Factor IX Padua AAV gene therapy in adolescents with hemophilia B: a phase 1 trial

Adeno-associated virus (AAV)-mediated factor IX Padua gene therapy has demonstrated good safety and efficacy in adult patients with hemophilia B, but its safety and efficacy in adolescent patients with hemophilia B remain unknown. Here we report a multicenter, single-arm, phase 1 study involving 11 adolescent participants (aged 12-18 years) with severe or moderately severe hemophilia B (factor IX coagulant activity (FIX:C) ≤2 IU dl^-1) in China. All of them received the AAV gene therapy BBM-H901 at a dose of 5 × 10¹² vector genomes per kilogram of body weight and were then enrolled in a 52-week follow-up. The primary endpoint was safety. No dose-limiting toxicity was observed throughout the study. The most common adverse events included elevation of white blood cell count, elevation of neutrophil count and rash associated with corticosteroid use. One serious adverse event and two grade 3 adverse events occurred. Abnormal liver function was observed in one participant, with elevated alanine aminotransferase (197.0 U l^-1) and aspartate aminotransferase (75.0 U l^-1) levels, which returned to normal 4 weeks after immunosuppression therapy. The key secondary endpoints included the mean (s.d.) FIX:C at week 52 following infusion, which was 41.8 (30.1) IU dl^-1 (one-stage SynthASil method), and the mean annualized bleeding rate, which decreased from 13.9 to 0.5. In this study, we show that the gene therapy drug BBM-H901 for hemophilia B is safe in adolescent patients, providing initial insights into its efficacy. ClinicalTrials.gov identifier: NCT05709288 .