Zelquistinel in the Treatment of Adults With Major Depressive Disorder

Inclusion Criteria

• Written informed consent from the participant has been obtained prior to any study-related procedures
• Meet DSM-5 (Diagnostic and Statistical Manual of Mental Disorders, 5th Edition) criteria (American Psychiatric Association, 2013). for MDD (based on confirmation from the modified SCID), with a current major depressive episode of at least 8 weeks and not exceeding 18 months in duration at Visit 1.
• Have a negative serum β-human chorionic gonadotropin (β-hCG) pregnancy test at screening (Visit 1) if a WOCBP (Women of Childbearing Potential).
• Female participants willing to minimize the risk of becoming pregnancy for the duration of the clinical study and follow-up period. A female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies:
• Not a WOCBP (Women of Childbearing Potential). OR
• A WOCBP (Women of Childbearing Potential). who agrees to follow the contraceptive guidance in during the treatment period and for at least 4 to 5 weeks after the last dose of study treatment.
• Male participants willing to minimize the risk of inducing pregnancy for the duration of the clinical study and follow-up period. A male participant must agree to use contraception during the treatment period and for at least 10 weeks after the last dose of study treatment and refrain from donating sperm during this period.
• Able, as assessed by the investigator, and willing to follow study instructions and likely to complete all required study visits.

Exclusion Criteria

Psychiatric and Treatment-Related Criteria

• DSM-5 (Diagnostic and Statistical Manual of Mental Disorders, 5th Edition) based diagnosis of any disorder other than MDD that was the primary focus of treatment within 6 months before Visit 1. Comorbid generalized anxiety disorder, social anxiety disorder, or specific phobias are acceptable provided they play a secondary role in the balance of symptoms and are not the primary driver of treatment decisions.
• Lifetime history of meeting DSM-5 (Diagnostic and Statistical Manual of Mental Disorders, 5th Edition)criteria for:
• Schizophrenia spectrum or other psychotic disorder
• Bipolar or related disorder
• Major neurocognitive disorder
• Neurodevelopmental disorder of greater than mild severity or of a severity that impacts the participant's ability to consent, follow study directions, or otherwise safely participate in the study
• Dissociative disorder
• Posttraumatic stress disorder
• MDD with psychotic features
• History of meeting DSM-5 (Diagnostic and Statistical Manual of Mental Disorders, 5th Edition) criteria for alcohol or substance use disorder (other than nicotine or caffeine) within the 6 months before Screening (Visit 1).
• History (based on participant report and/or medical records, and investigator judgment) of the following:
• Inadequate response to ECT, a monoamine oxidase inhibitor, ketamine, or adjunctive treatment with an antipsychotic
• Treatment with clozapine or any depot antipsychotic
• ECT, vagus nerve stimulation, transcranial magnetic stimulation, or any experimental central nervous system treatment during the current episode or in the 6 months before Screening (Visit 1) whichever is longer)
• Tardive dyskinesia, serotonin syndrome, or neuroleptic malignant syndrome
• Having received:
• Anticonvulsant/mood stabilizer, within 1 year prior to Screening (Visit 1)
• Antipsychotic in the current episode, with the exception of quetiapine given for insomnia ≤ 50 mg/day provided it can be safely discontinued prior to Visit 2
• Combination therapy of more than 2 ADTs in the current episode if given for depression at adequate dose and duration
• ADT augmentation agent in the current episode
• Lifetime history of nonresponse to ≥ 2 antidepressants after adequate trials (adequate treatment is defined as at least 6 weeks at an adequate dose(s) based on approved package insert recommendations).
• Positive result at Scre