The Associated Press reports that Impax Laboratories Inc. is challenging the patents on a heartburn drug from Takeda Pharmaceutical, just days after Takeda took the same action against Impax. The generic drugmaker said that it filed for marketing approval of generic versions of 30-milligram and 60-milligram capsules of Dexilant. Impax said it believes it is the first company to file for approval of a generic version of Dexilant, also called dexlansoprazole. If so, it could get six months of marketing exclusivity for its generic. The company said U.S. sales of Dexilant 30-milligram capsules totaled $20 million in the 12 months ended January 2011, and sales of the 60-milligram capsules totaled $261 million. Takeda Pharmaceutical Co. sued Impax for patent infringement on Friday, Impax said. The lawsuit prevents the Food and Drug Administration from approving Impax''s generic for 30 months after it filed for approval or until a court makes a ruling on the patents supporting Dexilant, whichever comes first. Dexilant was approved in January 2009, and the patents on the drug begin to expire in June 2020.