Inhibitory activity of camptothecin(CPT) on topoisomerase I (TopI). (A) CPT-induced DNA damage in A2780 ovarian carcinoma cells. Magnification, ×200. Cells were untreated, treated with DMSO, and CPT (25 μM) for 1 h, and were then analyzed by a neutral comet assay as described in "Materials and Methods." Upper panel, representative images. Lower panel, histogram of the tail moment plotted against each treatment condition. p values for comparisons (marked with *) were 0.005 as determined by two-tailed Student's t-test. (B) CPT prevented DNA from recombinant hTop I conversion of supercoiled DNA to relaxed closed circular DNA. pUC19 (0.2 μg) plasmid DNA was incubated at 37°C for 30 min with hTopI in the presence or absence of 2~8 μM of inhibitors.