An approach to estimating the levels of drug-related metabolites in human plasma in the absence of synthesized chemical standards has been developed. High-performance liquid chromatography/mass spectrometry (LC/MS) in combination with radiometric detection was used in this method. Biologically derived [(14)C] metabolites from preclinical in vitro and in vivo matrices are used as [(14)C] metabolite standards and their concentrations in matrices are calculated based on the corresponding radioactivity. The amount of drug-related metabolites in human plasma samples can be estimated by determining relative MS responses of metabolites between plasma and [(14)C] metabolite standards, and using the calculated concentrations of metabolite standards as calibrants. An example for the estimation of metabolites in human plasma was used to demonstrate the utility of this methodology.
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