By David B. Caruso
The Associated Press

PHILADELPHIA - A pair of lawsuits filed against GlaxoSmithKline this week are the latest in a string accusing pharmaceutical companies of improperly preventing cheaper, generic brands of top-selling medications from coming to market.

New York City filed federal antitrust lawsuits in Virginia and Pennsylvania on Tuesday claiming the pharmaceutical giant improperly extended its monopoly on the anti-anxiety drug Paxil and the antibiotic Augmentin by obtaining new patents for the drugs when old patents were set to expire, a practice critics call "evergreening."

Similar suits have been pending against GlaxoSmithKline for at least a year by groups that claim the company used the patents to tie up would-be competitors in court and delay the introduction of generic pills that would save consumers millions.

GlaxoSmithKline has denied any impropriety.

"We've obtained our patents in full compliance with the law," said company spokeswoman Mary Anne Rhyne. She said the company spent 13 years researching Paxil, and was entitled to patent protection for discoveries about its potential in treating anxiety and ways to make it more effective.

Other pharmaceutical companies have faced similar lawsuits brought on behalf of cities and states trying to recover money spent on drugs by the Medicaid program, and of individual patients who could have saved up to 50 percent if cheap, equally effective generics had been brought to market sooner.

Sarah Lock, a senior attorney with the AARP, said the drug companies have increasingly "pushed the edges of the law" to keep their monopolies from expiring on drugs they developed.

"No one would contest their right to a patent. But after a certain point, they have recouped their investment, plus a handsome profit; and there reaches a point where they need to give that up and let the generics come in," Lock said.

Under U.S. patent law, companies that invent a medication or discover a new use for it are generally rewarded with a temporary monopoly over sales of the drug. The system was created to prevent companies that spend time and money on research from having their discoveries exploited by copycats.