Proof of Concept Study Evaluating the Efficacy and Safety of MIJ821 in Patients With Treatment-resistant Depression

Key Inclusion Criteria: * Signed informed consent. * Male and female subjects, 18 to 65 years of age (inclusive) at screening. * SCID-based DSM-5 defined major depressive episode at the time of screening * Montgomery-Åsberg Depression Rating Scale (MADRS) score ≥ 24 at screening and baseline * Failure to respond to two or more antidepressant treatments, where two failed treatments are of two different antidepressants and at least one of which is in the current depressive episode, with adequate dose and duration (≥ 8 weeks duration, doses defined per agent), as identified by the Maudsley Treatment Inventory, and prior psychiatric history, assessed by the investigator, and further documented by medical records and/or third party report (family, friends, clinician-treaters) where available * If patients are taking any type of psychotropic drugs, a stable dose of psychotropic drugs at screening is defined as no changes in dose or type of antidepressants, antipsychotics, or mood stabilizers for at least 2 weeks prior to screening, if applicable. * No new antidepressant initiated 4 weeks or less before baseline, and 6 weeks or less before baseline if subject is initiated on fluoxetine * At least one prior clinical depressive episode (recurrent major depressive disorder), as identified by prior psychiatric history assessed by the investigator, and further documented by medical records and third party report (family, friends, clinician-treaters) where available. * Able to communicate well, and to understand and comply with study requirements Key Exclusion Criteria: * Any current diagnosis of bipolar disorder, schizophrenia, or schizoaffective disorder at screening. * Current alcohol or substance use disorder (including marijuana and prescribed amphetamine)) meeting DSM-5 criteria, within the past month at baseline. Nicotine and caffeine use disorders will not be considered as exclusionary. * Prior suicidality caused by or associated with ketamine, as identified by prior psychiatric history assessed by the investigator, and augmented by medical records and third party report (family, friends, clinician-treaters) where available. * Acute serious and/or imminent suicidal ideation and/or intent within the prior 2 weeks, or any suicide attempt within the prior 4 weeks at screening. Mild to moderate suicidal ideation, using the Sheehan Suicidal Ideation Scale and not meeting the above definition, is not an exclusion criterion. * Use of other investigational drugs within 30 days or 5 half-lives of randomization, whichever was longer; or longer if required by local regulations at baseline * Current pregnancy or lactation. * Positive HIV, Hepatitis B or C test. * Resting QTcF ≥450 msec (male) or ≥460 msec (female) at pre-treatment baseline * History of multiple and recurring allergies or allergy to the investigational compound/compound class being used in this study. * History of malignancy of any organ system (other than localized basal cell carcinoma of the skin or in-situ cervical cancer), treated or untreated, within the past 3 years, regardless of whether there is evidence of local recurrence or metastases. * Women of child-bearing potential, defined as all women physiologically capable of becoming pregnant, unless they are using highly effective methods of contraception during dosing and for 1 week after stopping of investigational drug. * History of hypersensitivity to any of the study treatments or excipients or to drugs similar to chemical classes that affect NMDA receptor. * Current diagnosis of borderline personality disorder or antisocial personality disorder, based on DSM-5 criteria. * Current acute depressive episode lasting longer than two years continuously, defined as no two week or longer period where depressive symptoms are subsyndromal in severity for a full DSM-5 acute major depressive episode. * Considered by the investigator, for any other reason, to be an unsuitable candidate for the study.