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BACKGROUND Congenital factor X deficiency (FXD) is a rare bleeding disorder that often presents with severe bleeding in the neonatal period. Long-term prophylaxis with infusions of FX-containing products is recommended in patients with FXD and a personal or family history of severe bleeding. A plasma-derived FX concentrate (pdFX) is approved for on-demand and prophylactic therapy in adults and children with FXD. The safety and efficacy of pdFX has been demonstrated in patients

Adeno-associated virus (AAV) vectors have been successfully applied in hemophilia clinical trials. However, this approach is limited to patients without AAV-neutralizing antibodies (NAbs). In this study, we explored the feasibility of AAV re-administration in hemophilia A dogs treated initially 8 years ago with AAV8.canine FVIII. After the re-administration in two NAb-negative dogs with AAV8 vectors carrying human factor VIII (hFVIII), along with the proteasome inhibitor bortezomib, we observed a phenotypic improvement in both dogs that persisted in one dog. Phenotypic improvement disappeared at 59 days after re-administration in the other dog, and specific cytotoxic T lymphocytes (CTLs) to the capsid were detected at day 17, but not to hFVIII. hFVIII inhibitors were observed at day 59 and gradually increased. Mechanistic studies demonstrated an increase in pro-inflammatory cytokines, a decrease in immunomodulatory cytokines, as well as lower Tregs after re-administration. These results suggest that hFVIII inhibitor development may contribute to the therapeutic failure via immune response activation. Interestingly, it takes about 30–50 days for AAV NAb titers to decrease by half. Collectively, this study suggests that re-administration of the same AAV serotype after long-term follow-up is feasible and that the study of AAV NAb kinetics will provide important information for predicating the efficacy of re-administration. Keywords: AAV, hemophilia, neutralizing antibodies, NAbs, re-administration, human factor VIII, hFVIII, hFVIII inhibitor