Pharmacological inhibition of UBASH3B induces mitotic arrest and selectively reduces tumor growth.

Cancer cells tolerate mitotic errors to sustain proliferation, yet the molecular dependencies enabling this tolerance remain poorly understood and largely unexploited therapeutically. UBASH3B ensures mitotic fidelity by regulating Aurora B localization and is overexpressed in tumors. We investigated whether pharmacological disruption of this pathway exposes a mitotic dependency in cancer. We demonstrate that the UBASH3B 2HP domain represents a druggable interface for disrupting UBASH3B-dependent Aurora B regulation. We identify UBASHIN, a small-molecule inhibitor targeting this domain. UBASHIN phenocopies UBASH3B depletion, causing Aurora B mislocalization, mitotic arrest and mitotic cell death. UBASHIN impairs UBASH3B localization to the mitotic spindle and selectively inhibits cancer cell proliferation while sparing non-cancerous cells. UBASH3B protein levels and the cellular ability to sustain mitotic arrest, but not global aneuploidy, predict UBASHIN sensitivity, supporting a cancer-specific on-target mechanism. In colorectal and triple-negative breast cancer mouse models, UBASHIN reduces tumor volume comparably to docetaxel, without detectable toxicity in wild-type animals. These findings identify the UBASH3B pathway as a cancer-specific mitotic vulnerability and highlight its pharmacological inhibition as a selective therapeutic strategy.