Pfizer Inc. says it is halting a key study of its experimental blood thinner apixaban because of dangerous bleeding among patients with a history of heart disease.

The drug, co-developed with Bristol-Myers Squibb Corp., has been touted as a potential blockbuster drug that could prevent heart attack and stroke without the bleeding side effects associated with older medications.

But Pfizer says it is halting a 10,000-patient study of the drug because of what it said was a clinically important increase in bleeding among patients.

The company is studying the drug for other uses to prevent blood clots and stroke in patients with irregular heartbeats.

Apixaban works by blocking a clotting protein called factor Xa. That is in contrast to drugs like Plavix, which work by preventing platelets from sticking together.