Stable Elevations in FIX Activity and Reductions in Annualized Bleeding Rate over up to 2 Years of Follow-up of Adults with Severe or Moderate-Severe Hemophilia B Treated with AMT-060 (AAV5-hFIX) Gene Therapy

Background: Gene transfer for hemophilia offers the potential to convert the disease from a severe to mild phenotype with a single treatment. AMT-060 consists of an adeno-associated virus serotype 5 (AAV5) vector containing a codon-optimized wildtype human factor IX (FIX) gene under control of a liver-specific promoter.

Aim: This phase 1/2 study investigates the safety and efficacy of AMT-060 at 2 dose levels in adults with moderate-severe or severe hemophilia B. Updated data with up to 2 years follow up will be presented.

Methods: Multi-national, open-label, dose-escalating study in patients (pts) with FIX activity ≤2 IU/dL of normal, and a severe bleeding phenotype (prophylactic exogenous FIX; or on-demand exogenous FIX, plus ≥4 bleeds/year or hemophilic arthropathy). Pts received either 5x10¹² gc/kg (Cohort 1, n=5) or 2×10¹³ gc/kg (Cohort 2, n=5) of AMT-060 iv. Efficacy assessments include FIX activity (measured ≥10 days after use of exogenous FIX); reduction of FIX use; and annualized spontaneous bleeding rates. Safety assessments include treatment related adverse events, immunological and inflammatory biomarkers.

Results: Nine pts with severe (FIX <1%) and 1 with moderate-severe (FIX 1.5%) hemophilia B were enrolled and received AMT-060. At study entry, 9 pts were on FIX prophylaxis, and 1 pt with severe hemophilia B in Cohort 2 used FIX on demand. Following AMT-060, mean FIX activity in the lower dose cohort was 4.6 IU/dL (95% CI 1.6-7.6) during 1.5 year of follow-up, and 7.1% (95% CI 3.2-11.1) in the higher dose cohort during 1 year of follow-up with stable FIX protein expression across both cohorts. Disease severity improved in all pts: severe to mild (n=5), severe to moderate (n=4), and from moderate to mild (n=1). Eight of 9 pts on FIX prophylaxis at study entry discontinued use after AMT-060 infusion. Across all pts, the total annualized reduction of exogenous FIX use post-AMT-060 and discontinuation of prophylaxis was 79% (85% in Cohort 1 and 68% in Cohort 2). Mean annualized spontaneous bleeds were reduced from 9.8 in the year prior to AMT-060 to 4.2 in the 1.5 years after treatment for pts in Cohort 1 (58% reduction), compared with a 54% reduction at the previously-reported 1-year follow up, while total bleeds were reduced by 54%. In 4 pts in Cohort 2, annualized spontaneous bleeds were reduced from a mean of 3.0 to 0.5 (84% reduction), compared with a 70% reduction at the previously-reported 6-month follow up, with no spontaneous bleeds reported in the last 6 months of follow up. Total bleeds were reduced by 64%. One pt in Cohort 2 was not included in the calculation as historical bleed data was not available; he experienced 1 traumatic bleed after discontinuation of prophylaxis. No pts developed inhibitors to FIX and there were no detectable signs of sustained AAV5 capsid-specific T-cell activation. Six pts (3 in each cohort) experienced a total of 14 treatment-emergent adverse events classified as possibly/probably related to treatment, most of which were classified as mild (n=11) and some as moderate (n=3). Mild, temporary elevations in ALT not associated with changes in FIX activity or capsid-specific T-cell responses were observed in 3 pts with higher mean FIX activity (6.4-12.7 IU/dL; 1 in the lower- and 2 in the higher-dose cohort). Each received a tapering course of prednisolone. ALT elevations have not recurred. Additional efficacy and safety data will be presented up to 2- and 1.5-years of follow-up for Cohorts 1 and 2, respectively.

Conclusions: Longer-term outcomes for up to 1.5 years following a single infusion of AMT-060 continue to indicate a positive safety profile coupled with stable and clinically meaningful elevations in FIX protein expression and activity without evidence of capsid-specific immune responses. Bleeding rates declined over time, with a near cessation of spontaneous bleeds in the high dose cohort.