Inclusion Criteria:  Adults 18 to 65 years, inclusive. Diagnosed with Major Depressive Disorder (MDD) based on Structured Clinical Interview for DSM-5 (SCID-5) for MDD. Current major depressive episode.  Exclusion Criteria:  Any current and primary psychiatric disorder other than Major Depressive Disorder. Severe alcohol or substance use disorder. History of bipolar I and II disorder, psychosis, and/or mania. Acute or chronic condition that, in the Investigator's opinion, would limit the subject's ability to complete or participate in this clinical study. Prior participation in a ketamine, esketamine, dextromethorphan or any other NMDAR- antagonist study, or received esketamine at any time.