AV-101 as Adjunct Antidepressant Therapy in Patients With Major Depression

Inclusion Criteria: * Diagnosed with MDD, single or recurrent, and currently experiencing a MDE of at least 8 weeks in duration. * Has a history of inadequate response to at least 1 approved antidepressant including at least 1 and no more than 3 during the current depressive episode. * Meet the threshold on the total HAMD-17 score of \> 20 * If female, a status of nonchildbearing potential or use of an acceptable form of birth control. * Body mass index between 18 to 40 kg/m2. * Other criteria may apply Exclusion Criteria: * History of bipolar disorder, schizophrenia or schizoaffective disorders, or any history of psychotic symptoms in the current or previous depressive episodes. * Any Axis I or Axis II Disorder, which at screening is clinically predominant to their MDD or has been predominant to their MDD at any time within 6 months prior to screening. * Women who are pregnant or breastfeeding or a positive pregnancy test at screening or baseline. * Currently taking a prohibited adjunct therapy. Such drugs must be washed out for at least 4 weeks prior to baseline. * Current diagnosis of moderate or severe substance use (including alcohol) disorder (abuse or dependence, as defined by DSM-5), with the exception of nicotine dependence, at screening or within 6 months prior to screening. * In the opinion of the investigator, the subject has a significant risk for suicidal behavior * Has received electroconvulsive therapy, or had repetitive transcranial magnetic stimulation in the current episode. * Has received vagus nerve stimulation at any time prior to screening. * Any current or past history of any physical condition, which in the investigator's opinion might put the subject at risk or interfere with study results interpretation.