The surprise finding, released at a medical meeting on Sunday, is good news for rival AstraZeneca, whose competitor product Brilinta did show an edge over Plavix in a comparable group of patients in an analysis published last year.

The battle among the different blood-thinning drugs, which are all designed to prevent dangerous clots in heart patients, has moved up a gear this year with the expiry of patents on Sanofi and Bristol-Myers Squibb's widely used Plavix.

That has opened the door to cut-price generic versions of Plavix, or clopidogrel, on both sides of the Atlantic, upping the pressure on the new rivals to prove they really offer a distinctive edge.

Lilly and its partner Daiichi Sankyo had hoped to prove that Effient, also known as prasugrel, was a better option for patients with acute coronary syndrome (ACS) who do not get a stent to open blocked arteries or bypass surgery.

ACS covers a range of serious conditions, including heart attacks and unstable angina, caused by a sudden reduction of blood flow to part of the heart.

There was good reason to think Effient might be better as it is a stronger inhibitor of platelets, which cause blood clots.

In fact, the large study funded by the two companies and involving more than 9,000 patients found no real difference between Effient and Plavix in preventing heart attacks, strokes and deaths. Both drugs were tested alongside aspirin.

Cardiovascular deaths, heart attacks and strokes among patients under the age of 75 - who made up most of the study population - were seen in 13.9 percent of those on Effient versus 16.0 percent on Plavix, but this difference was not statistically significant.

On the plus side, there was also no difference in serious bleeding complications, which is always a risk with blood-thinners and has been a problem for Effient in the past.

"The safety here is very comforting," said lead investigator Dr Magnus Ohman of Duke University Medical Center.

LONGER-TERM BENEFIT?

Intriguingly, there was a trend to lower risk of adverse events in patients on Effient after 12 months of treatment, which Ohman said needed further exploration.

Dr Elliott Antman of Brigham & Women's Hospital in Boston and a spokesman for the American Heart Association, who was not involved in the study, said this suggested more potent anti-platelet therapy probably had advantages for patients over time.

Most trials evaluating anti-clotting drugs for ACS have focused on patients undergoing invasive procedures, usually getting a stent - a tiny wire mesh tube that is left in the coronary artery to prevent it from closing up again.

But some 40 percent of patients who are not in urgent need of such invasive intervention are managed solely with drug therapy, and it had been thought that they should do better on a newer and more potent medicine like Effient.

Daiichi research head Glenn Gormley said the trial result was "not the outcome we anticipated" and medical experts at the European Society of Cardiology (ESC) annual congress, where the findings were reported, agreed the result was unexpected.

"The outcome is a bit surprising because we think usually that more aggressive therapy in the face of acute coronary syndrome would lead to less adverse outcome ... but the data is what the data is," said Dr William Zoghbi, president of the American College of Cardiology.

Effient was launched three years ago and the drug has taken off more slowly than many analysts expected, with worldwide sales last year of $302.5 million. Analysts, on average, expect sales to reach $1.1 billion by 2016, according to Thomson Reuters Pharma.

Dr Raffaele De Caterina of the University of Chieti, Italy, said the findings vindicated the current ESC guidelines, which recommend AstraZeneca's Brilinta, or ticagrelor, for ACS patients regardless of treatment strategy, while Effient is limited to patients getting invasive intervention like a stent.

The results of the Effient study were published simultaneously online by the New England Journal of Medicine.

(Editing by Hugh Lawson and Helen Massy-Beresford)

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