Topoisomerase II inhibitor 6 (Compound 5), a tryptanthrin derivative, is a potent and selective inhibitor of topoisomerase II. Topoisomerase II inhibitor 6 exhibits antiproliferative activity on different tumor cell lines. Topoisomerase II inhibitor 6 blocks the cell cycle of CCRF-CEM in the G2 phase and induces DNA DSB. Topoisomerase II inhibitor 6 has the potential for the research of cancer diseases.