Months after exiting clinical hold, Pfizer readies to resume dosing in phase 3 gene therapy trial

Pfizer is finally nearly ready to resume dosing in its phase 3 hemophilia A gene therapy trial. Six months after the FDA lifted the clinical hold on the study, trial sites are set to resume enrollment with a view to restarting dosing in October.

The FDA put the study of the Sangamo Therapeutics-partnered giroctocogene fitelparvovec on hold at the start of November, cementing in place the voluntary pause on screening and dosing that Pfizer had already implemented. Pfizer hit pause after seeing some patients experience activity of the blood clotting protein factor VIII greater than 150%. 

In March, the FDA lifted the clinical hold but Pfizer kept the voluntary pause in place while working to meet “all necessary conditions,” including approval of updated trial protocols by regulatory authorities. Pfizer now has everything in place to support the resumption of enrollment and dosing.

The situation has dragged on longer than anticipated. In February, Sangamo told its investors that Pfizer was aiming to reopen trial sites in the first half of 2022. By May, the target had shifted to restarting the trial in the third quarter of 2022 and delivering pivotal data in the second half of 2023. The target for the data drop had slipped again by August and now, after a further delay, sits in the first half of 2024. 

Pfizer was fairly deep into the clinical trial when it paused recruitment, having enrolled more than 50% of the targeted 63 participants. In the period of paused enrollment, BioMarin has worked to get its rival hemophilia A gene therapy back on track, aiming to resubmit its filing for FDA approval by the end of the month. 

Whether either gene therapy can make a major impact on the hemophilia A market remains an open question. Durability is a big concern, with studies of both the candidates tracking declines in factor VIII activity over time.