Indolocarbazole poisons of human topoisomerase I: Regioisomeric analogues of ED-110

All four “symmetrical” regioisomers of ED-110, an indolocarbazole derivative having potent activity against human topoisomerase I (Topo I) were synthesized. The isomer containing hydroxyl groups in the 3- and 9-positions was approximately ten-fold more active against Topo I, and 5- to 35-fold more active against human solid tumor cell lines in vitro, relative to ED-110.