Long-Term Durable FVIII Expression with Improvements in Bleeding Rates Following AAV-Mediated FVIII Gene Transfer for Hemophilia A: Multiyear Follow-up on the Phase I/II Trial of SPK-8011

Background: Hemophilia A (HA) is an inherited blood disorder caused by deficiency or dysfunction of factor (F)VIII. The disease is characterized by recurrent bleeding, particularly into joints, which often results in arthropathy and functional disability. HA is currently managed with intravenous (IV) infusions of replacement FVIII or non-factor replacement therapy such as emicizumab. More recently, gene therapy, involving the use of a recombinant adeno-associated viral (AAV) vector to target hepatocytes, with the aim of safely imparting long-term stable FVIII expression to prevent bleeding, is being investigated for HA. SPK-8011 is an investigational AAV vector optimized to achieve clinically relevant and durable hepatocyte-derived FVIII expression at the lowest possible vector dose.

Methods: This Phase I/II trial (NCT03003533/NCT03432520) of SPK-8011 is an open-label, multicenter, non-randomized trial, the full methodology of which has been reported previously (George et al. N Engl J Med 2021). Eligible participants were males ≥18 years of age, with FVIII levels ≤2%, >150 exposure days to FVIII concentrates, and no FVIII inhibitors. All participants were negative for neutralizing antibodies to SPK200. Participants were enrolled into four dose cohorts receiving a single IV dose of either 5×10¹¹, 1×10¹², 2×10¹², or 1.5×10¹² vector genomes (vg) per kilogram of body weight, with a maximum dose based on a BMI of 30 kg/m². Several immunomodulation (IM) approaches were evaluated. Trial objectives included evaluation of the safety and efficacy of SPK-8011 and the extent and durability of FVIII expression.

Results: At data cut-off (30 June 2022), 23 participants were included across dosing cohorts of 5×10¹¹ vg/kg (n=2), 1×10¹² vg/kg (n=3), 1.5×10¹² vg/kg (n=9), or 2×10¹² vg/kg (n=9). Median (range) observation time was 159.3 (6.9–259.9) weeks, based on both the primary study 8011-101 and the long-term follow-up study. A total of 49 treatment-related adverse events (AEs) were reported; 23 were considered vector-related, including one serious adverse event (Grade 2 alanine transaminase [ALT] elevation resulting in elective hospitalization for IV corticosteroid administration), and 26 were related to IM therapies. No FVIII inhibitors or thrombotic events were reported. Transient non-sustained asymptomatic ALT elevation occurred in 11 participants during the expected window of presumed capsid immune response and resolved with use of IM agents.

In 16 participants with ≥1 year follow-up, a one-stage FVIII assay showed no apparent decrease in FVIII activity over time, with a majority expressing in the mild HA range (Table). Sustained expression of FVIII was maintained in 21/23 participants with several methods of IM; two participants lost FVIII expression following a presumed capsid immune response (George et al. N Engl J Med 2021).

Across all 21 participants with sustained FVIII expression, a 91% (95% CI: 78–97%) reduction in annualized bleeding rate (ABR) for all bleeds was observed, with all-bleed ABR of 11.59 (7.39–18.17) before infusion vs. 0.99 (0.46–2.14) after infusion (Figure). The 16 participants on prophylactic treatment before gene therapy had an 82% (95% CI: 53–93%) reduction in ABR for all bleeds; 6.88 (4.16–11.36) before vs. 1.26 (0.57–2.77) after infusion. ABRs for spontaneous bleeds were 5.20 (2.99–9.04) and 0.52 (0.19–1.39) before and after infusion, respectively. The five participants using on-demand treatment before infusion had a 99% (95% CI: 98–100%) reduction in all-bleed ABR; 27.26 (18.80–39.53) vs. 0.20 (0.09–0.45) before and after infusion, respectively, and 20.98 (12.29–35.82) vs. 0.05 (0.01–0.29) for spontaneous bleeds. In total, 17/21 (81%) participants had an ABR of <1 for treated bleeds and 20/21 (95%) had an ABR of <1 for spontaneous treated bleeds following infusion of SPK-8011.

Median (IQR) annualized FVIII infusion rates (AIRs) of 0.2 (0.0–1.2) were reported across all participants 28 days post vector infusion vs. 82.0 (36.0–106.0) before vector administration.

Conclusions: With up to 5 years of follow-up, a single infusion of SPK-8011 resulted in durable year-to-year FVIII expression and clinically meaningful reductions in ABR and AIR. No major safety signals were reported, indicating SPK-8011 was well tolerated in this population of people with HA.