Abstract A62: A novel Eg5 inhibitor that causes mitotic arrest leading to rapid cancer cell death shows broad‐spectrum antitumor activity in preclinical xenograft tumor models

Antitubulin agents including taxanes and vincas that target mitosis of rapidly dividing cancer cells are among the most effective cancer therapies in current clinical use. However, these antitubulin agents also have debilitating side effects that are dose‐limiting, such as neuropathy, due to their disruption of the normal microtubule functions in resting cells including neuronal cells. Eg5 is an evolutionarily conserved mitosis‐specific kinesin essential for bipolar mitotic spindle formation and has no roles in microtubule functions of resting cells. Inactivation of Eg5 causes mitotic arrest of proliferating cells, resulting in formation of monopolar spindles. Targeting Eg5 for cancer treatment thus represents an attractive strategy that has the potential to maximize the anticancer efficacy by inhibiting cancer cell mitosis while minimizing debilitating side effects associated with antitubulins. Here we describe a selective ATP‐non competitive small molecule inhibitor of human Eg5 kinesin. The Eg5 inhibitor shows no effects on microtubule dynamics in cell‐free assays and arrests cells specifically at mitosis with monopolar spindles, resulting in rapid cancer cell death. Growth inhibition assays against a panel of 21 cancer cell lines shows that the Eg5 inhibitor has potent and broad‐spectrum activity with IC50 values ranged from 0.55 nM to 14.2 nM. Quantitative live cell imaging and high content imaging reveal that the Eg5 inhibitor has a threshold concentration activity and kills cancer cells specifically at mitosis in a time/cell cycle, but not concentration above the threshold,‐dependent manner. Consistent with the in vitro activities, the Eg5 inhibitor shows broad‐spectrum antitumor activity in preclinical xenograft tumor models representing major human cancer histologies also including drug resistant tumors and demonstrates superiority as compared to several chemotherapeutic agents targeting G2/M. Furthermore, its in vivo antitumor activity is highly schedule‐dependent with a clear threshold dose effect, as expected from in vitro observations. Indeed, the Eg5 inhibitor exhibits a robust PK/PD relationship in antitumor activity and its antitumor activity is associated with mitotic arrest of cancer cells and subsequent cell death. The Eg5 inhibitor is currently being evaluated in Phase I studies. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A62.