Assessment of joint bleeding and target joints in patients with severe or moderately severe hemophilia B (factor IX ≤2%) receiving prophylaxis with rIX-FP in the PROLONG-9FP clinical trial program.

Objective: To assess the impact of prophylaxis with rIX-FP, a fusion protein linking recombinant factor IX (FIX) with human albumin, on joint outcomes.

Methods: Joint outcomes were assessed in pediatric (<12 years) and adult/adolescent (≥12 years) patients receiving rIX-FP prophylaxis every 7, 10, or 14 days; patients (>18 years) well-controlled on a 14-day regimen could switch to a 21-day regimen. Target joints were defined as ≥3 spontaneous bleeds into a single joint within a 6-month period.

Results: For adult/adolescent (n = 63) and pediatric (n = 27) patients, median (Q1, Q3) annualized joint bleeding rate was 0.39 (0.00, 2.31), 0.80 (0.00, 2.85), 0.20 (0.00, 2.58), and 0.00 (0.00, 1.78) when treated with 7-, 10-, 14-, or 21-day prophylaxis. 50.0%, 38.9%, 45.5%, and 63.6% of adult/adolescent patients had no joint bleeds when treated with 7-, 10-, 14-, or 21-day prophylaxis, respectively, and 40.7%, 37.5%, and 37.5% of pediatric patients had no joint bleeds when treated with 7-, 10-, or 14-day prophylaxis. Ten adult and two pediatric patients developed target joints; all resolved by the end of the study.

Conclusion: Prophylaxis with rIX-FP produced low joint bleeding rates and provided excellent hemostatic efficacy in the treatment of joint bleeds. All target joints reported resolved with rIX-FP prophylaxis.