we do not yet appreciate. This is speculation, of course, but this is an unusual and enigmatic group of cells, and perhaps we’re missing something here that’s biologically important,” he said.
In addition to hemophilia, factor VIII deficiency may contribute to nonhemostatic pathologies, such as osteopenia/osteoporosis and hypertension, the latter possibly related to multiple renal bleeds or endothelial cell vasomotor dysfunction, he noted.
Despite decades-long experience with factor VIII concentrates, there are still uncertainties regarding optimal effective dosing, and about the mechanisms and management of factor VIII immunogenicity, both primary inhibitor development and immune tolerance induction, Dr. Lillicrap said.
Alternative therapies
Both factor VIII mimetics such as emicizumab (Hemlibra) and hemostasis rebalancing agents such as fitusiran, anti–tissue factor pathway inhibitor (TFPI) antibody and activated protein C serine protease inhibitor (APC serpin) require only infrequent subcutaneous administration, are efficacious in patients with factor VIII inhibitors, and are supported by either robust phase 3 data (in the case of mimetics) or evidence from late-phase clinical trials (in the case of the rebalancing agents).
However, “for the factor VIII mimetics we know that only partial factor VIII mimetic function, somewhere in the region of 10%-15% is obtained, and because of this, breakthrough bleeds do occur in these patients,” he said.
Additionally, the mimetics have been associated with rare, sometimes poorly explained thromboembolic complications, especially when they are given concurrently with activated prothrombin complex concentrate. Mimetic are also associated with infrequent development of antidrug antibodies, and “the fact that the factor VIII mimetic function is always ‘on’ is potentially a problem.”
For the rebalancing hemostasis agents, there are concerns about the ability to respond to dynamic challenges to the hemostatic system, such as sepsis or following trauma. These agents are also associated, albeit infrequently, with thromboembolic events, and they are somewhat difficult to monitor, he said.
Gene therapy
Gene therapy for hemophilia has the advantages of a single administration for a long-term effect, avoiding the peaks and troughs associated with substitution therapy, and the potential for being less immunogenic than factor VIII protein replacement.
The downside of gene therapy is that some patients may be ineligible for it because of preexisting immunity in about 50% of the population to the adeno-associated virus vectors used to carry the corrective gene.
Additional limitations are the occurrence in about 60% of patients of early although usually transient hepatotoxicity, significant variability in the factor levels ultimately attained, uncertainties about the durability of response, and the potential for long-term genotoxicity, Dr. Lillicrap said.
Tolerance for factor VIII
In the question and answer session following the presentation, session moderator Hervé Chambost, MD, from University Hospital La Timone and Aix-Marseille University, both in Marseille, France, asked whether there was a role for factor VIII and immune tolerance therapy (ITI) among patients who have been treated with non–factor replacement therapy.
“Is it important to have an antigenic pressure to maintain factor VIII or not for these patients?” he asked.
“I think this is a critical issue, and it’s an issue that we don’t yet have objective evidence for,” Dr. Lillicrap replied. “But the idea that we need to introduce some antigenic exposure to factor VIII in these individuals is a reasonable one, whether that be with intermittent exposure to factor VIII – weekly, monthly – we simply have no idea, but I think factor VIII will still be required in these patients because of breakthrough bleeds in patients who have been treated with non–factor replacement. So maintaining tolerance is a critical issue, and we need to develop maybe prospective trials to look at what those protocols are going to be to maintain tolerance in these patients.”
“As important, if not more so, is whether children should be tolerized at all,” commented Dan Hart, PhD, from Barts and the London School of Medicine and Dentistry, who also presented data during the session.
“The U.K. currently takes the view that, in children, new inhibitors arising may be delayed into the latter part of the first decade of their life if they have not had factors as their first choice but have had [replacement] on demand. I think we are heading into challenging times of understanding how to deliver ITI to larger children, how acceptable that is, and how we do it, but enabling [factor] VIII to be used long term rather than tolerating a chronic inhibitor I think is a really important issue where we need to head toward some consensus,” he said.
No funding source was reported. Dr. Lillicrap disclosed research funding from and advisory roles for several pharmaceutical companies. Dr. Hart disclosed grant/research support and speakers bureau activity for various companies. Dr. Chambost has previously reported no disclosures relevant to the topic at hand.
Factor VIII replacement therapies and gene therapy may soon reduce the need for factor VIII concentrate in hemophilia A, but concentrate, a staple of therapy for hemophilia A since the 1950s, will still likely have a role in certain circumstances, a hematology expert said.
“Factor VIII concentrate therapy should still be available for hemophilia A therapy in the future, for the treatment of breakthrough bleeds in non–factor substitution therapy cases, to obtain retain reliable levels of laboratory-measurable hemostatic activity, for enhanced global access to hemophilia A therapy, and finally – and somewhat speculatively – to treat nonhemostatic functions if these are better defined in future preclinical investigations,” said David Lillicrap, MD, from Queen’s University in Kingston, Ont.
He discussed factor VIII biology and the pros and cons of alternatives to factor VIII concentrate at the annual congress of the European Association for Haemophilia and Allied Disorders.
One factor, multiple sources
It has been known since at least the late 1960s and early ‘70s that the liver is a significant source of factor VIII, primarily through liver sinusoidal endothelial cells (LSECs), but more recent studies have revealed other, nonhepatic sites of factor VIII expression, including the kidneys, lungs, spleen, lymph nodes, heat, intestines, skin an pulmonary artery, he said.
Endothelial cells proven to express factor VIII included LSECs, lymphatic endothelium, glomerular endothelium, and high endothelial venules.
“This information suggests that maybe a site of factor VIII synthesis could be important for a function that we do not yet appreciate. This is speculation, of course, but this is an unusual and enigmatic group of cells, and perhaps we’re missing something here that’s biologically important,” he said.
In addition to hemophilia, factor VIII deficiency may contribute to nonhemostatic pathologies, such as osteopenia/osteoporosis and hypertension, the latter possibly related to multiple renal bleeds or endothelial cell vasomotor dysfunction, he noted.
Despite decades-long experience with factor VIII concentrates, there are still uncertainties regarding optimal effective dosing, and about the mechanisms and management of factor VIII immunogenicity, both primary inhibitor development and immune tolerance induction, Dr. Lillicrap said.
Alternative therapies
Both factor VIII mimetics such as emicizumab (Hemlibra) and hemostasis rebalancing agents such as fitusiran, anti–tissue factor pathway inhibitor (TFPI) antibody and activated protein C serine protease inhibitor (APC serpin) require only infrequent subcutaneous administration, are efficacious in patients with factor VIII inhibitors, and are supported by either robust phase 3 data (in the case of mimetics) or evidence from late-phase clinical trials (in the case of the rebalancing agents).
However, “for the factor VIII mimetics we know that only partial factor VIII mimetic function, somewhere in the region of 10%-15% is obtained, and because of this, breakthrough bleeds do occur in these patients,” he said.
Additionally, the mimetics have been associated with rare, sometimes poorly explained thromboembolic complications, especially when they are given concurrently with activated prothrombin complex concentrate. Mimetic are also associated with infrequent development of antidrug antibodies, and “the fact that the factor VIII mimetic function is always ‘on’ is potentially a problem.”
For the rebalancing hemostasis agents, there are concerns about the ability to respond to dynamic challenges to the hemostatic system, such as sepsis or following trauma. These agents are also associated, albeit infrequently, with thromboembolic events, and they are somewhat difficult to monitor, he said.
Gene therapy
Gene therapy for hemophilia has the advantages of a single administration for a long-term effect, avoiding the peaks and troughs associated with substitution therapy, and the potential for being less immunogenic than factor VIII protein replacement.
The downside of gene therapy is that some patients may be ineligible for it because of preexisting immunity in about 50% of the population to the adeno-associated virus vectors used to carry the corrective gene.
Additional limitations are the occurrence in about 60% of patients of early although usually transient hepatotoxicity, significant variability in the factor levels ultimately attained, uncertainties about the durability of response, and the potential for long-term genotoxicity, Dr. Lillicrap said.
Tolerance for factor VIII
In the question and answer session following the presentation, session moderator Hervé Chambost, MD, from University Hospital La Timone and Aix-Marseille University, both in Marseille, France, asked whether there was a role for factor VIII and immune tolerance therapy (ITI) among patients who have been treated with non–factor replacement therapy.
“Is it important to have an antigenic pressure to maintain factor VIII or not for these patients?” he asked.
“I think this is a critical issue, and it’s an issue that we don’t yet have objective evidence for,” Dr. Lillicrap replied. “But the idea that we need to introduce some antigenic exposure to factor VIII in these individuals is a reasonable one, whether that be with intermittent exposure to factor VIII – weekly, monthly – we simply have no idea, but I think factor VIII will still be required in these patients because of breakthrough bleeds in patients who have been treated with non–factor replacement. So maintaining tolerance is a critical issue, and we need to develop maybe prospective trials to look at what those protocols are going to be to maintain tolerance in these patients.”
“As important, if not more so, is whether children should be tolerized at all,” commented Dan Hart, PhD, from Barts and the London School of Medicine and Dentistry, who also presented data during the session.
“The U.K. currently takes the view that, in children, new inhibitors arising may be delayed into the latter part of the first decade of their life if they have not had factors as their first choice but have had [replacement] on demand. I think we are heading into challenging times of understanding how to deliver ITI to larger children, how acceptable that is, and how we do it, but enabling [factor] VIII to be used long term rather than tolerating a chronic inhibitor I think is a really important issue where we need to head toward some consensus,” he said.
No funding source was reported. Dr. Lillicrap disclosed research funding from and advisory roles for several pharmaceutical companies. Dr. Hart disclosed grant/research support and speakers bureau activity for various companies. Dr. Chambost has previously reported no disclosures relevant to the topic at hand.
Factor VIII replacement therapies and gene therapy may soon reduce the need for factor VIII concentrate in hemophilia A, but concentrate, a staple of therapy for hemophilia A since the 1950s, will still likely have a role in certain circumstances, a hematology expert said.
“Factor VIII concentrate therapy should still be available for hemophilia A therapy in the future, for the treatment of breakthrough bleeds in non–factor substitution therapy cases, to obtain retain reliable levels of laboratory-measurable hemostatic activity, for enhanced global access to hemophilia A therapy, and finally – and somewhat speculatively – to treat nonhemostatic functions if these are better defined in future preclinical investigations,” said David Lillicrap, MD, from Queen’s University in Kingston, Ont.
He discussed factor VIII biology and the pros and cons of alternatives to factor VIII concentrate at the annual congress of the European Association for Haemophilia and Allied Disorders.
One factor, multiple sources
It has been known since at least the late 1960s and early ‘70s that the liver is a significant source of factor VIII, primarily through liver sinusoidal endothelial cells (LSECs), but more recent studies have revealed other, nonhepatic sites of factor VIII expression, including the kidneys, lungs, spleen, lymph nodes, heat, intestines, skin an pulmonary artery, he said.
Endothelial cells proven to express factor VIII included LSECs, lymphatic endothelium, glomerular endothelium, and high endothelial venules.
“This information suggests that maybe a site of factor VIII synthesis could be important for a function that we do not yet appreciate. This is speculation, of course, but this is an unusual and enigmatic group of cells, and perhaps we’re missing something here that’s biologically important,” he said.
In addition to hemophilia, factor VIII deficiency may contribute to nonhemostatic pathologies, such as osteopenia/osteoporosis and hypertension, the latter possibly related to multiple renal bleeds or endothelial cell vasomotor dysfunction, he noted.
Despite decades-long experience with factor VIII concentrates, there are still uncertainties regarding optimal effective dosing, and about the mechanisms and management of factor VIII immunogenicity, both primary inhibitor development and immune tolerance induction, Dr. Lillicrap said.
Alternative therapies
Both factor VIII mimetics such as emicizumab (Hemlibra) and hemostasis rebalancing agents such as fitusiran, anti–tissue factor pathway inhibitor (TFPI) antibody and activated protein C serine protease inhibitor (APC serpin) require only infrequent subcutaneous administration, are efficacious in patients with factor VIII inhibitors, and are supported by either robust phase 3 data (in the case of mimetics) or evidence from late-phase clinical trials (in the case of the rebalancing agents).
However, “for the factor VIII mimetics we know that only partial factor VIII mimetic function, somewhere in the region of 10%-15% is obtained, and because of this, breakthrough bleeds do occur in these patients,” he said.
Additionally, the mimetics have been associated with rare, sometimes poorly explained thromboembolic complications, especially when they are given concurrently with activated prothrombin complex concentrate. Mimetic are also associated with infrequent development of antidrug antibodies, and “the fact that the factor VIII mimetic function is always ‘on’ is potentially a problem.”
For the rebalancing hemostasis agents, there are concerns about the ability to respond to dynamic challenges to the hemostatic system, such as sepsis or following trauma. These agents are also associated, albeit infrequently, with thromboembolic events, and they are somewhat difficult to monitor, he said.
Gene therapy
Gene therapy for hemophilia has the advantages of a single administration for a long-term effect, avoiding the peaks and troughs associated with substitution therapy, and the potential for being less immunogenic than factor VIII protein replacement.
The downside of gene therapy is that some patients may be ineligible for it because of preexisting immunity in about 50% of the population to the adeno-associated virus vectors used to carry the corrective gene.
Additional limitations are the occurrence in about 60% of patients of early although usually transient hepatotoxicity, significant variability in the factor levels ultimately attained, uncertainties about the durability of response, and the potential for long-term genotoxicity, Dr. Lillicrap said.
Tolerance for factor VIII
In the question and answer session following the presentation, session moderator Hervé Chambost, MD, from University Hospital La Timone and Aix-Marseille University, both in Marseille, France, asked whether there was a role for factor VIII and immune tolerance therapy (ITI) among patients who have been treated with non–factor replacement therapy.
“Is it important to have an antigenic pressure to maintain factor VIII or not for these patients?” he asked.
“I think this is a critical issue, and it’s an issue that we don’t yet have objective evidence for,” Dr. Lillicrap replied. “But the idea that we need to introduce some antigenic exposure to factor VIII in these individuals is a reasonable one, whether that be with intermittent exposure to factor VIII – weekly, monthly – we simply have no idea, but I think factor VIII will still be required in these patients because of breakthrough bleeds in patients who have been treated with non–factor replacement. So maintaining tolerance is a critical issue, and we need to develop maybe prospective trials to look at what those protocols are going to be to maintain tolerance in these patients.”
“As important, if not more so, is whether children should be tolerized at all,” commented Dan Hart, PhD, from Barts and the London School of Medicine and Dentistry, who also presented data during the session.
“The U.K. currently takes the view that, in children, new inhibitors arising may be delayed into the latter part of the first decade of their life if they have not had factors as their first choice but have had [replacement] on demand. I think we are heading into challenging times of understanding how to deliver ITI to larger children, how acceptable that is, and how we do it, but enabling [factor] VIII to be used long term rather than tolerating a chronic inhibitor I think is a really important issue where we need to head toward some consensus,” he said.
No funding source was reported. Dr. Lillicrap disclosed research funding from and advisory roles for several pharmaceutical companies. Dr. Hart disclosed grant/research support and speakers bureau activity for various companies. Dr. Chambost has previously reported no disclosures relevant to the topic at hand.
FROM EAHAD 2021