Children's Hospital of Philadelphia Researchers Find Gene Therapy Shows Long-Term Benefits for Hemophilia B Patients

PHILADELPHIA, April 17, 2025 ~ A recent study conducted by researchers at Children's Hospital of Philadelphia (CHOP) and Royal Prince Alfred Hospital in Sydney, Australia has shown promising results for the use of gene therapy to treat hemophilia B. The study, which was published in the New England Journal of Medicine, is the longest follow-up study to date for individuals receiving the gene therapy fidanacogene elaparvovec.

Hemophilia B is a rare, X-linked disorder that results from a deficiency in coagulation factor IX (FIX) activity. This inherited gene mutation impairs the body's ability to produce normal levels of the blood clotting factor IX, leading to disabling or life-threatening bleeding episodes. The current standard of care for hemophilia B involves intravenous factor replacement therapy to prevent or manage bleeding episodes and reduce complications caused by frequent bleeding.

However, researchers have been exploring alternative treatments such as gene therapy using recombinant adeno-associated virus (rAAV) vectors. Fidanacogene elaparvovec is a type of gene therapy that incorporates a hepatotropic AAV capsid and a high-activity FIX transgene encoding FIX-R338L (also known as FIX-Padua). This small natural change in the FIX protein makes it 8-12 times more effective, allowing patients to receive a lower dose of the gene therapy while still achieving the desired level of FIX activity.

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The initial successful trial for this gene therapy was led by Dr. Lindsey George, a hematologist at CHOP with expertise in gene therapy and Director of Clinical In Vivo Gene Therapy at CHOP. Published in 2017 in the New England Journal of Medicine, this trial showed therapeutic expression of factor IX after gene transfer in 10 participants with hemophilia B after one year of follow-up. The current study is a long-term progression of this initial research.

Dr. Benjamin J. Samelson-Jones, an attending physician in the Division of Hematology at CHOP and lead author of the study, stated, "Our findings mark the longest-ever follow-up for patients with hemophilia B who received gene therapy with FIX-Padua. These results offer hope that gene therapy for hemophilia B has the potential to transform the standard of care, offering a future with greater independence and improved quality of life for hemophilia patients."

The study followed 14 male participants aged 18 and older for a period of 3 to 6 years, with 8 participants still ongoing. The results showed long-term efficacy at the lowest intravenous dose of rAAV for any indication. Most patients maintained sustained levels of FIX activity and experienced fewer bleeding episodes, reduced joint pain, and an overall improvement in physical mobility.

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In terms of safety, there were no major adverse events directly linked to the therapy. Some patients did experience mild, transient elevations in liver enzyme levels, but these were effectively managed with medication. Post-treatment data also suggests that many participants were able to significantly decrease or even eliminate their reliance on FIX infusions, reducing the burden of treatment and healthcare costs.

This groundbreaking research was supported by Pfizer and was conducted by Professor John Rasko and his team at RPA Hospital in collaboration with Dr. Samelson-Jones and his team at CHOP. The study titled "Fidanacogene Elaparvovec for Hemophilia B: A Multi-Year Follow-Up Study" was published online on April 17th, 2025 in the New England Journal of Medicine (NEJM) with a DOI: 10.1056/NEJMoa2307159. These findings offer hope for a safe and effective long-term solution for individuals living with hemophilia B.

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