4. Has a history of treatment resistant depression (TRD) during the current MDE, as assessed by the investigator and remote centralized rater using the MGH ATRQ. TRD is defined as failure to achieve a satisfactory response (e.g., less than 50% improvement of depression symptoms), as perceived by the participant, to at least 2 "treatment courses" during the current episode of a therapeutic dose of an antidepressant therapy (ADT) of at least 8 weeks duration (including the current ADT). The adequacy of dose and duration of the antidepressant therapy will be determined as per the MGH ATRQ criteria. The TRD status will be confirmed by remote, independent raters from the MGH CTNI who will administer the MGH ATRQ, via teleconference, between the screening visit and the baseline visit. Participants must currently be on a stable (for at least 4 weeks) and adequate (according to the MGH ATRQ) dose of ongoing antidepressant therapy (any antidepressant therapy, with the exception of MAOIs), of which total duration must be at least 8 weeks.