BAY 1217389 is a potent and selective inhibitor of the mitotic kinesin KSP (kinesin spindle protein), which plays a critical role in cell division by facilitating the formation of the bipolar spindle during mitosis. By inhibiting KSP, BAY 1217389 effectively blocks mitotic progression, leading to cell cycle arrest and apoptosis in rapidly dividing cells. This mechanism makes it a candidate for cancer therapy, particularly in tumors with high proliferative rates. It has been investigated for its antitumor activity in preclinical models, showing efficacy in inducing tumor regression in various human cancer xenografts, including solid tumors. Due to its targeted action on a mitosis-specific protein, BAY 1217389 may offer a favorable safety profile by sparing non-dividing cells, potentially reducing side effects commonly associated with conventional microtubule-targeting chemotherapeutics like taxanes or vinca alkaloids. Its development highlights a strategy to overcome resistance mechanisms associated with traditional antimitotic drugs, as it does not target tubulin but instead acts upstream in the mitotic process. Research has focused on optimizing dosing regimens and evaluating combination therapies with other anticancer agents to enhance therapeutic outcomes.