Cell Cycle Arrest by Colcemid Differs in Human Normal and Tumor Cells¹

In the continuous presence of Colcemid, the mitotic index in cultures of nine human tumor cell lines began to increase immediately upon addition of the drug. For 12 human normal (nontumorigenic) cell lines, the mitotic index did not begin to increase for some 2 to 3 h after the addition of Colcemid. The effect was independent of whether the cells were of fibroblast or epithelial origin and occurred over a 1000-fold range of Colcemid concentrations. No such differential effect was seen with single concentrations of either Taxol or nocodazole, but a similar delayed effect was seen for two concentrations of vinblastine. These observations suggest a fundamental difference between human normal and human tumor cells involving a cell cycle checkpoint in G₂, about 1 to 2 h before mitosis.