By Ben Hirschler

BARCELONA, Sept 1 (Reuters) - AstraZeneca's blood-thinning drug Brilinta is safe to use in the ambulance when heart attack patients are being rushed to hospital but giving it early in this way does not provide any extra benefit.

That is the conclusion of a clinical study presented on Monday at the European Society of Cardiology congress, which suggests the drug is a flexible treatment option for doctors.

Some experts had hoped giving Brilinta plus aspirin - both of which are simple pills - before a patient arrives in hospital would further boost the chance of restoring healthy blood flow to the heart by minimising delays.

In the event, results proved comparable in the 1,862-patient ATLANTIC trial whether the drug was given before or after arrival. Encouragingly, early use was not associated with excess bleeding, a potential problem with blood thinners.

Gilles Montalescot of the Pitie-Salpetriere Hospital in Paris, who led the study, said there was also an indication that early Brilinta use could cut the risk of stent thrombosis, when blood clots form inside stents that are used to prop open coronary arteries.

The stent thrombosis result was statistically significant but it was a secondary finding in an overall neutral study and therefore "should not be interpreted as definitive", he added.

On the downside, there was a non-significant excess of deaths in the pre-hospital group, but the total mortality rate in the study was low and it was not clear if this finding meant anything.

Marc Ditmarsch, global development leader for Brilinta at AstraZeneca, said the study indicated it could be used flexibly, while the stent thrombosis findings were "encouraging and warrant further investigation".

Prospects for Brilinta brightened last month when the U.S. Department of Justice said it was dropping an investigation into the original clinical trial used to win marketing approval, lifting a shadow hanging over the product.

AstraZeneca flagged up the promise of Brilinta in its defence against an abortive $118 billion takeover bid by Pfizer earlier this year, forecasting annual sales of $3.5 billion for the drug by 2023.

That number assumes it works in a variety of other settings beyond its current indication for acute coronary syndrome, and a series of large trials are now under way to prove the case.

AstraZeneca hopes to report headline results from one of these trials, known as PEGASUS, in December, with full findings to follow at a medical meeting in 2015. PEGASUS is assessing the use of Brilinta in patients who experienced a heart attack 1-3 years ago - a group that could more than double the number patients eligible for the drug.

(Editing by Hugh Lawson)