By MICHAEL REID and SUSANNAH RODGERS DOW JONES NEWSWIRES LONDON -- GlaxoSmithKline PLC said Friday that U.S. regulators approved Wellbutrin XL, a new version of a GSK antidepressant whose earlier forms have been threatened by generic competition. The U.S. Food and Drug Administration approved Wellbutrin XL for treatment of major depressive disorder in patients aged 18 or over.

The approval has come slightly earlier than expected and will enable GSK to get the drug into pharmacies by mid-September. The new drug has two advantages for patients over other treatments: It's taken only once a day and there is less risk of side-effects, such as sexual function impairment and weight gain -- common problems with other treatments.

Company insiders told Dow Jones Newswires that GSK considers Wellbutrin XL to be one of its two most important drug launches this year -- the other being impotence treatment Levitra, GSK's answer to Pfizer Inc.'s multibillion-dollar drug Viagra. Wellbutrin XL has two dosages, 150mg and 300mg. "We think it has the potential to be a pretty big product," said one person at GSK familiar with the company's drug strategy. "We're going to put a lot of [sales] reps behind it."

For GSK, the approval is a key step in helping it defend Wellbutrin against generic competition. "Through the first half of 2003, our twice-daily formulation, Wellbutrin SR, has remained the No. 1 prescribed antidepressant among U.S. psychiatrists," said Chris Viehbacher, GSK's president of U.S pharmaceuticals. "Yet we know that for many doctors, twice-daily dosing is a major prescribing barrier." GSK hopes physicians who would otherwise opt for the generic companies' cheaper copies of the twice- and thrice-daily treatments (when they eventually launch them), will instead put their patients on the once-a-day XL version, even though it'll be more expensive. Williams de Broe analyst Navid Malik said the approval means GSK's Wellbutrin franchise looks "a little more stable than it did a few months ago." "Glaxo can now invoke a Paxil/Paxil CR strategy," Mr. Malik said, referring to GSK's attempt to defend its best-selling antidepressant, Paxil, by introducing a controlled-release version. Wellbutrin works differently from the more publicized class of antidepressants known as SSRIs -- selective serotonin reuptake inhibitors -- including Eli Lilly & Co.'s Prozac and GSK's own Paxil, and has different side effects. Paxil, also known as Seroxat, brought GSK sales of £2.1 billion last year. But it's also facing a possible generic threat. Like many drug companies, GSK is desperate to extend the life of established drugs by bringing out new, improved formulations with fresh patent protection, thereby preserving exclusive selling rights. The industry is at pains to milk established drugs for all their worth because of the fierce challenge generic drug makers is presenting to the pharmaceutical giants. Also, the big pharmaceutical companies are struggling to come up with new, big-selling medicines to replace the blockbusters that were the cornerstone of their earnings during the buoyant 1990s.