A Research Study to Compare the Treatments of a Combination of Elzasonan With Zoloft, to Zoloft Alone, or Placebo in People With Major Depressive Disorder (MDD)

Inclusion Criteria: * Adult subjects, 18 years of age or older with a diagnosis of recurrent, moderate-to-severe MDD without psychotic features (DSM-IV 296.3x, with a HAMD17 score \>= to 22 and CGI-S \>=4. * MDD must be the primary psychiatric disorder that motivated the subject to seek treatment and the current episode must be at least 1 month in duration but no longer than 6 months in duration. Exclusion Criteria: * Subjects who, in the investigator's judgement, would require treatment with electroconvulsive therapy (ECT), or antipsychotics, or would require a change in intensity of psychotherapy, or subjects who would require treatment with any other psychotherapeutic drugs during the course of the trial. * Subjects who have ever been diagnosed with Panic Disorder, Post-Traumatic Stress Disorder, Obsessive-Compulsive Disorder, Bipolar Affective Disorder, Schizophrenia, Schizoaffective Disorder or othe Psychotic Disorder, MDD with a seasonal pattern, or Dissociative Disorders per DSM-IV criteria.