July 13 (Bloomberg) -- Forest Laboratories Inc.'s patent for the antidepressant Lexapro is valid, a federal judge ruled. The shares of the company, which makes drugs that treat depression and Alzheimer's disease, surged as much as 18 percent after hours.

In a trial before U.S. District Judge Joseph Farnan Jr. in Wilmington, Delaware, Teva Pharmaceutical Industries Ltd.'s Ivax Corp. acknowledged that it infringes the 1994 patent, licensed by Forest from Denmark's H. Lundbeck A/S. Teva argued it should never have been issued.

Lexapro is New York-based Forest's biggest moneymaker, generating $2.08 billion in U.S. sales in the year ended November on 29.5 million prescriptions, according to court papers. Forest's sales in the year ended March 2005 were $3.11 billion.

``The court will enter judgment in favor of'' Forest Laboratories, Farnan wrote in his opinion today.

Forest initiated the legal dispute in 2003 with a lawsuit alleging infringement by Ivax. Invalidating Lexapro's patent would have allowed Ivax and Cipla Ltd. of India to sell a generic version of the drug.

Kevin Mannix, a spokesman for Petah Tikva, Israel-based Teva, didn't immediately return a call and e-mail message seeking comment. Forest spokesman Charles E. Triano didn't immediately return a call and e-mail message after business hours.

Forest shares rose $5.40, or 14 percent, to $43.80 in after hours trading. They fell 16 cents to $38.40 before the ruling in regular New York Stock Exchange composite trading.

The case is Forest Laboratories v. Ivax, No. 03-891, U.S. District Court in Wilmington, Delaware.

To contact the reporter on this story: Joel Rosenblatt in San Francisco at jrosenblatt@bloomberg.net.

Last Updated: July 13, 2006 20:54 EDT