CHICAGO (Reuters) - Johnson & Johnson's blood clot preventer Xarelto proved as effective as and safer than standard treatments against blood clots of the lung in a big study, possibly paving the way for it to become the eventual preferred treatment for the third most common cause of deaths in hospitals.

Researchers and J&J officials on Monday said the 4,833-patient study showed Xarelto was as effective as customary dual therapy -- injections of the clot-buster heparin given about the same time as the blood-thinning pill warfarin -- for treating the clots and preventing new lung clots or dangerous clots in the legs that can break free and cause lung clots. The vast majority of patients were treated for more than six months.

The largest clinical trial ever conducted among lung-clot patients also showed that those taking Xarelto, also known by its chemical name rivaroxaban, experienced only half the number of major bleeding incidents, largely brain hemorrhages, as patients receiving the heparin/warfarin combination.

Brain bleeding is one of the most worrisome side effects of warfarin, the active ingredient of rat poison that has been a mainstay anti-coagulant for more than half a century.

Rivaroxaban is just as good as standard treatment for pulmonary embolism -- these data are pretty convincing -- and this is an oral-only approach, which makes it very simple, said Dr. Harry Buller, a professor of vascular medicine at the Academic Medical Center in Amsterdam who led the trial.

In the J&J-sponsored trial, 10.3 percent of patients taking Xarelto had major or minor bleeding, compared with 11.4 percent of those taking heparin and warfarin, Buller said. He said 1.1 percent of patients taking Xarelto experienced major bleeding, versus 2.2 percent on standard dual therapy.

Buller presented the trial results on Monday at the annual scientific sessions of the American College of Cardiology in Chicago.

Xarelto has the potential to become the new standard of care, Paul Burton, cardiovascular medical leader at J&J's Janssen division, said in an interview. It may offer the opportunity for a single drug that doesn't require monitoring to be used in acute and long-term treatment of pulmonary embolism.

Based on the favorable findings, Burton said J&J plans in the second quarter to ask U.S. regulators to approve Xarelto for lung clots, which develop in an estimated 600,000 Americans a year and kill as many as 100,000 of them.

The clots, by blocking vital blood vessels, often kill within less than an hour after symptoms develop. Because immobility is a major cause of lung clots, hospitalized patients are at particular risk of developing them.

Xarelto, which J&J developed in partnership with German drugmaker Bayer AG, is already approved to reduce risk of blood clots in the legs and lungs of people who have had knee or hip replacement surgery. It is also approved to prevent strokes among people with an irregular heartbeat called atrial fibrillation.

Xarelto, like Eliquis from Bristol-Myers Squibb Co and Pfizer Inc, works by blocking a protein called Factor Xa involved in the clotting process. Neither of the new drugs carries the onerous demands of warfarin, such as the need for regular blood monitoring and strict avoidance of certain foods.

The reason people look for alternatives (to warfarin) is that it's a nightmare to give, Buller said. Rivaroxaban makes things easier for everybody -- patients and physicians.

Although Eliquis is approved in Europe to prevent blood clots after hip and knee replacements, it is not approved to treat lung clots. It is awaiting U.S. approval to prevent strokes in atrial fibrillation patients, by far the biggest commercial opportunity for the new crop of blood clot preventers.

(Editing by John Wallace)