The possibility of decreasing the gastrointestinal (GI) toxic effects of
5-fluorouracil (5-FU) on the digestive tract such as its injury of cells and induction of
diarrhea, without reducing its antitumor activity, was investigated in rats.
Oxonic acid was found to inhibit the phosphorylation of
5-FU to 5-fluorouridine-5'-monophosphate catalyzed by
pyrimidine phosphoribosyl-
transferase in a different manner from
allopurinol in cell-free extracts and intact cells in vitro. On p.o. administration of
5-FU (2 mg/kg) and a potent inhibitor of
5-FU degradation to
Yoshida sarcoma-bearing rats,
oxonic acid (10 mg/kg) was found to inhibit the formation of 5-fluorouridine-5'-monophosphate from
5-FU and its subsequent incorporation into the
RNA fractions of small and large intestine but not of
tumor and bone marrow tissues. This selective inhibition of
5-FU phosphorylation in the GI tract was due to the much higher concentrations of
oxonic acid in GI tissues than in other tissues and the blood. On p.o. administration with the
5-FU derivative, UFT, which is a combined form of 1 M
tegafur and 4 M
uracil and usually administered p.o. to
cancer patients in Japan,
oxonic acid (10-50 mg/kg) markedly reduced injury of GI tissues and/or severe
diarrhea without influencing the antitumor effect of UFT. These findings suggest that coadministration of
oxonic acid suppresses the GI toxicity of
5-FU and its derivatives without affecting their antitumor activity and thus prolongs the life span of
cancer-bearing rats.