LONDON, Oct 6 (Reuters) - AstraZeneca's blood-thinning drug Brilinta has won preferred status in the United States, where two medical groups have recommended it over the older drug Plavix, also known as clopidogrel.

The endorsement comes in an updated guideline from the American Heart Association and American College of Cardiology on the management of so-called non-ST-elevation acute coronary syndromes, or acute chest pain without certain electrocardiogram changes.

AstraZeneca said on Monday it was the first time the cardiology organisations had recommended one drug of this type over another in the treatment of acute coronary syndromes.

Brilinta has struggled so far to gain traction in a competitive market, where Plavix -- developed by Sanofi and Bristol-Myers Squibb -- is now available as a cheap generic.

However, prospects for the drug brightened in August when the U.S. Department of Justice dropped 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 syndromes, 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.

Commenting on the new U.S. guideline, Brilinta global vice president Tom Keith Roach said: "This is just another example of the growing momentum behind the brand, particularly in the U.S., but there is still much to do as we anticipate PEGASUS top line results at the end of the year." (Reporting by Ben Hirschler; Editing by Mark Potter)