UPDATE 2-Depression pill OK'd for kids but probe goes on

* Lexapro wins approval for adolescents

* Drug succeeded in one trial, failed in another

* FDA approval comes weeks after Justice Dept. charges (Adds closing stock price)

NEW YORK, March 20 (Reuters) - Just weeks after prosecutors accused Forest Laboratories Inc of illegally marketing its anti-depressants Celexa and Lexapro to children and paying pediatricians kickbacks, U.S. health regulators have approved Lexapro for depression in kids.

Forest FRX.N said Lexapro, its biggest product with annual sales of more than $2 billion, was approved by the U.S. Food and Drug Administration to treat major depressive disorder in adolescents aged 12 to 17 and as a maintenance therapy, meaning to maintain control of symptoms. It is already approved for adults.

But federal prosecutors have said that Lexapro and Celexa have long been used improperly to treat depression in children.

On Feb. 25, after a five-year probe, the U.S. Justice Department accused Forest of wrongly attempting to sway pediatricians to prescribe Celexa and Lexapro, including inducements like spa visits, fishing trips and tickets to sporting events and Broadway shows. Neither drug had been approved for kids until Lexapro won the FDA’s nod on Friday.

The federal complaint accused Forest of pushing aside a study showing that Celexa was ineffective for pediatric use and instead urging its sales staff to promote a second, more positive study.

The FDA on Friday approved Lexapro’s use for adolescents based on favorable results in two clinical trials, one involving adolescents taking Lexapro and another involving children and adolescents taking chemically similar Celexa, Forest said.

But effectiveness was not shown in another pair of studies, Forest said, one a Lexapro trial involving patients aged 7 to 17 and the other a study of Celexa in adolescents.

Moreover, the company said on Friday that Lexapro’s ability to maintain control of symptoms in adolescents had not been demonstrated.

But the FDA concluded that maintenance efficacy can be “extrapolated” from adult data and from comparisons of how Lexapro is absorbed among adults and adolescents, Forest said.

“A lot of these kinds of trials are not successful because it’s very difficult to do depression studies,” said Forest spokesman Frank Murdolo.

“But we have two studies that were successful” and will provide patients another treatment option, Murdolo said.

Lexapro, the 15th biggest drug in the United States by sales, is a derivative of Celexa, an older product that was once a blockbuster treatment but is now widely sold by generic drugmakers.

The FDA had approved Celexa to treat adult depression but declined to approve it for children.

The federal prosecutors, based in Massachusetts, last month said Celexa was no more effective than a placebo when taken by children or teenagers, and that more patients taking Celexa reported suicidal thoughts or attempted suicide.

They alleged that federal health care programs have paid “thousands of false and fraudulent claims for Celexa and Lexapro prescriptions that were not covered for off-label pediatric use and/or were ineligible for payment as a result of illegal kickbacks paid by Forest.”

New York-based Forest said depression affects about 2 million adolescents in the United States and that Lexapro is only the second anti-depressant approved for such younger patients.

Shares of Forest Labs closed down one cent at $21.13 on Friday on the New York Stock Exchange. (Reporting by Ransdell Pierson; Editing by Toni Reinhold)