Long-term durability of rAAV gene therapy in hemophilia: Factor expression, clinical outcomes and underlying molecular mechanisms

For decades, the management of hemophilia A and B has relied on frequent intravenous infusions of factor VIII (FVIII) or factor IX (FIX), a regimen that imposes substantial physical and emotional burdens on patients, and carries significant long-term costs despite advances in treatment. Although the advent of non-factor therapies has improved prophylaxis, these approaches still require regular administration and do not fully address the underlying factor deficiency. Adeno-associated virus (AAV)-mediated gene therapy represents a potentially transformative, one-time therapeutic alternative by enabling sustained, hepatocyte-driven production of FVIII or FIX. Approved therapies-valoctocogene roxaparvovec for hemophilia A and etranacogene dezaparvovec for hemophilia B-have demonstrated significant reductions in bleeding frequency and factor replacement requirements. Clinical experience indicates that FIX expression is generally stable over time, whereas FVIII levels often decline, highlighting ongoing challenges in achieving durable expression. These limitations are influenced by both immunological factors, including vector- and transgene-directed immune responses, and non-immunological factors such as hepatocyte turnover and epigenetic regulation of transgene expression. Pre-existing immunity and vector-induced immune responses may restrict the possibility of repeat administration. Long-term follow-up from pivotal trials has documented sustained factor expression for over 10 years in hemophilia B and up to 5 years in hemophilia A, though inter-individual variability remains substantial. Emerging strategies to enhance durability include next-generation vector design, codon optimization, gene editing technologies, and alternative delivery platforms. This review synthesizes current clinical evidence, biological understanding, and translational challenges, with an emphasis on strategies to optimize long-term efficacy of AAV-mediated gene therapy for hemophilia.