THURSDAY, Feb. 10 (HealthDay News) -- A new anti-clotting drug works better than aspirin for stroke prevention in some patients with the common, sometimes lethal, heart rhythm problem known as atrial fibrillation, according to research presented Thursday.

The new drug, apixaban, is not yet approved for use by the U.S. Food and Drug Administration. But study co-author Dr. Hans-Christoph Diener said the pill "reduced stroke risk [in patients with atrial fibrillation] by 55 percent, compared to aspirin." He believes that "the results of this clinical trial will change clinical practice."

Diener, of the department of neurology and the Stroke Center at University Hospital Essen in Germany, presented the findings at the annual meeting of the American Stroke Association's International Stroke Conference in Los Angeles. The findings are also published online Feb. 10 in the New England Journal of Medicine.

In atrial fibrillation, an irregular beating of the heart causes blood to pool in the heart's chambers. The heart can then "throw" clots up into the arteries supplying blood to the brain, greatly raising the risks for stroke.

Patients with atrial fibrillation are typically prescribed anticoagulants such as warfarin, which is notoriously hard to manage, Diener said at a news conference announcing the study results.

Anticoagulants taken orally can decrease stroke risk by up to 70 percent, according to Diener, but many patients don't comply with the regimen. "About half of all patients refuse to take [warfarin]," he noted, because its use is accompanied by dietary restrictions and the need for frequent blood tests to check blood levels of the drug. Some patients also fear the possibility of a known hazard of warfarin, an excess risk for bleeding.

Many patients who can't or won't take warfarin do take daily aspirin, which cuts the odds of stroke in atrial fibrillation by about 20 percent, according to background information in the study.

In the new study of apixaban, researchers assigned almost 5,600 patients with atrial fibrillation and an increased risk of stroke (due to age or prior stroke, for instance) to one of two groups: apixaban, at 5 milligrams taken twice daily; or aspirin, with doses ranging from 81 to 324 milligrams per day.

The study was done at 522 centers in 36 countries from late 2007 to late 2009. The researchers wanted to compare which drug was better at preventing stroke or blockages due to blood clots elsewhere in the body, called systemic embolism.

Among patients on apixaban, there were 51 strokes or embolisms, or 1.6 percent per year, compared to 113 such events, or 3.7 percent, among those on aspirin.

While apixaban patients experienced 44 major bleeding events, aspirin takers had 39, but the difference was not great enough to be significant from a statistical point of view, Diener said.

The study was funded by Bristol-Myers Squibb and Pfizer, who are working jointly to develop apixaban.

The drug has been shown in previous research to be better at preventing dangerous leg blood clots and lung clots after hip replacement surgery than an older drug, enoxaparin.

Apixaban works by blocking a crucial step in the formation of blood clots. The study of the drug's effects on stroke prevention was actually halted early after one year, Diener said, because of the huge difference found between the two drugs and the superiority of apixaban.

The new drug isn't yet approved by the FDA and Diener couldn't predict when that might happen. Results of another study, a head-to-head comparison of apixaban against warfarin, is due out in August, he said.

A 55 percent reduction in stroke risk compared to aspirin is impressive, said Dr. Larry Chinitz, professor of medicine at the New York University School of Medicine and director of the Heart Rhythm Center at NYU Langone Medical Center. He reviewed the study findings but was not involved in the research.

"I think it's a game-changer" for higher risk patients with atrial fibrillation, he said, such as those over age 70.

The new drug, if approved, ''will certainly improve the lifestyle of patients," Chinitz said, as it won't require, as warfarin does, frequent blood tests or dietary restriction.

Another new anti-clotting drug, Pradaxa (dabigatran), was approved by the FDA in October 2010 for stroke prevention in those with atrial fibrillation. It inhibits an enzyme involved in blood clotting.

More information

To learn stroke's warning signs, head to the American Stroke Association.