Stable Factor IX Expression and Sustained Reductions in Factor IX Use 9 Years After Gene Therapy With CSL220 in Adults With Haemophilia B

Highlights From the 19th Annual Congress of the European Association for Haemophilia and Allied Disorders 2026

Stable Factor IX Expression and Sustained Reductions in Factor IX Use 9 Years After Gene Therapy With CSL220 in Adults With Haemophilia B

Presenting Author: Wolfgang Miesbach, MD, PhD

W. Miesbach,1 K. Meijer,2 M. Coppens,3 R. Klamroth,4 P. E. Monahan,5 N. W. D. Jansen,5 S. Le Quellec,5 F. W. G. Leebeek6

Goethe University Hospital, Coagulation and Haemophilia Centre, Medical Clinic 2, Frankfurt Am Main, Germany
Department of Haematology, University Medical Center Groningen University of Groningen, Groningen
Amsterdam University Medical Centers, Department of Vascular Medicine, University of Amsterdam and Amsterdam Cardiovascular Sciences, Pulmonary Hypertension & Thrombosis, Amsterdam, Netherlands
Vivantes Klinikum im Friedrichshain, Berlin, Germany
CSL Behring, King of Prussia, PA, United States
Hematology, Erasmus University Medical Center, Rotterdam, Netherlands

Overview

At the EAHAD Congress, 9-year follow-up data from the Phase 1/2 extension study of CSL220, an AAV5-based gene therapy for hemophilia B and the predecessor to etranacogene dezaparvovec, were presented. The study followed 10 adults across two dose cohorts and showed stable factor IX activity through 9 years (median ~5–5.7 IU/dL), with sustained reductions in treated and spontaneous bleeding. No participant resumed continuous factor IX prophylaxis. The therapy maintained a favorable long-term safety profile, with no factor IX inhibitors, thrombotic events, late liver toxicity, or vector-related oncogenicity. These results demonstrate the durability, stability, and safety of AAV5-mediated gene therapy for hemophilia B and support its use in long-term hemophilia management.

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