Quantitation of simvastatin and its β-hydroxy acid in human plasma by liquid–liquid cartridge extraction and liquid chromatography/tandem mass spectrometry

A sensitive and reliable procedure for the simultaneous determination of simvastatin (SV) and its active β-hydroxy acid metabolite (SVA) in human plasma was developed and validated. The analytes were extracted simultaneously from 0.5 ml aliquots of human plasma samples by methyl tert-butyl ether (MTBE) via Chem Elut cartridge extraction [also called liquid–solid extraction (LSE) or liquid–liquid cartridge extraction (LLCE)], separated through a Kromasil C18 column (50 × 2 mm i.d. 5 µm) and detected by tandem mass spectrometry with a turbo ionspray interface. Stable isotope-labeled SV and SVA, 13CD3-SV and 13CD3-SVA, were used as internal standards. SV and SVA were detected in positive and negative ion modes, respectively, via within-run polarity switching. The use of Chem Elut cartridges not only provided a simple and efficient means of plasma sample extraction but also successfully reduced the interconversion between SV and SVA to an undetectable (for lactonization of SVA) or negligible (<0.07%, for hydrolysis of SV) level. The method showed excellent reproducibility, with intra- and inter-assay precisions <4.5% (RSD), and intra- and inter-assay accuracy between 94% and 107% of nominal values, for both analytes. The extraction recoveries were 78% and 87% on average for SV and SVA, respectively. The analyte was found to be stable in plasma through three freeze (−70 °C)–thaw (4 °C) cycles and for at least 3 h under bench-top storage condition in an ice-bath (4 °C), and also in the reconstitution solution at 4 °C for at least 24 h. The method has a lower limit of quantitation (LOQ) of 50 pg ml−1 with a linear calibration range of 0.05–50 ng ml−1 for both analytes, and has proved to be very reliable for the analysis of clinical samples. Copyright © 2000 John Wiley & Sons, Ltd.