Relationship of in vivo antitumor activities of fluorinated pyrimidines to thymidylate synthase activity and intratumoral concentrations of 5-fluorouracil and uracil.

MOPC-104E plasmacytomas were subcutaneously transplanted into BALB/c mice and after 7 days the mice were administered different fluorinated pyrimidines at 4 times the clinical doses, 5-fluorouracil (5-FU, 15 mg/kg), tegafur (FT, 100 mg/kg) or UFT (FT, 20 mg/kg + uracil, 44.8 mg/kg) daily for 7 days. Tumor growth was most effectively inhibited in the UFT group. The % inhibition of tumor growth on day 14, while not correlating with the concentration of 5-FU in the tumor, negatively correlated with the concentration of uracil, which was lowest in the UFT group. The activity of thymidylate synthase (TS) was measured using a 5-fluoro-deoxyuridine monophosphate (FdUMP) binding assay. The total and free TS activities in the tumor negatively correlated with the % inhibition of tumor growth, and were lowest in the UFT group. However, the % inhibition of TS activity in the tumor, which was about 80% in all 3 groups, did not correlate with the tumor-inhibitory effect. These results suggest that uracil in the tumor may play an important role in the metabolism of fluorinated pyrimidines, and that exogeneously administered uracil may decrease the amounts of uracil and TS in the tumor, and subsequently cause 5-FU accumulation.