Cases of High-Titer Inhibitor Formation After AAV Gene Therapy for Hemophilia A.

Gene therapy for hemophilia A (HA) aims to provide durable hemostatic control and reduce treatment burden, but AAV-based products to date have limitations: inconsistent peak expression, declining factor levels, and immunogenicity causing transaminitis and expression loss. ASC618 (ASC Therapeutics, Milpitas, CA, USA) was developed as a second-generation gene therapy product to overcome some of these limitations, using ET3, a bioengineered chimeric human/porcine B-domain-deleted FVIII transgene, paired with a minimal liver promoter and liver codon optimization. In a phase 1/2 trial (NCT04676048), two patients received ASC618. Despite initial factor expression, both developed high-titer FVIII inhibitors and are now maintained on emicizumab prophylaxis. Although inhibitor titers have declined, both remain high-titer nearly two years after a single infusion. The best explanation for inhibitor formation is that the novel FVIII with the human-porcine hybrid construct (ET3) contained epitopes for which these patients had not developed tolerance despite both having more than 1000 exposure days to exogenous FVIII in the past. Although this trial has been halted, these cases provide important lessons for the development of future hemophilia gene therapies. They highlight the need for careful immunogenicity assessment when using bioengineered transgenes, even in previously treated patients with extensive factor VIII exposure.