Double-blind, Placebo-controlled Study in Adults With Major Depressive Disorder

This study will evaluate the efficacy and safety of SPN-821 in adults with major depressive disorder

Inclusion Criteria: * Current diagnosis of MDD according to the DSM-5 for either single or recurrent MDE without psychotic features confirmed by the MINI * Duration of current MDE of at least 8 weeks * MADRS total score of ≥ 24 at the Screening Visit and Day 1 Visit * CGI-S score of ≥ 4 (moderately ill or worse) at the Screening Visit and Day 1 Visit * Stable, therapeutic dose of one of the following protocol-approved ADTs as a monotherapy for ≥ 8 weeks before the Screening Visit and ≥ 10 weeks at the Day 1 Visit. Additionally, inadequate response to the current ADT (less than 50% improvement in depressive symptoms) as confirmed by the ATRQ Investigator administered. Exclusion Criteria: * MADRS total score change of ≥ 25% from the Screening Visit to Day 1 Visit * History of treatment resistant depression (TRD) defined as 3 or more failed ADTs of adequate dose (per ATRQ) and duration (at least 8 weeks) for the current MDE * History of alcohol or substance use disorder according to DSM-5 criteria 6 months before the Screening Visit * Evidence of significant risk for suicidal behavior during participation in the study in the Investigator's opinion * Lifetime diagnosis of any psychotic disorder including MDD with psychosis, MDD with mixed features, bipolar I/II disorder, bipolar depression, schizophrenia, posttraumatic stress disorder, autism spectrum disorder, or any personality disorder or intellectual disability that would affect the ability of the participant to enroll in the study * Diagnosis less then 12 months before screening of severe obsessive-compulsive disorder, acute stress disorder, panic disorder, eating disorders, or any other psychiatric condition that has been the primary focus of treatment, or diagnosis of generalized anxiety disorder less then 6 months before screening. * History of cardiovascular, respiratory, gastrointestinal, renal, hepatic, and hematologic disorders, or other medical disorders that could impose undue risk or compromise the study in the Investigator's opinion * Clinically significant abnormal result prior to Day 1 Visit per Investigator's judgment or abnormal renal function. * Requires treatment with a medication or other substance that is prohibited by the protocol.