Previous studies have indicated that deoxycytidine kinase (dCK) is requisite and rate limiting in the phosphorylation of 1-β-d-arabinofuranosylcytosine (ara-C) and 9-β-d-arabinofuranosyl-2-fluoroadenine (F-ara-A) on the pathway to their respective cytotoxic 5′-triphosphates. In K562 cells, the rate of triphosphate accumulation was maximal during incubation with 10 µm ara-C (35 µm/h) and 300 µm F-ara-A (102 µm/h). Under these conditions, accumulation of cellular ara-CTP plateaued at about 110 µm after 3 h, whereas in separate cultures, F-ara-ATP continued to accumulate at a linear rate to cellular concentrations greater than 500 µm after 5 h. Other laboratories have demonstrated that dCK activity in cell-free extracts was inhibited by ara-CTP. To determine whether ara-CTP exhibited the same activity in whole cells, K562 cells were preincubated with ara-C to accumulate 110 µm ara-CTP. After washing into medium containing F-ara-A, the rate of F-ara-ATP accumulation was significantly decreased (37 µm/h). However, cells loaded with F-ara-ATP exhibited an increased rate of ara-CTP accumulation (110 µm/h) that resulted in cellular ara-CTP concentrations in excess of 400 µm after 5 h. This stimulation was proportional to the cellular concentration of F-ara-ATP, achieving a maximum effect between 75 and 100 µm. Phosphorylation of ara-C by cell-free extracts supplemented with physiological levels of ribo- and deoxyribonucleoside 5′-triphosphates was stimulated by addition of F-ara-ATP. The decreased rate of accumulation of products of dCK in intact cells containing 110 µm ara-CTP suggests that this active triphosphate may limit its own synthesis and phosphorylation of other substrates. In contrast, stimulation of the accumulation of ara-CTP in cells containing F-ara-ATP suggests new possibilities for the design of combination chemotherapy regimens.

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Supported in part by Grant CA 28596 from the National Cancer Institute, Department of Health and Human Services, and Grant CH-130 from the American Cancer Society.

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