Introduction: The Quest for Durability in Hemophilia B
For decades, the standard of care for severe or moderately severe hemophilia B has been prophylactic replacement therapy. While effective at reducing bleeds, this approach imposes a significant treatment burden, requiring lifelong, frequent intravenous infusions of factor IX (FIX) concentrates. The advent of gene therapy, specifically etranacogene dezaparvovec (an AAV5-vector-based therapy expressing the hyper-functional Padua variant of FIX), promised a ‘one-and-done’ solution. However, the critical question for clinicians and patients has always been the duration of this effect. The final five-year results of the HOPE-B study provide the most definitive answer to date regarding the long-term stability and clinical utility of this therapeutic modality.
Highlights of the HOPE-B Final Analysis
The HOPE-B trial has tracked the journey of etranacogene dezaparvovec from its initial efficacy signals to its long-term maintenance phase. Key highlights include:
Sustained FIX Activity
Endogenous factor IX expression remained remarkably stable, with a mean activity of 36.1 IU/dL at five years post-infusion.
Durable Bleed Control
The adjusted annualized bleeding rate (ABR) remained low at 1.52 during the 7-to-60-month period, representing a 63% reduction compared to the lead-in prophylaxis period.
Elimination of Prophylaxis
96% of participants remained free of routine FIX prophylaxis five years after a single dose of gene therapy.
Long-term Safety Profile
No new safety signals emerged in the later years of follow-up, with most treatment-related adverse events occurring within the first six months.
Evolution of Efficacy: From 6 Months to 5 Years
The trajectory of etranacogene dezaparvovec is best understood by examining the data chronologically across the three major reporting milestones of the HOPE-B trial.
The 6-Month Milestone: Rapid Onset and Superiority
In the initial phase of the study, 54 adult males with FIX activity ≤2% were enrolled. By the 6-month mark, the primary efficacy signal was clear: factor IX activity had increased from baseline by a least-squares mean of 36.2 percentage points. This period established the therapy’s ability to rapidly transition patients from a severe phenotype to a mild or even normal range of clotting factor activity. More importantly, it demonstrated that the therapy was effective regardless of the presence of pre-existing AAV5 neutralizing antibodies (NAb) at titers below 700, a significant advantage over other AAV-based therapies.
The 24-Month Milestone: Establishing the Steady State
At two years, the focus shifted to the maintenance of expression. The post-hoc analysis showed that mean factor IX activity was sustained at 36.7%. The clinical impact was profound: the mean adjusted ABR significantly reduced from 4.18 to 1.51. Perhaps most tellingly, during each 6-month window after the initial treatment, at least 67% of participants experienced zero bleeds, compared to only 26% during the lead-in period. By this point, 52 of the 54 participants were entirely free of prophylaxis, proving that the initial rise in factor levels was not a transient spike but a durable shift in the patient’s underlying physiology.
The 5-Year Final Analysis: Proof of Long-term Stability
The recently released 60-month data provides the ‘acid test’ for gene therapy. The mean factor IX activity level was 36.1 IU/dL, showing virtually no decay from the 2-year or 6-month levels. This stability is critical, as concerns regarding the gradual loss of episomal AAV DNA as liver cells turnover have been a theoretical hurdle for the field. In the HOPE-B cohort, the Padua variant’s high specific activity likely provides a robust buffer, ensuring that even minor fluctuations in expression do not compromise clinical protection. Exogenous factor IX consumption decreased by a staggering 96%, falling from an average of over 250,000 IU/year to just under 11,000 IU/year.
Safety and the Immunological Landscape
The safety profile of etranacogene dezaparvovec remained consistent throughout the five-year observation period. Most treatment-related adverse events, such as transient elevations in alanine aminotransferase (ALT) or aspartate aminotransferase (AST), occurred within the first six months and were managed with standard corticosteroid protocols. Beyond the 6-month mark, treatment-related adverse events were rare. Crucially, there were no reported cases of hepatocellular carcinoma (HCC) or the development of FIX inhibitors, which are major safety concerns in gene therapy and hemophilia management, respectively. The observation that efficacy did not differ based on baseline AAV5 NAb status remains one of the most clinically relevant findings, as it broadens the eligible patient population significantly compared to other gene therapy platforms.
Expert Commentary: Clinical and Translational Implications
The HOPE-B results represent a landmark in genomic medicine. From a clinical perspective, the stability of the FIX levels at 36% is ideal; it places the majority of patients in the ‘mild’ category or near-normal range, where spontaneous bleeding is rare and the need for prophylaxis is virtually eliminated. This not only improves quality of life but also offers a favorable economic profile by offsetting the high cost of lifelong factor concentrates.
However, long-term monitoring remains essential. While the 5-year data is highly encouraging, the hemophilia community will continue to watch for any signs of late-onset attrition in expression. Furthermore, while the AAV5 vector appears more ‘immune-stealthy’ than others, the management of the early inflammatory response in the liver remains a nuance of clinical administration that requires specialized hematological expertise.
Conclusion
The final analysis of the HOPE-B trial demonstrates that etranacogene dezaparvovec provides a durable, safe, and highly effective therapeutic option for patients with hemophilia B. With five years of stable endogenous factor IX expression and a sustained reduction in bleeding rates, this therapy successfully addresses the core unmet need in the hemophilia community: the transition from burdensome chronic treatment to long-term disease control with a single intervention. As the first gene therapy to provide such robust 5-year data in a Phase 3 setting, it sets a high bar for future developments in the field.
Funding and Clinical Registry
This study was funded by uniQure and CSL Behring. ClinicalTrials.gov number: NCT03569891.
References
- Pipe SW, et al. Final Analysis of a Study of Etranacogene Dezaparvovec for Hemophilia B. N Engl J Med. 2025 Dec 7. doi: 10.1056/NEJMoa2514332.
- Pipe SW, et al. Gene Therapy with Etranacogene Dezaparvovec for Hemophilia B. N Engl J Med. 2023 Feb 23;388(8):706-718.
- Coppens M, et al. Etranacogene dezaparvovec gene therapy for haemophilia B (HOPE-B): 24-month post-hoc efficacy and safety data from a single-arm, multicentre, phase 3 trial. Lancet Haematol. 2024 Apr;11(4):e265-e275.