On Sept. 28, 2023, Amicus Therapeutics’ treatment for a rare muscle disorder called Pompe disease was approved by the U.S. health regulator, the drugmaker said, ending its years-long efforts to launch the therapy. The concerns over the limited use of the drug, however, dragged the shares down to settle 8% lower.

The U.S. Food and Drug Administration’s (FDA) nod allowed the use of the therapy only in adults, weighing 40 kg and above, who had to switch from their current enzyme replacement therapy due to no signs of improvement. That makes the label narrower than that in Europe, which allows use in all adults with late-onset Pompe disease.

At least one analyst said the drug’s label, which restricts its use only to switch patients, weighed on the drugmaker’s shares. The late-stage trial also tested patients who had received no prior treatment. Amicus said it would launch the therapy immediately in the U.S. with a list price of around $650,000 for a patient weighing about 70 kilograms.

Its treatment consists of two components – Pombiliti, a lab-made protein alpha-glucosidase and the company’s oral drug migalastat under the brand name Opfolda, which work together to help break down glycogen.

Pompe disease is a rare genetic condition in which the body is unable to break down the complex sugar glycogen, leading to a buildup that causes muscular impairment. The condition may also lead to heart failure in young patients.