(Adds Merck comment, share movement)
By Ransdell Pierson
July 24 - Merck & Co. on Wednesday said U.S. regulators had accepted its marketing application for the blood clot drug vorapaxar, aimed at preventing heart attacks and strokes in patients who have already had a heart attack but who have no history of stroke.
The experimental drug, acquired by Merck through its 2009 acquisition of rival U.S. drugmaker Schering-Plough Corp., was once considered a potential blockbuster product.
But its prospects wilted in 2011 when a safety committee overseeing a 26,000-patient study, called TRA-2P, said the new type of anti-platelet drug was not appropriate for patients who had already suffered a stroke because the drug raised bleeding risk.
Merck was allowed to press ahead with the trial, one of the largest heart-drug studies ever conducted, among patients who were in stable condition after having heart attacks or being diagnosed with clogged leg arteries. Patients who had suffered strokes were excluded from the trial.
The study's aim was to see if vorapaxar could prevent heart attack and stroke in that more-limited patient population and whether it could do so safely, given the drug's established bleeding risk.
Despite mixed results of the now-completed trial and the drug's failure in a separate earlier study called TRACER, Merck last summer said it would seek approval of vorapaxar.
"Despite the use of standard therapy, there continues to be residual risk of additional heart attacks in these patients," Merck spokeswoman Pam Eisele said on Wednesday, citing the need for new drugs like vorapaxar, which works through a different mechanism than existing medicines.
But some analysts have cautioned that even if approved, it would likely be used by a limited number of patients because of the bleeding risk.
Merck shares slipped 0.6 percent to $47.63 in morning trading on the New York Stock Exchange.
(Reporting by Ransdell Pierson; Editing by John Wallace)