Gene therapy for hemophilia: a review on clinical benefit, limitations, and remaining issues.

In the last decade, enormous progress has been made in the development of gene therapy for hemophilia A and B. After the first encouraging results of intravenously administered adeno-associated virus (AAV)-based liver-directed gene therapy in patients with severe hemophilia B were reported in 2011, many gene therapy studies have been initiated. Most of these studies, using AAV vectors with various gene constructs, showed sufficient factor VIII and IX expression in patients to significantly reduce the number of bleeds and the need for prophylaxis in most patients with severe hemophilia. This resulted in great clinical benefit for nearly all patients. In this review, we will summarize the most recent findings of reported and ongoing gene therapy trials. We will highlight the successful outcome of trials with focus on the results of recently reported phase 1 trials and preliminary results of phase 2b/3 trials for hemophilia A and B. These new reports also reveal the impact of side effects and drawbacks associated with gene therapy. We will therefore also discuss the limitations and remaining issues of the current gene therapy approaches. These issues must be resolved before gene therapy will be widely available for the hemophilia patient population.