Hemophilia A is a condition in which blood is unable to clot effectively. It is caused by a mutation or deletion in the gene that is responsible for producing blood-clotting factor VIII protein. Individuals with hemophilia A suffer from repeated bleeding episodes, often into the joints, which can cause chronic joint disease and sometime results in death due to the inability of the blood to clot efficiently. This chronic joint disease can have significant physical, psychosocial, and quality-of-life effects, including financial burden. The current treatment is intravenous (i.v.) injections of factor VIII protein products, either 2-3 times weekly or in response to bleeding.

Recent preliminary clinical data of a hemophilia B gene transfer study (which is also being conducted by Spark Therapeutics) shows all study participants achieving therapeutic factor IX activity levels (average of maintaining factor IX activity levels around 30% of normal with no confirmed bleeds, after receiving Spark gene transfer, with the approach of using the novel bio-engineered recombinant adeno-associated viral (rAAV) vector carrying a high specific activity of a factor IX gene. The approach being tested in this clinical research study uses a further modified novel AAV vector (with a stronger attraction to the human liver) to deliver the human factor VIII (hFVIII) gene into liver cells so that they can produce factor VIII protein.