D-cycloserine for Relapse Prevention Following Intravenous Ketamine in Treatment-resistant Depression

Inclusion Criteria: * Patients aged 18-75 meeting DSM-V criteria for moderate-severe depression (MADRS≥25), who did not respond to two adequate antidepressant courses of treatment. Subjects will be required to continue on a stable dose of any psychotropic medication they are taking, for 8 weeks prior to ketamine infusion. Participants who respond to ketamine (reduction of 25% in symptoms) would be invited to participate in a second stage of the experiment, in which participants would consume DCS for 8 weeks (weeks 4-11). Exclusion Criteria: * Patients will be excluded if they have current or history of psychotic or dissociative symptoms, or severe personality disorder with psychosis or dissociative symptoms. Additional exclusion criteria will be a lifetime history of psychotic mania, a substance abuse or use of alcohol. Medical exclusion criteria will include - uncontrolled elevated blood pressure, non-sinus rhythm, unstable ischemic heart disease, uncorrected hyper thyroidism, and for women, pregnancy or the initiation of female hormonal treatment \<3 months. Before ketamine treatment women of childbearing age will be required to use a medical accepted contraceptive or abstain from sexual activity. In addition patients will be excluded if they suffer from chronic renal failure, epilepsy, organic brain disorder or neurological or an unstable medical condition. Due to neurotoxicity and convulsions, patients will be prohibited to consume alcohol.