Merck & Co. Inc. (MRK) said trial investigators reported an increase in bleeding in or around the brain among patients with a history of stroke who were participating in clinical trials of an experimental anti-clotting drug.

Merck had announced last week that researchers halted one study of the drug, vorapaxar, and altered the design of another study of the drug, casting doubt about the future of a drug that many thought had blockbuster potential.

Although the researchers in the Merck-sponsored study had suggested last week the drug might not be suitable for people with a history of stroke, the company didn't state specifically what led to the changes.

Merck said Wednesday that the chairman of one of the studies, Eugene Braunwald of Brigham & Women's Hospital in Boston, wrote to study investigators Tuesday with new information about the findings.

According to Merck, Dr. Braunwald wrote that a data safety monitoring board overseeing the study "has communicated to us that based on all of the data (safety and efficacy) available to them from both trials, they recommend that subjects with a history of stroke not receive vorapaxar. They have observed an increase in intracranial hemorrhage in patients with a history of stroke that is not outweighed by their considerations of potential benefit."

Dr. Braunwald also wrote that the DSMB has recommended that one of the trials continue to completion in the more than 20,000 subjects with history of heart attack or peripheral arterial disease, who haven't had a stroke, according to Merck.

Dr. Braunwald couldn't immediately be reached.

"We don't know the specific safety and efficacy results yet and so we're waiting to see what the data from these two studies show," Merck spokesman Ron Rogers said.

Last week, Merck said it would immediately discontinue patients who have suffered a stroke before or during the trial.

Vorapaxar, an anticlotting agent that had shown promise in preventing heart attacks and strokes without increased risk of bleeding, is one of the industry's most intensely followed new drugs.

Merck previously planned to file its application for the drug with the Food and Drug Administration later this year, and analysts have said sales would be above any of Merck's other drugs in development.

Merck's shares were up 2 cents to $33.93 after hours.

-Peter Loftus, Dow Jones Newswires; +1-215-656-8289; peter.loftus@dowjones.com

--John Kell, Ron Winslow and Jonathan D. Rockoff contributed to this report.

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