Trial of Centanafadine Efficacy and Safety as Monotherapy or as Adjunct to SSRI in Adults With Major Depressive Disorder

Key Inclusion Criteria: * Male and Female subjects between 18-65 years of age, with a primary diagnosis of major depressive disorder and in a current major depressive episode. * History of an inadequate response to at least 1 and no more than 3 antidepressant treatments in the current major depressive episode. Key Exclusion Criteria: * Subjects who report an inadequate response to more than 3 antidepressant treatments in the current episode. * Subjects with a lifetime history of schizophrenia spectrum or other psychotic disorder, bipolar or related disorder, neurocognitive disorder, or personality disorder. * Subjects with a current diagnosis of post-traumatic stress disorder, obsessive compulsive disorder, panic disorder, or eating disorder (including anorexia nervosa or bulimia). * Sexually active subjects, who could become pregnant, not agreeing to practice 2 sponsor approved methods of birth control or remain abstinent during the trial and for 30 days (females) or 90 days (males) after last dose of study drug.