In the course of our Heat Shock 90 program, certain carbazole compounds were identified which had an off-target antiproliferative activity. To understand the off-target activity, we studied one analog with strong activity. We discovered that it had an effect on tubulin polymerization kinetics and was competitive with colchicine. Additional analogs were made, and a number of potent compounds were identified. (C) 2009 Elsevier Ltd. All rights reserved.