UPDATE 3-US approves new drug for severe lung disease

(Updates with FDA decision on Encysive Pharmaceuticals drug)

By Lisa Richwine

WASHINGTON, June 15 (Reuters) - A new pill for a debilitating and often life-threatening lung disease won approval from U.S. health officials on Friday. Letairis from Gilead Sciences Inc. (GILD.O: Quote, Profile, Research, Stock Buzz) received Food and Drug Administration clearance for treating pulmonary arterial hypertension, or PAH.

Industry analysts estimate sales of the drug could top $1 billion a year. The generic name is ambrisentan.

PAH results from high blood pressure in the arteries around the lungs. Early symptoms of fatigue can progress to the point that minor exertion is extremely difficult.

About 100,000 people in the United States have PAH, the FDA said. The disease can occur as a complication of illnesses such as scleroderma and lupus, or for other unknown reasons.

In company studies of 393 patients, people who took Gilead's once-a-day pill were able to walk further on a standard six-minute walk test that measures endurance. Letairis also delayed the worsening of PAH, the FDA said.

The new drug is part of a family called endothelin receptor antagonists (ERAs), which block the action of a substance that narrows blood vessels and elevates blood pressure.