Despite being third to market, a new bloodthinner from Bristol-Myers Squibb and Pfizer

may become the biggest seller in an emerging multibillion-dollar class of drugs for preventing strokes in heart patients.

Eliquis, or apixaban, is set to steal the limelight at Europe's top medical meeting next week -- and if the data are as strong as many industry analysts expect, sales forecasts for the medicine are likely to jump.

Excitement over Eliquis has been growing since June when headline results showed it was better and safer than the decades-old warfarin in preventing strokes in patients with dangerously irregular heart rhythms.

Just how much better will become clear on August 28 when detailed data from a 18,000-patient clinical trial are presented at the European Society of Cardiology annual meeting in Paris.

In addition to reducing stroke and bleeding risk, there is a chance that Eliquis could be the first oral anticoagulant to show a statistically significant improvement in mortality over warfarin in atrial fibrillation (AF) patients.

That would be the icing on the cake, according to drug industry analysts at Jefferies, while the team at Barclays believes it would allow Eliquis to grab 50 percent of a likely $9 billion-a-year AF stroke prevention market by 2021.

Industry analysts, on average, currently expect 2015 sales of $1.6 billion for the product, which is due to be submitted for regulatory approval this year, according to Thomson Reuters Pharma.

That is less than the $3 billion forecast for Xarelto, from Bayer and Johnson & Johnson , suggesting the competitive landscape will realign if Eliquis lives up to its promise in Paris.

What's at stake is whether or not they are going to be best in class for atrial fibrillation, said Damien Conover, analyst at Morningstar.

Given that it's such a giant market, it's important to know whether or not they are going to be splitting it more equally or if they really do have best-in-class status which means they'll get the lion's share of the market.

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Confirmation of strong efficacy and low bleeding risk could boost Bristol-Myers shares some 5 percent and lift Pfizer by 2-3 percent, according to Chris Schott of JP Morgan.

On the flipside, confidence may start to ebb away in prospects for Xarelto and a third drug, Pradaxa, which is sold by privately held Boehringer Ingelheim and is currently the only approved oral alternative to warfarin in stroke prevention.

Xarelto and Pradaxa are both already approved for stopping clots after knee and hip surgery but stroke prevention in AF is the biggest opportunity for the new oral coagulants, which are also being studied for other heart conditions.

In all, the warfarin replacement market could be a $10-20 billion opportunity, analysts estimate, although low-cost warfarin -- a notoriously problematic drug -- is not going to disappear altogether, with patients who are well controlled on the old medicine likely to stay on it.

Success with Eliquis would be a boon for Bristol-Myers and Pfizer, which together face more than $30 billion in revenue loss due to patent expiries through 2016, according to Barclays.

It would underpin broader confidence that Big Pharma can still discover promising new medicines, despite the pipeline drought of recent years.

Also in focus at the Paris congress will be Roche's dalcetrapib, which is being developed to raise good HDL cholesterol.

The Swiss drugmaker is still more than a year away from being able to generate definitive data on the medicine, but doctors and analysts will be watching closely for news from two relatively small trials. A promising scientific abstract was released for one of these at the weekend.

(Additional reporting by Lewis Krauskopf in New York)

(Editing by David Cowell)

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