Shares in Wyeth tumbled more than 10% yesterday following news that the US Food and Drug Administration has requested additional clinical trial data for its serotonin-norepinephrine reuptake inhibitor Pristiq (desvenlafaxine), the follow-up to its blockbuster depression drug Effexor (venlafaxine), that could push back its launch for a year or more.

The reaction has been enhanced because Pristiq is one of Wyeth's most important medicines coming through the pipeline and was broadly expected to buffer declining sales of the $3.5 billion-a-year Effexor when it comes off patent in 2010.

Earlier this year, Pristiq received an approvable letter for use in major depressive disorder, but the concerns related to quality issues at Wyeth's manufacturing plant and the company was not required to submit additional data. Wyeth says it plans will submit a complete response at the end of August to address this approvable letter and the FDA is expected to make a decision at the beginning of next year.

However, for the indication of treating hot flashes and night sweats associated with menopause, the story is somewhat different. In its letter, the FDA said that before the application could be approved, Wyeth will need to provide additional data relating to Pristiq's potential for serious adverse cardiovascular and liver effects from a safety study of at least one year in postmenopausal women. The agency also requested that Wyeth address certain chemistry and manufacturing deficiencies prior to approval.

The news triggered Wyeth's share price to spiral, with some analysts suggesting they would not count on any revenues from Pristiq, anticipated to be in excess of $2 billion, in their forecasts.