Inclusion Criteria:
1. Participants who have completed 8 weeks of treatment in the preceding study (GLYX13-C-202, NCT01684163.
2. Participants who wish to continue treatment with GLYX-13 after the preceding study.
3. Meets Diagnostic and Statistical Manual, Fourth Edition, Text Revision (DSM-IV-TR) criteria for major depressive disorder (MDD).
4. Female subjects of childbearing potential with a negative serum pregnancy test prior to entry into the study and who are practicing an adequate method of birth control (eg oral or parenteral contraceptives, intrauterine device, barrier, abstinence) and who do not plan to become pregnant during the course of the study. Female subjects may be included without a negative serum pregnancy test if they are surgically sterile or at least 2 years post-menopausal.
5. Clinical laboratory values \<2 times the upper limit of normal (ULN) or deemed not clinically significant per the investigator and Naurex medical monitor.
6. Ability to understand the requirements of the study, provide written informed consent, abide by the study restrictions, and agree to return for the required assessments.
7. Based on the investigator and Naurex medical monitor's clinical judgment, subjects with eating disorders, obsessive compulsive disorder (OCD), panic disorder, post-traumatic stress disorder (PTSD), and generalized anxiety disorders secondary to major depressive episodes (MDEs) are permitted.
Exclusion Criteria:
1. Axis I diagnosis of delirium, dementia, dysthymia, amnestic or other cognitive disorder, schizophrenia or other psychotic disorder, bipolar I or II disorder, eating disorder (anorexia or bulimia nervosa), obsessive-compulsive disorder, panic disorder, acute stress disorder, agoraphobia, social phobia, attention-deficit hyperactivity disorder (ADHD), or PTSD.
2. A clinically significant current Axis II diagnosis of borderline, antisocial, paranoid, schizoid, schizotypal, or histrionic personality disorder
3. Experiencing hallucinations, delusions, or any psychotic symptomatology in the current episode; lifetime history of psychosis.
4. Huntington's, Parkinson's, Alzheimer's, Multiple Sclerosis, or a history of seizures or strokes.
5. Currently hospitalized or residing in an in-patient facility during study participation.
6. Substance abuse since the end of participation in GLYX13-C-202, including greater than or equal to 5 units of alcohol per day where 1 unit = ½ pint of beer, 1 glass of wine 4 oz, or 1 oz. of spirits consumed most weeks or in the opinion of the investigator
7. Women who are planning to become pregnant during the course of the study.
8. Allergy or intolerance to current antidepressant or other current medications.
9. Participation in any clinical trial of an investigational product or device within 30 days of enrollment in this trial with the exception of GLYX13-C-202.
10. Positive screen for drugs of abuse: cocaine, marijuana, PCP, ketamine, opioid or other agent that in the opinion of the investigator is being abused
11. Pose current (past 6 months) suicide risk based on administration of the C SSRS and the investigator's clinical judgment.
12. Human immunodeficiency virus (HIV) infection (based on the based on the HIV-1 \& HIV-2 antibody screen) or other ongoing infectious disease.
