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 degrees C
cyclostreptin was a strong competitive inhibitor of the binding of radiolabeled
paclitaxel to
tubulin polymer, with an apparent K(i) 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 degrees C versus temperatures over 30 degrees 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.