AstraZeneca said on Tuesday it had filed a lawsuit against Ranbaxy Laboratories in an attempt to prevent the Indian pharmaceutical company launching a generic version of Nexium, its top-selling ulcer drug.

Ranbaxy last month filed a petition with the US Food and Drug Administration challenging six patents on Nexium that had not been due to expire until dates ranging from 2014 to 2018. If successful, Ranbaxy could begin manufacturing a generic version of the drug as early as 2007.

Nexium is AstraZeneca’s biggest-selling drug, accounting for almost $4bn in sales last year, and the Anglo-Swedish pharmaceutical group had been widely expected to sue Ranbaxy.

A successful challenge to its Nexium patents would be a big setback for the company, which has been criticised on the strength of its drug pipeline. Earlier this month, David Brennan, AstraZeneca’s incoming chief executive, said his focus would be on strengthening the pipeline, by licencing in late-stage products.

The lawsuit was filed in the US District Court in New Jersey for “willful infringement” of AstraZeneca’s patents on Nexium, and bars the FDA from approving Ranbaxy’s request for 30 months or until an adverse court decision.

“AstraZeneca has full confidence in and will continue vigorously to defend and enforce its intellectual property rights protecting Nexium,” the company said.

Ranbaxy’s challenge to AstraZeneca’s patents protecting Nexium is the latest in a series of such moves by generic drug manufacturers.

Earlier this month, AstraZeneca also filed a lawsuit over plans by Israel’s Teva to try to prevent it launching a generic version of Seroquel, a treatment for schizophrenia with sales of more than $2bn annually.

Shares in AstraZeneca were up 4p at £26.02 by 10.00am London time.

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