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The pharmacokinetic profiles of clozapine and its main metabolite, norclozapine, were investigated in 18 chronic schizophrenic inpatients during long-term treatment.Patients received stable daily doses (between 300 and 900mg) for at least 1 month. Plasma drug concentrations were determined by high performance liquid chromatography. The pharmacokinetic parameters were calculated from both noncompartmental and compartmental approaches with zero-order input rate using a kinetic model for simultaneous fit of clozapine and norclozapine (active metabolite) concentrations.Large interpatient variations in pharmacokinetic parameters of the two drugs were observed. Plasma clozapine concentration peaked on average at 2 hours. The mean elimination rate constants from compartments 1 (k(10)) and 2 (k(20 ), elimination rate constant of norclozapine) were 0.087 and 0.156h(-1), respectively. The rate of formation of norclozapine, k(12), averaged 1.25h(-1). The mean fraction of the administered dose converted to norclozapine was estimated to be 66%. The apparent clearance of clozapine (CL/F) averaged 44.7 L/h and the volume of distribution (V(c)/F) was 7.00 L/kg. The pharmacokinetics of clozapine after multiple doses were linear over the range of clozapine plasma concentrations of 145 to 1411 microg/L.This is the first study assessing the pharmacokinetic profile of clozapine plus norclozapine in plasma during long-term treatment. This pharmacokinetic model can be used to determine the population pharmacokinetic parameters of clozapine and norclozapine in order to optimise individual dosage regimens using a Bayesian methodology. |
