US-based drugmaker Santarus has filed a lawsuit against Indian pharmaceutical company Dr Reddy’s Laboratories over the alleged patent infringement of heartburn medication Zegerid.

The lawsuit has been filed after Dr Reddy’s sought approval from the US Food and Drug Administration to market generic versions of Zegerid prior to the drug’s patent expiry in July 2016. Patents protecting the drug were ruled to be invalid by a US district court in April 2010.

Santarus appealed the ruling and is currently awaiting a decision, but has acted quickly to protect its drug from generic competition. Zegerid capsules achieved sales of $10.3m between April and June; 3% down on the same period in 2011.

The over-the-counter heartburn market in the US is thought be lucrative, with Santarus estimating its value at around $1.8bn. Santarus struck a deal with Schering-Plough HealthCare for the promotion of OTC Zegerid in April 2010, for which Santarus receives low double-digit royalties and could be entitled to up to $37.5m in sales milestones.

Zegerid capsules were first approved by the US Food and Drug Administration in 2006, however the a court ruling reached in Delaware, US, deemed five patents covering the drug were invalid, with the federal court citing "obviousness" as a reason. At the time, Santarus had been embroiled in a legal battle with Par Pharmaceutical regarding patent infringements.

Following the decision, Par Pharmaceutical was originally awarded 180 days of marketing exclusivity, after becoming the first to file an ANDA, and began shipping generic versions of Zegerid on 1 July 2010.