HAYWARD, Calif. (AP) — Drug developer Impax Laboratories Inc. said Monday it is being sued by Genzyme Corp. over a patent challenge to that company's kidney condition drug Renvela. The drug is approved in the U.S. to control serum phosphorus in patients with chronic kidney disease on dialysis. Serum phosphorus is a chemical used by the body to get energy from food. Hayward Calif.-based Impax asked the Food and Drug administration to approve its generic version of the 800-milligram tablet version of the drug. Cambridge, Mass.-based Genzyme, in turn, is suing Impax for patent infringement in U.S. District Court for the District of Maryland. Renvela is the next-generation version of Renagel and was launched in the U.S. in March 2008. The drugs had combined sales of $169 million during the fourth-quarter of 2008.