Forest Laboratories suffered a knock yesterday when the US Food and Drug Administration approved the first generic version of its antidepressant drug Lexapro developed by Israel’s Teva Pharmaceutical Industries.

But Teva still has to win a patent infringement lawsuit with Forest before it can go ahead and launch its copycat version of the Lexapro (escitalopram oxalate), which had sales of $464 million in the first quarter of this year. The FDA approved 5mg, 10mg and 20mg doses of the drug.

Forest claims it has patent protection on Lexapro out to 2012, but Teva is arguing that this protection is invalid. Teva said a trial in the case was held in March 2006 and the post-trial briefing period ended on May 19.

The US drugmaker has already settled litigation with another company seeking to market a version of the antidepressant, Alphapharm, which will result in the latter becoming an exclusive distributor of generic Lexapro.

Forest licenses escitalopram from Lundbeck, which also sells the antidepressant in Europe under the Cipralex brandname. Competition in the US market has already affected sales of the drug through Forest, causing it cut back on orders of bulk escitalopram from Lundbeck.

Forest’s sales of Lexapro in its fourth quarter (ended March 31) rose 16%, but were 3% down on the prior quarter.

Meanwhile, in another blow to Forest, Eli Lilly has reported the results of a head-to-head study of its Cymbalta (duloxetine HCl) antidepressant and Lexapro, claiming that its product is less likely to cause sexual dysfunction in men.

The Lilly-sponsored study, reported yesterday, showed that 37% of men treated with Cymbalta for depression exhibited a worsening of sexual functioning, compared with 59% of the Lexapro group. Among women there was little difference between the two drugs, with worsening sexual function reported in 36% of the Cymbalta group and 38% of those on Lexapro.

Disruption of sexual functioning is a common side effect of antidepressants, with up to 70% of men and women who experience the side effect stop taking their medication.

Cymbalta was launched onto the market in 2004, and achieved sales of $233 million in the first quarter of this year, a rise of 118%. It is approved for both depression and neuropathic pain, and has also just been submitted for approval in the USA to treat generalised anxiety disorder.