Paclitaxel (Taxol) is a taxane and a chemotherapeutic drug that stabilizes microtubules. While the interaction of paclitaxel with microtubules is well described, the lack of high-resolution structural information on a tubulin-taxane complex precludes a comprehensive description of the binding determinants that affect its mechanism of action. Here, we solved the crystal structure of baccatin III the core moiety of paclitaxel-tubulin complex at 1.9 Å resolution. Based on this information, we engineered taxanes with modified C13 side chains, solved their crystal structures in complex with tubulin, and analyzed their effects on microtubules (X-ray fiber diffraction), along with those of paclitaxel, docetaxel, and baccatin III. Further comparison of high-resolution structures and microtubules’ diffractions with the apo forms and molecular dynamics approaches allowed us to understand the consequences of taxane binding to tubulin in solution and under assembled conditions. The results sheds light on three main mechanistic questions: (1) taxanes bind better to microtubules than to tubulin because tubulin assembly is linked to a βM-loopconformational reorganization (otherwise occludes the access to the taxane site) and, bulky C13 side chains preferentially recognize the assembled conformational state; (2) the occupancy of the taxane site has no influence on the straightness of tubulin protofilaments and; (3) longitudinal expansion of the microtubule lattices arises from the accommodation of the taxane core within the site, a process that is no related to the microtubule stabilization (baccatin III is biochemically inactive). In conclusion, our combined experimental and computational approach allowed us to describe the tubulin-taxane interaction in atomic detail and assess the structural determinants for binding.

Tubulin has been shown to be an effective target for the development of cytotoxic agents against prostate cancer. Previously, we reported that Lx2-32c is an anti-tubulin agent with high binding affinity to tubulin. In this study, we investigated the potential of Lx2-32c to act as an effective cytotoxic agent in the treatment of prostate cancer. MTT assays showed that Lx2-32c was cytotoxic to all tested prostate cancer cell lines. The Lx2-32c-treated cells typically exhibited a rounded morphology associated with the onset of apoptosis, as evidenced by immunocytochemical staining. Human prostate cancer cell lines treated with Lx2-32c arrest in the G2/M phase of the cell cycle and the treatment is associated with an increased ratio of cells in the sub-G0/G1 phase as determined by flow cytometry. Furthermore, expression of the cleaved form of poly (ADP-ribose) polymerase in prostate cancer cell lines treated with Lx2-32c was shown by Western blotting assay. Xenograft implants of LNCaP and PC3-derived tumors in nude mice showed that Lx2-32c treatment significant inhibited tumor growth with effects equivalent to those of docetaxel. These findings demonstrate the potential of Lx2-32c as a candidate antitumor agent for the treatment of prostate cancer.

Structure activity relationship (SAR) and mechanism of paclitaxel and its analogues in recent years are discussed in the following areas: SAR of paclitaxel analogues toward “normal” and multi-drug resistance tumors; paclitaxel prodrugs with improved water solubitily and specificity; mechanism of paclitaxel related to tubulin binding and quest for its pharmacophore.