Wyeth shares dipped yesterday after the US drugmaker revealed that it would have to conduct additional clinical trials of its new antidepressant, desvenlafaxine, that will hold back the launch of the product until 2007.

Desvenlafaxine (also known as DVS-233) is one of the most important new drugs in Wyeth’s pipeline as it is designed to maintain the company’s antidepressant franchise now that Effexor (venlafaxine), from which it is derived, is facing generic competition.

News that desvenlafaxine would be delayed on the way to market had been predicted by analysts, although the expectation was that the delay was a result of compliance problems at one of the company’s manufacturing plants. But now it has emerged that the major issue is that Wyeth wants to test lower doses of desvenlafaxine to see if it can find a level that retains efficacy but improves its tolerability, particularly reducing the tendency of the drug to cause nausea.

Effexor also has nausea as a side effect, but Wyeth will have to show that DVS-233 is significantly better than its predecessor if it is to win market share for the new product and prevent it from being labeled a 'me-too' drug. In a Phase III trial reported at the American Psychiatric Association meeting earlier this year, around half the patients taking desvenlafaxine reported nausea.

Wyeth's Effexor franchise brought in $945 million in the first quarter of 2006, a 9% rise year-on-year, but saw its first competition when Teva Pharmaceutical Industries launched an authorised generic version in June.

The Guayama plant in Puerto Rico was found to be in violation of a number of Good Manufacturing Practice (GMP) standards earlier this year, including contamination control practices. Wyeth said yesterday it had submitted an action plan to the Food and Drug Administration and hopes to have the matter resolved by the end of the year.

Meanwhile, the company also reported that it expects to launch Lybrel (ethinyl estradiol and levonorgestrel), its new oral contraceptive, in 2007 subject to satisfying conditions laid out by the FDA in an approvable letter sent out last month, including the resolution of the compliance problems in Guayama.