The antitumor ribonucleoside analogue 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)cytosine (ECyd), synthesized in 1995, has strong antitumor activity. In mouse mammary tumor FM3A cells, ECyd was rapidly phosphorylated to ECyd-triphosphate (ECTP) as the final product, strongly inhibiting RNA synthesis. The ultimate metabolite of ECyd, ECTP, is stable in cultured FM3A cells with a half-life of 21 hr; ECyd is on a "closed" metabolic pathway to ECTP. Deaminated ECyd derivatives were minor metabolites in the cells treated with Ecyd; therefore cytidine forms probably were not converted to uridine forms at the nucleoside or nucleotide stage. The characteristics of ECyd may be important for the antitumor activity. RNA polymerase in the nucleus was inhibited competitively by ECTP; the ki value was 21 nM. ECyd induced DNA and 28S ribosomal RNA fragnetations. The cleavage pattern of rRNA resembled in that mediated by RNase L. The results suggested that RNase L related mechanisms might be involved in the antitumor activity of ECyd.
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