 The current study will evaluate the antidepressant effect of CERC-301 during 28 days of treatment in subjects with MDD who are currently experiencing a severe depressive episode despite stable ongoing treatment with selective serotonin- or serotonin-norepinephrine reuptake inhibitors SSRI or SNRI. The study population will be enriched for subjects that would benefit most from rapid onset, those with recent active suicidal ideation, but not a risk to themselves or others and are deemed appropriate for an out-patient study with careful safety surveillance. This will allow the study to focus on the antidepressant effects of CERC-301 but also explore effects on suicidal ideation. To explore rapid onset, the primary endpoint will be at 7 days, but effects over the 28 days of treatment will be examined as a secondary endpoint. 