Impax Laboratories Inc. said that it is challenging the patents on Purdue Pharma''s painkiller Oxycontin in order to be able to sell a generic version of the drug, according to the Associated Press. Impax has filed for Food and Drug Administration approval of its drug. However, Purdue and other stakeholders on the patents filed a lawsuit on Friday against Impax in U.S. District Court for the Southern District of New York to prevent the company from selling a generic version before current patents expire starting in 2013. Generic drug developers, such as Impax, often challenge patents on branded drugs as part of a strategy to be the first to bring a generic version of a blockbuster drug to the market, either through a settlement or a ruling. The lawsuit means that regulators will not approve Impax''s generic for up to 30 months, or until a court makes a ruling on the patents and the lawsuit. Other stakeholders involved in the Oxycontin patents include Rhodes Technologies, The Board of Regents of the University of Texas system, and Grunenthal GBMH. Impax said Oxycontin had U.S. sales of $2.8 billion during the 12-month period ending in January.