Effect of 5-Fluoro-2'-deoxyurldine on Deoxyribonucleotide Pools In Vivo 1.2

ABSTRACT-5-Fluoro-2'-deoxyuridine (FdUrd) lowered the dTIP levels in rapidly frozen 12-day W rat embryos and in a human neuroblastoma grown in nude N:NIH(S) mice to about 20 % of control values. This effect was associated with greatly increased dCTP levels and reduction of dGTP levels essentially to zero. Elimination of the dGTP pool correlated temporally with the cytotoxicity of FdUrd. Extremely rapid fixation of tissue was required to avoid artifactually high deoxyribonucleoside triphos-phate values.-JNCI 65: 603-605. 1980. It is generally accepted that the potent, cytotoxic anticancer agent FdUrd inhibits DNA synthesis in proliferating cells by interfering with the enzyme thymidylate synthetase and thus with the biosynthesis anew of thymidylate from deoxyuridine monophos-phate, leading to decreased amounts of dTTP, one of the four essential dNTP DNA precursors (1-7). We report here the effects of FdUrd on dNTP pools in the rapidly proliferating in vivo rat embryo and a preliminary study of a human neuroblastoma grown in the nude N:NIH(S) mouse. FdUrd significantly reduced dTTP concentrations, as anticipated, but it also caused an unexpected and profound decrease in dGTP concen-trations, which were lowered to zero or trace val ues. At the same time, dCTP increased severalfold, whereas dATP concentrations increased moderately. Apparently, the profound decrease in dGTP, rather than the partial decrease in the dTTP pool, may be the direct mecha-nism of FdUrd cytotoxicity. Extremely fast fixation of tissue prior to dNTP assay is necessary. Small samples of proliferating tissue, previously subject to FdUrd treatment and frozen in about 1 second by immersion in liquid nitrogen, showed zero or trace values for dGTP, whereas a procedure involving ice water and Dry Ice and requir-ing about 20 seconds for fixation resulted in artifactu-ally high values for all dNTP pools (8), including those for dGTP. One could expect that still slower fixation procedures in common use would also lead to discrepant val ues