Targeting Mitotic Checkpoints: The Therapeutic Potential of BUB1 Kinase Inhibitor BAY 1816032 in Antitumor Treatment

The spindle assembly checkpoint is a critical mechanism that ensures accurate chromosome distribution by delaying the anaphase phase until chromosomes are correctly aligned. While the non-enzymatic functions of the mitotic kinase BUB1 are crucial for the checkpoint's activation, its enzymatic activity is less significant for this process but is necessary for chromosome arm resolution and chromosomal passenger complex positioning. We introduce a new Bub1 kinase inhibitor, BAY 1816032, which has been characterized for its structure and functionality. This compound exhibits high potency, a prolonged target residence time, and favorable oral bioavailability. It effectively inhibits BUB1's enzymatic activity with an IC50 of 7 nM, and demonstrates slow dissociation kinetics, resulting in a target residence time of 87 minutes and high selectivity among 395 kinases. BAY 1816032 negates nocodazole-induced Thr-120 phosphorylation of histone H2A in HeLa cells, induces lagging chromosomes, and causes mitotic delay. When combined with paclitaxel, it leads to persistent lagging chromosomes and missegregation. As a single agent, BAY 1816032 inhibits the proliferation of various cancer cell lines with an average IC50 of 1.4 µM and shows synergistic or additive effects with paclitaxel or docetaxel in most evaluated cell lines. In tumor xenograft studies, BAY 1816032 modestly inhibits tumor growth when administered alone but significantly reduces tumor size when combined with paclitaxel or docetaxel. The compound also reduces intra-tumoral levels of phospho-Thr120 H2A and shows no signs of drug-drug interactions. Toxicological studies in rats and dogs indicate good tolerability. These results support the concept of disrupting mitotic checkpoints and suggest that BAY 1816032, in combination with taxanes, warrants clinical investigation to improve efficacy and potentially counteract resistance.