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Myogen (ticker: MYOG) announced positive top-line results today from the pivotal ARIES-2 trial comparing ambrisentan, a once-a-day oral endothelin receptor antagonist (NYSE:ERA), in pulmonary arterial hypertension.

ARIES-2 enrolled 186 patients in a randomized, double-blind, placebo-controlled trial evaluating two doses (2.5mg and 5mg) of ambrisentan. The primary endpoint was exercise capacity, which was defined as placebo-corrected mean change in six-minute walk distance at 12 weeks. In the study, 2.5mg dose of ambrisentan improved the six-minute walk distance by 32.3 meters (p=0.0219) while the 5mg dose improved 59.4 meters (p=0.0002). These results compared favorably with other oral PAH therapies, including Encysive's (ticker: ENCY) Thelin, which showed a 31.4 meter improvement in STRIDE-2, and Pfizer's (ticker: PFE) Rovitio, which showed a 50 meter improvement in SUPER-1.

Given these results, PAH will be extremely competitive in the coming years - Rovitio is already on the market for PAH while Thelin has a PDUFA date of March 24, 2006. Ambrisentan is about one year behind Thelin. Since ambrisentan and Thelin have the same mechanism of action, the real question is: Which drug will be the ERA of choice?

Ambrisentan appears to have a better profile than Thelin in terms of efficacy and safety. Ambrisentan's results in the six-minute walk test from ARIES-2 are significantly better than that of Thelin from STRIDE-2. From a safety standpoint, Thelin interacts with cytochrome P450 and thus interacts with Warfarin, a drug commonly prescribed to PAH patients, and to modest extent, Rovitio. Ambrisentan interacts with neither of these drugs. Results from the ARIES-1 study, which is evaluating higher doses of ambrisentan (5mg and 10mg), could further differentiate ambrisentan from Thelin. Based on the dose response in ARIES-2, ARIES-1 could show an improvement in six-minute walk significantly greater than that seen in the 2.5mg and 5mg doses.

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Source: PAH Market Heats Up: Myogen Reports Positive Data of Ambrisentan (MYOG)