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Inhibitory effects of the new mitotic inhibitor 5-chloropyrimidin-2-one and of vincristine on human cells in vitro.

By means of time-lapse microcinematography, metaphase arrest was studied in synchronized NHIK 3025 cells in contact with 5-chloropyrimidin-2-one (NY 3000) or vincristine. A dose-dependent prolongation of mitosis was found for both substances, and the fraction of cells able to escape mitotic arrest declined gradually as the concentration of NY 3000 or vincristine was increased. Significant prolongation of metaphase was observed after treatment with 0.5 mm NY 3000 or vincristine (0.5 ng/ml; 0.54 nm), while total block in metaphase was achieved with 8 mm NY 3000 or vincristine (16 ng/ml; 17.3 nm).

The inactivating effect was measured as loss of colonyforming ability after continuous exposure for 12 to 14 days. NY 3000 (0.75 mm) brought total inactivation, although the prolongation of metaphase still was small at this concentration of NY 3000. Vincristine at concentrations that cause great prolongation of metaphase still permitted formation of colonies.

Both drugs demonstrated dose-dependent prolongation of interphase at concentrations entailing complete metaphase block. The phase specificity of the interphase action on synchronized cells was measured by pulsed incorporation of [3H]thymidine combined with a registration of time of entry into metaphase. NY 3000 resulted in a prolongation of all stages of interphase. The interphase effect of vincristine was reflected in a delay in G2. This was confirmed by flow cytometric recording of DNA distributions.

These results reveal that the metaphase-arresting agents NY 3000 and vincristine act also in the interphases of proliferating cells. However, the mechanism behind the interphase effect seems to be different for the two drugs.