NEW YORK (CNN/Money) - Bristol-Myers Squibb can't afford to lose Plavix, an anti-stroke blood thinner and top-selling blockbuster, but the patent for the drug is being threatened by generic makers.

Sanofi-Aventis, the French patent holder that manufactures Plavix, and Bristol-Myers, its U.S.-based sales partner, are gearing up to defend the patent on one of their most profitable drugs in federal court this month.

"We, along with our alliance partner Sanofi-Aventis, believe in the validity of the patent and we will defend our intellectual property vigorously," said Bristol-Myers spokesman Brian Henry.

Sanofi-Snythelabo, a unit of Sanofi-Aventis, sued Dr. Reddy's Laboratories, an Indian generic drug firm, and Apotex, a Canadian generic firm, back in 2002 in response to the companies' patent challenge. Following a lengthy series of court actions, the case is getting closer to a jury trial. The outcome of the case, to be argued in the Southern District of New York, is particularly important to Bristol-Myers, which is already slated to lose the patent on blockbuster drug Pravachol in 2006.

A Plavix patent loss this year or next year could cause Bristol-Myers (down $0.13 to $25.39, Research) to cut up to half of its $1.12 annual dividend, said Le Anne Zhao, analyst for Caris & Co., who rates the company neutral. The Plavix patent is set to expire in 2011. (See correction.)

Bristol-Meyers is already bracing for next year's impending loss of Pravachol, a cardiovascular drug that accounted for $2.6 billion in 2004 sales.

Plavix sales in 2004 totaled 4.1 billion Euro, or $5.25 billion, for both companies worldwide, according to Sanofi spokesman Marc Greene. Of this amount, Bristol-Myers made $3.3 billion, said spokesman Brian Henry, accounting for more than one sixth of the company's sales, which totaled $19.4 billion in 2004.

Plavix represents nearly one tenth of all revenue for Sanofi (up $0.24 to $44.54, Research), which reported $20.4 billion in total sales for 2004. Sanofi announces first quarter earnings on May 13.

The companies involved in the suit were originally scheduled to file pre-trial documents this week, but they postponed the file date to May 27 before Judge Sidney Stein.

Citing a recent patent case involving the schizophrenia drug Zyprexa, Zhao said the odds are in favor of Sanofi.

In Indianapolis on April 27, District Court Judge Richard Young ruled in favor of Eli Lilly and Co., the patent holder for Zyprexa, which had global sales of $4.4 billion in 2004. The patent had been challenged by a trio of generic drugmakers: Dr. Reddy's, Israeli firm Teva Pharmaceuticals, and Zenith Goldline, a unit of Ivax Corp.

"The underlying patent holder usually prevails in court," said Zhao.

Undeterred by the Zyprexa ruling, a spokesman for Dr. Reddy's said his company was confident in the Plavix challenge.

"We believe that our argument in this case is strong," said the Dr. Reddy's spokesman, who did not want his name used. "We look forward to the hearing."

Dr. Reddy's just might have a chance of winning, said John Boris, analyst for Harris Nesbitt. Boris wrote in a May 10 report that the Plavix patent contained "ambiguous wording."

"An anticipation ruling would depend largely on how Judge Stein interprets certain wording within the patent," said Boris, who rates Bristol-Myers as neutral. "It adds some risk."

Boris said that Plavix is a "key growth driver" for Bristol-Myer and is projected to make up more than a quarter of the company's sales in 2005.

"A Plavix loss is not even hinted at in the stock," said Boris. "If they were to lose this, there's a very high probability that the dividend would be cut.

Correction: An earlier version of the story incorrectly quoted Le Anne Zhao, an analyst for Caris & Co., regarding a potential cut to Bristol-Myers' dividend if it loses patent protection on Plavix. CNN/Money regrets the error.

Analysts interviewed for this story do not own stock in the companies mentioned here and their firms do not conduct banking.