Possible pathways of the phosphorylation of 5-fluorouracil (FUra) were investigated in vitro and in vivo, using certain xenografts, of human gastric and colorectal cancer cell lines. The oxonic acid (Oxo), an inhibitor of orotate phosphoribosyltransferase (OPRTase) that catalyzes the direct production of 5-fluorouridine 5'-monophosphate from FUra (the first pathway) and 2, 6-dihydroxypyridine (DP), an inhibitor of uridine phosphorylase (UPase), the enzyme converting FUra to 5-fluorouridine (the second pathway), were employed to estimate a contribution of these two metabolic pathways in the anabolism of FUra. Ten out of 13 cancer cell lines tested were found to utilize the first route for the phosphorylation of FUra, as revealed by marked inhibition of the phosphorylation of FUra by Oxo in 4 of 5 and in 6 of 8 gastric and colorectal cancer cell lines, respectively. The phosphorylation of FUra in the xenografts of human AZ521 gastric adenocarcinoma and SNU-C2A colorectal carcinoma was also regulated by Oxo, the production of 5-fluoro-nucleotides after i.v. injection of FUra with the Oxo significantly decreased from 0.587 to 0.311 nmol/g and from 1.75 to 0.40 nmol/g in respective xenografts, suggesting that the nature of an anabolic pathway of FUra in the tumor cells in vitro reflects the metabolic pattern found in the in vivo conditions. Moreover, the intracellular concentrations of phosphoribosylpyrophosphate (PPRibP) in DLD-1 and SNU-C(2)A cells were much higher than those found in HCT-15 and MKN-28 cells, leading to the findings that FUra was phosphorylated by OPRTase in the former and by UPase and uridine kinase in the latter cells. These results also may suggest that the intracellular levels of PPRibP in the tumor cells are importantly related to the selection of a proper metabolic pathway of FUra by the cell.

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