Abbott Laboratories is suing India's Ranbaxy Laboratories as well as Israel's Teva and Sandoz of the Novartis group, in Chicago for allegedly violating its patents protecting Omnicef, a drug used to treat ear, throat and skin infections.

Ranbaxy, Teva and Sandoz had filed for permission to market their generic versions of Omnicef, which had originally been developed by Astellas Pharma of Japan and is marketed by Abbott, local daily Ha'aretz reported.

The three challengers have claimed that the patent protecting the drug, until 2011, is invalid,

the report said. The first one to file for marketing permission should get a six-month exclusive marketing window if it can prove that the patent is really invalid, it said.

Since Abbott has sued all the three it seems that they probably filed for marketing permission at about the same time and may receive joint 'exclusive' rights for marketing the product, the report said.

US sales of Omnicef was nearly $260 million in the fourth quarter of 2006 alone. The overall annual sale last year was registered at $637 million marking a 29 per cent increase.