Post hoc analysis of bleeding episodes and clinically relevant pharmacokinetic parameters among children <12 years old with severe haemophilia A receiving once-weekly efanesoctocog alfa prophylaxis in the XTEND-Kids phase 3 multinational trial

Background: Once-weekly efanesoctocog alfa (50 IU/kg) provided high-sustained factor VIII (FVIII) activity and robust bleed protection in previously treated children with severe haemophilia A. Objective: This post hoc analysis presents additional bleed treatment and pharmacokinetic (PK) outcomes from the XTEND-Kids study (NCT04759131). Patients/methods: XTEND-Kids was a Phase 3, open-label study in previously treated children &lt;12 years with severe haemophilia A. Participants received once-weekly efanesoctocog alfa (50 IU/kg) prophylaxis for 52 weeks. Annualised bleed rate (ABR), bleed treatment, response to treatment, PK, and safety outcomes, including immunogenicity, were assessed. Results: Once-weekly prophylaxis with efanesoctocog alfa (50 IU/kg) provided high-sustained FVIII levels in the normal to near-normal range (&gt;40 IU/dL) for 3 days and &gt;10 IU/dL for ∼7 days at steady state; PK parameters were similar in participants with blood groups O and non-O. Efanesoctocog alfa prophylaxis resulted in effective bleed protection with zero bleeds in 64% of participants and consistently low ABRs. The rate of spontaneous and traumatic bleeding episodes per week remained consistently low throughout the 52-week study period. A single dose of efanesoctocog alfa resolved 95% of bleeding episodes, regardless of type or location. The haemostatic efficacy of efanesoctocog alfa for the treatment of bleeding episodes was rated as excellent or good for nearly all evaluated first injections (97%). No inhibitors or unexpected safety findings were reported. Conclusion: Once-weekly prophylaxis with efanesoctocog alfa provided high-sustained FVIII levels and highly effective bleed treatment and prevention in previously treated children &lt;12 years of age with severe haemophilia A.