Gene transfer therapies utilizing adeno-associated virus (AAV) vectors involve a complex drug design with multiple components that may impact immunogenicity. Valoctocogene roxaparvovec is an AAV serotype 5 (AAV5)-vectored gene therapy for the treatment of hemophilia A that encodes a B-domain-deleted human factor VIII (FVIII) protein controlled by a hepatocyte-selective promoter. Following previous results from the first-in-human phase 1/2 clinical trial, we assessed AAV5-capsid- and transgene-derived FVIII-specific immune responses with 2 years of follow-up data from GENEr8-1, a phase 3, single-arm, open-label study in 134 adult men with severe hemophilia A. No FVIII inhibitors were detected following administration of valoctocogene roxaparvovec. Immune responses were predominantly directed toward the AAV5 capsid, with all participants developing durable anti-AAV5 antibodies. Cellular immune responses specific for the AAV5 capsid were detected in most participants by interferon-γ enzyme-linked immunosorbent spot assay 2 weeks following dose administration and declined or reverted to negative over the first 52 weeks. These responses were weakly correlated with alanine aminotransferase elevations and showed no association with changes in FVIII activity. FVIII-specific cellular immune responses were less frequent and more sporadic compared with those specific for AAV5 and showed no association with safety or efficacy parameters.

Introduction AAV-mediated gene therapy vectors represent a complex drug design with multiple components that may impact immunogenicity. Clinical trials monitor immunogenicity directed toward both the vector delivery system and the expressed transgene product. Valoctocogene roxaparvovec is an AAV5-mediated gene therapy under investigation for the treatment of hemophilia A and encodes a codon-optimized B-domain deleted human FVIII protein (hFVIII-SQ) under control of a liver-selective promoter. This report describes clinical immunogenicity monitoring data from up to 1 year of follow up from GENEr8-1, a Phase 3, single-arm, open-label study in 134 male participants with severe hemophilia A.