24-Hour Time Course of Striatal Dopamine D2 Receptor Occupancy of Ziprasidone: A PET Study

Ziprasidone is recommended to be dosed twice daily for the treatment of schizophrenia, based on peripheral pharmacokinetics and a knowledge of its half life in plasma level (5-10 hours). However, the plasma kinetics do not always mirror what occurs in the brain. Antipsychotics with a high-affinity at D2 receptors attach for a relatively long time to their binding sites even after plasma levels declined. Based on this observation, another antipsychotic with a similar high-affinity at D2 receptors, ziprasidone, would also be expected to keep a sufficiently high D2 receptor occupancy even 24 hours after the last dose. Given \>60% D2 occupancy is required to maximize chance of therapeutic efficacy, it would be valuable to assess the D2 receptor occupancy 24 hours postdose to predict the therapeutic effects of once-daily regimen. In this study, we will measure D2 receptor occupancy 6, 12, and 24 hours after the last dose of ziprasidone in patients with schizophrenia. The hypotheses are as follows: First, based on the known affinity of ziprasidone, the dopamine D2 occupancy 24 hours after the last administered dose of 80 mg will be \>60%. Second, the difference in dopamine D2 occupancy between scan at 6 hours and 24 hours will be less than 15%. Third, the difference in dopamine D2 occupancy between scan at 12 hours and 24 hours will be less than 10%. Fourth, ED50 24 hours post dose will be higher that those 6 and 12 hours postdose.

PET studies have demonstrated a therapeutic window of dopamine D2 receptor occupancy (60-80%) in patients with schizophrenia. This observation has been used to predict the therapeutic dose range. Ziprasidone is recommended to be dosed twice daily, based on a knowledge of its half life in plasma level (5-10 hours). However, the plasma kinetics do not always mirror the central kinetics. Antipsychotics with a high-affinity at D2 receptors like risperidone attach for a relatively long time to their binding sites even after plasma levels declined. Based on this observation, another antipsychotic with a similar high-affinity at D2 receptors, ziprasidone, would also be expected to keep a sufficiently high D2 receptor occupancy even 24 hours after the last dose. Given \>60% D2 occupancy is required to maximize chance of therapeutic efficacy, it would be valuable to assess the D2 receptor occupancy 24 hours postdose to predict the therapeutic effects of once-daily regimen. To date, there is no published report to examine D2 receptor occupancy of ziprasidone 24 hours after the last dose in patients with schizophrenia. This study will provide information on 24-hour time course of D2 occupancy of this drug, with which the dissociation between peripheral and central kinetics of this drug will be discussed. The results of this study will also test the feasibility of once daily dosing of ziprasidone, which will directly serve to guide physicians in clinical practice. Furthermore, the findings of this study will elucidate the relationship between D2 receptor occupancy and long-term outcome. The primary objective to determine the difference in dopamine D2 occupancy of ziprasidone at expected peak plasma levels (6 hours) and 12 and 24 hours postdose. The secondary objectives are to compare ED50 (the plasma levels of ziprasidone resulting in 50% maximal receptor occupancy) 24 hours postdose with those 6 and 12 hours postdose. Male or female patients aged 18-60 years suffering from schizophrenia or schizoaffective disorder will be eligible to participate in this study (Visit 1). Following the baseline clinical assessments (Visit 2), previous antipsychotics will be discontinued while initiating ziprasidone at 20mg BID and subsequently increasing the dose to 60mg BID (Visit 2-4). For patients already treated with ziprasidone 60 mg BID, no titration will be necessary. If patients are on a lower dose, they will only be included in the study if the treating clinician recommends an increase of the dose to 60 mg BID. Participants will undergo a total of 3 raclopride PET scans (6, 12, and 24 hours postdose) (Visits 5-7) after they have been on ziprasidone for at least 14 days. Psychopathology and side effects will also be assessed on these PET visits. MRI scan will be completed when possible (Visit 8). In the subsequent 6-month follow-up phase (Visit 9-17), participants will have clinical assessments biweekly in the first 3 months and monthly in the following three months. The dose will be titrated according to clinical response and tolerability in an open-labeled manner to a maximum dose of 80 mg BID.