Post hoc longitudinal assessment of efficacy and safety of recombinant factor IX Fc fusion protein in hemophilia B.

Long-term efficacy and safety of the extended half-life recombinant FIX Fc fusion protein (rFIXFc) has been established in previously treated patients with severe hemophilia B in 2 Phase 3 trials (B-LONG [NCT01027364] and Kids B-LONG [NCT01440946]) and a long-term extension study (B-YOND [NCT01425723]). Here we report post hoc analyses of pooled longitudinal data up to 6.5 years for rFIXFc prophylaxis. In B-LONG, subjects ≥12 years received weekly dose-adjusted prophylaxis (WP; starting dose 50 IU/kg), individualized interval-adjusted prophylaxis (IP; 100 IU/kg every 10 days initially), or on-demand dosing. In Kids B-LONG, subjects <12 years received 50-60 IU/kg every 7 days, adjusted as needed. In B-YOND, subjects received WP (20-100 IU/kg every 7 days), IP (100 IU/kg every 8-16 days), modified prophylaxis, or on-demand dosing; switching between treatment groups was permitted. A total of 123 subjects from B-LONG and 30 from Kids B-LONG were included, of whom 93 and 27, respectively, enrolled in B-YOND. Median (range) cumulative duration of treatment was 3.63 (0.003-6.48) years in B-LONG/B-YOND and 2.88 (0.30-4.80) years in Kids B-LONG/B-YOND. ABRs remained low, annualized factor consumption remained stable, and adherence was high throughout treatment. Low ABRs were also maintained in subjects with dosing intervals ≥14 days or with target joints at baseline. Complete resolution of evaluable target joints and no recurrence in 90.2% of baseline target joints during follow-up were observed. rFIXFc prophylaxis was associated with sustained clinical benefits for severe hemophilia B, including long-term bleed prevention and target joint resolution.