Why is AAV FVIII gene therapy not approved by the US Food and Drug Administration yet?

The prospect of a clinical strategy using an adeno-associated virus (AAV) vector for expression of therapeutic levels of factor VIII (FVIII) has been highly desirable. This was initially anticipated by promising data from clinical studies on AAV5-FVIII in men with severe hemophilia A. However, long-term follow-up showed a unique efficacy concern on the sustainability and durability derived from a continuous decline in the FVIII transgene levels starting 1 year after vector injection through year 5. Additional follow-up of early-phase studies and outcomes of an ongoing phase 3 study will likely provide evidence on the feasibility of this approach. Here, the potential underlying mechanisms of the FVIII declining levels, together with the revision of several unique early and late onset findings, are discussed. The lack of long-term preclinical studies in large animal models prevents the firm conclusion that FVIII levels decline was unexpected. It is possible that the combination of vector manufacturing platform and dose, accompanied with ectopic expression of supraphysiologic levels of FVIII at short-term follow-up, may all contribute to the sustainability and durability of the transgene levels. Notably, vector readministration to further improve the FVIII levels is not feasible at this time. Thus, the need of a one-and-done AAV strategy to achieve sustain FVIII levels of expression is sine qua non to impact favorably the disease phenotype.