Administration of the novel thiazole C-nucleoside, 2-β-d-ribofuranosylthiazole-4-carboxamide (NSC 286193), to BDF1 mice bearing subcutaneous implants of P388 leukemia provoked a sharp depression in the concentration of intratumoral guanine nucleotides and a correspondingly large expansion of the IMP pools. Measurements of IMP dehydrogenase in the tumors of treated mice revealed that this enzyme was inhibited in a dose-responsive way, with ∼50% inhibition engendered by the administration of the drug at a dose of 25 mg/kg and > 90% inhibition by all doses > 100 mg/kg. The inhibition of enzyme activity, seen after a dose of 250 mg/kg, reached a maximum 120 min after treatment and had subsided substantially 8 hr after dosing; by 24 hr, enzyme activity was fully restored. These results, coupled with the observation that the antitumor activity of the drug could be prevented in large part by the simultaneous administration of guanosine, support the conclusion that 2-β-d-ribofuranosylthiazole-4-carboxamide, after anabolism, exerts its antineoplastic effects via a state of guanine nucleotide depletion. In extracts of the tumors of mice given parenteral injections of the thiazole nucleoside, a potent dialyzable inhibitor of IMP dehydrogenase was demonstrable; its concentration fluctuated in parallel with enzyme inhibition. Although the chemical identity of the proximate inhibitory species has yet to be established, it is concluded on kinetic grounds that it is neither the native nucleoside nor its 5′-monophosphate.
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