Rival anti-clotting drugs from Bayer and Bristol-Myers Squibb both impressed in clinical trials, keeping them firmly in the race to displace the old, problematic heart drug warfarin.

Industry analysts believe the market for products to replace warfarin, which was originally developed as rat poison, could eventually be worth more than $10 billion a year and possibly as much as $20 billion.

Bristol's apixaban, which is partnered with Pfizer
, reduced the risk of stroke by 52 percent compared with aspirin, with no significant rise in major bleeding, researchers told the European Society of Cardiology congress on Tuesday.

Bayer's Xarelto delivered good results in dissolving potentially deadly clots in the legs and preventing new ones, without causing either excessive bleeding or liver problems, reassuring those worried about its side effects.

The German drugmaker had already announced on August 4 that Xarelto, being developed with Johnson & Johnson , was as effective as the standard treatment of injecting Sanofi-Aventis's Lovenox, followed by warfarin pills, but had not given precise data.

Unlisted German company Boehringer Ingelheim is leading the race with Pradaxa, which will be reviewed by a U.S. Food and Drug Administration regulatory panel on September 20 -- but it is clear competitors are on its heels.

It's still an open race for Bayer ... If Pradaxa gets FDA approval, I don't see why Xarelto shouldn't, said LBBW analyst Karl-Heinz Scheunemann.

Bayer shares were 2.5 percent higher by 1430 GMT, while Bristol had gained 1.5 percent.

EASY COMPARISON

The strongly positive results for apixaban were no great surprise, partly because Bristol and Pfizer had already announced in June that the trial was being stopped early because of its success and partly because aspirin is an easy comparison.

Tim Anderson, analyst at Sanford Bernstein, said halving stroke risk in patients with atrial fibrillation (AF), an irregular heartbeat, was broadly in line with the relative risk reduction seen in past trials comparing warfarin and aspirin.

An estimated 40 to 50 percent of AF patients are either intolerant to warfarin or refuse to take the medicine, leaving them dependent on less-effective aspirin -- opening an important initial commercial opportunity for the new product.

Apixaban is also being tested head-to-head against warfarin, with results from that trial expected in mid-2011.

Anderson is forecasting global apixaban sales of $1.7 billion by 2015, making it one of the biggest new drug prospects in development in the industry.

Bayer, meanwhile, has said Xarelto could eventually generate more than 2 billion euros ($2.5 billion) in annual sales.

Stuart Connolly of McMaster University in Canada, who presented the apixaban findings, said the results were truly impressive and it was highly likely the ongoing trial testing the drug against warfarin would also be positive.

Results of a second clinical study are also keenly awaited with Xarelto -- and in this case doctors and investors don't have as long to wait as with apixaban.

A key trial looking at Xarelto when given to significantly older patients as a stroke preventer will be presented at the American Heart Association annual meeting in November.

Xarelto researcher Harry Bueller of Amsterdam's Academic Medical Center said he could not speculate on the read-across from the deep vein thrombosis (DVT) trial presented in Stockholm to the stroke trial, but said the data so far was reassuring.

He noted that while the DVT study only showed Xarelto was as good as standard care in terms of efficacy, it came very close to statistically significant superiority, with recurrent clots occurring in 2.1 percent of patients on Xarelto compared with 3.0 percent in the control arm.

(Additional reporting by Ludwig Burger in Frankfurt; Editing by Michael Shields and Will Waterman)