NEW YORK (AP) – A federal judge Thursday blocked a Canadian company from selling more of its generic version of blood thinner Plavix, finding it unlikely that the manufacturer could prove that the patent on the drug used by 48 million Americans was invalid.

U.S. District Judge Sidney H. Stein in Manhattan issued his written ruling Thursday after a two-day hearing in which Sanofi-Aventis and partner Bristol-Myers Squibb Co. insisted the Canadian company, Apotex Inc., must be forced to stop selling its product.

The judge also denied demands by the companies that Apotex be forced to remove from the market the generic product it had already sold to distributors. In a statement, Apotex said it will appeal and it is filing an emergency motion with the Court of Appeals for the Federal Circuit to stay the injunction pending the appeal.

A large purchaser of Plavix said it had supply that would last into the fourth quarter.

Plavix is the world’s second best selling drug after Pfizer Inc.’s cholesterol-lowering agent Lipitor. It is Bristol-Myers’ best-selling product and analysts estimate it accounts for one-third of the company’s profits.

The drug is considered crucial to helping Bristol-Myers maintain its dividend, which props up the stock.

Bristol-Myers shares rose $1.75, or 8 percent, to $23.50 in extended trading on the New York Stock Exchange.

The ruling was issued after the close of regular trading.

U.S. shares of Sanofi-Aventis, which is based in France, rose $1.55, or 3.4 percent, to $44.95 on the NYSE.

“This is very good news,” said Jason Napodano, an analyst at Zacks Independent Research. Still, he said Bristol-Myers earnings would suffer because Apotex didn’t have to recall its product. Since it is unclear how much generic Plavix is on the market, determining the extent of the damage is difficult.

Napodano noted that Sanofi-Aventis and Bristol-Myers, which sells Plavix in the United States, still have to win the patent case before declaring victory. The case is set to begin in January.

However, the judge said Sanofi-Aventis, which owns a patent on Plavix, had adequately demonstrated that questions Apotex raised as to the validity and enforceability of its patent were without substantial merit.

He said he also concluded Sanofi-Aventis would suffer irreparable harm if Apotex were permitted to continue distributing its generic product and Apotex’s hardships primarily result from the company’s own calculated risk-taking.

The judge also considered the public interest of permitting competition to continue, noting that a large number of Americans have seen a significant cost savings since Apotex entered the market.

“Although there are competing – and substantial – public interests at stake on both sides of this litigation, the balance of those competing public interests slightly favors Sanofi,” the judge wrote. “The public interest in lower-priced drugs is balanced by a significant public interest in encouraging the massive investment in research and development that is required before a new drug can be developed and brought to market.”

“We’re pleased that the court has granted a preliminary injunction,” said Bristol-Myers spokesman Tony Plohoros.

“We’re not going to speculate on the amount of generic product introduced into the marketplace.’

Apotex had argued that a patent protecting sales of Plavix from competitors was no longer valid and that it was in the public interest to permit sales of its generic version of the drug because lower prices mean even more people can use a lifesaving drug.

Ontario-based Apotex began selling its product Aug. 8 after negotiations among the three companies failed to settle the dispute.

The judge said he based his decision on the chances that Apotex could ultimately prove it is entitled to sell its generic drug; the level of irreparable harm each side might suffer; the balance of hardships; and the impact on the public. He also ordered Sanofi to put up bond of $400 million, an amount meant to cover losses Apotex will have suffered if it eventually wins the case. Apotex said the amount was “grossly inadequate” to compensate for the damages it was in jeopardy of incurring.

Bristol-Myers disclosed last month that the Department of Justice was investigating a deal in which Bristol-Myers and Sanofi-Aventis agreed to pay Apotex at least $40 million to keep its version of Plavix off the market until 2011. The deal was rejected by state attorneys general.

At the hearing before Stein, attorney Evan Chesler of Sanofi-Aventis said that allowing Apotex to flood the market with its product and force prices lower would cripple the spirit of drug innovators, jeopardizing future pioneering medical breakthroughs.

“You can never put this Humpty-Dumpty back together if it’s not stopped,” Chesler said.

Chesler did not immediately return a telephone call for comment Thursday.

Frank Allen Bernatowicz, an accountant, testified for Apotex that the company stood to lose up to $4 billion in costs, future sales and lost opportunity if it was forced to withdraw its drug from the marketplace.

He said Sanofi-Aventis and Bristol-Myers stood to lose half as much and could withstand the hit much easier because they were much larger than Apotex, a company with $1 billion in annual sales.

It is a pharmaceutical product considered so important in preventing potentially fatal blood clots around metal stents and cholesterol deposits that lawyers on both sides referred frequently to lives being saved if they won the case.

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AP Business Writer Theresa Agovino contributed to this report.

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