Measurement of clozapine and norclozapine in plasma/serum by high performance liquid chromatography with ultraviolet detection.

A simple method for the measurement of clozapine and its N-desmethyl metabolite in human plasma or serum by high performance liquid chromatography is described. An internal standard (aqueous nortriptyline, 4 mg/L) (50 microL) and Tris buffer (2 mol/L, pH 10.6) (100 microL) are added to plasma/serum (200 microL) and the analytes and internal standard extracted into methyl tert-butyl ether (200 microL). The extracts are analysed on a 150 mm column containing Spherisorb S5SCX using methanol containing ammonium perchlorate (35 mmol/L, pH 6.7) as eluent at a flow rate of 1.5 mL/min. Detection is by ultraviolet absorption (215 nm). The limit of detection is better than 0.05 mg/L for both analytes and the intra-assay precision (CV) for clozapine and norclozapine was 5.3 and 7.3% at 0.5 mg/L and 2.6 and 2.8% at 1.5 mg/L, respectively. The method can be applied to the measurement of these compounds in plasma after acute overdosage, for the assessment of compliance in patients apparently refractory to therapy and to identify interactions between clozapine and other neuroleptic and antidepressant drugs which may effect toxicity.