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 EC50 between patients and healthy volunteers. Methods Ten healthy volunteers and eight patients with schizophrenia participated in the study. We measured dopamine receptor occupancy using [C-11]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 EC50. Results The elimination half-life was longer in healthy volunteers than in patients. Other pharmacokinetic parameters were not significantly different between two groups. The EC50 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 EC50 data of healthy volunteers.