FIXing Hemophilia B: Sustained Factor IX Expression of 30 Percent After Gene Therapy

Gene Therapy, Open-label, Dose-escalation Study of SPK-9001 [Adeno-associated Viral Vector With Human Factor IX Gene] in Subjects With Hemophilia B

NCT02484092

Spark Therapeutics and Pfizer

The Children’s Hospital of Philadelphia/University of Pennsylvania, University of Mississippi Medical Center, University of Pittsburgh, University of California Davis, Weill Cornell Medical College

To be determined

This is a phase I/II, open-label, dose-escalation clinical trial evaluating the safety, tolerability, and pharmacokinetics of SPK-9001 in subjects with hemophilia B. The target population is adult males (≥ 18 years) with hemophilia B (factor IX activity ≤ 2% of normal). Exclusion criteria include hepatitis C virus and HIV viremia, adeno-associated viral (AAV) -Spark 100 neutralizing antibody titers 1:5 or greater, or history of a factor IX (FIX) inhibitor. The primary objective of the study is to evaluate the safety and tolerability of SPK-9001. The secondary objective is to characterize the kinetics of SPK-9001 vector-derived FIX transgene expression. Exploratory objectives include evaluating changes from baseline in clinical outcomes, including number of bleeding events and factor consumption. The vector is administered intravenously over one hour. Subjects are observed for 52 weeks.

The SPK-9001 adeno-associated viral (AAV) vector was developed to achieve hemostatic levels of FIX activity at a dose low enough to avoid an immune response to the vector capsid, which has limited efficacy in prior clinical trials.¹  SPK-9001 consists of a bioengineered capsid, AAV-Spark100, with liver specific tropism analogous to AAV8. The seroprevalence of preexisting neutralizing antibodies to AAV-Spark100 is approximately 40 percent among sampled patients with hemophilia B.²  The vector expression cassette encodes a single-stranded FIX-Padua transgene. FIX Padua is a naturally occurring missense mutation in the protease domain that confers approximately eightfold greater specific activity than wild type FIX.³ 

Preliminary data on four subjects demonstrate that a single infusion of SPK-9001 at a dose of 5 × 10¹¹ vg/kg confers sustained FIX expression without need for immunosuppression. As of June 2016, four subjects have been followed for seven to 26 weeks following vector administration and achieved sustained mean (± SD) FIX activities of 31.3 percent ± 5.7 percent.⁴  There were no vector-related adverse events. Three of the four subjects were maintained on prophylaxis prior to study enrollment and safely stopped prophylaxis after vector infusion. Beyond a one-time factor infusion by a single subject two days following vector infusion, no subjects have experienced bleeding or required factor after vector administration. No subjects have experienced hepatic transaminase elevation more than 1.5-fold the normal level, a decrease in FIX activity, an immune response requiring immunosuppression, or development of an inhibitor to FIX.

Comment: Seminal data from Dr. Amit C. Nathwani and colleagues demonstrated long-term expression of FIX (1% to 7% of normal) in men with hemophilia B following AAV-mediated gene therapy.⁵  While the clinical impact of sustained FIX activity around 5 percent is indisputable, such levels of expression fall short of trough values achieved by prophylactic administration of extended half-life FIX products⁶  and of natural history data suggesting that approximately 15 percent factor activity eliminates spontaneous hemarthroses.⁷  Experience so far suggests that SPK-9001 at a dose of 5 × 10¹¹ vg/kg may consistently meet or exceed these thresholds. This early success requires confirmation in a larger number of patients and long-term monitoring of safety and efficacy. Nevertheless, the preliminary results suggest the possibility of a virtual “cure” for hemophilia B. Study recruitment and follow-up are ongoing. Challenges ahead for AAV gene therapy include developing strategies to overcome pre-existing AAV neutralizing antibodies and adapting gene therapy for the treatment of hemophilia A.