Concizumab prophylaxis in a 2-year-old patient with severe hemophilia B and inhibitor: a case report.

Hemophilia B is a rare X-linked congenital bleeding disorder characterized by a deficiency in coagulation factor IX (FIX). Standard management relies on exogenous factor replacement; however, the development of neutralizing alloantibodies (inhibitors) against infused clotting factor can significantly reduce therapeutic efficacy and complicate long-term management. Consequently, patients turn to bypassing agents or non-factor therapies to help achieve adequate hemostatic control. Concizumab is a novel subcutaneous non-factor therapy for hemophilia A and B that targets the tissue factor pathway inhibitor (TFPI). While it is approved for patients >12 years old, data remains sparse in younger children. Prophylactic treatment with concizumab reduces several limitations associated with inhibitor development and variability in treatment response with conventional therapies, especially in pediatric populations. We report a toddler with hemophilia B who developed a low-titer inhibitor following treatment with recombinant coagulation factor IX agents, resulting in recurrent bleeding complications and consistent subtherapeutic hemostatic control on standard bypassing agents. The patient was transitioned to concizumab for long-term prophylaxis, with dose adjustments based on clinical response and concizumab drug levels. Following the initiation of concizumab, the patient demonstrated a marked reduction in bleeding episodes. We highlight an individualized early drug-level-guided dose escalation approach in a very young child, demonstrating that pharmacokinetic monitoring can be utilized proactively to optimize therapeutic response in this age group, leading to effective drug levels and excellent clinical response. This case supports the role of concizumab in potentially reducing treatment burden and improving hemostatic outcomes in young patients with severe hemophilia B and inhibitors.