Pulse field gel electrophoresis using a contour-clamped homogeneous electric 
field was applied for the analysis of DNA-fragmenting activity of antitumor 
agents towards human uterine cervix carcinoma HeLa S3 cells. Duocarmycins 
(DUMs), novel antitumor antibiotics with ultrapotent cell growth-inhibitory 
activities, caused DNA fragmentation at 10 times their IC50 values at 2 h 
exposure. At 100 times their IC50 values, the size of the smallest fragments was 
about 245 kilobase pairs (kbp). DUMA, DUMB1 and DUMB2 exhibited similar DNA 
fragmentation patterns, suggesting similar action mechanisms. DNA fragmentation 
was also detected in cells treated with radical producers, intercalators and 
topoisomerase inhibitors. Two bands of about 1800 and 1500 kbp were commonly 
detected in the cells treated with DUMs and these agents. In addition, fragments 
of about 900 kbp were detected in the cells treated with a topoisomerase 
inhibitor, 4'-(9-acridinylamino)methane-sulfon-m-anisidine, and fragments in the 
broad size range between 700 and 245 kbp in the cells treated with radical 
producers, bleomycin and neocarzinostatin. DUMs showed a characteristic DNA 
fragmentation pattern, since both types of fragments induced by the 
topoisomerase inhibitor and the radical producers were simultaneously detected, 
suggesting a novel mode of interaction with DNA. DNA-crosslinking agents and 
mitotic inhibitors did not induce DNA fragmentation under these conditions. The 
pulse field gel electrophoresis is potentially useful for characterizing 
DNA-cleaving activity of various antitumor agents at the cellular level.