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
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 Ki values of 2.1 and 2.6 microM, 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 beta-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.