Interactions of tubulin with potent natural and synthetic analogs of the antimitotic agent combretastatin: a structure-activity study.

Combretastatin, an antineoplastic and antimitotic agent, was isolated from the bark of Combretum caffrum [Can. J. Chem. 60: 1374-1376 (1982); Biochem. Pharmacol. 32:3864-3867 (1983)]. Structurally, combretastatin consists of two substituted benzene rings linked by a saturated, hydroxy-substituted two-carbon bridge. A large number of combretastatin analogs have now been synthesized or obtained from C. caffrum. These vary in substituents on the phenyl rings or bridge carbons, bridge length, unsaturation of the bridge (i.e., stilbene derivatives, with the two phenyl rings oriented either cis or trans), and in precise ring structure (two major variants, with the bridge incorporated into a third six-member ring to form a phenanthrene structure or a methyl group eliminated from vicinal methoxy substituents to form a benzodioxole ring). Available analogs (17 natural products and 22 synthetic agents) were examined for antimitotic and cytotoxic activity and for effects on tubulin polymerization and colchicine binding. Nineteen compounds inhibited cell growth by 50% or more at concentrations of 1 microM or less, and 14 inhibited tubulin polymerization by at least 50% at stoichiometric drug concentrations. The most potent cytotoxic agents generally strongly inhibited both tubulin polymerization and the binding of colchicine to tubulin. The most promising compound is the (cis)-stilbene derivative (cis)-1-(3,4,5-trimethoxyphenyl)-2-(3'-hydroxy-4'-methoxyphenyl)ethene, which has been named combretastatin A-4. This compound inhibited cell growth by 50% at 7 nM, inhibited tubulin polymerization by 50% at 2.5 microM (1/4 molar equivalent), and competitively inhibited colchicine binding with an apparent Ki of 0.14 microM.