Pfizer Inc (NYSE:PFE, ETR:PFE)'s experimental gene therapy for haemophilia A has succeeded in a Phase 3 study, overcoming safety concerns that had halted the trial for almost a year.
The therapy, giroctocogene fitelparvovec, is a one-time infusion designed to enable patients to produce Factor VIII, a protein essential for normal blood clotting. Currently, haemophilia A patients rely on regular Factor VIII infusions to prevent bleeding episodes.
Fewer bleeding episodes, but how long will effects last?
In the AFFINE study, researchers monitored 75 patients for at least 15 months post-infusion. Pfizer reported that participants experienced fewer bleeding episodes and had higher Factor VIII levels compared to the standard prophylactic treatment prior to the infusion.
Only one patient resumed prophylactic infusions. The study results bring hope for patients looking to avoid the frequent burden of Factor VIII injections, though some questions about the treatment remain.
Like other gene therapies promising a one-time solution, it is uncertain how long the effects of Pfizer’s therapy will last and if it will permanently reduce the need for Factor VIII injections. Pfizer plans to continue monitoring patients for up to 15 years.
Regulators are likely to scrutinise the nearly 50% of patients in the study whose Factor VIII levels rose to 150% or more, raising concerns about potential blood clots. This issue led the Food and Drug Administration (FDA) to halt the trial in November 2021.
Pfizer addressed the FDA’s concerns and resumed the trial in 2022 with an adjusted protocol. On Wednesday, Pfizer stated that the elevated Factor VIII levels were transient and did not affect efficacy and safety results. The company intends to consult with regulators in the coming months regarding the next steps for the treatment.
FDA challenges
Gene therapy developers for haemophilia have faced challenges with the FDA. In 2020, the FDA rejected BioMarin's treatment for haemophilia, demanding more data, although it eventually approved the therapy, with sales falling short of expectations.
Pfizer developed its haemophilia A treatment in collaboration with Sangamo Therapeutics. While the company's gene therapy work has had mixed results, Pfizer received approval for a haemophilia B gene therapy in April and experienced a research setback in Duchenne muscular dystrophy in June.
In 2023, Pfizer sold its early gene therapy programs to AstraZeneca.