Indibulin (ZIO-301): An orally active tubulin polymerization inhibitor with a unique molecular mechanism of action

B276 Indibulin (N-(pyridin-4-yl)-[1-(4-chlorbenzyl)-indol-3-yl]-glyoxyl-amid; ZIO-301 or D-24851) is a novel synthetic, orally active anti-mitotic agent that binds tubulin, destabilizes microtubulin polymerization, and arrests tumor cell growth at the G2/M phase. Its tubulin binding site is distinct from that of other microtubulin inhibitors such as taxanes, colchicines, and vinca alkaloids. Indibulin does not bind acetylated (neuronal) tubulins and, in contrast to all other microtubulin inhibitors, has not caused neurotoxicity in animal models or in patients enrolled in ongoing Phase I clinical trials. Indibulin is active in a panel of human and rodent tumor cell lines in vitro and in human tumor xenografts including multidrug-resistant tumor cell lines. We have demonstrated in molecular mechanistic studies that indibulin affects microtubule-dependent processes such as the plasma recycling system in a manner different from other microtubule inhibitors. While taxol caused a shift of the apical recycling system from a central tubulovesicular location in the apical region to the perijunctional corner of polarized cells, indibulin dispersed the apical recycling system throughout the cell, similar to nocodazole. However, unlike nocodazole, indibulin did not disrupt the Golgi apparatus. While nocodazole disrupted all microtubules, indibulin did not affect acetylated microtubules in intact cells. These findings correlate with the lack of disruption of the Golgi apparatus, which is associated with the more stable acetylated microtubules. These results suggest that indibulin targets specific subsets in cellular microtubules. Indibulin has potent antitumor activity as a single agent in preclinical animal models. As we continue to translate its biology into the clinic, its cytotoxic activity in combination with approved chemotherapeutics in a series of cell lines is of interest. Strongest synergy was observed in a non-small cell lung cancer cell line, A549, with erlotinib, which is an epidermal growth factor receptor inhibitor. Indibulin also enhanced the effect of carboplatin in the same cell line. In the MCF-7 breast cancer cell line, indibulin had synergistic effects with both vinblastine and paclitaxel; this is of particular interest since indibulin has been previously shown to have antitumor activity in vinca alkaloid- and taxane-resistant tumor cell lines. Indibulin was also synergistic with 5-FU and tamoxifen in MCF-7 cells, and had additive effects with doxorubicin. In the SKOV-3 ovarian cancer cell line, indibulin showed additive effects with cisplatin, carboplatin, and gemcitabine, and in the PC3 prostate cancer cell line with estramustin and prednisol. These data suggest that indibulin has a unique molecular mechanism, targets specific subsets in cellular microtubules, and has potent oral single-agent antitumor activity that is highly synergistic with a number of widely used anticancer agents. When viewed with the lack of neurotoxicity in the Phase I trials to date, our results would strongly support successful clinical translation in upcoming Phase II studies both as a single agent and in combination.