Gilead Sciences (NASDAQ:GILD) received good news on Friday when the FDA approved its pulmonary arterial hypertension (PAH) treatment, Letairis (formerly ambrisentan), for marketing.

The success Gilead has had with Letairis at the FDA sharply contrasts with a rival compound by Encysive Pharmaceuticals (NASDAQ:ENCY) that failed to gain regulatory approval on Friday. With this head start on Thelin, if it can even get approved, Gilead now only has to face U.S. competition from Actelion's Tracleer in the oral endothelin receptor antagonist market.  

How well ambrisentan competes with Tracleer is still up in the air. Worldwide Tracleer sales were more than $700 million last year. Ambrisentan was approved to treat earlier-stage PAH patients with what is known as the World Health Organization class II or III symptoms, whereas Tracleer is approved to treat patients with the worse class III and IV symptoms (and is in testing for early-stage sufferers as well).

Ambrisentan may have some important safety advantages over Tracleer, with a lower incidence of liver-related adverse events in its clinical studies. But the drug also suffers from a very strict black box warning. Tracleer also carries a black box warning, and requires a patient access program.

After paying $2.5 billion to acquire Myogen last year, Gilead will now start to reap the rewards from this buyout, with sales of Letairis set to begin next week. With the release of positive clinical trial results for several pipeline compounds in the past two months and now the Letairis regulatory success, Gilead has had a strong first half of 2007.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy.