Randomized, Double-Blind, Placebo Controlled Study of Vilazodone's Efficacy, Safety, and Biomarkers of Response in Major Depressive Disorder (MDD)

Inclusion Criteria: * Patients 18-70 years of age. * A diagnosis of MDD, single episode or recurrent, according to DSM-IV-TR (296.2/296.3) with a current Major Depressive Episode of less than two year's duration with a minimum duration of at least 4 weeks. * Meets DSM-IV-TR criteria for Major Depressive Disorder. * HAM-D score ≥ 22 on the first 17 items of the 21-item HAM-D. * HAM-D item 1 (depressed mood) score ≥ 2. * Patients must be able to provide written informed consent * Patients must be able to speak, read and understand English Exclusion Criteria: * Patients with a current (or within 6 months prior to the Screening Visit) Axis I disorder of Post Traumatic Stress Disorder, Eating Disorder, Obsessive Compulsive Disorder. * Patients with a history of schizophrenia, schizoaffective disorder or bipolar I or II disorder (with a history of hypomanic or manic episodes). * Patients who meet DSM-IV-TR criteria for substance abuse (alcohol or drugs) within 3 months prior to the Screening Visit or substance dependence within 6 months prior to the Screening Visit. * Patients who meet criteria for any of the following DSM-IV-TR MDD Specifiers: \[a\] With Catatonic Features; \[b\] With Postpartum Onset; \[c\] With Seasonal Pattern \[d\]severe with Psychotic Features. * Patients who are receiving formal psychotherapy or have had psychotherapy within the 12 weeks prior to the Screening Visit. * Patients who have any one of the following: * In the month prior to screening, have had active suicidal ideation with some intent to act, without specific plan. * In the month prior to screening, have had suicidal ideation with specific plan and intent. * Have made a suicide attempt within the 6 months prior to the screening visit. * In the opinion of the Investigator, is currently at significant risk of suicide. * Patients who have had an inadequate response to at least 2 consecutive antidepressants from different classes given at adequate doses for an adequate duration. * Patients who have received electroconvulsive therapy within the 6 months prior to the Screening Visit. * Patients currently taking a psychotropic drug. Patients who have taken psychotropic drugs must have discontinued these prior to the Screening Visit. The minimum discontinuation periods are outlined in the study protocol. * Patients taking migraine medications with a serotonergic mechanism of action * Patients taking CYP3A4 inhibitors such as grapefruit juice, ketoconazole, diltiazem, and macrolide antibiotics or montelukast * Patients with known hypersensitivity to SSRIs (selective serotonin reyptake inhibitors) or 5-HT1a agonists. * Patients previously treated with vilazodone (also known as SB-659746-A or EMD 68 843). * Patients with a history of clinically significant cardiac, renal, neurologic, cerebrovascular, hepatic, hematologic, metabolic or pulmonary disorders. * Patients with any serious medical disorder or condition that would, in the investigator's opinion, preclude the administration of study medication. * Female patients must not be pregnant, lactating, or planning to become pregnant during the time of study participation. All female patients must be at least 1 year post menopausal or irreversibly surgically sterilized (by hysterectomy, oophorectomy, or bilateral tubal ligation with resection) or determined not to be at risk of pregnancy. * Patients with clinically significant abnormalities on electrocardiogram. * Patients having clinically significant abnormal laboratory findings. * Patients with a positive drug screen. * Patients who, in the opinion of the investigator, would be noncompliant with the visit schedule or study procedures. * Patients that have taken an investigational drug or participated in an investigational drug trial within the past 30 days.