This study will demonstrate the efficacy of agomelatine (AGO178) 25 mg and 50 mg in the prevention of relapse in patients with Major Depressive Disorder (MDD). Eligible patients will undergo open-label treatment for 20 to 26 weeks, depending on response to treatment. Patients demonstrating stable response at the end of the open-label treatment phase will be assigned to receive agomelatine or placebo for 52 weeks.

Inclusion Criteria:

• Male and female adults, 18 through 70 years of age, inclusive
• Diagnosis of Major Depressive Disorder, recurrent episode, according to Diagnostic and Statistical Manual of Mental Disorders - 4th Edition (DSM-IV) criteria
• A history of at least two previous episodes of Major Depression plus the current episode
• Hamilton Depression Rating Scale (HAM-D17) total score ≥ 22 at Screening and Baseline

Exclusion Criteria:

• History of bipolar disorder (I or II), schizophrenia, schizoaffective disorder, eating disorder, or obsessive compulsive disorder
• Any current Axis I disorder other than major depressive disorder which is the focus of treatment
• Substance or alcohol abuse in the last 30 days, dependence in the last 6 months
• Use of any psychoactive medication after the screening visit
• Patients who have been previously treated with agomelatine
• Female patients of childbearing potential who are not using effective contraception

Other protocol-defined inclusion/exclusion criteria may apply