Gene Therapy Supply Runs Out, Company Warns of Treatment Delays

Etranacogene dezaparvovec (Hemgenix), the only commercially available gene therapy for hemophilia B, is temporarily unavailable, according to maker CSL Behring.

In a letter addressed to the hemophilia B community, the company said it is experiencing a “temporary global stockout” of the one-time gene therapy that will result in treatment delays for some patients in countries where the product is approved, including the U.S.

“We want to be very clear that this situation is not related to the safety or effectiveness of Hemgenix,” Deborah Long, MD, the company’s senior vice president of medical affairs, wrote in the letter. “Rather, it reflects the complexity of manufacturing gene therapies, and our commitment to adhering to the highest regulatory and quality standards for the people we serve. We are working with regulatory authorities on strategies to ensure stable ongoing supply for Hemgenix while preserving our high-quality standards.”

The company’s product was one of two gene therapies approved for the inherited bleeding disorder, which is caused by a deficiency of clotting factor IX. However, Pfizer last year announced it was discontinuing its product, fidanacogene elaparvovec (Beqvez), citing “the limited interest patients and their doctors have demonstrated in hemophilia gene therapies to date.”

Etranacogene dezaparvovec, approved in 2022, is indicated for adults with hemophilia B who currently use factor IX prophylaxis therapy, have current or historical life-threatening bleeding, or have repeated serious spontaneous bleeding episodes.

The gene therapy was approved based on results from HOPE-B. The final analysis of that trial showed that patients continued to experience low annualized bleeding rates and stable endogenous factor IX expression 5 years after a single infusion.

Gene therapies are tremendously effective in treating hemophilia, but their hefty price tags have been a barrier to uptake.

When approved with its list price of $3.5 million, etranacogene dezaparvovec became the world’s most expensive drug. Despite that, CSL Behring reported “continued strong growth” in sales in the U.S. and Europe over the last half of 2025.

Other approved gene therapies have been less successful commercially.

Beyond the exit of Pfizer’s hemophilia B product, BioMarin last month announced it was pulling the only approved hemophilia A gene therapy from the market — valoctocogene roxaparvovec (Roctavian) — after failing to identify a company interested in buying it.