NEW YORK (CNNMoney.com) - Eli Lilly & Co. sued Barr Pharmaceuticals, a maker of generic drugs, for patent infringement over the antidepressant Prozac Weekly, said the companies on Thursday.

Lilly (down $0.47 to $51.19, Research) said its lawsuit was in response to the recent amendmentBarr (down $0.52 to $53.65, Research) made to its application to produce a generic version of Prozac Weekly, a drug produced by Lilly.

Barr originally filed this application in 2001 to the Food and Drug Administration, which has not yet made a decision. Lilly said its Prozac Weekly patent is valid until 2017, and considers Barr's amendment a challenge to its patent.

"They're challenging our patent on validity, and we believe that Barr's challenge to our patent is without merit," said Lilly spokeswoman Terra Fox.

The lawsuit specifically targets Barr Laboratories, a subsidiary of Barr Pharmaceuticals.

Barr said it is the first company to file an application to the FDA to produce a generic version of Prozac Weekly, which uses the compound fluoxetine hydrochloride. Lilly said its patent for Prozac Weekly is separate from the patent for Prozac, which expired in 2001. The patent for Prozac Weekly is specific to the coating that allows the drug to dissolve in the intestine, not the stomach, allowing the effects to last longer, says Lilly.

Lilly's sales for Prozac Weekly totaled $38 million in 2005.

Just days ago, a federal jury ruled that Lilly had infringed the patent of another drugmaker, Ariad Pharmaceuticals (down $0.31 to $5.51, Research), with its drugs Evista and Xigris. Lilly was ordered to pay Ariad $65 million in damages. Critics of the ruling said that Ariad's patent is too broad, covering a drug pathway in the human body that is used by many drugs that are currently on the market. Ariad said it would sell a license to any drug company willing to pay for it, and would supply licenses to academic organizations for free.

Lilly, based in Indianapolis, is a much larger company than Barr. Lilly's sales totaled $3.7 billion in the first quarter. Barr, based in Woodcliff Lake, N.J., reported $327 million for its third quarter for fiscal year 2006, its most recently reported quarterly sales.

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