Benzosuberene and Tetracyclic Analogues as Colchicine Site Inhibitors of Tubulin Polymerization

Colchicine site inhibitors of tubulin polymerization function as antiproliferative anticancer agents, with certain inhibitors demonstrating a dual mechanism of action as tumor-selective vascular disrupting agents (VDAs). Our previous studies yielded potent benzosuberene-based colchicine site inhibitors of tubulin polymerization. To expand structure-activity correlations, the seven-membered fused ring and the pendant ring were modified to generate a series of new compounds, 11 being strong inhibitors (IC₅₀ ≤ 5 μM) of tubulin polymerization. Structural modifications introduced a second benzylic olefin and, separately, a nitrogen atom to the fused seven-membered ring, along with conversion to unique tetracyclic fused ring systems. Two of the active inhibitors were synthetically converted to corresponding phosphate prodrug salts to increase aqueous solubility for <i>in vivo</i> studies. In a preliminary study of VDA efficacy, these prodrugs caused blood flow disruption in mice, as revealed by bioluminescence imaging with histological confirmation. The most effective analogues from this series offer promise as cancer therapeutic agents.