Determination of selected phenothiazines in human plasma by solid-phase extraction and liquid chromatography with coulometric detection

A new analytical method, based on a liquid chromatography with coulometric detection, has been developed and applied to the determination of selected phenothiazines (Chlorpromazine, Promazine, Fluphenazine and Levomepromazine) in human plasma. The drugs were separated on a Discovery HS F5 column, using a mobile phase composed of acetonitrile (32 %) and a pH 1.9 phosphate buffer (68 %). Promethazine was used as the internal standard. Detection was carried out at an oxidation potential of + 0.500 V. A novel clean-up procedure was developed by means of solid-phase extraction, using cyanopropyl cartridges, which allowed good extraction yield for all the analytes, with absolute recovery values higher than 91.0 %. The detector response was linear over a plasma concentration range of 0.5-250.0 ng/mL for Chlorpromazine, Promazine and Levomepromazine and of 0.2-4.0 ng/mL for Fluphenazine. Precision results, expressed by the intra-day and the inter-day relative standard deviation values, were good, being lower than 3.9 %. Accuracy data were satisfactory as well. The method has been successfully applied to the analysis of drug plasma levels of psychiatric patients undergoing therapy with selected phenothiazines.