AstraZeneca's new pill Brilinta for preventing heart attacks works better than Plavix, the world's second biggest selling drug, without increasing the amount of life-threatening bleeding, researchers said on Sunday.

The positive clinical trial result should ensure the blood thinner wins a slice of the Plavix market, worth $9 billion last year, and is likely to trigger upgrades in analyst forecasts for the drug, which AstraZeneca hopes to launch in 2010.

Some analysts believe it could be a $2 billion-plus seller.

Plavix is sold by Sanofi-Aventis and Bristol-Myers Squibb's. It already faces one rival in Eli Lilly and Daiichi Sankyo's recently launch Effient, which has also outperformed Plavix in tests.

But Effient, also known as prasugrel, is hampered by a strict warning on bleeding risks.

This drug appears to be even better than prasugrel, said Douglas Weaver of the Henry Ford Heart and Vascular Institute in Detroit, who was not involved in the study.

AstraZeneca believes its medicine has an edge in part because its action can be reversed. This means that heart patients who take Brilinta can be prepared for any necessary surgery much sooner, without excessive bleeding risk.

On the downside, it must taken twice a day, while Plavix and Effient are once-daily.

AstraZeneca said in May that Brilinta, or ticagrelor, had proved superior to Plavix in a head-to-head Phase III trial. But doctors and investors were only given details of the study at the European Society of Cardiology annual meeting in Barcelona.

These showed that patients with acute coronary syndrome (ACS) who took Brilinta were 16 percent less likely than those on Plavix to die from cardiovascular causes or suffer a heart attack or stroke.

That was a slightly weaker showing than for Effient, which showed a 19 percent advantage over Plavix in a different trial that analysts said was less onerous in design.

Brilinta also showed a 22 percent relative risk reduction in rates of death from any cause in the latest study.

The fact that ticagrelor was able to lower death rates is very powerful. We didn't see that with prasugrel, said Kirk Garratt, a cardiologist at Lenox Hill Hospital in New York.

While there was no significant difference in major bleeding risk, patients on Brilinta were more likely than those on Plavix to have spontaneous bleeding unrelated to heart bypass surgery, including intracranial and gastrointestinal bleeds.

Brilinta was also linked with shortness of breath, though less than 1 percent of patients discontinued treatment because of this, and there were more cases of slowing of heart rhythms, though this rarely caused any symptoms.

Lars Wallentin of the Uppsala Clinical Research Center in Sweden and colleagues, who carried out the one-year study involving more than 18,000 patients with ACS, also reported their results online in the New England Journal of Medicine.

ACS describes a range of conditions, including unstable angina, or acute chest pain, and heart attack.

AstraZeneca is relying on Brilinta and a clutch of other drugs in its pipeline to help offset a looming loss of patent protection on existing blockbusters such as Nexium and Seroquel.

Current consensus risk-adjusted forecasts for 2014 Brilinta sales are just $686 million, according to industry consultancy Evaluate Pharma. But some analysts think sales could reach $2.5 billion or more.

While Brilinta could dampen prospects for Effient, it will probably not have a significant impact on Sanofi or Bristol, since Plavix is expected to face U.S. generic competition in 2012. Generic copies of the drug are already on sale in Europe.