STRUCTURE-ACTIVITY ANALYSIS OF THE INTERACTION OF CURACIN-A, THE POTENT COLCHICINE SITE ANTIMITOTIC AGENT, WITH TUBULIN AND EFFECTS OF ANALOGS ON THE GROWTH OF MCF-7 BREAST-CANCER CELLS

Originally purified as a major lipid component of a strain of the cyan obacterium Lyngbya majuscula isolated in Curacao, curacin A is a poten t inhibitor of cell growth and mitosis, binding rapidly and tightly at the colchicine site of tubulin. Because its molecular structure diffe rs so greatly from that of colchicine and other colchicine site inhibi tors, we prepared a series of curacin A analogs to determine the impor tant structural features of the molecule. These modifications include reduction and E-to-Z transitions of the olefinic bonds in the 14-carbo n side chain of the molecule; disruption of and configurational change s in the cyclopropyl moiety; disruption, oxidation, and configurationa l reversal in the thiazoline moiety; configurational reversal and subs tituent modifications at C13; and demethylation at C10. Inhibitory eff ects on tubulin assembly, the binding of colchicine to tubulin, and th e growth of MCF-7 human breast carcinoma cells were examined. The most important portions of curacin A required for its interaction with tub ulin seem to be the thiazoline ring and the side chain at least throug h C4, the portion of the side chain including the C9-10 olefinic bond, and the C10 methyl group. Only two modifications totally eliminated t he tubulin-drug interaction. The inactive compounds were a segment con taining most of the side chain, including its two substituents, and an alogs in which the methyl group at the C13 oxygen atom was replaced by a benzoate residue. Antiproliferative activity comparable with that o bserved with curacin A was only reproduced in compounds that were pote nt inhibitors of the binding of colchicine to tubulin. Molecular model ing and quantitative structure-activity relationship studies demonstra ted that most active analogs overlapped extensively with curacin A but failed to provide an explanation for the apparent structural analogy between curacin A and colchicine.