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 stable dose of 1 or 2 antidepressants (e.g., SSRI, SNRI, bupropion, or gepirone) 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