Fidanacogene Elaparvovec for Hemophilia B — A Multiyear Follow-up Study
John E.J. Rasko, Benjamin J. Samelson‐Jones, Lindsey A. George, Adam Giermasz, Jonathan M. Ducore, Jerome Teitel, Catherine McGuinn, Katherine A. High, Ype P. de Jong, Amit Chhabra, Amanda C. O’Brien, Lynne M. Smith, Ian Winburn, Jeremy Rupon
Abstract
BACKGROUND: Treatment with fidanacogene elaparvovec, a recombinant adeno-associated virus (AAV) vector developed for the treatment of hemophilia B, led to sustained expression of the high-activity factor IX variant (FIX-R338L, or FIX-Padua) in a phase 1-2a study. The long-term safety and efficacy of this treatment are not known. METHODS: vector genomes (vg) per kilogram of body weight; thereafter, participants could enroll in a 5-year follow-up study. Safety end points included adverse events and changes in laboratory measures. Efficacy end points included the annualized rate of treated bleeding events (annualized bleeding rate) and factor IX activity. RESULTS: A total of 14 participants provided consent and completed at least 3 years of follow-up (median, 5.5; range 3 to 6); participation was ongoing among 8 at the data cutoff. None of the participants reported treatment-related adverse events after year 1. Throughout follow-up, nine serious adverse events were noted in 4 participants; none were thrombotic or treatment-related. No factor IX inhibitors were detected. Throughout follow-up, mean factor IX activity was in the mild hemophilia range; the mean annualized bleeding rate was less than 1, and 10 participants had no treated bleeding episodes. Surveillance liver ultrasounds obtained from year 1 onward showed no evidence of cancer but showed steatosis in 4 participants who had weight gain and elevated aminotransferase levels (maximum alanine aminotransferase level, 77 U per liter). One participant with a history of hepatitis C, hepatitis B, human immunodeficiency virus infection, and an elevated body-mass index had progression of underlying advanced liver fibrosis. A total of 13 surgical procedures were performed in 8 participants; exogenous factor IX was administered for 10 procedures, and no associated unexpected bleeding complications occurred. CONCLUSIONS: vg per kilogram, one of the lowest intravenous doses of AAV used for any indication. (Funded by Pfizer; ClinicalTrials.gov number, NCT03307980.).