Modulation of 5-fluorouracil by 5-ethyl-2 '-deoxyuridine on cell lines expressing different dihydropyrimidine dehydrogenase activities

The purpose of the present study was to clarify the significance of the inh ibition of dihydropyrimidine dehydrogenase (DPD) in the modulation of 5-flu orouracil (5-FU) action by 5-ethyl-2'-deoxyuridine (EUdR). Four human cell lines, which differed in their susceptibility to 5-FU and in their DPD acti vity, were selected as biological objects. Several other enzymes of pyrimid ine metabolism, i.e. thymidylate synthase (TS), thymidine kinase (TK) and p yrimidine nucleoside phosphorylase (PNP), which might be involved in the 5- FU action were also studied to elucidate their potential role in the modula tion of 5-FU cytotoxicity. Two out of the four cell lines, i.e. COLO1 and S W620, showed low (57 and 28 pmol/ min/mg protein) and the other two cell li nes, i.e. CAL51 and CAL33, showed high (235 and 184 pmol/min/mg protein) DP D activity, respectively. In our study, contrary to our expectation, no cor relation between the DPD and TS activity of the cell lines and their 5-FU s ensitivity could be observed. EUdR alone was cytotoxic only on CAL33 cells in a concentration below 1 mM (IC50=194 mu M) which might be due to the hig h TK activity (857 pmol/min/mg protein) measured in this cell line, favorin g the formation of the phosphorylated nucleotides EdUMP and EdUTP indispens able for the inhibition of TS and DNA polymerase, respectively. Surprisingl y, although EUdR by metabolizing to EUra was able to reduce the high activi ty of DPD in CAL33 and CAL51 cells by 47 and 55%, respectively, no potentia tion of the 5-FU action occurred on these cell lines. On the contrary, enha ncement of the 5-FU cytotoxicity was demonstrated on COLO1 and SW620 cells with low DPD activity. Our findings suggest that the 5-FU modulatory action of EUdR may be directed on other molecular targets than DPD as well, i.e. the augmentation of TS inhibition by EdUMP as demonstrated on SW620 cells m ight be one of these mechanisms. [(C) 1999 Lippincott Williams & Wilkins.].