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.