Stable Factor IX Activity Following AAV-Mediated Gene Transfer in Patients with Severe Hemophilia B

We are conducting a phase I/II clinical trial of factor IX gene transfer for severe hemophilia B. In the trial we are using a serotype-8 pseudotyped self-complementary adeno-associated virus (scAAV) vector expressing a codon-optimized coagulation factor IX (FIX) transgene (scAAV2/8-LP1-hFIXco). We have previously reported the early safety and efficacy of our novel gene transfer approach in six patients with severe hemophilia B following a single peripheral vein infusion of one of three vector doses (low [2×10¹¹ vector particles (vp)/kilogram weight (kg)], intermediate [6×10¹¹ vp/kg], or high dose [2×10¹² vp/kg]) (Nathwani et al, NEJM 365:2357–65, 2011). AAV-mediated expression of FIX at 1–6% of normal was established in all six participants with an initial follow-up of between 6–14 months following gene transfer. We now report longer follow-up of these participants, as well as data from two additional participants recently enrolled at the high dose level.

We have now infused scAAV2/8-LP1-hFIXco in eight subjects with severe hemophilia B (FIX activity, <1% of normal values). Vector was administered without immunosuppressive therapy, and participants have now been followed for 3 months to 2½ years. FIX activity, serum transaminases, vector genomes in secretions/excretions, antibodies to FIX and AAV8, and AAV8 capsid-specific T-cells were monitored during the follow-up.

Each of the participants currently has AAV-mediated activity of FIX at 1 to 6% of normal levels. These levels have been stable in each during the follow-up period which is now greater than 1½ years for the first six participants. Five of the eight participants have discontinued FIX prophylaxis and remain free of spontaneous hemorrhage; in the other three, the interval between prophylactic injections has increased. None of the participants in the low or intermediate dose cohorts had evidence of transaminitis; each currently has FIX activity of 1–3% for over 1½ years. Of the four participants who received the high dose of vector, one had a transient, asymptomatic elevation of serum aminotransferase levels, which was associated with the detection of AAV8-capsid-specific T cells in the peripheral blood; two others had a slight increase in liver-enzyme levels, the cause of which was less clear. Each of these three participants received a short course of glucocorticoid therapy, which rapidly normalized their aminotransferase levels and maintained FIX levels in the range of 4 to 6% of normal values. The fourth participant has not had transaminitis three months after vector administration.

This represents the first successful, long-term, gene therapy-mediated expression of a therapeutic protein from an AAV vector delivered to human liver. Although immune-mediated clearance of AAV-transduced hepatocytes remains a concern, this process may be controlled with a short course of glucocorticoids without loss of transgene expression. Larger numbers of patients followed for longer periods of time are necessary to fully define the benefits and risks and to optimize dosing. However, this gene therapy approach, even with its risk of mild, transient transaminitis, has the potential to convert the bleeding phenotype of patients with severe hemophilia B into a mild form of the disease or to reverse it entirely for a prolonged period of time following vector administration. (ClinicalTrials.gov number, NCT00979238).

Chowdary:Novo Nordisk: Consultancy. High:Amsterdam Molecular Therapeutics: ; Baxter Healthcare: Consultancy; Biogen Idec: Consultancy; bluebird bio, Inc.: Membership on an entity's Board of Directors or advisory committees; Genzyme, Inc.: Membership on an entity's Board of Directors or advisory committees; Novo Nordisk: ; Sangamo Biosciences: ; Shire Pharmaceuticals: Consultancy.