NEW YORK (CNN/Money) - Bristol-Myers Squibb and Sanofi-Aventis are preparing to defend their patent on blockbuster Plavix, an anti-stroke blood thinner, in a New York district court this week against two generic drug makers: Dr. Reddy's and Apotex.

Who's going to win?

Generally, generics exploit legal loopholes in order to claim that a patent is invalid. Dr. Reddy's Laboratories Ltd. (up $0.05 to $16.32, Research), an Indian maker of generic drugs, has been involved in at least 26 patent challenges. But Dr. Reddy's has lost most of its court battles in recent years, eroding its chances of winning this Plavix challenge, according to Merrill Lynch analyst Visalakshi Chandramouli.

A 2001 win giving Dr. Reddy's the right to manufacture a generic form of Prozac is the company's only successful patent challenge since the company was founded in 1984, according to Chandramouli.

The Canadian Federal Court of Ottawa rejected a separate challenge of the Plavix patent, held by French drug-making giant Sanofi-Aventis (down $0.23 to $45.61, Research), by Apotex Inc., a Canadian generic manufacturer. Chandramouli believes this precedent will make it harder for the challengers to succeed in a U.S. court.

"With the Canadian patent for Plavix being recently upheld, we remain less confident on a positive U.S. ruling for the generic companies," wrote Chandramouli, who rates Dr. Reddy's as a "sell," in an April 15 report.

Chandramouli also expressed doubts that Dr. Reddy's would win in a separate district court challenge for the patent on Zofran, an anti-nausea drug produced by GlaxoSmithKline (up $0.32 to $49.72, Research). In her report, Chandramouli cited the 2004 loss of Teva Pharmaceutical Industries (up $0.15 to $34.08, Research), an Israeli maker of generic drugs, in a challenge to the same drug patent.

But Ram Rammohan, spokesman for Dr. Reddy's, said that judging his company's future success based on lost patent challenges is "speculative in nature."

"We are confident about the merits of our case and, as it is in all patent litigation cases, it is only proper to await the court judgment," said Rammohan in an e-mail.

Sanofi-Synthelabo, a unit of Sanofi-Aventis, counter-sued Dr. Reddy's and Apotex in 2002. Following a lengthy series of court actions, the case is getting closer to a jury trial. The companies involved in the case are required to submit all pre-trial hearing statements to the Southern District of New York by Friday, May 27.

Bristol-Myers (down $0.07 to $25.53, Research), the U.S.-based sales partner for Sanofi, cannot afford to lose control of the patent for Plavix, which made $3.3 billion for Bristol-Myers last year, especially since its patent on Pravachol, a $2.6 billion cardiovascular drug, is set to expire in 2006.

Bristol-Myers is not using Apotex's failed challenge in Canada as a testing ground for the U.S. battle over Plavix, said Brian Henry, spokesman for Bristol-Myers. "That case was decided in Canada with Canadian law and this case will be decided in a U.S. court with U.S. law," Henry said. "One can't make the comparison between the two."

Apotex Inc., a privately held firm, had better success last year, when its U.S. subsidiary won a challenge against the patent for Paxil, an antidepressant made by GlaxoSmithKline.

Apotex spokesman Elie Betito and a lawyer representing the company declined to comment on the new case.

William West, a patent litigation attorney and partner with Howrey LLP, said the generic companies "win occasionally." But patent law tends to favor the patent holders, allowing them to file multiple patents from a single drug while its patent is still in litigation, increasing the burden for the challengers.

"In theory it's supposed to be balanced," said West. "In practice, I think it's been skewed a little bit in favor of the patent holders."

Mark Banner, patent litigation attorney and partner with Banner & Witcoff, said that drug patents are more difficult to challenge that many other forms of patents.

"[Drug patents] are complicated, as opposed to a patent on a railroad car or a patent on a new bicycle accessory," said Banner. "The standards for proving any patent invalid are higher than a normal civil case. The more complicated the technology, the more difficult it's going to be for the challenger to prove it's invalid."

Dr. Reddy's recent challenges have resulted in a string of failures.

Last month, the company lost in a patent challenge over Zyprexa, a treatment for schizophrenia. A district court judge in in Indianapolis ruled in favor of patent holder Eli Lilly and Co., which made $4.4 billion from Zyprexa in 2004. Dr. Reddy's had challenged the patent with two other generic firms, Teva and Zenith Goldline, a unit of Ivax Corp (up $0.01 to $19.51, Research).

In 2002, a U.S. court rejected Dr. Reddy's challenge to AstraZeneca's (up $0.09 to $42.91, Research) patent on Prilosec, a medication for ulcers and heartburn. In 2004 a U.S. court shot down Dr. Reddy's patent challenge to Pfizer's (down $0.67 to $28.23, Research) Norvasc, a treatment for hypertension. In 2002, a U.S. appeals court upheld Bayer's (up $0.09 to $34.30, Research) patent for antibiotic Ciprofloxacin.

Chandramouli does not own stock in the companies mentioned in her report and Merrill Lynch does not conduct banking with them.