This study is designed to assess the use of pramipexole dihydrochloride and quetiapine (Seroquel) XR as combination therapy for bipolar depression. The proposed benefit of the combination therapy investigated in this study is improved treatment of bipolar depression.

Quetiapine is a first-line option in Canadian guidelines for the treatment of bipolar depression; however, there is room for improvement as the remission rate is approximately 50% and the response rate is approximately 60%. Pramipexole, which is currently used to treat Parkinson's disease and restless legs syndrome, is reported to have antidepressant effects in patients with unipolar or bipolar depression. There are no other clinical studies to evaluate the efficacy of pramipexole combined with an atypical antipsychotic, such as quetiapine XR, in the treatment of bipolar depression.

This study is a multicentre, randomized, double-blind and placebo-controlled exploratory study in which patients will receive one of three treatment arms for a treatment period of 16 weeks: quetiapine XR plus placebo, quetiapine XR plus 0.25mg pramipexole, or quetiapine XR plus 0.50mg pramipexole.

Inclusion Criteria:

• You must be between 18 years and 65 years of age.
• You must have been diagnosed with bipolar depression.
• You must (for women who are able to become pregnant) have a negative urine human chorionic gonadotropin (HCG) pregnancy test at enrolment and be using a reliable form of birth control for the entire duration of the study. The study staff will inform about what is considered an acceptable method of birth control.
• You must provide consent.
• You must be able to understand and comply with the requirements of the study

Exclusion Criteria:

• You are pregnant or lactating (breast-feeding),
• Your symptoms are due to the direct physiological effects of a substance (e.g. drug of abuse, medication, or other treatment) or a general medical condition,
• You have a primary psychotic disorder (e.g., schizophrenia),
• You have a personality disorder diagnosis which in the study doctor's opinion is the focus of clinical concern.
• You have a history or presence of any psychotic illness, including major depression with psychotic features.
• In the opinion of the study doctor, you pose an imminent risk of suicide or a danger to yourself or others,
• You have known allergies to quetiapine or to components of the medication capsule,
• You use any "prohibited" medications. You must inform the study coordinator or study doctor of all the medications that you are taking, and he/she will tell you if any of those medications exclude you from the study.
• You have a substance dependence (drug or alcohol dependence) which in the study doctor's opinion is the focus of clinical concern,
• You have a medical condition that would affect absorption, distribution, metabolism, or excretion of the medication,
• You have an unstable or inadequately treated medical illness (e.g. unstable diabetes, angina pectoris, hypertension) as judged by the study doctor,
• You have diabetes mellitus (DM) which, in the opinion of the study doctor, would exclude you from the study,
• You have an absolute neutrophil count (ANC) of ≤1.5 x 109 per liter
• You are involved in the planning and conduct of the study ,
• You were previously enrolled or randomized in this present study,
• You participated in another drug trial within 4 weeks prior enrolment into this study