Incorporation of 5-fluorodeoxycytidine and metabolites into nucleic acids of human MCF-7 breast carcinoma cells.

Several mechanisms of action have been proposed for the antitumor agents, 5-fluorouracil (FUra) and 5-fluorodeoxyuridine (FdUrd), including their incorporation into both cellular RNA and DNA. Another fluorinated pyrimidine, 5-fluorodeoxycytidine (FdCyd), has been shown to be even more active than FdUrd against certain experimental tumors. Although FdCyd is deaminated to FdUrd, the precise mechanism of action of this agent has remained unclear. We have therefore monitored the incorporation of FdCyd and its metabolites into the nucleic acids of human MCF-7 breast carcinoma cells. The results demonstrate the internucleotide incorporation of FdCyd in MCF-7 DNA. The results also demonstrate that FUra residues are detectable in both MCF-7 DNA and RNA following treatment with FdCyd. Cytidine and deoxycytidylate deaminase inhibitors increased the extent of (FdCyd) DNA synthesis, but they had little if any effect on formation of (FUra) RNA. In contrast, deoxyuridine increased incorporation of FdCyd into DNA and blocked the formation of FUra RNA. Deoxyuridine also enhanced the cytotoxicity associated with FdCyd treatment. The present results further demonstrate that FdCyd inhibits postsynthetic methylation of MCF-7 DNA. These findings would suggest that FdCyd has multiple mechanisms of action and that incorporation of this agent into DNA distinguishes its effects from those of FUra and FdUrd.