Importance of Inhibitor Surveillance During Emicizumab Prophylaxis in Young Children With Hemophilia: An Illustrative Case Series.

Hemophilia is an X-linked inherited bleeding disorder associated with bleeding, which starts in infancy. The age of initiation of prophylaxis with clotting factor concentrate is limited by the intravenous mode of administration. Emicizumab, a Factor VIII (FVIII) mimetic, may be initiated for prophylaxis in persons with hemophilia A (HA) in infancy, given the subcutaneous route of administration. Bleeds that occur while on emicizumab prophylaxis are treated with clotting factor concentrate. The primary risk of clotting factor concentrate is inhibitor development, with the highest risk occurring within the first 10-20 exposure days. Individuals on emicizumab who develop inhibitors may still use emicizumab for prophylaxis but require a change in bleed management. We report bleeding and inhibitor outcomes in six infants with severe HA, who were effectively treated with emicizumab prophylaxis starting at a median age of 8 months old. Two cases were diagnosed with high-titer inhibitors during surveillance testing performed after initiation of emicizumab. They continued emicizumab for prophylaxis and changed bleed management to Recombinant factor VIIa (rFVIIa). This report highlights the importance of ongoing inhibitor surveillance during emicizumab prophylaxis to ensure inhibitor detection, which requires a change in bleed management.