Capillary electrophoretic chiral determination of mirtazapine and its main metabolites in human urine after enzymatic hydrolysis

Capillary electrophoresis and liquid-phase microextraction using porous polypropylene hollow fibers were employed for the enantioselective analyses of mirtazapine and its metabolites demethylmirtazapine and 8-hydroxymirtazapine in human urine. Before the extraction, urine samples (1.0 mL) were submitted to enzymatic hydrolysis at 37 degrees C for 16 h. Then, the enzyme was precipitated with trichloroacetic acid, the pH was adjusted to 8 with 0.5 mol/L phosphate buffer solution (pH 11) and 15% sodium chloride was further added. The analytes were transferred from the aqueous donor phase, through n-hexyl ether (organic solvent immobilized in the fiber), into 0.01 moL/L acetic acid solution (acceptor phase). The electrophoretic analyses were carried out in 50 mmol/L phosphate buffer solution (pH 2.5) containing 0.55% w/v carboxymethyl-beta-cyclodextrin. The method was linear over the concentration range of 62.5-2500 ng/mL for each mirtazapine and 8-hydroxymirtazapine enantiomer and 62.5-1250 ng/mL for each demethylmirtazapine enantiomer. The quantification limit was 62.5 ng/mL for all the enantiomers. Within-day and between-day assay precision and accuracy were lower than 15% for all the enantiomers. Finally, the method proved to be suitable for pharmacokinetic studies.