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The FDA on Tuesday the first gene therapy for treating hemophilia B, a genetic bleeding disorder resulting from missing or insufficient levels of factor IX.

Etranacogene dezaparvovec (Hemgenix) is indicated for adults with the condition who currently use factor IX prophylaxis therapy for blood clotting, those who have or have had life-threatening hemorrhage, or those who have repeated serious spontaneous bleeding episodes.

"Gene therapy for hemophilia has been on the horizon for more than two decades. Despite advancements in the treatment of hemophilia, the prevention and treatment of bleeding episodes can adversely impact individuals' quality of life," said Peter Marks, MD, PhD, director of FDA's Center for Biologics Evaluation and Research, in a statement. "Today's approval provides a new treatment option for patients with hemophilia B and represents important progress in the development of innovative therapies for those experiencing a high burden of disease associated with this form of hemophilia."

Most individuals who experience symptoms of hemophilia B -- which makes up 15% of patients with hemophilia -- are men, though women may have moderate or severe symptoms in rare cases.

Etranacogene dezaparvovec's approval was supported by results from two trials involving 57 adult men with hemophilia B.

In the ongoing HOPE-B trial, which enrolled 54 men with moderately severe to severe hemophilia B, a single dose of etranacogene dezaparvovec produced mean factor IX activity of 39% at 6 months post-infusion and 37% at 24 months.

The mean adjusted annualized bleeding rate at 7 to 18 months after infusion was reduced by 54% when compared with a lead-in period where trial participants received factor IX prophylactic replacement therapy. In addition, 94% of patients treated with the therapy discontinued use of prophylaxis and remained free of previous continuous routine prophylaxis therapy.

Etranacogene dezaparvovec "is unique in its approach to increasing mean factor IX activity and hemostatic protection in those with hemophilia B, and today's approval could fundamentally transform the treatment paradigm for this life-long condition," lead HOPE-B investigator Steven Pipe, MD, of the University of Michigan in Ann Arbor, said in a from CSL, the therapy's developer.

The most common side effects (incidence of 5% or more) associated with the gene therapy were liver enzyme elevations, headache, elevated levels of a certain blood enzyme, flu-like symptoms, infusion-related reactions, fatigue, nausea, and feeling unwell.

The therapy will be manufactured by UniQure NV, which sold the commercial rights to CSL in 2020.

CSL said the treatment would be available "as soon as possible" for eligible patients. It will be expensive, however, with the company announcing the one-time infusion will , making it the world's most expensive treatment.

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