The difference in mechanism of action of 5-fluorouracil and its nucleosides in L5178y cells.

The mechanism of antitumor activity of 5-fluorouracil (FU) was studied in mouse leukemia L5178Y cells in vitro. FU increased labeled-thymidine incorporation into acid-insoluble fraction and inhibited labeled-deoxycytidine incorporation as did 5-fluorouridine (FUR) and 5-fluoro-2'-deoxyuridine (FUdR). FU and FUR inhibited labeled-uridine incorporation but FUdR did not. For reversal method at the equieffective concentration of FU, FUR or FUdR, antiproliferating effects of FU and FUR were partially reversed by thymidine and deoxyuridine though FUdR toxicity was completely abolished by both compounds. These results demonstrate that FU and FUR affect not only thymidylate synthesis as a consequence of the conversion to deoxymononucleotide, but also the site concerning functioning RNA synthesis in L5178Y cells, and the FUdR is a specific inhibitor of thymidylate synthesis.