Gene therapy for hemophilia: past, present and future

After numerous preclinical studies demonstrated consistent success in large and small animal models, gene therapy has finally seen initial signs of clinically meaningful success. In a landmark study, Nathwani and colleagues reported sustained factor (F)IX expression in individuals with severe hemophilia B following adeno-associated virus (AAV)-mediated in vivo FIX gene transfer. As the next possible treatment-changing paradigm in hemophilia care, gene therapy may provide patients with sufficient hemostatic improvement to achieve the World Federation of Hemophilia's aspirational goal of "integration of opportunities in all aspects of life… equivalent to someone without a bleeding disorder." Although promising momentum supports the potential of gene therapy to replace protein-based therapeutics for hemophilia, several obstacles remain. The largest challenges appear to be overcoming the cellular immune responses to the AAV capsid; preexisting AAV neutralizing antibodies, which immediately exclude approximately 50% of the target population; and the ability to scale-up vector manufacturing for widespread applicability. Additional obstacles specific to hemophilia A (HA) include designing a vector cassette to accommodate a larger cDNA; avoiding development of inhibitory antibodies; and, perhaps the greatest difficulty to overcome, ensuring adequate expression efficiency. This review discusses the relevance of gene therapy to the hemophilia disease state, previous research progress, the current landscape of clinical trials, and considerations for promoting the future availability of gene therapy for hemophilia.