Age and Sex Impact Clozapine Plasma Concentrations in Inpatients and Outpatients With Schizophrenia

Background Although clozapine is primarily used in a younger to mid-life population of patients with psychosis, there are limited data on the clinical pharmacology of clozapine later in life. The objective of this study was to assess the magnitude and variability of plasma concentrations of clozapine and norclozapine across the lifespan in a real-world clinical setting. Design A population pharmacokinetic study using nonlinear mixed effect modeling (NONMEM). Age, sex, height, weight, and dosage formulation were covariates. Setting Inpatients and outpatients at the Centre for Addiction and Mental Health, Toronto, from 2001 to 2007. Participants Patients ranging in ages from 11 to 79 with schizophrenia spectrum disorders and prescribed clozapine (Clozaril). Measurements A total of 1142 plasma clozapine and norclozapine concentrations (2,284 concentration measurements) from 391 patients with schizophrenia spectrum disorder. Results A one-compartment model with first-order absorption and elimination best described the data. The population predicted clearance of clozapine for females was 27.1 L/h (SE 11.1%) and 36.7 L/h (SE 9.7%) for males. For norclozapine, clearance in females was 48.6 L/h (SE 10.8%) and 63.1 L/h (SE 9.3%) in males. The only covariates with a significant effect on clearance were age and sex: clearance for both parent and metabolite decreased exponentially with age at least 39 years. Conclusions Decreased clearance of clozapine and norclozapine with age results in increased blood concentrations and, hence, the potential for adverse drug reactions. These findings have particular clinical relevance for the dosing and safety monitoring of clozapine in older adults, highlighting a need for increased vigilance.