Quantitative whole-body autoradiography (QWBA) is increasingly being used to determine drug tissue distribution. Application of the technique would be enhanced if it could also be used to distinguish between the parent drug and metabolites. A method is described for drug ADME analysis, including characterization and quantification of tissues metabolites, using microplate scintillation counting and capillary LC/MS in conjunction with QWBA. Frozen whole-body sections as well as plasma, urine and bile were obtained from rats dosed with a radioactive drug. Tissues of interest were excised from the sections, followed by radioactivity profiling by HPLC-microplate scintillation counting with a detection limit of 18 DPM. Fractions containing unknown components were recovered and further structurally characterized by capillary LC/microspray ion trap MS. This integrated method provided a tool for complete ADME analysis in a few rats, including quantitative determination of radioactivity and metabolite distribution in tissues, characterization of unknown metabolites, and correlation with metabolite profiles in the plasma, urine and bile. The technique may prove particularly useful for the study of tissue- or organ-specific drugs (e.g. CNS compounds) as well as tissue-specific toxicity.
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