A Study of Aticaprant as Adjunctive Therapy in Adult Participants With Major Depressive Disorder (MDD) With Moderate-to-severe Anhedonia and Inadequate Response to Current Antidepressant Therapy

Inclusion Criteria: * Be medically stable on the basis of physical examination, medical history, vital signs, and 12-lead electrocardiogram (ECG) performed at screening and baseline * Have a Hamilton Depression Rating Scale 17 item (HDRS-17) total score of 20 or higher at the first and second screening interviews and must not demonstrate a clinically significant improvement between the first and the second independent HDRS-17 assessments * Meet Diagnostic and Statistical Manual of Mental Disorders-5th edition (DSM-5) diagnostic criteria for recurrent or single episode major depressive disorder (MDD), without psychotic features, based upon clinical assessment and confirmed by the structured clinical interview for DSM-5 Axis I disorders-clinical trials version (SCID-CT). Participants 65 years of age or older must have had the first onset of depression prior to 55 years of age * Have had an inadequate response to at least 1 oral antidepressant treatment, administered at an adequate dose (at or above the minimum therapeutic dose per Massachusetts General Hospital Antidepressant Treatment Response Questionnaire \[MGH ATRQ\]) and duration (at least 6 weeks) in the current episode of depression. An inadequate response is defined as less than(\<) 50% reduction in depressive symptom severity but with some improvement (\>0%) (ie, there may be minimal to moderate symptomatic improvement since the initiation of treatment, but some of the initial symptoms are still present, troubling to …