Pfizer Inc. won U.S. approval for its first gene therapy, expanding into a promising field of medicine that has proved to be a challenging business.

The treatment, called Beqvez, gained clearance for patients with hemophilia B, a rare bleeding disorder, Pfizer said Friday in a statement. Pfizer’s medicine is a one-time therapy designed to insert a correct copy of the mutated gene that causes hemophilia B, preventing the dangerous bleeding episodes that mark the disease.

Drug companies have struggled to turn gene therapies, which offer potentially curative benefits at a high up-front cost, into profitable products. Hemophilia B affects about 40,000 people worldwide, and the first gene treatment for it, sold by the Australian biotech company CSL Behring, took six months to find its first commercial patient. Demand has remained tepid, physicians have said. BioMarin Pharmaceutical’s gene therapy for the more common hemophilia A reached only one paying patient in its first 19 months on the market.

Pfizer said it will charge $3.5 million for Beqvez and establish a warranty program that would refund payers if patients don’t benefit from the gene therapy. CSL Behring’s treatment, sold as Hemgenix, has the same price, while BioMarin’s Roctavian costs $2.9 million.

Those prices aren’t “outrageous” in the context of hemophilia treatment, said Ellis Neufeld, a hematologist at St. Jude Children’s Research Hospital in Memphis. Traditional therapy for hemophilia B, which requires twice-monthly injections of proteins called factor IX that boost blood clotting, can cost about $750,000 a year for an adult patient, Neufeld said.

“The issue is that patients are pretty satisfied with their current therapy,” said Neufeld, who has consulted for Pfizer and other drugmakers. “It’s awfully easy to get a dose of factor IX. Come to the clinic, get a short infusion, and you’re done.”

Beqvez reduced the annualized rate of bleeds by 71% compared to standard treatment for hemophilia B, according to a key clinical trial. Patients treated with the gene therapy saw a 92% decrease in their need for Factor IX infusions.

Pfizer, whose shares have lost about a third of their value in the past 12 months, is looking for new drugs that might replace the plummeting sales of its medicines for Covid-19. The company fell to its lowest point in a decade last year after forecasting 2024 revenue well below Wall Street expectations.

Pfizer is also developing gene therapies for hemophilia A and Duchenne muscular dystrophy, a rare degenerative disorder. Each is in late-stage development.