CSL Behring Reports Durable Five-Year Efficacy for AAV Gene Therapy Hemgenix in Hemophilia B

December 7, 2025 — CSL Behring today announced long-term data from its pivotal Hope-B trial, demonstrating that the therapeutic benefits of Hemgenix (etranacogene dezaparvovec-drlb), the first AAV (adeno-associated virus) vector-based gene therapy for hemophilia B, remain durable five years post-infusion.

The five-year results, presented at the American Society of Hematology (ASH) annual meeting and simultaneously published in The New England Journal of Medicine, provide crucial evidence regarding the sustained efficacy of this one-time AAV treatment. Hemgenix is an AAV gene therapy indicated for the treatment of adults with Hemophilia B.

Sustained Bleeding Reduction and AAV Durability

The analysis included 51 adult male patients five years after receiving a single dose of the AAV gene therapy. The data confirmed a dramatic and sustained reduction in annual bleeding rates (ABR), addressing long-term durability questions regarding AAV therapies.

Key efficacy findings at Year Five:

• Mean Adjusted ABR Reduction: The mean adjusted ABR for all bleed types diminished by approximately 90% from the lead-in rate (from 4.2 to 0.4).

• Specific Bleed Reductions: Joint bleeds were reduced by 93%, and spontaneous bleeds were reduced by 94%.

• Prophylaxis Freedom: 51 (94%) of patients no longer required continuous prophylaxis treatment following the AAV infusion.

“To be able to move to a space where we have the opportunity to offer people living with hemophilia B a single infusion, one-time therapy and transition their lives from this burden of treatment… is really incredible,” said Deborah Long, M.D., Senior VP and Head of Medical Affairs at CSL Behring.

Sustained Factor IX Activity

The AAV gene therapy successfully maintained therapeutic levels of Factor IX, the key blood-clotting factor deficient in hemophilia B patients.

• Factor IX Activity: Mean Factor IX activity levels were sustained at greater than 36% over years one through five post-infusion.

• Year Five Level: The mean Factor IX activity level stood at 36.1 IU/dL at year five.

The safety profile remained manageable, with most of the 100 total treatment-related adverse events occurring within the first four months. Safety and efficacy largely matched or improved upon the original data that secured Hemgenix’s FDA approval in 2022. The company plans an extended study to track patients for up to 15 years post-treatment to further assess AAV longevity.

Navigating Access and Adoption

CSL Behring is navigating the complexities of access and adoption for the high-priced AAV gene therapy, which launched at $3.5 million per dose. CSL has so far dosed more than 75 commercial patients globally.

The company’s commitment stands in contrast to rivals, with Pfizer discontinuing its competing hemophilia B AAV gene therapy, Beqvez, and BioMarin moving to divest its hemophilia A gene therapy, Roctavian. CSL emphasized that the sustained five-year data and real-world evidence will be used to build patient and physician confidence in the AAV therapeutic option.