Abstract 5406: Evaluation of antitumor effects of plogosertib, PLK1 inhibitor in biliary tract cancer with BUBR1 as a potential biomarker

Abstract Background: Polo-like kinase 1 (PLK1) has been emerging as a promising therapeutic target in solid tumors due to its central role in cell cycle regulation and cancer cell proliferation. In biliary tract cancer (BTC), cell cycle-related factors such as PLK1, CDKs, and Aurora kinase are frequently overexpressed or aberrantly activated, driving tumor progression. However, studies targeting these proteins in BTC remain limited. BUBR1, a critical mitotic checkpoint protein, is essential for maintaining chromosomal stability by ensuring proper chromosome alignment and segregation during cell division. This study evaluated the antitumor effects of a PLK1 inhibitor plogosertib in BTC and identified BUBR1 as a potential biomarker to predict treatment response. Methods: Total 9 BTC cell lines (SNU-245, SNU-308, SNU-478, SNU-869, SNU-1196, HuCCT-1, TFK-1, SNU-2670, SNU-2773) were used. We used plogosertib (PLK1 inhibitor), ceralasertib (ATR inhibitor) and panobinostat (HDAC2/3 inhibitor). The anticancer effects of plogosertib, both alone and in combination with ceralasertib and panobinostat, were evaluated through MTT assay, Colony formation assay, Annexin-V assay, and Cell cycle analysis. The effect of plogosertib on mitotic arrest was assessed using phospho-histone H3 assay and Immunofluorescence. Results: Plogosertib monotherapy showed higher anticancer effects in BUBR1-high BTC cells (SNU2773, SNU478) compared to BUBR1-low cells (SNU869, SNU245). Plogosertib promoted mitotic checkpoint complex (MCC) formation in prometaphase, arresting mitosis and ultimately leading BTC cells to undergo apoptosis. However, plogosertib also induced ATR activation, which may limit its efficacy by initiating DNA repair pathways. ATR inhibition by ceralasertib further enhanced the anticancer effects of plogosertib in BUBR1-high cells. In BUBR1-low cells, plogosertib reduced BUBR1 protein expression, possibly through ubiquitination-proteasome-mediated degradation, limiting MCC formation and its therapeutic effects. Among several candidates regulating ubiquitination processes, HDAC2/3 inhibition by panobinostat enhanced the responsiveness of BUBR1-low cells to plogosertib. Conclusion: BTC cells with high BUBR1 expression are sensitive to the PLK1 inhibitor plogosertib, which shows synergistic effects when combined with an ATR inhibitor. In contrast, BTC cells with low BUBR1 expression which are relatively insensitive to plogosertib, HDAC2/3 inhibition increases the sensitivity to plogosertib. These findings suggest that targeting PLK1 could be an effective strategy for BTC treatment, especially with BUBR1 expression as a potential biomarker to inform optimal combination therapies. Citation Format: Yoojin Jeong, Ah-Rong Nam, Kyoung-Seok Oh, Jae-Min Kim, Ju-Hee Bang, Sea Young Choo, Hyo Jung Kim, Jeesun Yoon, Tae-Yong Kim, Do-Youn Oh. Evaluation of antitumor effects of plogosertib, PLK1 inhibitor in biliary tract cancer with BUBR1 as a potential biomarker [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5406.