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Identification and Characterization of a New Tubulin-Binding Tetrasubstituted Brominated Pyrrole

Department of Physiology and Medicine, Southwest Foundation for Biomedical Research, San Antonio, Texas (S.L.M., K.N.W.); Lombardi Comprehensive Cancer Center, Drug Discovery Program, Department of Oncology, Georgetown University Medical Center, Washington DC (S.D., M.L.B.); Toxicology and Pharmacology Branch, Developmental Therapeutics Program, Division of Cancer Treatment and Diagnosis, National Cancer Institute at Frederick, National Institutes of Health, Frederick, Maryland (E.H.); and Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia (E.J.B., A.K., J.H., J.T.G.)

We studied the mechanism of action of 3,5-dibromo-4-(3,4-dimethoxyphenyl)-1H-pyrrole-2-carboxylic acid ethyl ester (JG-03-14) and found that it is a potent microtubule depolymerizer. JG-03-14 caused a dose-dependent loss of cellular microtubules, formation of aberrant mitotic spindles, accumulation of cells in the G₂/M phase of the cell cycle, and Bcl-2 phosphorylation. These events culminated in the initiation of apoptosis, as evidenced by the caspase 3-dependent cleavage of poly(ADP-ribose) polymerase (PARP). JG-03-14 has antiproliferative activity against a wide range of cancer cell lines, with an average IC₅₀ value of 62 nM, and it is a poor substrate for transport by P-glycoprotein. JG-03-14 inhibited the polymerization of purified tubulin in vitro, consistent with a direct interaction between the compound and tubulin. JG-03-14 potently inhibited the binding of [³H]colchicine to tubulin, suggesting that it bound to tubulin at a site overlapping the colchicine site. JG-03-14 had antitumor effects in the PC3 xenograft model, in which it caused greater than 50% reduction in tumor burden after 14 days of treatment. Molecular modeling studies indicated that the dimethoxyphenyl group of JG-03-14 occupies a space similar to that of the trimethoxyphenyl group of colchicine. However, the 2,3,5-trisubstituted pyrrole group, which is connected to the dimethoxyphenyl moiety, interacted with both and tubulin in space not shared with colchicine, suggesting significant differences compared with colchicine in the mechanism of binding to tubulin. Our results suggest that this tetransubstituted pyrrole represents a new, biologically active chemotype for the colchicine site on tubulin.

Address correspondence to: Dr. Susan L. Mooberry, Department of Physiology and Medicine, Southwest Foundation for Biomedical Research, P.O. Box 760549, San Antonio, Texas 78245-0549. E-mail: smooberry{at}sfbr.org

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