Originally purified as a major lipid component of a strain of the cyanobacterium Lyngbya majuscula isolated in Curaçao, curacin A is a potent inhibitor of cell growth and mitosis, binding rapidly and tightly at the colchicine site of tubulin. Because its molecular structure differs so greatly from that of colchicine and other colchicine site inhibitors, we prepared a series of curacin A analogs to determine the important structural features of the molecule. These modifications include reduction and E-to-Z transitions of the olefinic bonds in the 14-carbon side chain of the molecule; disruption of and configurational changes in the cyclopropyl moiety; disruption, oxidation, and configurational reversal in the thiazoline moiety; configurational reversal and substituent modifications at C13; and demethylation at C10. Inhibitory effects on tubulin assembly, the binding of colchicine to tubulin, and the growth of MCF-7 human breast carcinoma cells were examined. The most important portions of curacin A required for its interaction with tubulin seem to be the thiazoline ring and the side chain at least through C4, the portion of the side chain including the C9-C10 olefinic bond, and the C10 methyl group. Only two modifications totally eliminated the tubulin-drug interaction. The inactive compounds were a segment containing most of the side chain, including its two substituents, and analogs in which the methyl group at the C13 oxygen atom was replaced by a benzoate residue. Antiproliferative activity comparable with that observed with curacin A was only reproduced in compounds that were potent inhibitors of the binding of colchicine to tubulin. Molecular modeling and quantitative structure-activity relationship studies demonstrated 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.

The successful synthesis of dolastatin 11, a depsipeptide originally isolated from the mollusk Dolabella auricularia, permitted us to study its effects on cells. The compound arrested cells at cytokinesis by causing a rapid and massive rearrangement of the cellular actin filament network. In a dose-and time-dependent manner, F-actin was rearranged into aggregates, and subsequently the cells displayed dramatic cytoplasmic retraction. The effects of dolastatin 11 were most similar to those of the sponge-derived depsipeptide jasplakinolide, but dolastatin 11 was about 3-fold more cytotoxic than jasplakinolide in the cells studied. Like jasplakinolide, dolastatin 11 induced the hyperassembly of purified actin into filaments of apparently normal morphology. Dolastatin 11 was qualitatively more active than jasplakinolide and, in a quantitative assay we developed, dolastatin 11 was twice as active as jasplakinolide and 4-fold more active than phalloidin. However, in contrast to jasplakinolide and phalloidin, dolastatin 11 did not inhibit the binding of a fluorescent phalloidin derivative to actin polymer nor was it able to displace the phalloidin derivative from polymer. Thus, despite its structural similarity to other agents that induce actin assembly (all are peptides or depsipeptides), dolastatin 11 may interact with actin polymers at a distinct drug binding site.