As the debate over the effectiveness on antidepressants rumbles on, Wyeth has got the green light from US regulators for Pristiq, a derivative of the firm’s blockbuster Effexor.

The US Food and Drug Administration has approved Pristiq (desvenlafaxine), a structurally novel, once-daily serotonin-norepinephrine reuptake inhibitor, to treat adult patients with major depressive disorder. The thumbs-up was based on four eight-week, randomised, double-blind, placebo-controlled, fixed-dose studies in adults and at the recommended dose of 50mg, the discontinuation rate due to an adverse experience for Pristiq (4.1%) was similar to placebo (3.8%).

However, Wyeth noted that FDA approval was subject to “several post-marketing commitments”, including conducting and submitting data from a new long-term maintenance study. The agency also wants the firm to carry out a sexual dysfunction study, paediatric trials, a study exploring lower doses and an additional non-clinical toxicity trial.

Wyeth, which expects to begin shipping Pristiq to wholesalers beginning in the second quarter, will be hoping that sales of the new treatment should help compensate for the loss of earnings once Effexor (venlafaxine) faces generic competition in 2010. The approval of Pristiq comes just over a year after the firm submitted the results of two studies that the FDA had asked for when it issued only an approvable letter for Pristiq.

In the past, Wyeth has claimed that Pristiq could bring in sales of around $1 billion but that enthusiasm is not shared by many in the investment community. Timothy Anderson, an analyst at Sanford Bernstein, issued a research note pointing out that the data Wyeth has produced fails to distinguish Pristiq from other antidepressants. He added that “payers are not likely to widely cover Pristiq in our view, and we forecast only low levels of sales,” some $500 million by 2012.