Their companies just survived a big challenge in court, so investors in Sanofi-Aventis ( SNY) and Bristol-Myers Squibb ( BMY) can now focus on a challenge coming from the lab. At the center of both matters is Plavix, the anticoagulant that produced $6.3 billion in sales in 2005, making it the world's second-best-selling drug behind only Pfizer's ( PFE) Lipitor. A U.S. federal appeals court on Dec. 8 upheld a preliminary injunction blocking Canada's Apotex from selling generic versions of the drug. The injunction protects Plavix until a court decides a patent infringement suit filed against Apotex, a case that many analysts predict the generic-drug maker will lose. Oral arguments start Jan. 22, but a decision will take many months. Meanwhile, a scientific challenge that's no less important is being mounted by Eli Lilly ( LLY) and its experimental clot-preventer Prasugrel. A Lilly-sponsored clinical trial, whose results are due in mid-2007, will have a profound effect on all three companies because it directly compares Plavix and Prasugrel in preventing heart attacks, stroke and death. Bristol-Myers, which licenses Plavix for marketing in the U.S. and Canada, needs this drug to maintain a revenue foundation for the next four years, assuming its U.S. patent remains in force until mid-2011. For the first half of last year, Plavix accounted for 22% of Bristol-Myers' revenue and 8% of Sanofi-Aventis' sales.