PRETREATMENT OF COLON-CARCINOMA CELLS WITH TOMUDEX ENHANCES 5-FLUOROURACIL CYTOTOXICITY

The cytotoxic effect of sequence and dose of Tomudex (TX) and 5-fluoro uracil (FUra) on an HCT-8 colon carcinoma cell line using a clonogenic assay was evaluated, Synergistic cell kill was obtained with 24 h of exposure to TX followed by 4 h of exposure to FUra, Marginal synergy w as obtained with the same sequence but with a 5-day exposure to FUra, The reverse sequence, FUra (either 4 h or 5 days), followed by TX (24 h), resulted in less-than-additive cell kill, The synergistic effect w as not due to augmented inhibition of thymidylate synthase, as determi ned by the measurement of thymidylate synthase activity by tritium rel ease from [5-H-3]2'-deoxyuridine. Surprisingly, an increase in intrace llular levels of phosphoribosylpyrophosphate was observed after 24 h o f exposure to TX, suggesting the possibility of an indirect effect of TX and/or its polyglutamates on purine biosynthesis, Moreover, we obse rved an increased formation of FUra nucleotides in the cells preexpose d to TX, likely due to the increased intracellular levels of phosphori bosylpyrophosphate, that as a consequence led to an enhanced incorpora tion of FUra into RNA and increased cell killing.