(Reuters) -Shares of United Therapeutics surged over 42% in premarket trading on Tuesday after its lung disease treatment, treprostinil, met the main goal in a late stage study.

The inhaled drug, marketed as Tyvaso, demonstrated substantial improvement in lung function in patients with Idiopathic Pulmonary Fibrosis, a debilitating and irreversible condition characterized by stiff and scarred lung tissue, making breathing increasingly difficult.

In the 597-patient study, treprostinil significantly improved forced vital capacity (FVC) — the amount of air a person can forcibly exhale after a deep breath — compared with placebo after 52 weeks, the company said.

FVC is a widely used indicator to assess disease progression in IPF patients.

The drug is already approved in the U.S. to treat an

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