Liquid chromatography-accurate radioisotope counting and microplate scintillation counter technologies in drug metabolism studies

The present study involves an analysis of the performance of liquid chromatography (LC)-accurate radioisotope counting (ARC) and microplate scintillation counter (TopCount) technologies in drug metabolism studies. For the purpose of evaluating these systems, biological samples resulting from the metabolism of a radiolabeled [14C] compound, known as compound B, are analyzed using LC-ARC and TopCount under similar high-performance LC conditions. Counting efficiency is 83% for LC-ARC, 77% for TopCount, and linearity is R2 of 0.9998 versus 0.9984, respectively. The limit of detection for LC-ARC is 12 disintegrations per minute (dpm) with 1-min/fraction counting, yet for TopCount it is 8.7 dpm with 5-min/fraction counting. Under optimal conditions for each, the total run time of LC-ARC is approximately half that of TopCount. These results indicate that there is no significant difference between these two systems in terms of efficiency, linearity, and limit of detection. However, the LC-ARC system does not involve any manual operations, yet TopCount requires manual sample transfer and data import. This study shows that impressive progress has been made in the technology of radioisotope counting in drug metabolism using LC-ARC. This system enhances the resolution of radiochromatograms and is able to measure volatile metabolites that TopCount cannot detect at all. The ability to acquire mass spectra online is also a major advancement. The overall results suggest that the combination of LC-ARC with radioactivity detection and mass spectrometry has great potential as a powerful tool for radioisotope measurement in metabolite identification studies during drug discovery and development.