A Study of Aticaprant 10 Milligrams (mg) as Adjunctive Therapy in Adult Participants With MDD With Moderate-to-severe Anhedonia and Inadequate Response to Current Antidepressant Therapy

Inclusion Criteria:

1. Be medically stable on the basis of physical examination, medical history, vital signs, and 12-lead electrocardiogram (ECG) performed at screening and baseline
2. 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 (that is, an improvement of more than 20 percent [%] on their HDRS-17 total score) between the first and the second independent HDRS-17 assessments
3. 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 structural 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
4. Is currently receiving and tolerating well any one of the following selective serotonin reuptake inhibitor (SSRI) or serotonin-norepinephrine reuptake inhibitor (SNRI) for depressive symptoms at screening, in any approved formulation and available in the participating country/territory: citalopram, duloxetine, escitalopram, fluvoxamine, fluoxetine, milnacipran, levomilnacipran, paroxetine, sertraline, venlafaxine, desvenlafaxine at a stable dose (at or above the minimum therapeutic dose per Massachusetts General Hospital Antidepressant Treatment Response Questionnaire [MGH-ATRQ] for at least 6 weeks. The current antidepressant cannot be the first antidepressant treatment for the first lifetime episode of depression
5. Participant's current major depressive episode, and antidepressant treatment response in the current depressive episode, must all be confirmed by the Site Independent Qualification Assessment

Exclusion Criteria:

1. Have had in the current depressive episode, no response (treatment failure) to 5 or more antidepressant treatments including the current SSRI/SNRI (that is, the one presumed to be continued in the treatment phase) assessed using the MGH-ATRQ
2. Has a history or evidence of clinically meaningful noncompliance with current antidepressant therapy
3. Has a history of moderate-to-severe substance use disorder including alcohol use disorder according to diagnostic and statistical manual of mental disorders-5th edition (DSM-5) criteria within 6 months before screening
4. Has had in the current episode an inadequate response to adequate course of intravenous or intranasal ketamine or esketamine, electroconvulsive therapy (that is, at least 7 treatments), vagal nerve stimulation, or deep brain stimulation device
5. Has current, or a history (past 6 months), of seizures
6. Has a current homicidal ideation/intent, per the investigator's clinical judgment, or has suicidal ideation with some intent to act within 3 months prior to the start of the Screening Phase, per the investigator's clinical judgment or based on the Columbia Suicide Severity Rating Scale (C-SSRS), corresponding to a response of "Yes" on Item 4 (active suicidal ideation with some intent to act, without specific plan) or Item 5 (active suicidal ideation with specific plan and intent), or a history of suicidal behavior within the past 6 months prior to the start of the Screening Phase. Participants reporting suicidal ideation with intent to act or suicidal behavior at baseline should be excluded
7. Has one or more of the following diagnoses: a) A diagnostic and statistical manual of mental disorders-5th edition (DSM-5) diagnosis (which has been the primary focus of psychiatric treatment within the past 2 years) of any of the following: panic disorder, generalized anxiety disorder social anxiety disorder, specific phobia; b) A current (in the past year) DSM-5 diagnosis of: obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD), anorexia nervosa, bulimia nervosa; c) A current or prior (lifetime) DSM-5 diagnosis of: a psychotic disorder or major depressive disorder (MDD) with psychotic features, bipolar or related disorders, intellectual disability, autism spectrum disorder, borderline personality disorder, antisocial personality disorder, histrionic personality disorder, narcissistic personality disorders, somatoform disorders