Studies on the mechanism of action of a new oncolytic nucleoside, 2-β-d-ribofuranosylthiazole-4-carboxamide, have been undertaken using P388 murine leukemia cells growing in culture. The title compound was cytotoxic at micromolar levels, but a number of simple substitutions of both the ring and sugar moieties nullified cytotoxicity. Cytofluorimetric analysis revealed that the drug arrests cells in the "S phase" of the cell cycle. At antiproliferative concentrations, the agent inhibited the synthesis of both RNA and DNA. The macromolecular incorporation of preformed pyrimidines, including thymidine, was inhibited by the drug but, among the purines, this effect extended only to members of the adenine family and, in fact, the utilization of guanine and its congeners was reproducibly stimulated. When an examination was made of the ability of a comprehensive series of preformed purines and pyrimidines to overcome the inhibition of thymidine incorporation provoked by exposure to the thiazole nucleoside, the guanines were notably effective, but xanthosine also was shown to be an active antidote. Confirmation that the drug was producing a state of guanine deprivation was provided by high performance liquid chromatography (HPLC) analysis of acid-soluble extracts: a time-dependent fall in the concentrations of GMP and GTP ensued upon exposure to the drug; on the other hand, IMP concentrations increased by ∼15-fold. Pursuant to these findings, an examination was made of the enzymologic steps unique to the biosynthesis of guanine nucleotides in cells exposed to cytotoxic concentrations of the drug. No prominent inhibition of GMP synthetase could be demonstrated in vitro or in culture, but the specific activity of IMP dehydrogenase underwent substantial reductions in both of these cases. HPLC analyses of extracts of cultures exposed to supralethal concentrations of the title compound provided evidence of modest anabolism to the 5′-monophosphate among other products; in vitro a chemically synthesized sample of 2-β-d-ribofuranosylthiazole-4-carboxamide-5′-monophosphate was twenty times more potent than the parent nucleoside in inhibiting IMP dehydrogenase. On kinetic analysis, this inhibition was non-competitive with IMP as the variable substrate.
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