Dopamine D3 Receptor Occupancy in Bipolar Depression

Bipolar disorder is a common and disabling condition with a high rate of suicide. The depressed phase of the disorder accounts for most of the morbidity and mortality. Only four medications are FDA approved to treat bipolar depression. They all include a second generation antipsychotic (SGA), yet the antidepressant mechanism of SGA's is unknown. Not all bipolar patients respond to these treatments, and they can have problematic side effects. There is therefore a great need to understand these medications' antidepressant mechanism of action in order to design more effective treatment options. This will be an exploratory occupancy study with cariprazine (Vraylar) in unmedicated participants with bipolar depression (n=8). Participants will have \[11C\]-(+)-PHNO scans before treatment starts, after three weeks of cariprazine, and after six weeks. Participants will be randomized to two dose groups, 1.5 mg daily (n=4) and 3 mg daily (n=4). These are typical doses of the medication for bipolar depression. Serum levels of cariprazine will be measured at each PET scan.