* Xarelto as effective as warfarin in trial -Bayer
* Bleeding risk shown to be comparable to warfarin -Bayer
* Detailed results to be presented at Nov. 15 conference
* Bayer shares up 3 percent, J&J shares gain in Frankfurt
By Ludwig Burger
FRANKFURT, Nov 1 (Reuters) - Shares in Bayer jumped to the top of Germany's stock benchmark after preliminary study results lifted the odds for its new stroke prevention drug to enter a $12 billion plus market.
The anti-clotting pill Xarelto, co-developed with Johnson & Johnson, proved a safe alternative to the complicated standard treatment warfarin, the German drugmaker said late on Sunday, giving Bayer's shares a 3 percent boost on Monday.
Shares in partner J&J rose 1 percent on the Frankfurt stock exchange ahead of the U.S. open. Both outperformed the STOXX Europe 600 Health Care's 0.4 percent gain at 0936 GMT.
A late-stage Phase III study called Rocket-AF showed that Xarelto was as good as the generic warfarin pill at reducing the risk of strokes due to a form of irregular heartbeat that is common among the elderly, Bayer said in its brief statement.
The preliminary results also revealed that patients on Xarelto, an anti-blood-clotting drug based on the active ingredient rivaroxaban, ran a risk of bleeding that was comparable to those on warfarin.
The danger of haemorrhaging -- a common side effect in anti-clotting drugs -- was a main focus of the trial whose participants were in their mid-70s.
Xarelto's efficacy had largely become evident in previous studies, where it was used to prevent thrombosis in patients bed-ridden after surgery.
Detailed results of the Rocket-AF trial are scheduled to be presented at the American Heart Association conference in Chicago on Nov. 15.
"(Bayer's) healthcare unit is very well positioned for long-term earnings growth. However, the exact competitive positioning (of Xarelto) is not clear yet," said Kepler Capital Markets analyst Tero Weckroth.
He said he expects annual sales up to 3.2 billion euros ($4.5 billion) from the oral drug. Before Sunday's results, analysts had on average forecast sales of $2.2 billion in 2014, according to Thomson Reuters Pharma.
Weckroth and other analysts pointed out that while the study was only designed to show Xarelto to be statistically non-inferior to warfarin, the Bayer pill could even prove to have superior effectiveness in the detailed analysis on Nov. 15.
On the other hand, worrisome side effects beyond bleeding such as liver damage cannot be ruled out when regulators take a fine-tooth comb to the study data, WestLB analyst Cornelia Thomas cautioned.
Bayer declined to provide details ahead of the conference.
The drugmaker has said it is eyeing more than 2 billion euros in annual sales from Xarelto in a market worth an estimated $12 billion to $15 billion in total, a commercial potential only matched by its cancer treatment Nexavar.
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Analysts' projections of that market range between $10 billion and $20 billion in annual sales.
Xarelto is already approved in Europe to prevent thrombosis in patients bed-ridden after surgery.
But prevention of strokes in patients with a type of irregular heartbeat called atrial fibrillation (AF) is a far bigger opportunity for the new anti-coagulants.
In that market, Bayer's unlisted German rival Boehringer Ingelheim is in the lead with a product called Pradaxa that won U.S. regulatory approval on Oct 19 and which will be priced at $6.75 per day.
Bristol-Myers Squibb and Pfizer are also vying to enter the stroke prevention market with their experimental apixaban pill.
Warfarin interacts badly with food as well as other medicines and requires regular blood tests, making it a particularly onerous therapy for the elderly -- the patient group most affected by AF. ($1=.7158 Euro) (Additional reporting by Frank Siebelt and Ben Hirschler in London; Editing by Hans Peters)