Inclusion Criteria:

1. Males or females between 18 and 65 years of age (inclusive).
2. Either outpatients or inpatients diagnosed with major depressive disorder (MDD) according to Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR, see Appendix 3) and MINI International Neuropsychiatric Interview (MINI).
3. Patients with recurrent depressive episode of at least 2 months in duration. Patients must have previously responded (significant clinical improvement judged by the Principal Investigator) to at least one antidepressant treatment.
4. HAMD-17 total score * 22 with a severity score of at least 2 on Item 1 at the Placebo Run-In Visit and the Baseline/Day 1 Visit.
5. HAMD-17 score reduction ≤ 15% between the Placebo Run-In Visit and the Baseline/Day 1 Visit.
6. HAM-A total score < 17 at the Screening Visit.

Exclusion Criteria:

1. Patients with a HAMD-17 total score reduction of more than 15% between the Placebo Run-In Visit and the Baseline/Day 1 Visit (placebo responders).
2. Patients with a medical history of MDD that consistently did not respond significantly to an adequate treatment regimen of a monoamine oxidase (MAO) inhibitor.
3. Patients who are known to be antidepressant treatment-resistant. Patients are defined as treatment-resistant if in the past they have failed adequate antidepressant treatments (dose level approved in the product labeling and was administered for at least 4 weeks) from two or more different pharmacological classes (e.g., TCA, SSRI, SNRI, MAO-I, etc). Failure to respond to an adequate antidepressant treatment is defined as the absence of at least a 50% improvement in symptoms by patient report or documented history, or lack of significant clinical improvement at the Principal Investigator's discretion.
4. Patients with a medical history of MDD who consistently did not respond significantly to electroconvulsive shock therapy (ECT) or had ECT within a year prior to the Screening Visit regardless of outcome.
5. Patients with psychotic depression