By Ben Hirschler

LONDON (Reuters) - AstraZeneca's (AZN.L) blood-thinning drug Brilinta cut the risk of death or heart attack in patients with a history of previous attacks, according to a major clinical trial that will boost hopes for the closely watched product.

Giving Brilinta to patients who had a heart attack over a year ago could more than double the number of people eligible for the medicine, which the company has predicted could eventually generate sales of $3.5 billion (2.3 billion pounds) a year.

Results from the 21,000-patient study, known as PEGASUS, were released on Wednesday, lifting shares in AstraZeneca 1.5 percent by 1000 GMT, while the wider London market fell one percent (.FTSE).

The trial, which studied subjects who had a heart attack within the past one to three years, was designed to prove Brilinta's value beyond its current approved use in patients with acute coronary syndromes, or recent attacks.

Brilinta has struggled so far to gain traction in a competitive market for blood thinners, where Plavix -- developed by Sanofi (SASY.PA) and Bristol-Myers Squibb (BMY.N) -- is now available as a cheap generic. Brilinta also competes with Eli Lilly's (LLY.N) Effient.

AstraZeneca Chief Executive Pascal Soriot has made Brilinta a top priority and the company flagged its potential when he fought off a $118 billion bid by Pfizer (PFE.N) last year, forecasting annual sales of $3.5 billion for the drug by 2023.

That makes Brilinta a critical component in AstraZeneca's wider goal of reaching overall sales of $45 billion by 2023, although many analysts remain sceptical.

The drug's sales in the first nine months of 2014 were $343 million and consensus forecasts point to them hitting only $1.23 billion by 2019, according to Thomson Reuters Cortellis.

Tom Keith-Roach, vice president for Brilinta, told Reuters he expected sales to strengthen through 2015 and the latest positive trial results should lead to expanded approvals in major markets over the next 18 months.

"This is very important news for the franchise," he said.

Analysts agreed the study was a clear win for AstraZeneca but said they wanted to see full details on the magnitude of clinical benefit, as well as data on bleeding. All blood-thinning drugs can cause bleeds.

So far, AstraZeneca has only said that the PEGASUS data was positive for both the 60 mg and 90 mg doses of Brilinta, when given alongside low-dose aspirin, and that preliminary analysis did not reveal any unexpected safety issues.

Complete results from the study will be submitted to a scientific meeting in 2015.

AstraZeneca is also conducting further studies looking at Brilinta's use in stroke, peripheral artery disease and diabetes.

(Editing by Mark Potter and Keith Weir)