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