Safety and Efficacy of MK-6096 as Adjunctive Therapy in Participants With Major Depressive Disorder And Partial Response to Antidepressant Monotherapy (MK-6096-022)

The purpose of this study is to evaluate the safety and efficacy of filorexant (MK-6096) versus placebo as adjunctive treatment for major depressive disorder (MDD), in participants who are partial responders to antidepressant monotherapy with one of identified selective serotonin reuptake inhibitors (SSRIs) or serotonin norepinephrine reuptake inhibitors (SNRIs), or bupropion. The primary hypothesis of the study is that filorexant is superior to placebo as augmentation therapy with respect to change from baseline to Week 6 in the Montgomery Asberg Depression Rating Scale (MADRS) total score.

Participants will continue to take their pretrial antidepressant medication as prescribed throughout the trial. Participants will be randomized in a 1:1 ratio to receive filorexant or placebo for a 6-week treatment period. Following completion of the treatment period, participants will enter a 2-week double-blind run-out period. During the run-out period, participants who received placebo in the 6-week treatment period will continue to receive placebo and participants who received filorexant in the 6-week treatment period will be randomized to receive either filorexant or placebo in a 1:1 ratio.

Eligibility Criteria:
Inclusion Criteria:

* Participant is (a) male or (b) female and not of reproductive potential, or (c) female of reproductive potential has a serum beta-hCG level consistent with the nongravid state at screening and agrees to use acceptable contraception;
* Current primary diagnosis of recurrent major depressive disorder, without psychotic features, with a current moderate or severe depressive episode;
* Duration of the current major depressive episode must be at least 2 months but no more than 18 months at Screening;
* Participant has undergone an adequate trial of an antidepressant (one of identified SSRIs or SNRIs, or bupropion) for the current depressive episode.

Key Exclusion Criteria:

* Current primary psychiatric diagnosis other than major depression;
* Lifetime diagnosis of bipolar disorder, schizophrenia, schizoaffective disorder, or other psychotic disorder;
* Alcohol or other substance abuse or dependence (excluding nicotine);
* Clinically significant abnormality or disease of the central nervous system (including dementia and other cognitive disorders or traumatic brain injury);
* Imminent risk of self-harm or of harm to others;
* Participant is a psychiatric inpatient;
* Participant has been on continuous antidepressant treatment for \>18 months prior to Screening visit;
* Inadequate response to more than 3 adequate antidepressant trials (including the current antidepressant treatment trial) for treatment of the current depressive episode;
* Participant ever received electroconvulsive therapy, transcranial magnetic stimulation, or vagal nerve stimulation for treatment of depression;
* History of narcolepsy, cataplexy, circadian rhythm disorder, parasomnia, sleep-related breathing disorder, restless legs syndrome, periodic limb movement disorder, excessive daytime sleepiness or difficulty sleeping due to a medical condition;
* Clinical, laboratory, or electrocardiogram (ECG) evidence of significant systemic disease;
* Cardiovascular event (e.g., myocardial infarction) or procedure (e.g., coronary artery bypass surgery) within 3 months of study;
* History of malignancy ≤5 years prior to study, except for adequately treated basal cell or squamous cell skin cancer or in situ cervical cancer;
* Body Mass Index \>40 kg/m\^2;
* Pregnancy, breast-feeding, or expecting to become pregnant.