By Archi Damania, CNBC-TV18

The world's largest drug maker Pfizer has stepped up its offensive against Ranbaxy to block it from launching a generic version of its blockbuster heart drug Lipitor. Pfizer has sued Ranbaxy alleging that it has infringed two of Lipitor's  process patents that expire only after 2016. The patent infringement relates to Lipitor and Caduet. The case has been filed at a federal court in Wilmington, Delaware.

Ranbaxy has already won a US court verdict that allows it to market generic versions of Lipitor from March 2010.

Lipitor is a USD 12 billion anti-cholesterol drug and Ranbaxy has challenged its patents in over ten countries.

Lipitor is a big drug and Ranbaxy could have had a one-time upside of USD 200-250 million dollars during the period of exclusivity if this would have been launched. This converts into a per share upside of Rs 40-45 in 2010, when this product would have been launched. This is a big generic opportunity for Ranbaxy in terms of launching a product of this size.