Dangerous Bleeding Halts Pfizer Blood Thinner Study

Pfizer Inc. said Thursday it is halting a late-stage study of its highly anticipated blood thinner apixaban, due to 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 for its potential to prevent heart attacks and strokes without the bleeding side effects associated warfarin, a drug used since the 1950s.

But Pfizer said in a statement late Thursday it halted a 10,000-patient study of the drug because of "a clinically important increase in bleeding among patients."

The study was designed to find out whether adding apixaban to older blood-thinning medications would decrease rates of heart attack and related problems. The companies halted the study after a panel of medical experts said the drug's bleeding side effect outweighed its benefits on the heart.

"We took our guidance from the data monitoring committee that has been monitoring this study, and their read out to us was that they saw an excess of bleeding with apixaban," said Dr. Jack Lawrence, head of apixaban development at Bristol-Myers Squibb.

All patients in the trial were already taking two other blood-thinning medications: Plavix and aspirin, the latter of which carries its own risks of bleeding.

"I think with any anti-thrombotic drug, bleeding is an unavoidable side effect. But in the successful therapies the efficacy benefit outweighs the bleeding risk," Lawrence said.

More than two years ago, the drugmakers reported that patients taking apixaban had significantly more bleeding than those taking aspirin.

Pfizer and Bristol-Myers are studying the drug in nine other trials for several uses, including: stroke prevention in patients with irregular heart rhythms and blood clot prevention in hip replacement patients.

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

In October, the FDA approved the first drug in the factor Xa-blocking class — Pradaxa, made by the German firm Boehringer Ingelheim.

Pfizer is racing several other companies, including Johnson & Johnson, to bring the second drug in the class to the U.S. market.

Pfizer's shares fell 41 cents, or 2.4 percent, to $16.42 in aftermarket trading following the apixaban announcement. Bristol Myers shed 28 cents, or 1 percent, to $26.04.