A Study of Seltorexant as Adjunctive Therapy to Antidepressants in Adolescents With Major Depressive Disorder Who Have an Inadequate Response to Selective Serotonin Reuptake Inhibitor (SSRI) and Psychotherapy

Prior treatment

• Has current the diagnostic and statistical manual of mental disorders, fifth edition (DSM-5) diagnosis of conduct disorder, intellectual disability, autism spectrum disorder, borderline personality disorder, somatoform disorders, or fibromyalgia. A prior history of one or more of these disorders is allowed as long as the disorder(s) are currently stable and major depressive disorder (MDD) is not secondary to the original diagnosis. Has a current or prior DSM-5 diagnosis of a psychotic disorder, or bipolar disorder

Other illness

• Has a history of liver or renal insufficiency, significant cardiac (example, congenital heart disease, cardiomyopathy, or tachyarrhythmias), vascular, pulmonary, gastrointestinal, endocrine (including uncontrolled hyperthyroidism), neurologic (including seizure disorder), hematologic, rheumatologic, psychiatric, or metabolic disturbances. Stable medical conditions are allowed
• Has a significant primary sleep disorder (example, obstructive sleep apnea, parasomnias) at screening but participants with insomnia or hypersomnia disorders are allowed
• At significant risk of committing suicide based on history or according to the investigator's experience, or based on active suicidal ideation, intent or plan, item 4 or 5 of the Columbia-Suicide Severity Rating Scale (C-SSRS) within the past 3 months or a history of suicidal behavior within the last 6 months
• Has known allergies, hypersensitivity, or intolerance to seltorexant or its excipients

• Has inadequate response to trial of at least 1, but no more than 2 antidepressant treatments during the current major depressive episode including their current antidepressant fluoxetine or escitalopram (SSRI). Inadequate response is determined as less than (<) 50 percentage (%) symptom reduction after adequate antidepressant treatment for at least 6 weeks at or above the minimum therapeutic dose prior to screening
• Has had access to adequate psychotherapy in the current depressive episode (based on investigator judgement/local guidance) prior to randomization
• Must have Children's Depression Rating Scale-Revised (CDRS-R) total score greater than or equal to (>=) 48 at screening and >=40 at the baseline visit
• Participants weighing between fifth and ninety-fifth percentile for age and sex. Obese participants greater than ninety-fifth percentile and underweight participants below fifth percentile may participate following medical clearance, as long as their baseline weight is >=30 kilograms (kg)
• A female participant of childbearing potential must have a negative urine pregnancy test at screening and baseline