The antitumor activity of 1-(3-C-ethynyl-beta-D-ribopentofuranosyl) cytosine (ECyd), designed as a potential multifunctional antitumor nucleoside that could be expected to inhibit RNA and DNA syntheses, was examined. ECyd inhibited the growth of 47 kinds of cultured human cells in vitro, and also showed strong antitumor effects on 15 human solid cancers xenografted into nude mice at a dose of 0.25 mg/kg by intravenous administration for 10 consecutive days. The in vitro cytotoxic effect of ECyd was prevented dose dependently by cytidine and uridine, suggesting that ECyd may require phosphorylation by uridine/cytidine kinase for antitumor activity. ECyd strongly inhibited RNA synthesis and also slightly inhibited DNA synthesis. ECyd has shown potent antitumor activity against human experimental solid type tumors with minimal toxic effects in vivo, suggesting that ECyd will be a promising agent with a unique mechanism of action for the treatment of cancer.

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