Different Modes of Cell‐killing Action between DNA Topoisomerase I and II Inhibitors Revealed by Kinetic Analysis

We compared the modes of cell‐killing by DNA topoisomerase I and II inhibitors. The effects of camptothecin (CPT), KT‐6528 and UCE6 upon colony formation by inhibiting DNA topoisomerase I, and of etoposide (VF‐16), teniposide, amsacrine and UCT4‐A as inhibitors of DNA topoisomerase II were analyzed based upon a kinetic method that distinguishes between cell cycle phase‐specific and ‐nonspecific agents. Human colorectal cancer WiDr cells were exposed to several concentrations of each agent for various periods and 90%‐inhibitory concentrations (IC(90)) at each time were determined by means of a clonogenic assay. When exposure times and corresponding IC(90)s were plotted on a log‐log scale, all inhibitors of DNA topoisomerase II gave curves including a linear portion with a slope of −1, which is characteristic of cell cycle phase‐nonspecific agents. In contrast, the curves for all inhibitors of DNA topoisomerase I had a much steeper slope than −1, which is typical of cell cycle phase‐specific agents. In agreement with this finding, the cells were remarkably accumulated in the G(2‐)M phase when exposed to VP‐16, but in late S‐phase when exposed to CPT as determined by a flow cytometric assay. These results indicated that the two classes of agents kill cells in a quite different manner although they are inhibitors of similar enzymes.