A study evaluating efanesoctocog alfa (EFA) for children with severe hemophilia A met its primary endpoint, demonstrating no occurrence of inhibitor development. The final study results were presented as a late-breaking abstract at the International Society on Thrombosis and Haemostasis (ISTH) 2023 Congress by Lynn Malec, MD, of the Versiti Blood Research Institute and Medical College of Wisconsin in Milwaukee, Wisconsin, and colleagues.

The open-label, multicenter phase 3 XTEND-Kids study (ClinicalTrials.gov Identifier: NCT04759131) enrolled previously treated children younger than 12 years of age with severe hemophilia A (<1 IU/dL endogenous factor VIII [FVIII] activity).

Patients received once-weekly prophylaxis with EFA given intravenously at a dose of 50 IU/kg over 52 weeks. The study’s primary endpoint was the occurrence of inhibitor development, based on an inhibitor result of ≥0.6 Bethesda units/mL with confirmation in a separate test 2 to 4 weeks later.

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The overall population included 74 patients with a median age of 5.0 years (range, 1.4-11.0). The population included 38 patients below 6 years of age and 36 patients from 6 to <12 years of age. Most patients in this study did not have a family history of inhibitors.

In this study, no FVIII inhibitors were detected. The incidence of neutralizing antibodies to FVIII was 0.0% (95% CI, 0.0-4.9). Additionally, the median annualized bleeding rate (ABR) was 0.00, with an estimated mean ABR of 0.89 (95% CI, 0.56-1.42). The majority of patients (64%) had 0 bleeding episodes, 82% of patients had 0 joint bleeds, and 88% of patients had 0 spontaneous bleeds.

Among 73 patients receiving 1 injection per week per protocol, the estimated mean ABR was 0.61 (95% CI, 0.42-0.90). Overall and joint ABRs were lower among these 73 patients than in the full analysis set of 74 patients, which included 1 patient receiving an intensified consolidation treatment.

Regarding treatment of bleeding episodes, the use of a single injection of 50 IU/kg of EFA resolved 95% of bleeding episodes, or 41 of 43 bleeds, and 97% of evaluable responses were considered excellent/good. Major surgeries occurred in 2 patients, and both patients were considered to have excellent hemostatic responses with EFA given in the perioperative period.

In a safety analysis, 4.1% of patients were considered to have at least 1 treatment-emergent adverse event (TEAE) considered related to study drug. Reportedly, no TEAEs led to patient death or treatment discontinuation.

TEAEs present in >10% of the overall population included SARS-CoV-2 test positivity (14.9%), upper respiratory tract infection (14.9%), and pyrexia (12.2%). There were no inhibitors or antidrug antibodies detected or reports of serious allergic reactions, anaphylaxis, or embolic or thrombotic events.

“To conclude, there was no development of inhibitors, the primary outcome, to factor VIII or any antidrug antibodies that were detected following treatment in the XTEND-Kids study,” Dr Malec said in her presentation. She also highlighted high, sustained FVIII activity with weekly dosing of EFA and the effectiveness of EFA in bleed protection and treatment, and she indicated that reported TEAEs were generally consistent with expectations.

Disclosures: This research was supported by Sanofi and Sobi. Some authors have declared affiliations with or received grant support from the pharmaceutical industry. Please refer to the original study for a full list of disclosures.


Malec L, Peyvandi F, Chan AKC, et al; for the XTEND-Kids Trial Group. Efanesoctocog alfa prophylaxis for previously treated patients <12 years of age with severe hemophilia A. ISTH 2023 Congress; June 24-28, 2023. Abstract LB01.1.