Depression market poised to decline

Despite the fact that the U.S. antidepressant market grew at a 20% annual rate between 1995 and 2000, the loss of patent exclusivity of selective serotonin reuptake inhibitors - beginning with Indianapolis-based Eli Lilly and Co.'s Prozac® (fluoxetine) last August - will likely cause the market for SSRIs and similar drugs to decline for the first time in over a decade, according to Decision Resources Inc., Waltham, MA. Also, novel therapies now under development for depression face greater challenges than previous treatments did.

The arrival of generic fluoxetine will have a profound impact on the depression market in the decade ahead. In addition to eroding the market for branded SSRIs, generic fluoxetine will likely cannibalize sales from other classes of antidepressants. The extent to which prescribers will switch from other branded SSRIs to generic fluoxetine is not yet known; however, a recent Internet survey by Decision Resources of 72 U.S. primary care physicians who are top prescribers of antidepressants revealed that generic switching will impact some branded SSRIs, such as New York-based Pfizer Inc.'s Zoloft® (sertraline HCl), more than others. The survey also showed that physicians were unlikely to prescribe generic fluoxetine if a patient previously had a good response to another SSRI or requested another SSRI.

Multiple receptors

Growing interest in antidepressants acting at more than one receptor, like serotonin and norepinephrine reuptake inhibitors, suggests that these agents will provide increasingly important alternatives to SSRIs, especially in treatment-refractory patients. This trend will likely be of greatest benefit to Madison, NJ-based American Home Products Corp.'s SNRI venlafaxine, as well as investigational compounds like Eli Lilly's duloxetine.

New classes

Two other promising new drug classes include NK1 antagonists and corticotropin-releasing factor antagonists. Though years away from approval, both classes of drugs could theoretically compete in a variety of depression and anxiety segments with relatively favorable side-effect profiles. However, although NK1 antagonists are further along in development, both of these emerging classes have yet to show evidence of clinical efficacy sufficient to drive significant sales in what is fast becoming a very crowded market.

Whether any of these new agents will ultimately be successful at fending off encroachment by generics will largely be determined by an ability to successfully treat a broader base of patients (especially those with comorbid anxiety), a "clean" side-effect profile and a reduced time to onset of action, Decision Resources said. The drugs currently in clinical development, though promising in some respects, have yet to display all these desired characteristics. PR