Improved solid-phase extraction and HPLC measurement of torasemide and its important metabolites

Torasemide is a “loop type” diuretic drug. For pharmacokinetic studies sensitive analytic methods are essential for authentic qualitative and quantitative information. A robust, selective and sensitive HPLC method is described for the simultaneous determination of torasemide, its major metabolite M5 and its active metabolites M1 and M3 and an internal standard within 17 min. Solid-phase extraction with C2-cartridges was used for the clean-up of plasma samples. The chromatographic separation was carried out on a CN-column with a mobile phase consisting of perchloric acid (0.02 M, pH 2.5)/acetonitrile (90/10, v/v)). The calibration range used reached from 20 to 1000 ng/ml for all analytes. Coefficients of variation were less than 10% at every calibration point for each analyte. Plasma concentrations in samples obtained from volunteers in the course of a clinical study could be reliably measured with this method. Median maximum concentrations in plasma after a 10 mg oral dose during a 24 h study interval were located at 1 h for torasemide, 1 h for M1 and 2 h for M5. Concentrations between 2226 and <20 ng/ml for torasemide, between 159 and <20 ng/ml for M1 and between 420 and <20 ng/ml for M5 were observed.