To evaluate the efficacy of a fixed dose of SR58611A(350 mg q12) compared to placebo in patients with MajorDepressive Disorder (MDD) using escitalopram (10 mgqd) as positive control. The study is a multicenter, US and Canadian, randomized, double-blind, 3-parallel-group, placebo- and escitalopram-controlled, Phase III study consisting of four segments (A, B, C and D). Segment A is a 1-week, placebo, single-blind period. Segment B is an 8-week, double-blind period. Segment C is an optional 18-week double-blind extension period. Segment D is a 1-week safety follow-up period after study drug discontinuation or early termination (during Segment B or C).

To evaluate the efficacy of a fixed dose of SR58611A(350 mg q12) compared to placebo in patients with MajorDepressive Disorder (MDD) using escitalopram (10 mgqd) as positive control. The present study is an 8-week, double-blind, placebo- and escitalopram-controlled, randomized, parallel-group study. A 1-week, placebo, single-blind period precedes the 8-week randomized treatment period and an optional 18-week, double-blind extension period follows the randomized treatment period. A Safety Follow up Visit is scheduled 1 week after the acute and extension period, or early termination. Escitalopram, a selective serotonin reuptake inhibitor (SSRI), an approved treatment for MDD, is chosen as a positive control agent in this study. The dose of 10 mg is within the approved dose range with no need for dose adjustment in elderly patients. This trial is designed to formally compare the efficacy, safety, and tolerability of SR58611A to placebo. Escitalopram is used as a positive control.

Inclusion Criteria:

Main inclusion criteria:

• 1. Out-patients, 18 year and older.
• 2. Major Depressive Disorder (MDD) with a recurrentMajor Depressive Episode (MDE) according to DSMIV-TR criteria
• 3. Duration of current episode is at least of 6 weeksunless severity of symptoms justifies shorter duration.
• 4. Patients have been treated or hospitalized for aprevious episode, or a previous episode requiredantidepressant treatment(s) at the recommended doselevel for a continuous total duration of at least 2months.

Exclusion Criteria:

Main exclusion criteria:

• 1. Patients at immediate risk for suicidal behavior
• 2. Patients with a MDE with psychotic features, catatonic features, seasonal pattern or postpartum onset
• 3. The duration of the current depressive episode is greater than 2 years
• 4. Patients whose current depressive episode is secondary to a general medical condition
• 5. Patients with a lifetime history of (1) bipolar disorder, (2) psychotic disorder, (3) antisocial personality disorder
• 6. Patients who have received non-pharmacologic, somatic treatments for psychiatric disease
• 7. Patients who have initiated, stopped, or changed the frequency or nature of psychotherapy within 3 months prior to screening