By Kara Ngo
A newly discovered gene therapy for Hemophilia B has demonstrated sustained safety and effectiveness over a 13-year period in June 2025. Conducted by researchers at St. Jude Children’s Research Hospital and University College London, this study is the longest follow-up reported for gene therapy targeting this rare bleeding disorder.
Hemophilia B is a genetic condition caused by a mutation in the F9 gene, resulting in a deficiency of factor IX, which is a protein essential for blood clotting. Individuals with severe forms of the disorder have less than 1% of normal factor IX activity, and so they often experience spontaneous bleeding that can lead to joint damage or life-threatening events.
The gene therapy, delivered through a dose of an adeno-associated virus (AAV) vector, allowed the body to produce its own factor IX. Ten adult men with severe Hemophilia B received the treatment between 2010 and 2012. Over the 13-year follow-up, all participants maintained stable factor IX levels, with no new safety concerns. Their annual bleeding rate dropped from a median of 14 episodes to just 1.5, and seven of the ten participants no longer required preventive treatment.
“For these 10 patients, the factor levels are stable and have been at the same level across these 13 years,” added Dr. Ulrike Reiss, principal investigator at St. Jude. “Further, we have not seen any side effects or toxic events in the long-term follow-up.”.
Dr. Andrew Davidoff, co-investigator and chair of surgery at St. Jude, explains the impact of this study’s results.
“The key benefit is that gene therapy is a one-time, simple intravenous infusion that’s very straightforward to do and potentially has positive effects for a lifetime,” said Dr. Davidoff.
“I remember being told, ‘Within your lifetime, there may be a cure.’ It always seemed like a magic bullet or wishful thinking. But it’s starting to prove true. This is one step closer. So science, let’s keep making it happen,” says Terence Blue, one of the first patients in New England to receive gene therapy for Hemophilia B.
Building on this patient perspective, leading researcher from University College London, Dr. Amit Nathwani, weighed in on what this breakthrough means for the future of Hemophilia B treatment.
“It’s incredibly rewarding to see the sustained safety and efficacy, which truly validates the potential of gene therapy as a one-time treatment for this condition. Our findings answer critical questions about the long-term durability of gene therapy, offering profound hope and a significantly improved quality of life for patients,” Dr. Nathwani says.
While the therapy has proven to be safe and effective, researchers noted that high levels of antibodies to the AAV virus have persisted for years, which may limit the ability to repeat this treatment. Continued monitoring is necessary to evaluate rare risks and explore strategies to overcome these immune system challenges.
As gene therapy continues to evolve, this study marks the start of a new future, one where these inherited afflictions may no longer define a person’s life.
Reflecting on this transformation from the past, 74-year-old gene therapy recipient and pharmacist Chip Hailey says, “It’s freedom from the need. It’s freedom from having to sit down with a calendar and figure out when your next infusion is going to be... I don’t take any [factor IX] with me on vacation. That anxiety that I had, I can still relive it today. I don’t miss that one bit.”
