UPDATE 1-Astellas sues Impax over generic version of Flomax

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TOKYO, July 22 (Reuters) - Astellas Pharma Inc (4503.T) said on Tuesday it had filed a suit against unlisted Impax Laboratories Inc over the U.S. company's application for a generic version of the Japanese drug maker's prostate drug Flomax.

Astellas and Germany's Boehringer Ingelheim Pharmaceuticals Inc, which co-promotes Flomax in the United States, jointly filed the suit at the U.S. District Court, Northern District of California on Friday.

Flomax, one of Astellas' mainstay drugs, is used to treat urinary problems in men from enlarged prostates.

Flomax, known as Harnal in Japan, was the second-biggest product of Astellas, generating 122 billion yen ($1.15 billion) in sales or 13 percent of the company's consolidated revenues in the year ended March.

Flomax is sold in 91 countries under different names.

Astellas' patent on Flomax in the United States will expire in October, 2009, an Astellas spokesman said.

In November last year, Astellas and Boehringer reached a settlement agreement with Ranbaxy Laboratories (RANB.BO) over the alleged patent infringement on Flomax by the Indian generic drug maker. (Reporting by Yumiko Nishitani)