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Enzymatic conversion of 5-fluoro-2'-deoxyuridine to 5-fluorouracil or 5-fluoro-2'-deoxyuridine 5'-monophosphate in human tissues.

The relative activities and Km values of pyrimidine nucleoside phosphorylase (PNPase) and thymidine (TdR) kinase were compared in postsurgical human tissues. TdR kinase, which activates 5-fluoro-2'-deoxyuridine (FUdR), was higher in normal liver than in other tissues examined. PNPase, which cleaves FUdR, was higher in normal spleen and liver and lowest in kidney. In normal lung tissue, almost all of the FUdR was cleaved at a high concentration since PNPase activity was much higher than TdR kinase activity. The affinity of TdR kinase as indicated by Km was greater in normal than in cancerous lung tissue. At a low concentration, FUdR is relatively well activated in the normal tissue; thus it may not be suitable for chemotherapy of lung cancer. When comparing normal versus cancerous colon tissues, no difference was seen in either TdR kinase affinities or TdR kinase and PNPase activities. However, the PNPase affinity for FUdR was significantly higher in normal than in cancerous colon tissue. When FUdR is maintained at low concentrations, it might have a selective cytotoxicity for colon cancer tissue since it is well cleaved in normal colon.