Spk-9001: Adeno-Associated Virus Mediated Gene Transfer for Hemophilia B - 1 Year Follow up and Impact of Baseline Characteristics on Transgene-Derived Factor IX Activity and Persistence

Background: Prior work demonstrated long term expression of factor IX (mean FIX:C ~5.1%) after AAV8-mediated gene transfer in hemophilia B (Nathwani et al., 2014), but transgene-derived FIX:C fell short of trough values obtained by long-acting FIX prophylaxis (Santagostino et al . 2016) and natural history data supporting levels of ~12% sufficiently eliminate spontaneous hemarthroses (den Uijl et al. 2011). We developed an efficient capsid and expression cassette that could be administered at low doses to achieve FIX:C adequate to prevent bleeding without use of exogenous FIX. Here we report 1 year follow up data in 7 subjects following SPK-9001 infusion at a dose of 5 x 10¹¹vg/kg. These data represent the largest cohort of hemophilia B subjects treated with gene transfer using the same vector and dose with available 1 year follow up data. We additionally report observations of subject baseline pre-vector characteristics relative to steady-state transgene-derived FIX:C and clinical outcomes.

Objective: To obtain long term transgene-derived FIX:C > 12% able to prevent spontaneous bleeding without the need for prophylactic FIX infusions with consistent and predictable results among all subjects, irrespective of baseline characteristics.

Methods: The study vector, SPK-9001, consists of a liver specific, bioengineered AAV capsid (Spark100) and a single-stranded codon optimized expression cassette encoding FIX-Padua. FIX-Padua is a naturally occurring variant that confers ~8-12-fold greater specific activity compared to wild-type FIX (Simioni et al. 2009, Crudele et al. 2015). Subject baseline characteristics were determined at study screening. FIX:C and clinical data were prospectively collected after vector infusion.

Results: To date, 7 subjects completed 1 year follow up after SPK-9001 infusion at a dose of 5 x10¹¹ vg/kg. No subjects developed a FIX inhibitor or experienced vector related adverse events (AEs). Subject mean FIX:C following vector infusion through 52 weeks is outlined in figure 1. Subjects were males ages 18-52 yrs with FIX:C ≤2%. One subject had a Spark 100 neutralizing antibody (NAb) of 1:1 and the remaining were <1:1. Five patients were CRM+ and 2 were CRM-. The FIX:C at steady state, defined as the average of all values measured starting at week 12, for the 5 CRM+ subjects was 32.3±11.0% (mean±SD) and 21.0±5.7% for the 2 CRM- subjects. However, the analysis is confounded by a 1:1 NAb in one CRM- subject and partial loss of FIX:C due to immune response in one of the CRM+ subjects. Steady state FIX:C for the 6 subjects with <1:1 NAb was 31.1±10.3% and 17% for the one subject with NAb 1:1. FIX:C in subjects with a history of HCV and stage 1-2 liver fibrosis (29.7±12.7%), was comparable to that of subjects with no history of HCV (28.2±10.2%). FIX:C in the one patient with HCV exposure on HIV anti-retroviral therapy did not differ from other participants (25% vs. 29.7%±11.7%). All subjects discontinued prophylaxis at vector infusion. One subject self-reported bleeding that was incongruent with the remaining cohort, including those with the similar vector-derived FIX:C and baseline hemophilic arthropathy, who did not experience bleeding events or require factor (Table 1). The 1 subject (subject 7) who experienced a suspected capsid immune response had no known baseline characteristics that differed from the cohort; the subject completed a course of steroids, had a stable, sustained FIX:C of 14% and did not experience bleeding post vector. All subjects reported significantly improved quality of life (QoL) at 1-year (p<0.023).

Conclusion: As of 8/1/17, we report the highest, most consistent and sustained vector derived FIX:C following gene transfer. Despite heterogenous baseline characteristics, all subjects achieved consistent clinical results, including no vector related AEs, improved QoL, termination of prophylaxis, near complete elimination of bleeding and factor use. Low grade liver fibrosis and prior HCV exposure did not appear to affect steady-state FIX:C. Individual subject data may imply effects of CRM status and NAb titer on achieved FIX, but the current dataset is too small to define the relationship. In summary, these preliminary data suggest SPK-9001 safely, consistently, and independent of subject baseline characteristics, achieved FIX:C able to substantially ameliorate clinical manifestations of hemophilia.

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