The purpose of this research study is to determine whether AZD6765 has an effect on the patient's depression when taken together with current depression medication. In addition, information will be gathered on how well AZD6765 is tolerated, investigate the levels of AZD6765 and the levels of the current depression medication in the blood. In addition, the research staff will determine if AZD6765 has any mood or calming effects (how you feel).

Inclusion Criteria:

• Signed informed consent before any study-related procedures start.
• The patient is previously diagnosed with Major Depressive Disorder (MDD) and currently taking an antidepressant for at least 6 weeks.
• Patient has a history of poor response to 1 or more antidepressants (in addition to the antidepressant the patient is taking at enrollment) after exposure at adequate doses or maximum tolerated doses for ≥4 weeks.

Exclusion Criteria:

• Patient has a lifetime history of schizophrenia, bipolar, psychosis or psychotic depression.
• Patient has a lifetime history of failure to ECT therapy.
• Patient is pregnant or breast feeding.
• Length of current episode of depression exceeds ≥2 years.