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 (VP-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 (IC90) at each time were determined by means of a clonogenic assay. When exposure times and corresponding IC90s 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 G2-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.