ROLE OF DEOXYCYTIDINE KINASE IN AN IN-VITRO MODEL FOR ARAC-RESISTANCEAND DAC-RESISTANCE - SUBSTRATE-ENZYME INTERACTIONS WITH DEOXYCYTIDINE, 1-BETA-D-ARABINOFURANOSYLCYTOSINE AND 5-AZA-2'-DEOXYCYTIDINE

Deoxycytidine kinase activity (dCk) was monitored in cell lines from a rat acute myeloid leukemia model of acquired resistance to cytosine a rabinoside (AraC) and decitabine (DAC). In both AraC-resistant cell li nes (RCL/A and its subclone RA/7), as well as in a DAC-resistant cell line (RCL/D) which we generated from the drug-sensitive RCL/0 cell lin e, a total deficiency of dCk activity and a cross-resistance for AraC and DAC was demonstrated. Furthermore, the metabolization of deoxycyti dine (dC) was severely impaired in all these cell lines. K(m) values f or dC (9.4 muM in RCL/0 cells) had increased 70- to 100-fold in RCL/D (K(m) = 673.2 muM), in RCL/A (K(m) = 947.2 muM) and in RA/7 (K(m) = 81 7.5 muM). V(max) values were unaltered in RCL/D and RA/7, and twofold increased in RCL/A. Addition of hydroxyurea (HU) to cell cultures stim ulated dCk salvage pathway activity in RCL/0 cells for dC, AraC, and D AC by increasing V(max) values approximately 160% leaving K(m) constan ts unchanged. In all resistant cell lines, HU pre-incubation did not i nfluence the level of dCk activity, leaving K(m) and V(max) values una ltered. These data indicate that deficiency of dCk activity is crucial in the mechanism of drug resistance in this model.