The lawsuit, filed in a U.S. district court, is in response to Dr. Reddy's application with the U.S. Food and Drug Administration, seeking approval to market generic versions of Zegerid before July 2016, when Santarus' patents on the drug expire.

Zegerid capsules, which treat heartburn and other symptoms related to gastroesophageal reflux disease, brought in sales of $10.3 million in the April-June period, about 3 percent less than a year earlier.

A U.S. district court in April 2010 ruled that Santarus' patents on Zegerid were invalid.

The California-based company appealed the ruling and is currently awaiting a decision on the case.

Santarus' shares, which have risen 85 percent this year, closed at $6.14 on Wednesday on the Nasdaq.

(Reporting by Prateek Kumar in Bangalore; Editing by Saumyadeb Chakrabarty)

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