The DNA unwinding reaction mixtures contained 0.3 μg of relaxed plasmid DNA, 2 units of human topoisomerase I, and 100 μM of the various drugs. Two independent experiments for each assay exhibited essentially identical results; representative results are shown. Drugs are denoted as follows: 1, compound 1; 2, compound 2; 3, compound 3; 4, compound 4; A, amsacrine; and E, etoposide.