Sustained and Complete Phenotype Correction of Hemophilia B Mice Following Intramuscular Injection of AAV1 Serotype Vectors

We previously reported that direct intramuscular injection of non-serotype-2 AAV vectors, especially AAV serotype 1 (AAV1), resulted in expression of supranormal levels of canine F9 in immunodeficient mice. Here we test the ability of the AAV1-F9 vector to deliver sustained expression and correction of factor IX (FIX) deficiency in genetically engineered hemophilic mice. Intramuscular injection of AAV1-F9 resulted in 100-1000 times more canine F9 in plasma of recombinant AAV1-F9 mice compared with injection of AAV2-F9. Assessment of clotting activity by activated partial thromboplastin time confirmed that circulating canine FIX was indeed functional. Moreover, phenotypic correction assayed by tail clip challenge resulted in survival of all AAV1-F9 treated animals, in contrast to naive mice and 50% of AAV2-treated hemophilia B mice, which failed to survive. Administration of cyclophosphamide (CTX) was required to suppress formation of anti-canine FIX antibodies for AAV2-treated animals, whereas it was dispensable for those treated with AAV1-F9. This difference in immunogenicity further emphasizes the usefulness of serotype-specific vectors. Finally, we report that correction of the hemophilia phenotype using AAV1-F9 was complete and persistent (over 8 months), a result that underscores the value of continued exploration of alternative AAV serotype vectors.