Pfizer, the world's biggest pharmaceutical company, has sued Indian drug maker Dr Reddy's Laboratories for allegedly infringing its patents on Norvasc, a top-selling hypertension medicine.

The case comes after Dr Reddy's filed for US regulatory approval to sell a slightly altered version of Norvasc in the American market.

At issue are one of Pfizer's two patents on the drug, which is also used to treat angina, and which had US sales of almost $2.5bn last year.

Conflicts between Indian and Western drug makers are on the rise, as firms in developing countries are capable of producing generic copies of patented remedies extremely cheaply.

Patent problems

Dr Reddy's is challenging Pfizer patents listed in the so-called "Orange Book", which allows branded drug makers at least 30 months of additional marketing exclusivity over a drug, unless a court clears generic challengers.

The Indian firm claims that it uses a different salt to combine with the active ingredient, thereby allowing its copy to bypass Orange Book restrictions.

Whatever the outcome of the case, Dr Reddy's version could be classified as a generic drug if it is approved after Pfizer's patent expires.

Big drug firms like Pfizer have suffered from the expiry of key patents, as developing new products is time-consuming and expensive.