Subjects must meet all of these criteria for inclusion in the study:  The subject has completed written informed consent. At the time of signing the informed consent, subject must be 18 to 65 years of age, inclusive. The subject has a primary diagnosis of Major Depressive Disorder (MDD), without psychotic features, meeting the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) criteria. Subject must have had inadequate response to antidepressant treatment. Subject is currently on stable pharmacological treatment for depression. Subject must have a total Hamilton Depression Rating Scale-17 Item (HAMD17) score ≥ 22 at screening. Subjects must have been taking current antidepressant medication(s) for ≥ 8 weeks. Subjects must be willing to comply with all study procedures and restrictions.  Exclusion Criteria:  Subjects will be excluded from the study if they meet any of the following criteria:  Subject is pregnant or breastfeeding. Subject has an unstable medical condition or chronic disease. Subject has a history of neurological abnormalities. Subject is currently diagnosed with or prior diagnoses of psychiatric disorder which was the primary focus of treatment other than MDD. The subject's depressive symptoms have previously demonstrated nonresponse to an adequate course of treatment with electroconvulsive therapy (ECT). The subject has an alcohol or substance use disorder. In the Investigator's opinion, the subject is not capable of adhering to the protocol requirements.