Cyclostreptin (FR182877), a bacterial natural product, was reported to have weak paclitaxel-like activity with tubulin but antitumor activity in vivo. We used synthetic cyclostreptin in studies of its mechanism of action. Although less potent than paclitaxel in several human cancer cell lines, cyclostreptin was active against cells resistant to paclitaxel and epothilone A. At equitoxic concentrations with paclitaxel, cyclostreptin was more effective in arresting MCF-7 cells in mitosis and equivalent in bundling microtubules in PtK 2 cells. Tubulin assembly with paclitaxel occurs at low temperatures and in the absence of GTP or microtubule-associated proteins (MAPs). Brisk assembly with cyclostreptin required MAPs, GTP, and higher reaction temperatures. On the basis of turbidimetry, cyclostreptin-induced microtubules were more stable in the cold than the paclitaxel-induced polymer. Moreover, at 37 °C cyclostreptin was a strong competitive inhibitor of the binding of radiolabeled paclitaxel to tubulin polymer, with an apparent Ki value of 88 nM. Competition studies versus a fluorescent taxoid across a temperature range, in comparison with paclitaxel and docetaxel, showed that only the binding of cyclostreptin to microtubules was markedly reduced at 4 °C versus temperatures over 30 °C. The binding of cyclostreptin to microtubules was characterized by a relatively greater endothermic and entropic profile as compared with those of the taxoid binding reactions, which are characterized more by exothermic and enthalpic interactions. Molecular modeling showed that cyclostreptin formed a pharmacophore with taxoids but formed hydrogen bonds only with the S9-S10 and M loops in the taxoid site. Initial studies also indicate that, relative to paclitaxel, cyclostreptin is more deficient in nucleation than elongation of polymer.
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