The suits were filed in the District Court of Delaware on April 29. Pfizer has accused Lupin of infringing three patents on Lyrica, while others have allegedly attempted to infringe one of the patents.

Pfizer's decision to sue the three Indian companies has come after the latters' Abbreviated New Drug Applications seeking marketing  approvals for generic versions of Lyrica challenged the validity of several Lyrica patents that are expected to expire in 2018.

Under US rules for selling generic drugs, the first company to challenge the patent of the innovator drug -- through what is known as a Paragraph IV certified marketing application -- gets 180 days of marketing exclusivity once the patent expires.  The innovator has to sue the generic company within 45 days of the Paragraph IV challenge to get a 30-month stay on final marketing approval.

Lyrica is one of Pfizer's top-selling medicines that treat central nervous system disorders. It had sales of $ 2.57 billion globally last year. In US, revenues from Lyrica were $1.45 billion in 2008. Lyrica is prescribed for the treatment of epilepsy, and diabetic neuropathy pain and is available in all major markets including India.

Pfizer has accused Sun and Wockhardt of violating a 2001 patent (No 6,197,819) granted to Lyrica. The petition also said Lupin's ANDA infringes three patents granted in 1996, 1999 and 2001.

In addition to the three Indian firms, Pfizer is also known to have sued generic firms such as Teva, Sandoz and Actavis for alleged attempts to infringe Lyrica patent rights in the US.