Indian drugmaker Dr Reddy's Laboratories said on Thursday it had filed an application with the US Food and Drug Administration to market a generic form of GlaxoSmithKline's anti-nausea drug Zofran.

It said in a statement that it had filed the abbreviated new drug application for 16-mg ondansetron hydrochloride tablets under Para IV on the four Orange Book patents.

This book lists patents from branded drugmakers other than that for the original chemical.

"GlaxoSmithKline...(has)...filed a lawsuit against the company (Dr Reddy's) in the United States District Court for the District of New Jersey alleging patent infringement on three of the four Orange Book patents as well as a process patent not listed in the Orange Book," Dr Reddy's statement said.

The US sales of all the dosage forms of Zofran totalled $900 million in 2002, it said.

New York Stock Exchange-listed Dr Reddy's said that Glaxo did not now market the 16 mg dose of the drug.

GlaxoSmithKline's Zofran already faces challenges in the United States from other generic manufacturers.

"It's a positive development but a little off in the future, since the earliest patent expiry is in 2005," said Richard D'Souza, pharma analyst with Sunidhi Consultancy.

Dr Reddy's shares were down 3.64 per cent at Rs 844 at the Bombay Stock Exchange in morning trade, while the Bombay Stock Exchange benchmark index was down 0.38 per cent.

A notice on the US Food and Drug Administration's Web site, www.fda.gov/cder/ogd/ppiv.htm, said an unnamed generic firm had filed for permission to market a cheap version of the injectable medicine.