Considerable optimism over gene therapy
“There is now repeated proof of concept success for hemophilia A and B gene therapy. I think this supports the considerable optimism that’s really driving this field,” said Lindsey A. George, MD, of the University of Pennsylvania and Children’s Hospital of Philadelphia.
She reviewed adeno-associated virus (AAV) vector and AAV-mediated gene transfer approaches for hemophilia A and B.
There are currently four clinical trials of gene therapy for patients with hemophilia B, and five for patients with hemophilia A.
Because AAV efficiently targets the liver, most safety considerations about systemic AAV-mediated gene therapy are focused around potential hepatotoxicity, Dr. George said.
“Thankfully, short-term safety in the context of hemophilia has really been quite good,” she said.
Patients who undergo gene therapy for hemophilia are typically monitored twice weekly for 3 months for evidence of a capsid-specific CD8 T cell response, also called a capsid immune response. This presents with transient transaminase elevations (primarily ALT) and a decline in factor VIII and factor IX activity.
In clinical trials for patients with hemophilia, the capsid immune response has limited the efficacy of the therapy in the short term, but has not been a major cause for safety concerns. It is typically managed with glucocorticoids or other immunomodulating agents such as mycophenolate mofetil or tacrolimus.
There have also been reported cases of transaminase elevations without evidence of a capsid immune response, which warrants further investigation, she added.
Regarding efficacy, she noted that across clinical trials, the observed annualized bleeding rate has been less than 1%, despite heterogeneity of vectors and dosing used.
“That’s obviously quite optimistic for the field, but it also sort of raises the point that the heterogeneity at which we’re achieving the same phenotypic observations deserves a bit of a deeper dive,” she said.
Although hemophilia B gene transfer appears to be durable, the same cannot be said as yet for hemophilia A.
In canine models for hemophilia A and B, factor VIII and factor IX expression have been demonstrated for 8-10 years post vector, and in humans factor IX expression in patients with hemophilia B has been reported for up to 8 years.
In contrast, in the three hemophilia A trials in which patients have been followed for a minimum of 2 years, there was an approximately 40% loss of transgene vector from year 1 to year 2 with two vectors, but not a third.
Potential explanations for the loss of expression seen include an unfolded protein response, promoter silence, and an ongoing undetected or unmitigated immune response to AAV or to the transgene.
Regarding the future of gene therapy, Dr. George said that “we anticipate that there will be licensed vectors in the very near future, and predicted that gene therapy “will fulfill its promise to alter the paradigm of hemophilia care.”
Dr. Lim disclosed honoraria from several companies and travel support from Novo Nordisk. Dr. Ma disclosed honoraria and research funding from Takeda. Dr. George disclosed FVIII-QQ patents and royalties, research funding from AskBio, and consulting activities/advisory board participation with others.
A version of this article first appeared on Medscape.com.