Pfizer Inc. and Bristol-Myers Squibb Co. said they have halted a late-stage study of their experimental blood-thinner apixaban because of safety concerns.

An independent data safety monitoring committee found a clinically important increase in bleeding in the 10,800-patient study of apixaban, the companies said.

There was clear evidence of a clinically important increase in bleeding among patients randomized to apixaban. This increase in bleeding was not offset by clinically meaningful reductions in ischemic events, the companies said in a statement.

The halted trial is one of nine late stage trials currently investigating apixaban.

Apixaban is an investigational oral Factor Xa inhibitor, part of a class of agents being studied for the potential to treat blood clots in the veins and arteries.

In 2007, the companies agreed to collaborate on the development and commercialization of apixaban, which was discovered by Bristol-Myers.

We remain committed to the development of apixaban in other patient populations, said Brian Daniels, senior vice president for Global Development and Medical Affairs at Bristol-Myers Squibb.

Apixaban is being evaluated within the EXPANSE Clinical Trials Program, which is projected to include nearly 60,000 patients worldwide across multiple indications and patient populations.

The program is evaluating the prevention of venous thromboembolism, prevention of stroke and other thromboembolic events in patients with atrial fibrillation and the treatment of venous thromboembolism.

Shares of Pfizer, which closed Thursday's regular trading session at $16.83, fell 2.4 percent to $16.42 in after hours trading. Bristol-Myers, which ended the day at $26.32, slipped 1 percent to $26.04 in after hours.