The interaction between fluoropyrimidines and methotrexate,and [14C]-formate incorporation into nucleic acids and protein

Summary Changes are reported in [¹⁴C]-formate incorporation into nucleic acids and protein of Ehrlich ascites tumor cells during exposure to methotrexate (MTX) and fluoropyrimidines. The rate of [¹⁴C]-formate incorporation into RNA, DNA, and protein in the presence of only MTX was inhibited by 82%, 91%, and 75% respectively, when compared with control rates. However, in the presence of 5-fluorodeoxyuridine (FdUrd) plus MTX, formate incorporation into RNA, DNA, and protein was inhibited by 67%, 85%, and 66%. Incubation of cells in vitro with [³H]-dihydrofolate (DHF) results in its rapid conversion to [³H]-tetrahydrofolate (THF). The THF/DHF ratio from the soluble fraction of cells that were incubated with [³H]-DHF was 43% greater in the presence of FdUrd and MTX than in the presence of MTX alone. As the rate of [³H]-dUrd incorporation into DNA was reduced by 88% and 99% by pretreating cells with 0.1 M and 1 M FdUrd, respectively, the inhibitory effect of MTX on [¹⁴C]-formate incorporation into (a) RNA was decreased by 63% and 46%; (b) DNA was decreased by 74% and 61%; and (c) protein was decreased by 63% and 32%. These data suggest that fluoropyrimidines can antagonize the effects of MTX on purines or nucleic acid synthesis and protein synthesis by preventing the consumption of THF for dTMP synthesis.The abbreviations used are MTX methotrexate - FdUrd 5-fluorodeoxyuridine - Fura 5-fluorouracil - FdUMP 5-fluorodeoxyuridine monophosphate - dUrd deoxyuridine - dUMP deoxyuridine monophosphate - dTMP thymidylate - DHF dihydrofolate - THF tetrahydrofolate - 5,10-CH₂THF 5,10-methylene tetrahydrofolate - DHFR dihydrofolate reductase