A simple, specific, and sufficiently sensitive liquid chromatography-tandem mass spectrometry (negative-ion electrospray ionization) methodology to determine mevalonic acid (MVA) in human plasma is described, and its application to the analysis of rat plasma MVA levels after rosuvastatin administration is demonstrated. The method was validated over the linearity range of 0.5-50.0 ng/ml (r 2 . 0.99) using deuterated MVA as an internal standard. The lower limit of quantification was 0.5 ng/ml. The assay procedure involved the isolation of MVA from plasma samples using solid-phase extraction. Chromatographic separation was achieved on a HyPurity Advance column with a mobile phase consisting of ammonium formate buffer (10 mM, pH 8.0) and acetonitrile (70:30, v/v). Excellent precision and accuracy were observed. MVA and deuterated mevalonolactone were stable in water and plasma under different storage and processing conditions. The recovery observed was low, which was attributable to a significant matrix effect. A significant decrease (30-40%; P , 0.05) was observed in rat plasma MVA levels after rosuvastatin administration. In the biosynthesis of cholesterol, the conversion of HMGCoA to mevalonic acid (MVA) by HMG-CoA reductase is an early and rate-limiting step (1-3). The statin class of drugs, such as simvastatin, atorvastatin, and rosuvastatin, act on HMG-CoA reductase, resulting in the inhibition of MVA biosynthesis (Fig. 1) (4, 5). Understanding the reason for increased cholesterol levels and interindividual variability in response to statin therapy can lead to better and monitored pharmacotherapy (6). Because the reduction of MVA levels is an indirect measure of decreased cholesterol levels, MVA can be used as a biomarker to measure the extent of statin activity.A large variety of methods have been published for MVA estimation in urine and plasma. These involve enzyme immunoassay (7), radioimmunoassay (2), and GC-MS methods (8-10). However, there are very few methods reported for liquid chromatography-tandem mass spectrometry (LC-MS/MS) (11, 12).The main challenge in developing and validating a method for determining MVA in human plasma was that MVA is a polar, endogenous moiety that circulates in the blood stream at nanogram levels. In most methods, the extraction of MVA from plasma was carried out using ionexchange resins in the form of mevalonolactone (MVAL) (11,12). Complicated procedures such as column switching and gradient flow with long run times were followed (11). In a modified assay procedure, a polar-end-capped reverse-phase liquid chromatography column was used for the quantification of plasma MVA over a calibration range of 0.5-50 ng/ml in human plasma (12). This assay had the advantages of shorter run time and isocratic flow.These methods have reported recovery to be 50-87%. The procedure followed does not capture the effect of any constant impurity/substance that may suppress ionization. The exact recovery can be obtained by comparing the response of processed spiked plasma with that o...

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