Protocol Lay Person Short Title: A Trial of the Safety and Efficacy of Flexible Doses of SEP-363856 as Adjunctive Therapy in the Treatment of Adults with Major Depressive Disorder.

Other

• Any subject who, in the opinion of the clinical trial sponsor, investigator, or medical monitor, should not participate in the trial will be excluded.

• The study population includes subjects 18 to 65 years of age inclusive at the time of consent with MDD as the main diagnosis and who are currently experiencing a major depressive episode (MDE) as determined by the DSM-5 criteria (Diagnostic and Statistical Manual of Mental Disorders, 5th Edition) and confirmed by the Structured Clinical Interview for DSM-5, Clinical Trials Version (SCID-5-CT) and appropriate clinical psychiatric assessment. The current MDE must have a duration of ≥ 8 weeks and < 2 years. In addition, subjects must have reported a history for the current MDE of an inadequate response to at least 1 and no more than 3 adequate ADTs. Subjects must also have a total score ≥ 18 at the screening and baseline visit on the 17-item Hamilton Depression Rating Scale (HAM-D17).