The U.S. Food and Drug Administration last week approved a drug made by Marathon Pharmaceuticals to treat Duchenne muscular dystrophy (DMD), a devastating muscle-wasting disease that mainly affects young boys.

The drug, Emflaza, known generically as deflazacort, belongs to a class of anti-inflammatory drugs known as corticosteroids that are frequently used to treat DMD and other conditions.

Emflaza is the first steroid to win formal FDA approval to treat DMD. Prednisone, another steroid, is frequently prescribed for the disease though it does not have official FDA approval. Emflaza is thought to have fewer side effects than Prednisone.

Deflazacort has been available outside the United States for decades.

The big issue facing closely held Marathon will be how to price the drug. Marathon did not invent deflazacort and patients have been able to import it inexpensively for personal use for years because there was not an approved U.S. treatment.

That will now change. Patients will no longer be allowed to import the drug now that an FDA-approved product is available. Northbrook, Illinois-based Marathon said the drug will carry a list price of $89,000 a year but that the company will provide a “robust” assistance program for patients who are not covered by insurance.

Duchenne’s is a rare genetic disorder affecting some 15,000 U.S. patients that causes progressive muscle deterioration and kills most sufferers by the age of 30. Last year Dr. Janet Woodcock, head of the FDA’s pharmaceuticals division, ordered the approval of a Duchenne drug made by Sarepta Therapeutics despite opposition from the agency’s top scientists and a negative recommendation from a panel of outside advisers.

The FDA approved Emflaza for all DMD patients aged 5 and older. Sarepta’s drug treats a subset of DMD patients representing about 13 percent of the total.

Sarepta’s shares closed down 5.2 percent at $27.83 on Friday.