Characterizing a Cohort of Patients with Hemophilia B Treated with Fidanacogene Elaparvovec from the Phase 3 Benegene-2 Study Who Returned to Factor IX Prophylaxis

Background: Fidanacogene elaparvovec (PF-06838435, formerly SPK-9001) is an adeno-associated virus (AAV)-based gene therapy designed to deliver a high-activity human factor IX (FIX) variant transgene, FIX-R338L, resulting in endogenous FIX production in people with hemophilia B. To date, 45 participants with moderately severe to severe (FIX:C ≤2%) hemophilia B have received fidanacogene elaparvovec as part of the ongoing phase 3 study, BENEGENE-2 (NCT03861273). Of these 45 participants, 6 returned to prophylaxis (RTP) of FIX after initially responding to treatment. We describe the characteristics of the RTP participants.