Australian biotech giant CSL will bring the first gene therapy for haemophilia B to market in the United States after gaining regulatory approval for a new drug called Hemgenix.

CSL confirmed on Wednesday that the US Food and Drug Administration has granted approval for the therapy, which it has been developing since inking a deal worth $US655 million with US biotech UniQure back in 2020.

Patients with haemophilia B have insufficient levels of a blood protein called Factor IX, and require regular infusions throughout their lifetime to produce the blood clotting proteins they need to stay healthy.

Hemgenix is designed as a one-time treatment administered via an intravenous infusion that teaches a patient’s cells to make Factor IX, removing the need for more regular treatments.

Its approval marks a world-first for gene therapies, an emerging treatments sector which aims to remove faulty genes or add new genes to help fight disease.

Because gene therapies are designed as one-off treatments that replace regular treatments across a patient’s lifetime, they often come with steep price tags.

CSL will list Hemgenix at $US3.5 million ($5.3 million), making it one of the most expensive single treatments in the history of the United States.

When making pricing decisions, drugmakers look at the economic benefits of a treatment across a patient’s lifetime and what the treatment will save the healthcare system compared with current standards of care, as well as what insurers will be willing and able to pay.

Earlier this year, the independent non-profit group Institute for Clinical and Economic Review (ICER), which models drug pricing in the United States, looked at the treatment with the hypothetical “placeholder price” of $US2.5 million and said it would be worth the money.