Abstract
Hemophilia A and B are monogenic bleeding disorders resulting from loss of functional coagulation factors VIII or IX, respectively. Prophylactic treatment requires frequent intravenous injections of exogenous factor VIII (F.VIII) or factor IX (F.IX), due to the short half-life of both factors. Hemophilia patients are at risk of developing neutralizing antibodies to F.VIII (~25-30%) or F.IX (~2-4%), which require the use of expensive bypass agents and immune tolerance induction protocols. Viral vector mediated liver gene transfer of F.VIII or F.IX offers an alternative treatment for hemophilia with easily defined clinical endpoints and no need for strict regulation of coagulation factor expression, as both proteins circulate as inactive zymogens. Adeno-associated viral (AAV) vectors are derived from a non-pathogenic human virus that efficiently transduce non-dividing cells, such as hepatocytes, and provide stable transgene expression. In vivo liver gene transfer of AAV-F.VIII and -F.IX vectors has restored hemostasis in murine and canine hemophilia models long-term, and has also been shown to induce immune tolerance. Consequently, two Phase I/II clinical trials have been conducted, based on hepatic AAV-FIX gene transfer to patients with severe hemophilia B. The first trial, utilizing serotype 2, demonstrated transient correction, which was limited by a cellular immune response against the viral capsid. However, sustained therapeutic expression has been achieved in a second trial, using AAV8 for expression of a codon-optimized F.IX transgene. Translation of F.VIII gene transfer studies into the clinic may require additional optimization of gene transfer and vector to effectively express the larger cDNA of F.VIII.