A validated HPLC–MS/MS assay for quantifying unstable pharmacologically active metabolites of clopidogrel in human plasma: Application to a clinical pharmacokinetic study

► Assay to quantify four chemically reactive thiol metabolites of clopidogrel. ► Stabilization of the thiol group by alkylation with 2-bromo-3′-methoxyacetophenone. ► Separation of the four derivatized diastereomeric metabolites by HPLC. ► Application of the assay to a clinical pharmacokinetic study. Clopidogrel is prescribed for the treatment of Acute Coronary Syndrome and recent myocardial infarction, recent stroke, or established peripheral arterial disease. A sensitive and reliable high performance liquid chromatography–tandem mass spectrometry (HPLC–MS/MS) assay was developed and validated to enable reliable quantification of four diastereomeric and chemically reactive thiol metabolites, two of which are pharmacologically active, in human plasma. The metabolites were stabilized by alkylation of their reactive thiol moieties with 2-bromo-3′-methoxyacetophenone (MPB). Following organic solvent mediated-protein precipitation in a 96-well plate format, chromatographic separation was achieved by gradient elution on an Ascentis Express RP-amide column. Chromatographic conditions were optimized to ensure separation of the four derivatized active metabolites. Derivatized metabolites and stable isotope-labeled internal standards were detected by positive ion electrospray tandem mass spectrometry. The HPLC–MS/MS assay was validated over concentration ranges of 0.125–125ng/mL for metabolites H1–H3 and 0.101–101ng/mL for H4. Intra- and inter-assay precision values for replicate quality control samples were within 14.3% for all analytes during the assay validation. Mean quality control accuracy values were within ±6.3% of nominal values for all analytes. Assay recoveries were high (>79%). The four derivatized analytes were stable in human blood for at least 2h at room temperature and on ice. The analytes were also stable in human plasma for at least 25h at room temperature, 372 days at −20°C and −70°C, and following at least five freeze–thaw cycles. The validated assay was successfully applied to the quantification of all four thiol metabolites in human plasma in support of a human pharmacokinetic study.