['depressive disorder, treatment-resistant']
['avp-786 (d6-dextromethorphan hydrobromide and quinidine sulfate combination)', 'placebo']

        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.
      