Abstract 5711: Anti-tumor effects of fadraciclib, CDK2/9 inhibitor, in biliary tract cancer

Abstract Background: Targeting CDKs has emerged as a significant strategy in the development of new drugs. The CDK4/6 inhibitors have proven their efficacy in several tumor types, while the CDK2 and CDK9 inhibitors are currently undergoing clinical trials. In biliary tract cancer (BTC), gene expressions of CDK2 and CDK9 are elevated compared to normal tissue. CDK9, a transcriptional CDK, regulates RNA polymerase II (RNAP II), leading to enhanced transcription of oncogenes, such as MCL1. Aberrant activation of these CDKs is associated with cancer progression and evading apoptosis in BTC. Notably, MCL1 is frequently amplified in intrahepatic cholangiocarcinoma (16-21%). This finding underscores the potential significance of CDK2 and CDK9 as therapeutic targets in BTC. However, in BTC, targeting CDK2 or CDK9 has not yet been studied. In this study, we aimed to test CDK2/9 targeting and develop possible biomarker strategies in BTC. Method: A total 9 BTC cell lines (SNU245, SNU308, SNU478, SNU869, SNU1196, SNU2670, SNU2773, TFK1, and HUCCT1) were used. Fadraciclib (CDK2/9 dual inhibitor), Olaparib (PARP inhibitor), JQ1 (BRD4 inhibitor) was used. MTT assay, Colony formation assay, Annexin-V assay, and Cell cycle analysis were performed for evaluation of anti-cancer effects. The effect of Fadraciclib on homologous recombination (HR) -mediated DNA damage repair was assessed using focus formation assay and DRGFP assay. Combination of Fadraciclib with Olaparib or JQ1 was evaluated in vitro and in vivo. Result: The anti-tumor effect of Fadraciclib monotherapy was relatively higher in MCL1-High BTC cells (SNU869, SNU2773) compared to MCL-Low cells (SNU245, SNU2670). In MCL1-High cells, Fadraciclib inhibited CDK9, leading to downregulation of RNAP II phosphorylation at Serine 2 and MCL1. Fadraciclib also interfered with HR-mediated DNA damage response by inhibiting transcription of HR-related genes. The combination of Fadraciclib and Olaparib exhibited synergistic anti-tumor effects in vitro in MCL1-High cells and in vivo xenograft model. Conversely, in MCL1-Low cells, Fadraciclib increased BRD4 mRNA expression, leading to the restoration of CDK9-RNAP II activity. This reactivation of RNAP II restored cellular transcription homeostasis, thereby reactivating the transcription of oncogenes such as MCL1. Combination of Fadraciclib and JQ1 showed synergistic effects in vitro in MCL1-Low cells and in vivo xenograft model by inhibiting restored CDK9-RNAP II activity. Conclusion: MCL1-High BTC cells are sensitive to CDK2/9 inhibitor and show synergism between CDK2/9 inhibitor and PARP inhibitor. In MCL1-Low BTC, a combination of a CDK2/9 inhibitor and a BRD4 inhibitor might represent an optimal strategy for new drug development. Citation Format: Jae-Min Kim, Ah-Rong Nam, Kyoung-Seok Oh, Ju-Hee Bang, Yoojin Jeong, Sea Young Choo, Su In Lee, Hyo Jung Kim, Jeesun Yoon, Tae-Yong Kim, Do-Youn Oh. Anti-tumor effects of fadraciclib, CDK2/9 inhibitor, in biliary tract cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 5711.