Last Friday, drugmaker Gilead Sciences (NASDAQ:GILD) received good news from the Food and Drug Administration, which granted Gilead's recently acquired pulmonary arterial hypertension (PAH) drug ambrisentan a priority review.

When the FDA decides to review a drug candidate for possible marketing approval, it grants the drug's application either a standard 10-month review period or a priority six-month review. To get a six-month review, a drug needs to provide a possible "significant improvement compared to marketed products," according to the FDA.

After the FDA granted competitor Encysive Pharmaceuticals (NASDAQ:ENCY) a standard review for its PAH drug Thelin back in 2005, I predicted that ambrisentan would receive a similar review. Considering that both drugs have exhibited similar efficacy and safety results in improved six-minute walk tests and liver toxicity tests in many of their studies, it's interesting that the FDA would grant ambrisentan a priority review and Thelin a standard review.

Gilead's priority review for ambrisentan means that the FDA's target date for ruling on its possible approval is just three days after June 15, when the FDA will rule on Thelin for the third time. This is a stunning turnaround from the more-than-a-year head start that Encysive would have had marketing Thelin in the U.S., had it been approved the first go-round last March.

With Gilead now in control of ambrisentan after its acquisition of Myogen, this battle of the endothelin receptor antagonists may turn out to be a story of David versus Goliath for tiny Encysive. If Thelin has to face ambrisentan competition so soon in the U.S., there's no way that Encysive, with its tiny $200 million market cap, will outmarket Gilead, more than 160 times its size. Even Thelin's proven advantage over the current standard of care won't help it much.

Encysive already received Thelin approval in the EU, and it will reveal its first sales numbers from some of the European countries in its March fourth-quarter financial announcement. A marketing application for ambrisentan in the European Union hasn't been mentioned yet, but it can't be far behind.

Actelion has proven that there is a large and growing market for endothelin receptor antagonists, raking in Tracleer sales in excess of $550 million in just the first nine months of this year. With Thelin and ambrisentan on the verge of gaining regulatory approval in the U.S., competition in the marketplace for PAH drugs is about to heat up.

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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.