ACP-211 Monotherapy for Major Depressive Disorder With Inadequate Antidepressant Response

Inclusion Criteria: * Adults ≥18 and ≤65 years of age * Provides written informed consent * Clinical diagnosis of MDD * History of inadequate response to at least two antidepressants, with at least one inadequate response documented during the current episode * Currently treated with an approved antidepressant at a stable dose prior to Screening * MADRS total score ≥28, CGI-S score ≥4 , and QIDS-SR16 score ≥16 at Screening and Baseline * Females of childbearing potential must have a negative pregnancy test and agree to use acceptable contraception; males must agree to use barrier protection and refrain from sperm donation Exclusion Criteria: * Current diagnosis of certain personality disorders or persistent depressive disorder * Recent substance use disorders, excluding caffeine or nicotine * Active suicidal risk or recent suicidal attempt * History of schizophrenia, psychotic disorders, bipolar disorder, or MDD with psychotic features * Current treatment requirement for PTSD, acute stress disorder, panic disorder, or OCD * History of neuroleptic malignant syndrome, serotonin syndrome, or epilepsy (except single febrile seizure in infancy) * Documented non-response to ADT, including ketamine or esketamine * Allergy or sensitivity to ketamine or esketamine * Significant cardiovascular disease * Positive history of hepatitis B, hepatitis C, or HIV infection * Unstable diabetes or uncontrolled medical conditions * Positive urine drug test for an illicit drug or cannabis * Received neuromodulation therapies (ECT,TMS, VNS, DBS) in the current depressive episode * Recent initiation or change in psychotherapy Additional inclusion/exclusion criteria apply. Participants will be evaluated at Screening to ensure that all criteria for study participation are met.