Atsushi Azuma

1-(3-C-ETHYNYL-β-D-RIBO-PENTOFURANOSYL)CYTOSINE (ECYD, TAS-106)1: ANTITUMOR EFFECT AND MECHANISM OF ACTION

The antitumor activity, cellular metabolism and mechanism of action of the antitumor nucleoside analog, 1-(3-C-ethynyl-β-D-ribo-pentofuranosyl)cytosine (ECyd) are described.

Cellular and biochemical mechanisms of the resistance of human cancer cells to a new anticancer ribo-nucleoside, TAS-106.

We have established variants of DLD‐1 human colon carcinoma and HT‐1080 human fibrosarcoma cells resistant to the new anticancer ribo‐nucleosides, 1‐(3‐C‐ethynyl‐β‐D‐ribo‐pentofuranosyl)‐cytosine (ECyd, TAS‐106) and 1‐(3‐C‐ethynyl‐p‐D‐ribo‐pentofuranosyl)uracil (EUrd). Both variants were shown to have decreased (3‐ to 24‐fold decrease) uridine‐cytidine kinase (UCK) activity, and exhibited cross‐resistance to EUrd and TAS‐106. Based on the IC50 values determined by chemosensitivity testing, a 41‐ to 1102‐fold resistance to TAS‐106 was observed in the resistant cells. TAS‐106 concentration‐dependently inhibited RNA synthesis, while its effect on DNA synthesis was negligible. The degree of resistance (14‐ to 3628‐fold resistance) calculated from the inhibition of RNA synthesis tended to be close to the degree of chemoresistance of tested cells to TAS‐106. The experiments on the intracellular metabolism of TAS‐106 in the parental cells revealed a rapid phosphorylation to its nucleotides, particularly the triphosphate (ECTP), its major active metabolite. The amount of TAS‐106 transported into the resistant cells was markedly reduced and the intracellular level of ECTP was decreased from 1/19 to below the limit of detection; however, the unmetabolized TAS‐106 as a percentage of the total metabolite level was high as compared with the parental cells. The ratio of the intracellular level of ECTP between parental and resistant cells tended to approximate to the degree of resistance calculated from the inhibitory effect on RNA synthesis. These results indicate that the TAS‐106 sensitivity of cells is correlated with the intracellular accumulation of ECTP, which may be affected by both the cellular membrane transport mechanism and UCK activity.

Intracellular metabolism and action of an antitumor nucleoside, 2'-C- cyano-2'-deoxy-1-β-D-arabinofuranosylcytosine (CNDAC)

Abstract The mechanism of antitumor activity of 2′-C-cyano-2′-deoxy-1-β-D-arabinofuranosylcytosine (CNDAC) has been examined. Intracellular metabolism of CNDAC using human leukemia cell lines are described. Incorporation of CNDAC triphosphate into DNA and the consequence of this incorporation have been evaluated in vitro using DNA primer extension assay with purified human DNA polymerase α and defined DNA primer/templates.