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The Food and Drug Administration is expected to approve Merck’s drug to treat a rare lung disorder called pulmonary arterial hypertension on Tuesday, making available a novel treatment for a deadly condition that’s long been challenging to treat.

In a large trial published last year, the drug, called sotatercept, exceeded expectations in significantly increasing the distance that patients could walk and cutting the risk that their condition would worsen, that they would die, and that they would need new treatments.

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Pulmonary arterial hypertension, or PAH, is estimated to occur in 15 to 50 per million people, mostly in women. It’s characterized by a proliferation of cells in the blood vessels that connect the right side of heart to the lungs, making it harder for blood to pass through and leading the heart to have to work harder to pump blood into the lungs. The disease, which causes difficulty breathing, fatigue and dizziness, has a high mortality rate, with one study finding that one-fifth of patients died at around three years after diagnosis.

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