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 …