Study of ALTO-300 in MDD

Inclusion Criteria: * Have a diagnosis of moderate to severe major depressive disorder (MDD) * At Visit 1, currently taking a single SSRI, SNRI, or bupropion for at least 6 weeks with no dose modifications in the past 2 weeks by Visit 2 * 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 unstable medical condition * Nightly use of sleep medication * Diagnosed bipolar disorder, psychotic disorder, or dementia * Current moderate or severe substance use 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