ALTO-207 in Adults With Treatment-resistant Depression (TRD)

Inclusion Criteria: * Male and female participants, ages 18 to 75 years, inclusive, at the time of signing the ICF. * Prior diagnosis of MDD without psychotic symptoms (in the current episode) and have confirmatory diagnosis of current MDD (moderate to severe). * Failure to respond (\<50% improvement) to at least 2-5 antidepressant treatments (including the current treatment) * Currently taking a stable dose of at least 1 but no more than 2 oral antidepressants at baseline Exclusion Criteria: * Evidence of unstable medical condition * Concurrent use of any prohibited medications or substance use disorder * Diagnosed bipolar disorder or a psychotic disorder or symptoms * Significant current PTSD symptoms or history of PTSD * Clinically significant current impulse control difficulties * Has a history of hypersensitivity or allergic reaction to ALTO-207 or any of its components/excipients * Concurrent or recent participation in another clinical trial for mental illness involving an investigational product or device