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.