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

This is a multicenter, randomized, double-blind, placebo-controlled study to assess the efficacy and safety of SPN-821 administered twice a week as adjunctive therapy to an approved antidepressant therapy in adult participants with MDD. The study includes a 2-week Screening Period, a 4-week Double blind Treatment Period, and a Safety Follow-up phone call one week after the completion of the Treatment Period.

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.