WASHINGTON (AP) - The government approved a pill that could dramatically transform treatment for hundreds of people left gasping by a rare but often fatal type of high blood pressure.

Tracleer becomes the first oral medication to treat pulmonary hypertension, a disease that causes dangerously high blood pressure in the artery connecting the heart and lungs. If not alleviated quickly, it can destroy the heart.

Today's only treatment forces patients to wear for life a battery-powered pump that infuses a medication through a permanently implanted tube in the chest.

This intravenous prostacyclin relaxes blood vessels and buys an extra five years of life for about 60 percent of patients. But it causes some troubling side effects - and patients can die if the tube gets blocked or the pump breaks down.

Tracleer, made by Swiss pharmaceutical company Actelion Ltd., is a pill that works by blocking a chemical named endothelin that is involved in constricting blood vessels.

"It's pretty important," Food and Drug Administration drug chief Dr. Robert Temple said of Tracleer. "There's considerable excitement" among specialists and patients awaiting the drug.

But Tracleer may cause serious liver toxicity and could cause birth defects, the FDA warned. So patients need monthly blood tests to catch early signs of liver damage, and female patients need monthly pregnancy tests, the agency decided.

To be sure patients get the required tests, the FDA will not let Tracleer sell in general drugstores. Instead, Actelion will hire a central pharmacy to mail patients pills once their doctors enroll them in the Tracleer access program - and remind both patients and doctors before each refill if anyone misses a test.

Tracleer offers enough benefit to people who "have a terrible disease" to surmount those big risks, Temple said. "We think this is going to be manageable for them."

Actelion said Tracleer sales will begin in December, and the drug will cost $28,500 a year. That's far less than today's intravenous treatment, which costs about $60,000, Actelion said.

Primary pulmonary hypertension strikes one or two people per million, primarily women in their 30s and 40s. Its cause is usually unknown, although it was a rare side effect of the now-banned diet pills Redux and fen-phen. Another form of the disease, called secondary pulmonary hypertension, occurs as a complication of such disorders as lupus.

First symptoms are subtle - mild shortness of breath or difficulty exercising - but the disease can progress rapidly, leaving patients gasping for air as they walk across a room. Untreated, only about half survive two years.

Researchers gave 213 severely ill patients either Tracleer for three months or a dummy pill. At the end of that period, scientists measured how far the patients could walk in six minutes. Tracleer recipients could walk 35 feet farther, an improvement that Temple called "extremely impressive."

Today's pulmonary hypertension treatment, prostacyclin, is proved to extend many patients' lives. Tracleer, known chemically as bosentan, hasn't been studied long enough to know if it will extend life, too.

But in studies, 6 percent of Tracleer patients either died or significantly worsened, compared with 20 percent of patients taking a placebo.

If patients do worsen on Tracleer, the pumped drug remains an option they could try, Temple noted.

Copyright 2001 The Associated Press. All rights reserved.