• Meets the DSM-5 criteria for Major Depressive Disorder, with a current major depressive episode duration of \> 6 weeks and ≤ 18 months.

• A score of ≤ 22 (midrange mild/moderate) on the Hamilton Anxiety Rating Scale.

• Montgomery-Asberg Depression Rating Scale total score of \> 24 \[indicating moderate to severe depression\] at Screening and at Baseline.

• Subject is compliantly using a single selective serotonin reuptake inhibitors / serotonin norepinephrine reuptake inhibitor antidepressant for at least 6 weeks for their current episode of depression, with an adequate dose, and with an inadequate response as defined by the Antidepressant Treatment Response Questionnaire. The dosage of the current antidepressant must have been stable for the past 4 weeks, and the dosage and specific antidepressant used should remain the same from Screening through the end of the Follow-up Period.