REGULATION OF PHOSPHORYLATION OF DEOXYCYTIDINE AND 2',2'-DIFLUORODEOXYCYTIDINE (GEMCITABINE) - EFFECTS OF CYTIDINE 5'-TRIPHOSPHATE AND URIDINE 5'-TRIPHOSPHATE IN RELATION TO CHEMOSENSITIVITY FOR 2',2'-DIFLUORODEOXYCYTIDINE

Deoxycytidine kinase (dCK) and deoxycytidine deaminase (dCDA) are two key enzymes in the activation and inactivation, respectively, of deoxy cytidine and its antiviral and anticancer analogues. One purpose of th is study was to determine whether or not the deoxycytidine-converting activity of both enzymes would correlate with growth inhibition by 2', 2'-difluorodeoxycytidine (dFdC), a deoxycytidine analogue with establi shed antitumour activity in solid rumours. Another aim of this work wa s to determine the effects of normal nucleotides on dCK. dCK and dCDA activities were measured with both deoxycytidine and dFdC as substrate s in 5 solid tumour cell lines, but no correlation with cellular sensi tivity to dFdC was found with either substrate. The normal dCK activit ies with deoxycytidine as substrate varied between 0.8 and 13 nmol/hr/ 10(6) cells. The activities determined with dFdC as substrate were rem arkably similar in all 5 cell lines (1.1-1.6 nmol/hr/10(6) cells). dCD A activities varied considerably with both substrates (20-30-fold). Be cause dFdC markedly affected intracellular concentrations of cytidine 5'-triphosphate (CTP) and uridine 5'-triphosphate (UTP), we studied th eir effects on deoxycytidine- and dFdC-phosphorylating activities in 3 cell lines (i.e., A2780, WiDr and C26-10) with a similar dCK activity but major differences in dFdC sensitivity. 1 mM CTP inhibited deoxycy tidine phosphorylation (at 230 mu M) by 20-30% in A2780 and C26-10 cel ls, but increased that of WiDr cells by approximately 70%. CTP did not affect dFdC phosphorylation (at 230 mu M) in A2780 cells, but did inc rease it by 40% in WiDr cells. At 1 and 10 mu M of deoxycytidine the e ffects of CTP on dCK activity in A2780, C26-10 and WiDr cells were les s pronounced. 1 mM UTP enhanced deoxycytidine phosphorylation at 230 m u M in WiDr cells by approximately 40%, whereas dFdC phosphorylation w as increased 40% by UTP in C26-10 cells but decreased by 70-80% in WiD r cells. UTP caused a more pronounced increase in dCK activity at 1 an d 10 mu M deoxycytidine in C26-10 cells, but provoked a higher inhibit ion in A2780 and WiDr sells at 10 mu M. Because of these complex resul ts, dCK kinetics were studied in greater detail. Biphasic kinetics for deoxycytidine were observed in all 3 cell lines, with K-m values of 2 3.2 and 0.4 mu M for A2780 cells, 15.9 and 1.5 mu M for C26-10 cells, and 27.2 and 0.9 mu M for WiDr cells. In all 3 cell lines, adenosine 5 '-triphosphate (ATP) was the optimal phosphate donor, as compared to C TP and UTP. in conclusion, the efficiency of dCK (V-max/K-m ratio) see ms to correlate with accumulation of dFdCTP, the active metabolite of dFdC, and with cellular sensitivity. UTP and CTP, which are seriously affected in cells exposed to dFdC, display varying effects in these so lid tumour cell lines. Both activation and inhibition have been observ ed; the physiologically low CTP Cools and the relatively minor effect on dCK in A2780 cells seem to favour dFdC phosphorylation in these cel ls, which are the most sensitive.