The Coral-Derived Natural Products Eleutherobin and Sarcodictyins A and B: Effects on the Assembly of Purified Tubulin with and without Microtubule-Associated Proteins and Binding at the Polymer Taxoid Site

Abstract

We examined interactions with purified tubulin of synthetic sarcodictyins A and B and eleutherobin (coral-derived antimitotic agents) and of compound 1, an analogue of sarcodictyin A methylated at the C-3 oxygen atom (i.e., the methyl ketal analogue of sarcodictyin A and thus structurally similar to eleutherobin but lacking the C-3 sugar moiety). Eleutherobin was much more active than sarcodictyins A and B, which were somewhat more active than compound 1. Effects of eleutherobin did not differ greatly from those of paclitaxel and epothilone A. Eleutherobin and epothilone A were competitive inhibitors of the binding of radiolabeled paclitaxel to tubulin polymer (apparent K_i values of 2.1 and 2.6 μM, respectively). Tubulin assembly reactions induced by all compounds were similar to the paclitaxel-driven reactions in being enhanced by the addition of microtubule-associated proteins and/or GTP to the reaction mixture and by progressively higher reaction temperatures. Antiproliferative activity was studied in six human cancer cell lines, including two paclitaxel-resistant lines with point mutations in a β-tubulin gene. Except for compound 1, effects on cell growth were generally in accord with effects on purified tubulin. Thus, sarcodictyins A and B had IC₅₀ values in the 200−500 nM range; paclitaxel, <10 nM (except in the resistant lines); and eleutherobin and epothilone A, 10−40 nM. The antiproliferative activity of compound 1 was more comparable to that of eleutherobin than sarcodictyin A, despite its weak interaction with tubulin. The activities of the sarcodictyins, eleutherobin, and compound 1 in the mutant ovarian lines were similar to their activities in the parental line.