NEW YORK (CNNMoney.com) - The stock price for drug makers Bristol-Myers Squibb and Sanofi-Aventis surged Wednesday, after reaching a tentative settlement the previous day with generic manufacturer Apotex that could protect the patent on anti-blood clot drug Plavix until 2011.

Shares of New York City-based Bristol-Myers (up $2.41 to $25.24, Research) rose nearly 12 percent, while the French company Sanofi-Aventis (up $4.20 to $47.88, Research) stock jumped nearly 10 percent. The companies, which make brand-name drugs, announced Tuesday night that they had reached a tentative agreement with Apotex Inc., a privately held firm, to settle a patent infringement lawsuit in federal court in the Southern District of New York over the drug Plavix.

The agreement, if accepted by the Federal Trade Commission, lifts the black cloud of generic drug pressure from Plavix, at least for the next five and half years. The agreement would protect the patent until Sept. 17, 2011 for Plavix, a drug with 2005 sales totaling $3.8 billion for Bristol-Myers and $2.4 billion for Sanofi-Aventis. As part of the deal, Apotex is granted a royalty-bearing license to sell clopidogrel bisulfate, the active ingredient in Plavix, in the U.S. after that date of expiration. This means that Apotex would get to sell the drug eight months ahead of the original patent expiration date in May, 2012.

Generic drug makers often sue branded drug makers to win the rights to produce and sell generic, lower-cost versions of patented drugs. These lawsuits are efforts by generic companies try to move up the patent expiration dates and to secure the right to be the only generic manufacturer of that drug for the first six months after expiration. After that, open season is declared on the drug for all generic manufacturers to produce and sell the product.

Bristol-Myers, which totaled $19.2 billion in 2005 sales, and Sanofi-Aventis, which totaled $33 billion in 2005 sales, said they are also trying to reach a deal with Dr. Reddy's Laboratories (up $2.15 to $32.75, Research), an Indian maker of generic drugs that has also been locked in a patent battle over Plavix and is known to aggressively challenge patents.

Analyst reaction to the deal was cautious.

"The agreement, in our view, lifts a significant overhang surroundings shares of [Bristol-Myers,] as we believe the market had discounted a significant risk for the potential loss of Plavix revenues," wrote David Moskowitz and other Friedman Billings Ramsey analysts, who reiterated their "outperform" rating with a 12-month price target of $28.

However, the FBR analysts noted that Dr. Reddy's could still challenge the patent, posing risks to Plavix sales.

Bear Stearns analyst John Boris wrote that "settlement exuberance" was tempered by the "significant risk" posed by the FTC, which has yet to decide on the settlement.

"We expect [Bristol-Myers] shares to react positively to the proposed settlement but view regulatory approval as a substantial hurdle," wrote Boris, who rates the company "peer perform." "Therefore, we recommend taking profits on the news as a share price could deflate as FTC risk is digested."

Bear Stearns analyst Alexandra Hauber also noted that the FTC review poses risks to Sanofi-Aventis, which she rates a "peer perform," and wrote that "this is a not a done deal yet."

"If the FTC views the settlement as uncompetitive, it may file objections to the court to stop it," wrote Hauber. "Whilst this seems unlikely, we cannot rule out such a scenario."

Bristol-Myers and Sanofi-Aventis said they would continue to defend the Plavix patent if the deal falls through.

What would happen to Bristol-Myers if it lost Plavix? Click here.