Although generic fluoxetine immediately cannibalized sales of Prozac, its long-term effect on other branded agents in the selective serotonin reuptake inhibitor (SSRI) class-GlaxoSmithKline's Paxil (paroxetine), Pfizer's Zoloft (sertraline), and Forest Laboratories' Celexa (citalopram)-remains unknown. (See "SSRI Sales.")
Sales of class-leader Zoloft are likely to be most affected because its marketing messages failed to create a widely recognized, indication-specific niche for the product, according to Decision Resources. Conversely, Paxil will probably be least affected by Prozac's expiration because GSK succeeded in creating a strong anti-anxiety position and has successfully disseminated its messages to physicians. In fact, industry analysts predict Paxil will be the best selling SSRI by 2006.Forest Labs also forecasts an increase in prescriptions for Celexa. "The elimination of detailing and sampling of Prozac by Eli Lilly is a key factor here," says Chuck Triano, a Forest spokesperson. "The Celexa message is being heard more loudly and clearly with one less competitor."
Forest is also currently awaiting FDA approval of escitalopram, the single isomer version of Celexa.
"The launch of the single isomer in mid-2002 will be the real story," says Triano.
However, the market may move away from SSRI treatments and toward therapies that act on more than one receptor and offer better side effect profiles. American Home Products' currently marketed serotonin and norepinephrine reuptake inhibitor Effexor (venlafaxine); Eli Lilly's duloxetine, in Phase III trials; and the new classes of NK1 antagonists and corticotropin-releasing factor antagonists are expected to reap rewards for pharma companies in the growing antidepressant market.