Sales in seven major markets of unipolar depression drugs - which include products for the treatment of major depressive disorder, minor depression and dysthymia - will slide from $11.6 billion in 2010 to $9.8 billion in 2020, according to new market forecasts.

The market decline over the next decade will be due to generic competition for blockbuster products, and mainly because of the loss of market exclusivity in the US for just two leading antidepressants - escitalopram (Lundbeck's Cipralex/Seroplex and Forest Laboratories' Lexapro) in 2012 and Eli Lilly/Boehringer Ingelheim's Cymbalta/Xeristar (duloxetine) in 2013, according to the report, from Decision Resources.

In addition, the market is still reacting to the rapid generic erosion of Pfizer's Effexor (venlafaxine XR), which lost market exclusivity in the US in 2010, the study adds.

The report, which looks at the unipolar depression drug market in the US, France, Germany, Italy, Spain, the UK and Japan, also points out that Bristol-Myers Squibb/Otsuka's Abillify (aripiprazole) and AstraZeneca's Seroquel (quetiapine) are continuing to gain patient share as adjunctive major depressive disorder treatments, and that this will contribute to the stability of the market in the near term.

Moreover, the authors note that only one emerging therapy, AstraZeneca/Targacept's nicotinic channel blocker TC-5214, has the potential to demonstrate an advantage in tolerability over other agents approved for adjunctive treatment of major depressive disorder.

Most of the emerging antidepressants which are expected to reach the market will offer only subtle clinical differences compared with selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine inhibitors (SNRIs) and will not, therefore, penetrate first-line therapy, they say.

"The unipolar depression market continues to be lucrative for drug developers owing to both the high prevalence of depressive disorder and the low patient response rate, as more than one-third of depression patients do not respond to first-line antidepressants," according to Decision Resources analyst Natalie Taylor.

However, she adds: "the growing number of generic products will make it increasingly difficult for marketers to gain blockbuster status for branded emerging therapies."