Mutation-dependent sensitivity to the dual TTK/PLK1 inhibitor BAL0891 in patient-derived gastric cancer organoids

Background: BAL0891 is a first-in-class dual inhibitor of threonine tyrosine kinase (TTK, Mps1) and polo-like kinase 1 (PLK1), which cooperatively regulate the spindle assembly checkpoint (SAC) to maintain mitotic fidelity. Dysregulated SAC signaling contributes to chromosomal instability and tumor progression, particularly in gastric cancer. Targeting the SAC through dual kinase inhibition has emerged as a promising approach to exploit mitotic vulnerabilities in solid tumors. Methods: The pharmacologic effects of BAL0891 were evaluated across 40 patient-derived gastric cancer organoids representing distinct molecular subtypes. Cytotoxicity (IC₅₀) was analyzed in relation to TTK and PLK1 protein expression and compared across mutation-defined subgroups, including KRAS , SMAD4 , PTEN , PIK3CA , and BRAF alterations identified by WGS. LC/MS-MS was additionally performed to characterize SAC-associated signaling and mitotic regulatory networks linked to BAL0891 response. Results: Among the 40 patient-derived gastric cancer organoids analyzed, six harbored KRAS alterations, while the remaining samples were wild-type. In the KRAS-mutant, TTK expression was significantly higher in drug-sensitive organoids than in resistant ones. In contrast, PLK1 expression showed no significant association with drug response, suggesting that KRAS-driven mitotic signaling preferentially relies on TTK-mediated regulation. Excluding KRAS-mutant cases, additional alterations were detected in PTEN (n = 3), PIK3CA (n = 2), and BRAF (n = 1). In these mutation contexts, both TTK and PLK1 expression tended to be higher in resistant samples (p = 0.078 and p = 0.087, respectively), implying that enhanced PLK1/TTK activity stabilizes the SAC and promotes survival through PI3K-AKT or MAPK signaling, thereby conferring relative resistance. Overall, these findings demonstrate that the relationship between mitotic kinase expression and drug response is context-dependent: KRAS-altered tumors with high TTK expression exhibit increased sensitivity due to heightened SAC dependency, whereas PTEN, PIK3CA, and BRAF mutant tumors with elevated kinase activity maintain checkpoint stability and survival signaling, resulting in reduced responsiveness to BAL0891. Conclusion: BAL0891 effectively disrupts mitotic checkpoint signaling through simultaneous inhibition of TTK and PLK1. The therapeutic outcome is genotype-dependent: KRAS-mutant tumors exhibit TTK-driven SAC dependency and higher sensitivity, whereas PTEN, PIK3CA, and BRAF mutants show relative resistance due to compensatory survival signaling. These findings identify SAC dysregulation as a predictive biomarker for BAL0891 responsiveness in gastric cancer.