Forest Laboratories ( FRX) won a major legal victory, repelling an attempt to invalidate its patent on the anti-depressant Lexapro, which accounts for 63% of its revenue. Forest announced Thursday night that a federal court judge rejected a challenge by Teva Pharmaceutical Industries ( TEVA), the world's biggest generic-drug company. A federal judge in Delaware ruled in favor of Forest and H. Lundbeck, a Danish company that licences Lexapro to Forest, according to the statement. The decision protects the patent on escitalopram, the active ingredient in Lexapro, until March 2012. In premarket trading Friday, Forest's stock rose $5.20, or 13.5%, to $43.60. Teva, which can appeal, offered no immediate comment. In May, Teva received approval from the Food and Drug Administration to begin marketing three dosages of Lexapro; but it refrained from acting until the lawsuit was resolved. Forest joins Eli Lilly ( LLY) and Pfizer ( PFE) in winning lawsuits against generic-drug companies seeking to break the U.S. patents on their biggest drugs. Lilly defeated a challenge to its schizophrenia drug Zyprexa in April 2005 and Pfizer won in December to protect the cholesterol drug Lipitor. Zyprexa accounted for 29% of Lilly's revenue last year; Lipitor contributed 24% of Pfizer's revenue. The litigation isn't over for Forest, which last year settled a Lexapro patent dispute with the Australian generic-drug company Alphapharm. Earlier this week, Forest and Lundbeck sued another generic-drug company, Caraco Pharmaceutical Laboratories ( CPD), for infringing on the Lexapro patent. Caraco has asked the FDA to approve its generic versions of Lexapro.