Looks like Pfizer (NYSE: PFE) might be a little tense about its foothold in the pulmonary arterial hypertension (PAH) market; the drug giant licensed another PAH drug today.

Pfizer already sells Revatio for the treatment of PAH and bought Encysive Pharmaceuticals a few years ago for its PAH drug, Thelin. After Encysive's snafu with the Food and Drug Administration, Pfizer decided to run an additional phase 3 trial to satisfy regulators.

Now Pfizer has licensed another PAH drug, terguride, from Ergonex Pharma. Pfizer is taking over development outside of Japan -- terguride is currently in a phase 2 trial -- and will pay Ergonex undisclosed milestone payments and royalties if the drug works.

Without knowing any financial details it's hard to know whether Pfizer got a good deal, but the bigger question is why it felt the need for another PAH drug in the first place.

Revatio has the same active ingredient as Pfizer's erectile dysfunction drug Viagra. Pfizer says the U.S. patent on Viagra will expire in 2012, so presumably Revatio will see generic competition at the same time. Thelin is approved in Europe, but Pfizer doesn't break out sales. Perhaps it figures Thelin won't be a big seller in the U.S. even if it passes its phase 3 trial.

Pfizer may also be worried about the competition. Sales of Gilead Sciences' (Nasdaq: GILD) Letairis were up 40% in the first quarter. Outside the U.S., where GlaxoSmithKline (NYSE: GSK) sells the drug as Volibris, sales more than doubled in the first quarter, albeit from a small base. Revatio also got new competition last year from United Therapeutics' (Nasdaq: UTHR) Adcirca, which is the active ingredient in Eli Lilly's (NYSE: LLY) erectile dysfunction drug Cialis.

When it comes to pipelines, the more options the better, but only to a point. There's not much reason to develop a drug that's just going to eat at sales of your current offerings. Only time will tell whether that's the case for Pfizer's newest PAH addition.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool owns shares of Glaxo and has a disclosure policy.