Metabolism and cytotoxicity of 5-azacytidine in cultured Novikoff rat hepatoma and P388 mouse leukemia cells and their enhancement by preincubation with pyrazofurin.

5-Azacytidine transport into cells was measured in the absence of metabolism in adenosine triphosphate-depleted and uridine kinase-deficient Novikoff cells. Azacytidine is transported with about the same efficiency as uridine and cytidine by the facilitated nucleoside transport system of these cells. The phosphorylation of azacytidine in untreated, wild-type cells, however, is much more inhibited by uridine and cytidine than is its transport into the cell. This inhibition seems to be responsible for the specific protection of cells by these nucleosides from azacytidine toxicity. Azacytidine is incorporated by Novikoff and P388 cells into both RNA and DNA, and this incorporation seems to be responsible for its cytotoxicity; an inhibition of de novo pyrimidine nucleotide synthesis is not a major contributory factor. Incorporation of azacytidine into nucleic acids is relatively slow, but it is enhanced 3 to 4 times when cells are preincubated with pyrazofurin. Pyrazofurin inhibits de novo pyrimidine synthesis and thus causes a depletion of cellular pyrimidine nucleotides. Azacytidine is largely cytostatic for Novikoff and P388 cells, but a sequential treatment with pyrazofurin and azacytidine markedly increases the cytotoxicity over that observed with drug alone or when administered together with drug, even at higher concentrations. Increased cytotoxicity correlates with the increased incorporation of azacytidine into nucleic acids.