Double-blind, Placebo-controlled Study 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.