To determine the safety, efficacy and tolerability of SEP-225289 in subjects with Major Depressive Disorder

This is a randomized, placebo-controlled, double-dummy, multi-center study of the safety, efficacy and tolerability of SEP-225289 in male and female subjects with MDD. Subjects meeting DSM-IV criteria for Melancholic or Atypical Features specifier are eligible for participation. The study will consist of a screening period, which may last up to 2 weeks, an eight week (56 day) double-blind treatment period, a two week (14 day) wash-out, and a one week (7 day) follow up. Total subject participation will be approximately 91 days (13 weeks). This study was previously posted by Sepracor Inc. In October 2009, Sepracor Inc. was acquired by Dainippon Sumitomo Pharma., and in October 2010, Sepracor Inc's name was changed to Sunovion Pharmaceuticals Inc.

Inclusion Criteria:

• The duration of the current episode must be at least 1 month but not longer than 12 months.
• Subjects must have a primary diagnosis of Major Depressive Disorder.
• Subjects must have had at least one previous, diagnosed episode of MDD in the past 5 years.
• MDD must be the condition that was chiefly responsible for motivating the subject to seek treatment.
• Subject is in general good health.

Exclusion Criteria:

• Subject is participating in, has participated in, or plans to participate in any investigational drug study.
• Subject who has donated blood within the last 30 days or plans to donate blood during and 30 days following participation.
• Known failure to respond (in the past 5 years) to two adequate (dose and duration) antidepressant medications with distance mechanisms of action including tricyclics.
• Subjects who have undergone Electroconvulsive Therapy treatment.
• Treatment with fluoxetine, in the 6 weeks before baseline.
• Subject with psychotic disorders, anorexia nervosa, bulimia or post-traumatic stress disorder.
• Subject with a history or presence of bipolar disorder (i.e., current or past history of manic episode).
• Subjects with Obsessive Compulsive Disorder.
• Subjects with a lifetime diagnosis of Panic Disorder.
• Subject received treatment with antidepressants within 2 weeks.
• Subject with lifetime history of suicidal attempts, alcohol dependence or abuse, drug(s) dependence or abuse (excluding nicotine and caffeine) or has a positive urine drug screen.
• Subject has a history of significant risk of suicide or homicide.
• Bereavement - Defined as death of a loved one within 3 months.
• Subject has a documented history of HIV, hepatitis B or hepatitis C.