Memantine Augmentation of Antidepressants

Inclusion Criteria:

* Male or female patients between 18 and 85 years of age at screening.
* Patients must provide written informed consent prior to study entry.
* Patients must meet DSM-IV-TR (Diagnostic and Statistical Manual IV Text Revision) criteria for Major Depressive Episode of a severity mild, moderate or severe or in partial remission, as confirmed by the MINI.
* Patients must have a HAM-D (17-item) score of 16 or higher.
* Patients must have been on 1 of the following medications for 4 or more weeks at or above the listed dose with no psychiatric medication dose changes for the past 25 days:

  * 20 mg qD of fluoxetine (Once Daily)
  * 50 mg qD of sertraline
  * 20 mg qD of paroxetine
  * 200 mg qD of fluvoxamine
  * 20 mg qD of citalopram
  * 10 mg qD of escitalopram
  * 150 mg qD of venlafaxine or venlafaxine sustained release
  * 300 mg qD of bupropion or bupropion sustained or extended release
  * 15 mg qD of mirtazapine
  * 60 mg qD of duloxetine
* Participants must agree to keep the dose of their existing antidepressant(s) constant throughout the 8-week trial.

Exclusion Criteria:

* Diagnosis of bipolar disorder or schizophrenic or schizoaffective disorder.
* History of alcohol or drug abuse or dependence within 6 months of enrollment.
* Patients who have received ECT (Electroconvulsive Therapy) in the past 3 months.
* History of seizures.
* Moderate dementia (MMSE score of 20 or less).
* Active suicidal ideation: endorsing a 3 (most severe score) on QIDS-SR (Quick Inventory of Depression Symptomatology Self Reports) suicide item OR a score of 2 or higher for the past week on Suicide Scale items 4 or 5 (current suicidal ideation moderate or strong or would avoid taking steps to save life).
* Currently taking a mood stabilizer or antipsychotic (except lithium clearly used as an augmenting agent).
* Patients who, in the opinion of the investigator, might not be suitable for the study.