Rapid and selective liquid chromatographic/tandem mass spectrometric method for the determination of fosfomycin in human plasma

A rapid and selective liquid chromatographic/tandem mass spectrometric method for determination of fosfomycin was developed and validated. Following protein-precipitation, the analyte and internal standard (fudosteine) were separated from human plasma using an isocratic mobile phase on an Ultimate™ XB-CN column. An API 4000 tandem mass spectrometer equipped with Turbo IonSpray ionization source was used as detector and was operated in the negative ion mode. Multiple reaction monitoring using the precursor to product ion combinations of m/z 137 → 79 and m/z 178 → 91 was performed to quantify fosfomycin and fudosteine, respectively. The method was linear in the concentration range of 0.10–12.0 μg/mL using 50 μL of plasma. The lower limit of quantification was 0.10 μg/mL. The intra- and inter-day relative standard deviation over the entire concentration range was less than 10.6%. Accuracy determined at three concentrations (0.25, 1.00 and 8.00 μg/mL for fosfomycin) ranged from −1.0% to −4.2% in terms of relative error. Each plasma sample was chromatographed within 5.0 min. The method was successfully used in a bioequivalence study of fosfomycin in human plasma after an oral administration of capsules containing 1.0 g fosfomycin (∼1.3 g calcium fosfomycin).