Competitive Binding Radioassay for the Determination of 5‐Fluorodeoxyuridine and 5‐Fluorodeoxyuridine‐5′‐monophosphate Levels in Plasma and Tumor Tissue

A competitive binding radioassay was developed to measure 5‐fluoro‐2′‐deoxyuridine (FUDR) as well as 5‐fluoro‐2′‐deoxyuridine monophosphate (FdUMP). FdUMP has been measured by a competitive binding radioassay with thymidylate synthase as the binding enzyme (TS assay). FUDR was enigmatically converted to FdUMP by thymidine kinase, and then the converted FdUMP was measured by the competitive binding assay to determine the concentration of FUDR in plasma and tumor tissue. As little as 100 pg/ml of FUDR or 50 pg/ml of FdUMP can be detected quantitatively by this method. When TS assay and high‐performance liquid chromatography were compared for the measurement of FUDR and FdUMP levels in plasma and tumor tissue of Ehrlich carcinoma (EQ‐bearing mice following administration of FUDR, a close agreement was observed for FUDR levels, though low FdUMP levels were detectable only by the TS assay method. The examination of intracellular metabolism of FUDR in EC cells by this method showed that metabolic conversion of FUDR into FdUMP or 5‐fluorouracil is rapid. Thus, we have established a highly sensitive method for measuring not only FdUMP but also FUDR with TS assay. This should be very useful for experimental and clinical studies on fluoropyrimidines.