A quantitative analysis of the conformational properties of colchicine important for its antimitotic and antitumor activity

Colchicine binds to tubulin, prevents the polymerization of microtubules, and inhibits cell division. Endogenous cellular molecules have been shown to react with this same binding site. Thus, a better understanding of the finding site may lead to the synthesis of drugs which could safely control cell division and cancer. It is known that isocolchicine is completely inactive as a mitotic inhibitor, so the binding site must be selective enough to distinguish between colchicine and isocolchicine. However, certain other molecules that do not appear to be structural analogs of colchicine bind to the site and inhibit microtubule polymerization. Since isocolchicine contains the same chemical functional groups as colchicine, its inactivity may be the result of more subtle, conformational differences. A Quantitative Conformational Analysis (QCA) of colchicine and isocolchicine was done using classical molecular mechanics and approximate molecular orbital theoretical techniques. Several steric and electronic differences between colchicine and isocolchicine are revealed. The most important difference with regard to tubulin binding is in the geometric relationship of three electronegative atoms. No energetically feasible conformation of isocolchicine can match any energetically feasible conformation of colchicine with respect to the triangle formed by the 1-methoxy, the acetamide and the keto oxygen moieties. The results of this conformational analysis also provide support for the hypothesis that tubulin binding requires a more planar conformation of colchicine than its "buckled" crystal form. Colchicine is predicted to be flexible enough to achieve planarity of its conjugated ring system and the energy difference between the planar and buckled conformations is calculated to be less than the colchicine-tubulin free energy of binding. The triangle formed by the 1-methoxy, the acetamide and the keto oxygen for this planar conformation of colchicine defines a "pharmacophore geometry" which is observed in other mitotic inhibitors such as podophyllotoxin and steganicin. This pharmacophore geometry, in combination with information from previous structure-activity relationship (SAR) studies, helps to indirectly define the colchicine-tubulin binding site.