CSL Limited (ASX: CSL; USOTC: CSLLY) has announced the five-year results from its pivotal HOPE-B study evaluating Hemgenix (etranacogene dezaparvovec-drlb), the world’s first and only commercially available gene therapy for adults with haemophilia B. The results, published in the New England Journal of Medicine and presented at the American Society of Hematology (ASH) 2025 Annual Meeting, confirm the long-term efficacy and safety of the one-time treatment.
According to CSL’s five-year analysis, 94 percent of patients who received Hemgenix remained free of continuous prophylaxis treatment five years after a single infusion. Mean factor IX activity levels remained stable at 36.1 percent, offering durable bleed protection. The results mark a milestone in gene therapy for bleeding disorders, with many experts now viewing Hemgenix as a potential paradigm shift in haemophilia B management.
Analysts tracking CSL’s gene therapy portfolio believe these data significantly strengthen confidence among healthcare providers, payers, and patients, especially as concerns about long-term durability had previously slowed commercial uptake of gene therapies in rare disease markets.
How do the HOPE-B results compare to baseline prophylaxis?
The open-label, Phase 3 HOPE-B trial enrolled 54 adult male participants with severe or moderately severe haemophilia B. These individuals, who previously relied on regular prophylactic infusions of factor IX, received a one-time intravenous administration of Hemgenix. After five years, 50 patients completed follow-up, and the outcomes have exceeded early expectations.
Mean factor IX activity remained consistently above 36 percent from year one to year five. Specifically, year-one levels averaged 41.5 IU/dL, with subsequent yearly averages of 36.7, 38.6, 37.4, and finally 36.1 IU/dL in year five. These levels are considered sufficient to reduce the frequency and severity of spontaneous bleeding episodes.
The annualized bleeding rate (ABR) for all bleeds dropped by approximately 90 percent from the pre-treatment lead-in phase. In the lead-in period, patients experienced an average ABR of 4.16. By year five, this fell to just 0.40. Joint bleeds were reduced by 93 percent, and spontaneous bleeds by 94 percent, further reinforcing the therapy’s long-term protective effect.
Importantly, 94 percent of patients remained off continuous prophylaxis throughout the five-year period. Only one participant resumed prophylaxis at month 30 post-infusion due to lower-than-expected factor levels.
What do the safety data reveal about long-term gene therapy use?
The five-year safety profile of Hemgenix remained favorable. Across the duration of the study, there were no serious treatment-related adverse events reported. A total of 100 treatment-related adverse events were recorded, with the majority occurring within the first four months post-infusion.
Between years four and five, only five treatment-related adverse events were documented. The most commonly observed issue was elevated alanine transaminase (ALT), a liver enzyme. Nine participants required supportive corticosteroid therapy to manage these ALT increases, with a mean treatment duration of 81.4 days.
Two deaths occurred during the trial, but both were deemed unrelated to Hemgenix. One patient died of cardiogenic shock and urosepsis at 15 months post-dose, while another passed away due to cardiac amyloidosis at approximately 54 months. A separate incident of myelodysplastic syndrome was initially thought to be treatment-related but was later reclassified as unrelated following molecular risk assessment.
No participants developed inhibitors to factor IX, a complication that has historically limited the effectiveness of factor replacement therapies.
What does Hemgenix’s mechanism of action reveal about its long-term potential?
Hemgenix works by delivering a functional copy of the F9 gene via an adeno-associated virus 5 (AAV5) vector. This allows the liver to produce functional factor IX proteins, specifically a highly active variant known as FIX-Padua. The gene therapy is designed to remain in the target cells without integrating into the patient’s genome, thereby reducing long-term safety risks.
Once inside the liver cells, the introduced gene provides instructions to generate factor IX proteins that are roughly eight times more active than the body’s natural version. This sustained expression translates into significantly reduced bleeding episodes without the need for regular factor IX infusions.
The trial included patients with and without pre-existing antibodies to AAV5, making Hemgenix the only available gene therapy approved for use regardless of prior viral exposure status.
How does this data impact real-world use and regulatory confidence?
More than 75 individuals across eight countries have received Hemgenix in real-world settings since its approval. Regulatory authorities in the United States, Canada, the European Union, the United Kingdom, Switzerland, Australia, Saudi Arabia, South Korea, Taiwan, Singapore, and Hong Kong have already granted approvals or conditional authorizations for its use.
The release of five-year data is expected to further accelerate adoption in clinical practice. For regulators and payers, the durability of factor IX expression combined with a strong safety record provides reassurance around the cost-benefit ratio of a one-time treatment priced in the million-dollar range.
For clinicians, the ability to offer a treatment that eliminates the burden of weekly or bi-weekly infusions — and significantly improves patient quality of life — is a powerful motivator.
What is next for patients enrolled in the Hemgenix program?
Although the five-year data represent the final analysis for the pivotal HOPE-B study, CSL has confirmed that patients who opt in will continue to be monitored in an extended follow-up study known as IX-TEND 222-3003. This study will track long-term safety, efficacy, and durability for up to 15 years post-treatment.
In addition, CSL has established a global post-marketing registry to collect real-world evidence from diverse populations. This ongoing data collection is expected to inform future guidelines, reimbursement frameworks, and possibly support label expansion in pediatric or other subpopulations if future trials permit.
CSL acquired global commercialization rights to Hemgenix from uniQure N.V. (NASDAQ: QURE), which led early-stage development. The licensing deal transferred responsibility for future trials and commercial strategy to CSL, which has since positioned Hemgenix as a flagship gene therapy in its pipeline.
How do analysts and patient groups view the five-year durability update?
The release of long-term data appears to mark a major inflection point for gene therapy in rare diseases. For years, the promise of a one-time, curative treatment was hampered by uncertainty around durability, manufacturing costs, and patient access.
With Hemgenix now demonstrating stable factor expression and freedom from prophylaxis five years post-dose, institutional sentiment is shifting toward a more bullish stance.
Healthcare analysts covering the biotech sector believe this data will enhance investor confidence in both CSL and the broader gene therapy market, particularly as competing therapies for haemophilia A and other genetic disorders advance through trials.
Patient advocacy groups have also welcomed the results. Many organizations representing people with haemophilia have long argued for treatments that reduce the psychological toll and logistical burden of lifelong infusions. The HOPE-B trial data show that Hemgenix may allow for a more normal lifestyle with dramatically fewer clinical interventions.
What are the broader implications for gene therapy as a treatment category?
Hemgenix is now positioned as a proof point for the broader field of gene therapy, which has faced multiple headwinds including cost concerns, immune responses, and complex manufacturing logistics.
Success stories like Hemgenix could encourage more innovation and investment in gene therapies targeting other monogenic diseases. Moreover, the five-year HOPE-B results provide compelling evidence for healthcare systems to consider value-based payment models or long-term outcome-based reimbursement frameworks.
As CSL’s medical leadership has emphasized, the significance of Hemgenix lies not only in its scientific innovation but also in its real-world ability to deliver long-lasting benefit with a single dose. If longer-term registry follow-up continues to confirm these outcomes through year ten and beyond, Hemgenix could serve as the archetype for next-generation therapies across rare diseases.
What are the key takeaways from the five-year Hemgenix trial data?
- Hemgenix achieved a 94 percent rate of freedom from prophylaxis over five years after a single infusion
- Mean factor IX activity levels remained above 36 percent, indicating sustained therapeutic benefit
- The annualized bleeding rate dropped by approximately 90 percent compared to baseline
- No serious treatment-related adverse events were reported over five years
- The trial included patients with and without AAV5 antibodies, enhancing clinical applicability
- Real-world adoption is growing, with more than 75 patients dosed across eight countries
- Extended 15-year follow-up is underway via the IX-TEND study and global registry
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