Inclusion Criteria:  Have a diagnosis of moderate to severe major depressive disorder Currently taking a SSRI, SNRI, or bupropion for at least 6 weeks with no dose modifications in the past 2 weeks Must have failed to adequately respond to the current antidepressant medication Willing to comply with all study assessments and procedures Must not be pregnant or breastfeeding at time of enrollment or throughout study  Exclusion Criteria:  Evidence of liver impairment or disease Active suicidal ideation Moderate to severe Alcohol Use Disorder Diagnosed bipolar disorder or psychotic disorder Has a history of hypersensitivity or allergic reaction to ALTO-300 or any of its components/excipients Concurrent or recent participation in another clinical trial for mental illness involving an investigational product or device