CYT 997 : a novel orally active tubulin polymerization inhibitor with potent cytotoxic and vascular disrupting activity in vitro and in vivo

CYT997 is a wholly synthetic compound that possesses highly potent cytotoxic activity in vitro through inhibition of microtubule polymerization. CYT997 blocks the cell cycle at the G2-M boundary, and Western blot analysis indicates an increase in phosphorylated Bcl-2, along with increased expression of cyclin B1. Caspase-3 activation is also observed in cells treated with CYT997 along with the generation of poly(ADP-ribose) polymerase. The compound possesses favorable pharmacokinetic properties, is orally bioavailable, and is efficacious per os in a range of in vivo cancer models, including some refractory to paclitaxel treatment. CYT997 exhibits vascular disrupting activity as measured in vitro by effects on the permeability of human umbilical vein endothelial cell monolayers, and in vivo by effects on tumor blood flow. CYT997 possesses a useful combination of pharmacologic and pharmacokinetic properties and has considerable potential as a novel anticancer agent. [Mol Cancer Ther 2009;8 (11):3036–45] Introduction Microtubules play an essential role in cell division, intracellular transport, and motility. Several clinically important anticancer drugs such as paclitaxel and the Vinca alkaloids, vincristine and vinblastine (1–3), bind to microtubules thereby altering normal microtubule dynamics (4), which in turn, causes cells to arrest at the G2-M boundary of the cell cycle and ultimately leads to apoptotic or nonapoptotic cell death (5). Although microtubule binding agents such as the taxanes and the Vinca alkaloids are frontline treatments for breast, ovarian, and hormone-refractory prostate cancer, they have substantial drawbacks, notably development of resistance over time (6), the need for intravenous administration and, in the case of paclitaxel, formulation in the lipid-based solvent Chremophor EL, which itself can cause hypersensitivity and neuropathy (7). In recent years, there has been considerable research toward the identification of new tubulin-binding compounds that address the shortcomings of the classic agents (8, 9). Thus, various analogues of the taxanes and Vinca alkaloid classes have been reported (10, 11), as have a range of other natural product-derived compounds, such as the epothilones (12). Several small molecule tubulin-binding drugs, chemically distinct from the Vinca and taxanes class, have also been described. These compounds generally possess improved physicochemical and pharmacokinetic properties compared with paclitaxel and the Vinca alkaloids, although few possess oral activity. Whereas the clinical efficacy of some of these new agents has been mixed, it has become apparent that several of these small molecule microtubule-binding agents exhibit disruptive effects on the tumor vasculature at concentrations well below theirmaximum tolerated dose (13). We describe herein CYT997, a microtubule-disrupting agent that possesses potent cytotoxic activity in vitro, an attractive pharmacokinetic profile, oral activity in two separate xenograft models, and in addition, a clear indication of antivascular effects in vitro and in vivo. Materials and Methods CYT997 was prepared as previously described (14). Paclitaxel, colchicine, vincristine, and vinblastine were purchased from Sigma. Animal studies were conducted in accordance with the Australian and New Zealand Council for the Care of Animals in Research and Training Guidelines and with the approval of the institutional animal experimentation ethics committees.