The purpose of this study is to evaluate the safety and effectiveness of vilazodone for the treatment of major depressive disorder versus citalopram. Doctors want to determine if vilazodone is effective for the treatment of major depressive disorder in those who have not responded to generic selective serotonin reuptake inhibitors (SSRI), which is a class of anti-depressant drugs such as Prozac, Lexapro, Paxil, or Zoloft. Both vilazodone and citalopram have been approved for the treatment of major depressive disorder. This research is being done because the researchers want to find out if vilazodone works in reducing the symptoms of depression significantly more than a generic SSRI.

The goal of the proposed study is to evaluate the efficacy and safety of switching to Vilazodone in patients with major depressive disorder (MDD) who are unresponsive to, only partially responsive to, or cannot tolerate a trial of the generic SSRI, citalopram (e.g., "partially responsive" means patients who report that their depressive symptoms have improved through the use of citalopram but that significant depressive symptoms persist; "cannot tolerate" refers to patient report of intolerable side effects that result in a desire to discontinue the medication). Seventy-two subjects with major depressive disorder who are still symptomatic or report intolerable side effects after a 6-week open-label trial of citalopram 20mg/day ( i.e. who are not classified as responders) will be randomized to receive a higher maximum dose of citalopram (40mg/day) or switch to vilazodone during the randomization phase of the trial for 6 weeks. The hypothesis to be tested is that vilazodone will result in greater rates of treatment response and be better tolerated compared to being titrated up to a higher maximum dose (40mg/day) of citalopram. The proposed study will provide needed data on the efficacy of switching antidepressants when individuals do not fully respond to previous treatment or have intolerable side effects with a generic SSRI.

Inclusion Criteria:

1. Men and women age 18-60;
2. Primary diagnosis of MDD. Diagnosis of MDD will be made with the Structured Clinical Interview for DSM-IV
3. Score of at least 23 on the Montgomery-Åsberg Depression Rating Scale
4. Treatment with citalopram at a dose no higher than 20mg/day for no longer than 4 weeks (subjects not currently taking an antidepressant will be started on citalopram 20mg/day for the 6-week open-label phase)
5. Ability to understand and sign the consent form.

Exclusion Criteria:

1. Unstable medical illness based on history or clinically significant abnormalities on baseline physical examination (e.g., congestive heart failure, bradyarrhythmias).
2. Current pregnancy or lactation, or inadequate contraception in women of childbearing potential
3. Subjects considered an immediate suicide risk based on the Columbia Suicide Severity rating Scale (C-SSRS)
4. Past 3-month DSM-IV substance abuse or dependence
5. Illegal substance use based on urine toxicology screening
6. Initiation of psychotherapy or behavior therapy specifically for MDD from a mental health professional within 3 months prior to study baseline
7. Initiation of any other psychotropic medication within 2 months prior to study inclusion
8. Concomitant use of any antidepressant (except low dose doxepin, amitriptyline, trazodone when used PRN as a hypnotic).
9. Concomitant use of medications that prolong the QT interval or are CYP2C19 inhibitors (e.g., cimetidine)
10. Previous treatment with vilazodone
11. Diagnosis of bipolar I or II disorder or any psychotic disorder (anxiety disorders will be allowed as long as MDD is considered the primary psychiatric disorder)
12. Cognitive impairment that interferes with the capacity to understand and self-administer medication or provide written informed consent

The adverse events reported were during the double-blind phase of the study

The sample size was likely too small to make comments about predictors of outcome and generally the study was under-powered, which would make differences difficult to detect. The study design enrolled only non-responders into the double-blind phase.