Early Clinical Trial Results Following Administration of a Low Dose of a Novel Self Complementary Adeno-Associated Viral Vector Encoding Human Factor IX In Two Subjects with Severe Hemophilia B

Abstract 248

We have developed a unique approach for the treatment of hemophilia B (HB) that is currently being tested in the clinic. This open-label Phase I/II clinical trial entails peripheral vein administration of a single dose of our novel self complementary AAV vector encoding a codon-optimised human FIX transgene (scAAV2/8-LP1-hFIXco) into adult subjects with severe HB. Our plan is to evaluate three dose levels, progressing to the intermediate and high doses only in the absence of toxicity in a minimum of two subjects each. Vector is being administered in the absence of immunosuppression. Thus far, two subjects have received peripheral vein infusion at the low dose, each without any side effects. Importantly, there were no adverse reactions during vector infusion and no subsequent evidence of hepatotoxicity. Overall, there were no significant changes in the complete blood count and serum chemistry panel. The longest follow-up is in the first subject, in whom plasma FIX levels increased from a baseline of <1% to between 1.5–2% of normal levels within 2 weeks. This level of transgene expression has been maintained for a period that extends beyond 5 months following vector infusion. Importantly, this subject has not required any treatment or prophylaxis with FIX concentrate over this period and remains free of spontaneous joint bleeds. A robust primary humoral response to AAV8 capsid protein was observed following vector infusion. Capsid-specific cytotoxic T cells were monitored by IFN-g ELISPOT and by staining for IFN-g, TNF-a, and CD107a followed by flow cytometry; this analysis showed no increase in AAV8 specific CD8+ T cells at any of the time points evaluated. Following vector administration there was transient, self-limited shedding of scAAV in some body fluids/excretions but vector was not detected in the semen. The same dose has recently been administered to a second patient without toxicity. The FIX level in this subject, who is on regular prophylaxis, is currently 2% of normal, 13 days after his last dose of prophylaxis which was given the same day as the vector and raised his level to 26%. His levels will be carefully monitored to more conclusively establish evidence of expression of transgenic FIX. A third patient is scheduled for treatment at the intermediate dose in the fall. These early data are highly encouraging and suggest that low doses of scAAV vector, when pseudotyped with serotype 8 capsid can mediate therapeutic levels of FIX for at least several months without provoking an immunological response of the type seen in the previous trial.