Efficacy, Safety, and Tolerability Study of AVP-786 as an Adjunctive Therapy in Patients With Major Depressive Disorder With an Inadequate Response to Antidepressant Treatment

Inclusion Criteria: * Clinical diagnosis of major depressive episode ≤ 24 months in duration * HAM-D17 score ≥ 20. * Documented to not have a significant (25% or greater) change in QIDS-SR16 score between Screening and Baseline visits. * Patients have been deemed to have an inadequate response (less than 50% symptom reduction) to at least 1 but no more than 3 adequate antidepressant trials during the current depressive episode. * Patients must be receiving ongoing treatment with an adequate dose of antidepressants. * Body Mass Index (BMI) of 18-35 kg/m². Exclusion Criteria: * History of myasthenia gravis. * Have cardiovascular concerns such as: * History of complete heart block, QT interval corrected for heart rate (QTc) prolongation, or torsades de pointes. * QTc using the Fridericia's formula (QTcF) at screening \> 450 msec for males and \> 470 msec for females based on central review at the screening visit, unless due to ventricular pacing. * Any family history of congenital QT interval prolongation syndrome. * Known hypersensitivity/intolerance to DM, Q, opiate drugs (codeine, etc.), or any other ingredient of the study medication. * Pose a current suicide risk, as evidenced by any of the following: * It is the judgment of the investigator that the subject may be at risk for suicide. * The subject is rated a "yes" to question 4 or question 5 on the Baseline C-SSRS, if the most recent episode occurred within the past 12 months. * The subject has attempted suicide within the past 6 months * Presence of any other current DSM-IV-TR Axis I disorders with the exception of: generalized anxiety disorder (GAD: 300.02), social anxiety disorder (300.23), dysthymic disorder (300.4), or specific phobia (300.29). Patients with co-morbid GAD, social anxiety disorder, or specific phobia are ineligible if the co-morbid condition is clinically unstable, or has been the primary focus of treatment within the 6 month period prior to screening * Axis I diagnosis of: * Delirium, dementia, amnestic, or other cognitive disorder; * Schizophrenia or other psychotic disorder, based on the M.I.N.I.; * Bipolar I or II disorder, based on the M.I.N.I. * Clinically significant Axis II (DSM-IV-TR) diagnosis of borderline, antisocial, paranoid, schizoid, schizotypal or histrionic personality disorder.