Engineered Factor VIII: A Comprehensive Review of Molecular Biology, Design Strategies, and AAV-Mediated Gene Therapy

Coagulation factor VIII (FVIII) is a key therapeutic protein for hemophilia A, an inherited bleeding disorder caused by FVIII deficiency or dysfunction. The development and production of FVIII-based therapeutics remains challenging due to the intrinsic complexity of FVIII biosynthesis, folding, secretion, and functional stability. Advances in the understanding of the FVIII life cycle, as well as studies of its natural homologs, have created opportunities to develop improved FVIII variants. In parallel, therapeutic strategies to restore FVIII activity have evolved from replacement therapy with exogenous FVIII to gene-based delivery of a functional F8 copy. Among these approaches, AAV-mediated gene therapy has emerged as a clinically viable platform with a generally favorable safety profile, prompting the development of multiple therapeutic candidates for hemophilia A. This review summarizes established and current knowledge of FVIII structure, biosynthesis, and immunology in the context of developing effective and clinically applicable AAV-mediated therapies for hemophilia A. Particular attention is given to engineered FVIII variants and the latest clinical findings in AAV gene therapy, as well as to long-term efficacy and safety.