Determination of Clozapine, Norciozapine, and Clozapine-N-oxide in Serum by Liquid Chromatography

We report a new assay to measure the serum concentra-tions of the atypical antipsychoticdrug clozapine and two major metabolites, norclozapine and clozapine-N-oxide. The analytes and an internal standard (triprolidine)were extracted from alkalinized samples into ethyl acetate and back-extracted into 0.1 mauL HCI. The acid extracts were chromatographed on a reversed-phase liquid chromato-graphic column with photodiode array detection (210-340 nm). With the 254-nm signal, between-run imprecision (CV) was <2 % for clozapine and norclozapine at 400.Lg/L,and 4.1 % for clozapine-N-oxide at 100 g/L Abso-lute recovery exceeded 65%, and the detection limitwas-3-4 g/L In 25 patients at steady state at a mean daily clozapine dosage of 269 mg (3.09 mg/kg), clozapine averaged 231 ± 144 zg/L (mean ± SD); norclozapine and clozapine-N.oxide concentrationsaveraged 84 % and 23 % that of clozapine. Analyte concentrations were sig