Tiazofurin (2-β-d-ribofuranosylthiazole-4-carboxamide, NSC-286193) has shown potent cytotoxic and antitumor activity against hepatoma 3924A carried in the rat [Lui et al. J. biol. Chem. 259, 5078 (1984)]. However, eventually the tumor emerged, proliferated and killed the host. To throw light on the factors that play a role in the resistance to this drug, a tiazofurin-induced resistant hepatoma 3924A line in culture was produced, and its biochemical and pharmacological pattern was examined. Resistance in hepatoma cells was expressed by a reprogramming of gene expression that entailed the display of a program of multiple biochemical alterations. In the resistant cells the activity of IMP dehydrogenase, the target enzyme of tiazofurin, was increased 2- to 3-fold. The steady-state guanylate pools were elevated 3-fold, and there was a decrease in the de novo synthesis of guanylate. There was an expansion of guanylate salvage, which could circumvent inhibition of de novo guanylate synthesis by tiazofurin. For the first time in studies on the resistance of different cell lines to tiazofurin, reduced tiazofurin transport (to 50%) in resistant hepatoma cells was identified which might account for the decreased concentration (50%) of the active metabolite, thiazole-4-carboxamide adenine dinucleotide (TAD), in these cells. NAD pyrophosphorylase activity also decreased to 53% of that of the sensitive line, which was responsible, in part at least, for the decreased TAD concentration of the resistant cells. When resistant cells were cultured in the absence of tiazofurin, resistance to the drug gradually decreased, and by 50 passages sensitivity returned. Resistance to tiazofurin in hepatoma cells appears to be a drug-induced metabolic adaptation which involves alterations in the activity of the target enzyme, in the transport and concentration of the drug and the active metabolite, and an increase of guanylate concentration and guanine salvage capacity.
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