More than half the states in the United States have brought an anti-trust lawsuit over a breast cancer treatment against giant drug maker Bristol-Myers Squibb.

The filing alleges the US-based firm illegally prevented breast cancer suffers from having access to cheaper, generic versions of the drug Taxol.

The lawsuit, which seeks unspecified damages, was filed in the US District Court for the District of Columbia.

"Bristol made billions off this single drug while untold numbers of women were being denied access to a more affordable treatment," Michigan Attorney General Jennifer Granholm said in a statement.

Patent extension

Bristol-Myers spokesman Robert Laverty said: "We will continue to deal with the issues raised by this new suit as we have been doing with other litigation related to these matters."

The US Food and Drug Administration (FDA) granted Bristol-Myers exclusive marketing rights to Taxol in 1992 for five years.

Bristol-Myers later obtained extra patents that protected its exclusive position for another three years until October 2000.

The states argue that by doing so it blocked rival firms from developing alternative, generic versions of the drug which would have cost 33% less.

"It is morally and legally reprehensible for this country's companies to block competition and overcharge consumers, especially cancer patients," said Ms Granholm.

The FDA licensed generic versions of the drug in the autumn of 2000.

Yew bark

The states have also objected that the active ingredient in Taxol was developed using public funds.

Paclitaxel, the compound's active ingredient, was developed by the National Cancer Institute from a substance found in yew tree bark.

Bristol-Myers took the drug through clinical trials and brought it to market.

Taxol is also used to treat ovarian cancer and other cancers.

Bristol-Myers's shares slid 4% on Tuesday to close at $28.65 on the New York Stock Exchange.

In recent weeks, Bristol-Myers Squibb has held talks with UK rival GlaxoSmithKline to explore the possibility of a merger to create the world's largest pharmaceutical firm.

Analysts estimate a successful tie-up would create a firm worth $190bn (�129.7bn; 202.8bn euros).