Topoisomerase I inhibitor 5 is an effective topoisomerase inhibitor with IC 50 value of. Topoisomerase I inhibitor 5 can interfere with DNA and significantly inhibit the activity of Topoisomerase I. Topoisomerase I inhibitor 5 can arrest cell cycle at the G1 phase and induce MCF-7 cells apoptosis. Topoisomerase I inhibitor 5 has potency in reversing P-gp-mediated resistance to Adriamycin.