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Watson Pharmaceuticals Inc. said late Monday it is seeking clearance to sell a generic version of Endo Pharmaceutical Holdings Inc.'s painkiller Opana ER. Endo filed a patent infringement suit to block the approval.

Watson is hoping to get approval to market a generic version of 40-milligram Opana ER tablets. The company said it filed for Food and Drug Administration approval on Jan. 19 and challenged the patents on the drug. Endo and its partner Penwest Pharmaceuticals responded by filing a suit against Watson.

The lawsuit prevents the FDA from approving the Watson generic until the companies resolves their disagreement or a U.S. court makes a ruling on the generic drug and Endo and Penwest's patents.

Opana ER, or oxymorphone, is an extended-release drug used to treat moderate to severe pain in patients who need long-term treatment. The drug was approved in 2006. Watson said U.S. sales totaled $245 million in 2009. The first patents on Opana ER drug are scheduled to expire in September 2013.

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