The use of healthy volunteers instead of patients to inform drug dosing studies: a [¹¹C]raclopride PET study.

RATIONALE: Receptor occupancy study has been performed to evaluate pharmacokinetic profiles in new antipsychotic drug development. While these findings highlight the value of positron emission tomography (PET) for dose-finding study, what is unclear is if it is necessary to conduct these studies in patients with schizophrenia or whether studies in healthy volunteers are adequate.

OBJECTIVES: To determine if it is necessary to conduct dopamine receptor occupancy studies in patients with schizophrenia or whether studies in healthy volunteers are adequate for dose-finding study, we compared the concentration-occupancy relationship in terms of EC(50) between patients and healthy volunteers.

METHODS: Ten healthy volunteers and eight patients with schizophrenia participated in the study. We measured dopamine receptor occupancy using [(11)C]raclopride PET and plasma concentration of YKP1358, a novel antipsychotic drug under clinical development, at a number of time points after the administration of YKP1358. Pharmacokinetic data including area under the plasma concentration versus time curve, elimination half-life, maximum observed plasma concentration, and the time to reach the maximum observed plasma concentration were obtained. We explored the relationship between plasma concentration and dopamine D(2) receptor occupancy using E (max) model and calculated EC(50).

RESULTS: The elimination half-life was longer in healthy volunteers than in patients. Other pharmacokinetic parameters were not significantly different between two groups. The EC(50) was 7.6 ng/ml (95% confidence interval (CI) 6.2-9.0) in healthy volunteers and 8.6 (95% CI 7.4-9.9) in patients.

CONCLUSIONS: The antipsychotic concentration-occupancy relationship in patients can be estimated from the EC(50) data of healthy volunteers.