An observational, long-term, postauthorization, follow-up study is set to evaluate the short- and long-term effectiveness and safety of etranacogene dezaparvovec (Hemgenix®) in patients with hemophilia B.

The study plan also involves a cohort of patients with hemophilia B who receive only factor IX (FIX) prophylaxis. The study, set to start in September 2023, is not yet recruiting.

The researchers plan to enroll approximately 500 patients with hemophilia B who have been treated with etranacogene dezaparvovec in countries where it is approved for commercial use and are enrolled in the sponsor’s study or the ATHN Transcends study (hemophilia cohort, gene therapy outcomes arm), as well as the patients who receive routine prophylaxis treatment with FIX replacement therapy that are enrolled in the American Thrombosis & Hemostasis Network (ATHN) Transcends study (hemophilia cohort, natural history arm) or a similar registry.

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Those who have been treated with etranacogene dezaparvovec in a clinical trial will not be considered eligible for participation in this observational study.

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The primary outcome measure for this study will be the annualized bleeding rate (ABR) for all bleeds, measured during the 52 weeks following stable FIX expression in the follow-up period and 6, 24, 36, 48, 60, 72, 84, 96, 108, 120, 132, 144, 156, 168, and 180 months after treatment with etranacogene dezaparvovec.

The researchers will measure the number of patients with zero bleeds, the ABR for all bleeds, the mean uncontaminated endogenous FIX activity, the mean annualized consumption of FIX replacement therapy, the number of patients remaining free of previous continuous routine prophylaxis, the annualized occurrence of spontaneous bleeding into target joints, and the resolution of target joints bleeding. They will also perform a correlation analysis of ABR as a function of mean FIX activity as a secondary outcome measure in the etranacogene dezaparvovec cohort.

In addition, they will analyze the ABR for total, spontaneous, FIX-treated, traumatic, and joint bleeds as well as the mean annualized consumption of FIX replacement therapy and the annualized occurrence of spontaneous bleeding into target joints as secondary outcome measures in the FIX prophylaxis cohort.

The secondary outcome measures will also include the incidence of related serious adverse events and adverse events of special interest.

The study, sponsored by CSL Behring, is estimated to last until August 2043.


An observational cohort study to characterize the effectiveness and safety of HEMGENIX® in patients with hemophilia B (IX-TEND 4001). Published online August 24, 2023. Accessed September 3, 2023.