The purpose of this study is to assess the safety and effect of TC-5214 as a single therapy in patients with major depressive disorder who exhibit inadequate response to antidepressants.

Inclusion Criteria:

• Provision of signed and dated informed consent before initiation of any study-related procedures.
• The patient must have a clinical diagnosis of major depressive disorder (MDD) with inadequate response to no more than one antidepressant.
• Women of child-bearing potential must have a negative urine pregnancy test and confirmed use of a highly effective form of birth control before enrollment and until 3 months after their last dose of study drug.
• Outpatient status at enrollment and randomization.

Exclusion Criteria:

• Patients with a lifetime history of bipolar disorder; psychotic disorder or post-traumatic stress disorder.
• Patients with a history of suicide attempts in the past year and/or seen by the investigator as having a significant history of risk of suicide or homicide.
• Patients with any significant unstable hepatic, renal, pulmonary, cardiovascular, ophthalmologic, neurologic, or any other medical conditions that might confound the study or put the patient at greater risk during study participation.
• History of stroke or transient ischemic attack, seizures or seizure disorder, head trauma including closed head injury.
• Pregnancy or lactation.