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) 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.

Swiss biotechnology company Actelion ATLN.S sells a drug in the same class called Tracleer, a twice-a-day pill. Actelion receives nearly 95 percent of its revenue from Tracleer sales.

"Letairis is similar to an existing drug, but offers the potential for fewer drug interactions," Dr. John Jenkins, director of the FDA's Office of New Drugs, said in a statement.

Pfizer also sells a version of Viagra for PAH under the name Revatio.

On Friday, Encysive Pharmaceuticals Inc. ENCY.O said the FDA again declined to approve its proposed PAH drug called Thelin.

The most common side effects from Letairis included leg and ankle swelling, nasal congestion, facial flushing and sinusitis, the FDA said.

Patients should have monthly blood tests to check for potential liver injury, the FDA said. The drug should not be used by pregnant women or women who may become pregnant because it may cause birth defects. Tracleer carries similar warnings. A one-month supply will cost $3,940, said Kevin Young, Gilead's executive vice president of commercial operations. The drug will be available in the United States early next week.

Because of the risks of birth defects and liver injury, Letairis will be available only through prescribers, pharmacies and patients who sign up for a restricted distribution program, Gilead said.