CSL, including CSL Behring in the Drug Discovery & Development Pharma 50FDA has approved CSL Behring’s (ASX: CSL) Hemgenix (etranacogene dezaparvovec), an adenovirus-associated virus–based gene therapy for adults with hemophilia B.

The price tag for the gene therapy — the first for hemophilia B — is $3.5 million. The drug is administered as a single infusion.

Hemophilia B is a genetic bleeding disorder caused by missing or defective blood coagulation factor IX, a clotting protein. The condition affects men more often than women.

People with moderate to severe hemophilia B typically receive regular prophylactic infusions of factor IX replacement therapy, frequently several times per week.

In the Phase 3 HOPE-B study, Hemgenix demonstrated “a profound and impressive decrease in bleeding rates compared to standard of care,” said Dr. Steve Pascoe, CSL Behring’s senior vice president of clinical and therapeutic strategy, in an R&D investor briefing.

“We also saw that factor IX levels rose dramatically above and beyond what you would expect with prophylactic replacement therapy,” Pascoe added.

In the HOPE-B trial, Hemgenix lowered the rate of annual bleeds. A total of 94% of patients discontinued factor IX prophylaxis and were prophylaxis-free.

The FDA approval came earlier than expected. It covers patients on Factor IX prophylaxis therapy or with a history of life-threatening hemorrhage or serious spontaneous bleeding episodes.

Hemophilia B affects roughly one in 25,000 male births, according to the National Organization for Rare Disorders (NORD). It is less prevalent than hemophilia A, which occurs in about one in 5,000 male births.

CSL Behring is optimistic about the durability of the treatment. The company had observed that factor IX levels were maintained up to 24 months after initiation of the therapy. In addition, following a Hemgenix infusion, study participants had mean factor IX activity of 39% at six months and 36.7% at 24 months.

The 24-month data gives the company “enhanced confidence in the durability of etranacogene,” Pascoe said.

Before CSL Behring announced the price of the gene therapy, Pascoe said that etranacogene represented “a true step up in value proposition in the treatment benefits conferred to patients.” To support that assertion, he pointed to the increased protection from bleeding.

“What is less obvious is the impact of the disease and medicalization has on patients,” Pascoe said. “Currently, even well-treated hemophilia B patients are recommended to avoid strenuous sports and, indeed, to avoid occupations that involve some degree of risk.”

In the HOPE-B clinical trial, 51 out of 54 patients (94%) who received Hemgenix discontinued prophylaxis therapy.

“We believe that, with the Factor IX levels we’re seeing, a large number of patients will actually be able to normalize their lives,” Pascoe added. “That, we believe, is a substantive benefit that has not been seen with any other treatments.”