Study of ALTO-300 in MDD

Inclusion Criteria:

• Have a diagnosis of moderate to severe major depressive disorder (MDD)

• At Visit 2, currently taking a single SSRI, SNRI, or bupropion for at least 6 weekswith no dose modifications in the past 2 weeks

• 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 itscomponents/excipients

• Concurrent or recent participation in another clinical trial for mental illnessinvolving an investigational product or device