Use of concizumab in children with severe hemophilia B and factor IX-inhibitors

Introduction Patients with severe hemophilia B complicated by high-titer inhibitors and allergic reactions to factor IX concentrate have very limited therapeutic options. Concizumab, a subcutaneous anti–TFPI antibody, represents a novel prophylactic approach for these patients. We report our experience with use of concizumab in three pediatric patients with severe hemophilia B, recurrent inhibitors, allergic reactions to factor IX, and frequent bleeds. Method Clinical data were collected retrospectively from medical records of the patients treated from 2023 -to 2025 with concizumab. Parameters analyzed included genetic findings, treatment history, bleeding events, immune tolerance induction, and outcomes after concizumab initiation. The primary endpoint was the annualized bleeding rate, secondary endpoints were on demand factor use and adverse events. Results Patient A was diagnosed with severe hemophilia B (FIX activity