A novel online method is developed, using liquid chromatography (LC)-accurate radioisotope counting dynamic-flow (ARC) coupled with a radioactivity detector and mass spectrometer, for metabolite identification in drug discovery and development. This method offers the advantages of improved sensitivity for detecting radiolabeled drugs as well as streamlining the process of identifying and characterizing metabolites. For the purposes of evaluating this method, in vitro human liver microsomal

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... ations with [ 14 C]dextromethorphan are conducted. Online separation and identification of [ 14 C]dextromethorphan metabolites are achieved without intensive sample preparation, concentration, or fraction collection. Mass spectrometric analysis identified and characterized the metabolites of dextromethorphan formed by N-and O-dealkylation, correlated well with previously published results. Chromatographic peaks for [ 14 C]dextromethorphan and its metabolites are collected online, then infused for extended periods of time at a flow rate of 10 µL/min while maintaining the column pressure. The continuous analytical signal input allowed acquisition of a higher order of multistage fragmentation for both major and minor metabolites. The multistage MS fragmentation pattern obtained for the metabolites allowed defining the sites of metabolism for dextromethorphan. Further evaluations of this method are also conducted using a [ 14 C]compound A to check the linearity and sensitivity of the dynamic-flow method. The R 2 value is 0.996 for the dynamic-flow method between 50 and 600 disintegrations per minute (dpm); the limit of detection for LC-ARC is 20 dpm, which is approximately 10 times more sensitive than conventional continuous-flow radioactivity detection techniques. The overall results suggest that the combination of LC-ARC with radioactivity detection and mass spectrometry has great potential as a powerful tool for enhancing the sensitivity of radioisotope measurement in metabolite identification studies during drug discovery and development. Recently, a detection method combining online LC-accurate radioisotope counting dynamic-flow (ARC) stop-flow coupled