Abstract 517: Targeting chromosomally unstable tumors with a selective KIF18A inhibitor

Chromosome instability (CIN) is an attribute of cancer cells that has yet to be fully exploited therapeutically, despite its prevalence across many tumor types and lack of occurrence in normal cells. To identify genes uniquely essential to CINhigh cells, we mined the Cancer Dependency map (DepMap) for genes required for proliferation of tumor cell lines with high levels of copy number aberrations, an indicator of CIN. This analysis identified KIF18A, a mitotic kinesin also identified by others as a vulnerability of aneuploid and whole genome doubled cells. Genetic disruption of KIF18A with RNAi confirmed its necessity in CINhigh cells, and these findings translated to in vivo reduction in tumor growth, illustrating the potential of KIF18A as a therapeutic target. High-throughput screening of a chemical library for inhibitors of KIF18A motor domain’s ATPase activity identified an ATP-noncompetitive hit that was optimized for potency, selectivity, and pharmacokinetic properties. These optimized inhibitors are orally bioavailable, exhibit sub-nM potency, and are highly selective for KIF18A vs. other kinesins. They also display a long drug-target residence time, a unique property that allows cells to be trapped in mitosis long enough to result in cell death. Mechanistically, inhibition of KIF18A’s ATPase activity renders KIF18A unable to localize to the metaphase plate, instead becoming trapped at the spindle poles. This inability to translocate results in defects in chromosome congression, evidenced by a less compact metaphase plate, and an accumulation of cells in G2/M. Treatment of CIN cancer cells with these inhibitors results in a proliferative defect due in part to induction of programmed cell death. In contrast to the effect in cancer cells, KIF18A inhibition has no effect on rapidly proliferating non-transformed epithelial cells and primary T cells. The differential impact between normal and cancer cells sets KIF18A inhibitors apart from other anti-mitotic therapies, which have had limited clinical benefit due to their detrimental impact on both cancer and highly proliferative normal cells. The therapeutic potential of KIF18A inhibitors was assessed in human tumor xenograft models. Treatment of multiple tumor models resulted in substantial, dose-dependent inhibition of tumor growth. KIF18A inhibition in the tumor was confirmed through evaluation of KIF18A localization changes, chromosome congression and an increase in mitotic index. The strong biologic