2'-C-Cyano-2'-deoxy-1-beta-D-arabino-pentofuranosylcytosine (
CNDAC) is an antitumor
nucleoside with a novel chemical structure that exerts potent antitumor activity against various human
tumor cells in vitro and in vivo. In order to be active it needs to be phosphorylated by
deoxycytidine (dCyd)
kinase. We induced resistance to
CNDAC in the human
fibrosarcoma cell line HT-1080 by exposure to increasing concentrations of
CNDAC. The resistant cells showed over 560 times higher resistance as compared to that of the parental HT-1080 cells and were cross-resistant to the other 2'-deoxycytidine derivatives. The dCyd
kinase mRNA expression of the resistant cells decreased and there was the expression of aberrant
mRNA of dCyd
kinase which contained a 116-nucleotide deletion within the coding region, corresponding to the fifth exon of the gene. The dCyd
kinase enzymatic activity of the resistant cells was deficient. The initial uptake of
CNDAC into the resistant cells was similar to that of the parental cells. However, the incorporation of
CNDAC into the
DNA fraction of the resistant cells was significantly less than that of the parent cells. These results led us to conclude that the acquired resistance to
CNDAC can be attributed to a deficiency of dCyd
kinase activity, which should be based on a remarkable decrease in
mRNA expression and genetic mutation of the dCyd
kinase gene, but not on cellular
CNDAC accumulation.