Determination of Tiopronin in Human Plasma and Pharmacokinetics by LC-ESI-MS

A rapid, sensitive and specific method based on high performance liquid chromatography-electrospray ionization-mass spectrometry was developed for the determination of tiopronin in human plasma. In this study, vitamin C and dithiothreitol were used as the reducer and to release and stabilize tiopronin from dimeric mixed forms with endogenous thiols encountered during pretreatment of plasma samples. The separation was successfully achieved on an Agilent SB-Aq column packed with 5 mu m C-18 silica, using an aqueous formic acid solution (pH 4.5-4.7), 0.5 mM tris(hydroxymethyl)aminomethane and methanol (95:5, v/v) as the mobile phase. Mass spectra were acquired in selective ion monitoring mode, using the [M - H] (-) ions of tiopronin at m/z 162.0 and the [M - H](-) of the internal standard sodium cyclamate at m/z 178.0, respectively. This quantitative assay was fully validated with respect to precision, repeatability and accuracy. The correlation coefficients were > 0.9995 in the range 0.025-8.15 mu g mL(-1) in human plasma. The mean recoveries were above 85%. The limit of quantitation was 0.012 mu g mL(-1) with a relative standard deviation of inter-day and intra-day accuracy of less than 15%. This LC-ESI-MS method was also successfully applied to a pharmacokinetic study after oral administration of formulated tiopronin to healthy volunteers. The elimination half-life (T (1/2)) was 21.5 +/- A 11.1 h.