Rapid and selective HPLC analysis of olanzapine and desmethylolanzapine in plasma using coulometric detection coupled to a novel SPE procedure

A rapid and sensitive liquid chromatographic method was developed for the simultaneous determination of olanzapine and its metabolite N-desmethylolanzapine in human plasma. A chromatographic run on a C8 (150x4.6 mm, 5 µm) column lasts about 8 minutes, using a mobile phase composed of methanol (30%) and a pH 3.5 phosphate buffer (70%). A coulometric detector was used; the first coulometric cell was set at +350 mV and the second at -200 mV. A careful solid-phase extraction procedure, based on diol cartridges, was implemented for the pre-treatment of plasma samples; only 250 µL of plasma are needed for a complete analysis. Linear responses were obtained between 0.4 and 40.0 ng/mL for both analytes, with a detection limit of 0.1 ng/mL. Extraction yield values for the analytes exceeded 97%, with relative standard deviation < 2.2%. Thus, precision was good; accuracy was also satisfactory. Due to its high selectivity and sensitivity, the proposed liquid chromatographic method seems to be suitable for therapeutic drug monitoring of patients treated with Zyprexa tablets undergoing polypharmacy and also for pharmacokinetic studies.