Inhibition of topoisomerase by compounds 1-8. Native pBR322 plasmid DNA (-lane) was incubated with 4 units of topoisomerase I (Topo I) and II (Topo II) without (+lane), with control (CT-camptothecin or ET-etoposide), or with drug (lanes 1-8). DNA was analyzed by 1% agarose gel electrophoresis. The gel was stained with ethidium bromide and photographed under UV light.