A Study of Seltorexant as Adjunctive Therapy to Antidepressants in Adult and Elderly Participants With Major Depressive Disorder With Insomnia Symptoms Who Have Responded Inadequately to Antidepressant and Long-term Safety Extension Treatment With Seltorexant

Inclusion Criteria

• Meet Diagnostic and Statistical Manual of Mental Disorders-5th edition (DSM-5) diagnostic criteria for 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) diagnosed with first depressive episode prior to age 60. The length of the current depressive episode must be less than or equal to ( ] 0%) in depressive symptom severity with residual symptoms beyond insomnia present, and overall good tolerability, as assessed by the Massachusetts General Hospital-Antidepressant Treatment Response Questionnaire (MGH-ATRQ)
• Is 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 formulation and available in the participating country: citalopram, duloxetine, escitalopram, fluvoxamine, fluoxetine, milnacipran, levomilnacipran, paroxetine, sertraline, venlafaxine, desvenlafaxine, vilazodone, or vortioxetine at a stable dose (at therapeutic dose level) for at least 6 weeks, and for no greater than 18 months in the current episode
• Body mass index (BMI) between 18 and 40 kilograms per meter square (kg/m^2), inclusive (BMI = weight/height^2)
• Participant must be medically stable on the basis of the following performed at screening: physical examination (including a brief neurological examination), vital signs (including blood pressure), and 12-lead electrocardiogram (ECG) performed at screening and baseline

Exclusion Criteria

• Has a recent (last 3 months) history of, or current signs and symptoms of, a) severe renal insufficiency (creatinine clearance [CrCl] <30 milliliter per minute [mL/min]); b) clinically significant or unstable cardiovascular, respiratory, gastrointestinal, neurologic, hematologic, rheumatologic, immunologic or endocrine disorders; c) uncontrolled Type 1 or Type 2 diabetes mellitus
• Has a current or recent history of homicidal ideation or serious suicidal ideation within the past 3 months, corresponding to a positive response 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) for ideation on the Columbia Suicide Severity Rating Scale (C-SSRS), or a history of suicidal behavior within the past 6 months, as validated by the C-SSRS at screening or Day 1. Participants with prior suicidal behavior in the past year, or prior serious suicidal ideation/plan within the past 6 months, should be carefully screened. For current suicidal ideation, only participants with non serious items (1-3 of the suicidal ideation section of the C-SSRS) may be included at the discretion of the investigator
• Has a history of treatment-resistant MDD, defined as a lack of response to 2 or more adequate antidepressant treatments in the current episode, as indicated by no or minimal (<25% improvement in symptoms) when treated with an antidepressant of adequate dose (per MGH-ATRQ) and duration (at least 6 weeks)
• Has history or current diagnosis of a psychotic disorder, bipolar disorder, intellectual disability, autism spectrum disorder, borderline personality disorder, or somatoform disorders
• Has any significant primary sleep disorder, including but not limited to obstructive sleep apnea, restless leg syndrome, or parasomnias. Participants with insomnia disorder are allowed
• Has a history of moderate to severe substance use disorder including alcohol use disorder according to DSM-5 criteria within 6 months before screening