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