A Study of Aticaprant 10 mg as Adjunctive Therapy in Adult Participants With Major Depressive Disorder (MDD) With Moderate-to-severe Anhedonia and Inadequate Response to Current Antidepressant Therapy and Long-term Extension Treatment With Aticaprant

Inclusion Criteria: * Be medically stable based on physical examination (including a brief neurologic examination), medical history, vital signs (including blood pressure), and 12-lead electrocardiogram (ECG) performed at screening and double blind (DB) baseline * 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 (DSM-5 296.22, 296.23, 296.32, or 296.33), based upon clinical assessment and confirmed by the structured clinical interview for DSM-5 axis I disorders-clinical trials version (SCID-CT) * Have symptoms of anhedonia based on clinical assessment and confirmed by presence of anhedonia (positive response to MDE module symptom Item 2) on the SCID-CT at screening * Have had an inadequate response to at least 1 and up to 5 (inclusive) oral antidepressant treatments, administered at an adequate dose * Is currently receiving and tolerating well any one of the following selective serotonin reuptake inhibitor or serotonin-norepinephrine reuptake inhibitor (SSRI or SNRI) antidepressants at screening, in any approved formulation and available in the participating country/territory: citalopram, duloxetine, escitalopram, fluvoxamine, fluoxetine, milnacipran, levomilnacipran, paroxetine, sertraline, venlafaxine, or desvenlafaxine at a stable dose (at or above the minimum therapeutic dose per Massachusetts general hospital antidepressant treatm…