2′‐Deoxy‐2′‐azidocytidine, a New Inhibitor of DNA Replication in Mammalian Cells

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Abstract
Cell growth is reversibly inhibited by the nucleoside analogue, 2''-deoxy-2''-azidocytidine; the inhibition is a result of interference with DNA replication. The 5''-diphosphate of the analogue specifically inactivates the enzyme ribonucleotide reductase [EC 1.17.4.1] in vitro. Measurements of the pools of deoxyribonucleoside triphosphates in cells incubated in azidocytidine showed only minor changes which appeared to result from, and not to be the cause of, the inhibition of DNA replication. The DNA synthesized in polyoma-infected [mouse 3T6 fibroblast and Chinese hamster ovary CHO] cells after incubation in azidocytidine showed a sedimentation pattern quite different from that seen after inhibition of DNA synthesis with arabinosyl cytosine or hydroxyurea. Experiments with nuclei isolated from azidocytidine-inhibited, polyoma-infected cells indicated that the number of replicating molecules is decreased during the inhibition and upon incubation of the nuclei there is a rapid synthesis of DNA occurring in a new class of DNA molecules which are at a very early state of replication. Neither the 5''-triphosphate of azidocytidine nor the nucleoside itself inhibit DNA synthesis in vitro in isolated nuclei from polyoma-infected cells and at present the nature of the DNA-synthesis-inhibiting compound acting in the cells after addition of azidocytidine is unknown. Taken together the results suggest that azidocytidine inhibits DNA synthesis at an early stage, possibly by blocking the initiation of DNA synthesis at the origin or by interfering with the elongation of newly initiated DNA molecules.