Gene therapy at $3.5m a dose approved for US adults with hemophilia B

US drug regulators have approved a one-off gene-therapy treatment for adults with the genetic blood disorder hemophilia B that frees patients from repeated treatments but costs $3.5m a dose, making it the most expensive medicine in the world.

Hemophilia B represents 15% of hemophilia patients, occurs mostly in men and is prevalent in about one in 40,000 of the population. Women who carry the disease often have no symptoms.

New gene therapies are notoriously expensive and the cost of Hemgenix raises questions about whether it will be widely adopted.

Traditional treatments directly infuse missing proteins, or clotting factors, that the body needs to form clots and stop bleeding. It is estimated that about 16 million people in the US and Europe have hemophilia B, while the more common hemophilia A affects five times as many.