Study of Safety & Tolerability of OPC-34712 as Adjunctive Therapy in Treatment of Adult Patients With Major Depressive Disorder

Inclusion Criteria: * Male or female subjects between 18 and 65 years of age, with a diagnosis of a single or recurrent, non-psychotic episode of major depressive disorder, as defined by DSM-IV-TR criteria and confirmed by the Mini International Neuropsychiatric Interview (M.I.N.I.) which is equal to or greater than 8 weeks in duration. * Subjects must currently be taking allowable antidepressant therapy at an adequate dose for a minimum of six weeks by the end of the screening period (ie at the time of the Baseline visit). * Subjects must report a history for the current depressive episode of an inadequate response to at least one and no more than four adequate antidepressant treatments. Exclusion Criteria: * Females who are breast-feeding and/or who have a positive pregnancy test result prior to receiving study drug. * Subjects who report an inadequate response to more than three adequate trials of antidepressant treatments during current depressive episode at a therapeutic dose for an adequate duration. * Subjects with a current Axis I (DSM-IV-TR) diagnosis of: Delirium, dementia,amnestic or other cognitive disorder Schizophrenia, schizoaffective disorder, or other psychotic disorder Bipolar I or II disorder, eating disorder (including anorexia nervosa or bulimia), obsessive compulsive disorder, panic disorder, post-traumatic stress disorder. * Subjects with a clinically significant current Axis II (DSM-IV-TR) diagnosis of borderline, antisocial, paranoid, schizoid, schizotypal or histrionic personality disorder.