DETERMINATION OF CLOZAPINE, NORCLOZAPINE, AND CLOZAPINE-N-OXIDE IN SERUM BY LIQUID-CHROMATOGRAPHY

We report a new assay to measure the serum concentration of the atypic al antipsychotic drug clozapine and two major metabolites, norclozapin e and clozapine-N-oxide. The analytes and an internal standard (tripro lidine) were extracted from alkalinized samples into ethyl acetate and back-extracted into 0.1 mol/L HCl. The acid extracts were chromatogra phed on a reversed-phase liquid chromatographic column with photodiode array detection (210-340 nm). With the 254-nm signal, between-run imp recision (CV) was <2% for clozapine and norclozapine at 400 mug/L, and 4.1% for clozapine-N-oxide at 100 mug/L. Absolute recovery exceeded 6 5%, and the detection limit was approximately 3-4 mug/L. In 25 patient s at steady state at a mean daily clozapine dosage of 269 mg (3.09 mg/ kg), clozapine averaged 231 +/- 144 mug/L (mean +/- SD); norclozapine and clozapine-N-oxide concentrations averaged 84% and 23% that of cloz apine. Analyte concentrations were significantly correlated with daily dose. The method's ability to quantify clozapine and two major metabo lites simultaneously with precision and sensitivity makes it useful in pharmacokinetic studies and therapeutic monitoring.