The effect of 3-deazauridine pretreatment on 5-azacytidine metabolism was studied in suspension cultures of L5178Y murine leukemia. A 3-hr exposure to 2 µm 3-deazauridine followed by a 1-hr exposure to 5 µm [14C]-5-azacytidine resulted in a 2-fold increase in total intracellular 5-azacytidine accumulation compared to untreated controls. Under the same conditions, incorporation of 5-azacytidine into the acid precipitable fraction of L5178Y cells was increased 3-fold. Incorporation of 5-azacytidine into RNA increased 85% following 3-deazauridine pretreatment, but 5-azacytidine incorporation into DNA did not change significantly. In cells pretreated with 3-deazauridine, there was an 80% reduction of intracellular cytidine triphosphate, the natural feedback inhibitor of uridinecytidine kinase, the rate-limiting enzyme in the phosphorylation of 5-azacytidine. Intracellular levels of 5-azacytidine triphosphate, the presumed lethal metabolite of 5-azacytidine, increased from 28.8 pmol/106 cells in control cells to 56.4 pmol/106 cells following 3-deazauridine treatment. The sequence of 3-deazauridine followed by 5-azacytidine demonstrated synergistic cell killing when measured by an in vitro soft-agar cloning assay. Similar biochemical alterations were also seen in human leukemic myeloblasts. It appears that 3-deazauridine-induced alterations in 5-azacytidine metabolism may account for the enhanced cytotoxicity of this drug sequence.

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Supported by National Cancer Institute Grants CA 24187 and CA 09200, a Swebilius Cancer Award from the Yale Comprehensive Cancer Center, and Grant CH-145 from the American Cancer Society.

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