Abstract A032: Alisertib, a newly defined CBX2 inhibitor, overcomes cisplatin resistance and tumor progression in high grade serous carcinoma (HGSC)

Abstract Objectives: Platinum-based chemotherapy is the primary treatment for high grade serous carcinoma (HGSC); however, 10-20% of chemo naïve tumors demonstrate innate resistance, and 70% of initial responders will develop resistance with recurrent disease. Overcoming therapeutic resistance is an urgent clinical need. In patients with HGSC, increased expression of Chromobox 2 (CBX2), an epigenetic reader and subunit of polycomb repressor complex 1 (PRC1), is associated with poor survival. Increased CBX2 expression is associated with chemoresistance and loss of CBX2 resensitizes HGSC cells to cisplatin. We have found that alisertib, an aurora kinase A inhibitor, also functions as a CBX2 inhibitor. Methods: We used two syngeneic murine models of HGSC: the ID8 model (Tp53-/-, BRCA2-/-) and the SO model (Tp53-/-; Myc+; Hras+). In the ID8 model, tumor-bearing mice received cisplatin, alisertib, or a combination of both. We evaluated tumor dissemination, tumor weight, and the tumor microenvironment. In the SO model, which is innately cisplatin resistant and homologous recombination proficient (HRP), tumor-bearing mice were treated with cisplatin or alisertib, and we assessed the same parameters. In parallel, we performed an ultra-low dilation assay (ULDA) in 3D culture using ID8-cisR and SO cells, treating them with a combination of cisplatin and alisertib. We developed the cisplatin-resistant ID8 cells (ID8-cisR) in vitro with both CBX2 wildtype and CBX2 knockdown (KD) and then conducted an ULDA in 3D culture. After treating the cells with cisplatin, alisertib, or a combination, we measured stem cell frequency. Results: In the ID8 mouse model, single agent treatment with alisertib or cisplatin lead to reduced tumor weight (Ctrl vs alisertib p=0.171, vs cisplatin, p=0.083), number of dissemination sites (Ctrl vs alisertib p=0.083, vs cisplatin, p=0.06), and total weight of disseminated tumors (Ctrl vs alisertib p=0.082, vs cisplatin, p=0.056). Notably, the combination treatment with cisplatin/alisertib showed an additive effect on the tumor weight (p=0.025), number of dissemination sites (p=0.024), and total weight of disseminated tumors (p=0.027). In the SO model, both cisplatin and alisertib significantly reduced tumor weight (Ctrl vs alisertib p=0.013, vs cisplatin, p=0.001), dissemination sites (Ctrl vs alisertib p=0.002, vs cisplatin, p>0.001) and lowered PD-L1 positive immune-suppressive macrophages. In ID8-cisR cells grown in 3D culture, cisplatin increased stem cell frequency; however, this increase was reversed by alisertib. In contrast, alisertib did not have an effect on CBX2 KD cells. In 3D culture, treatment of SO (HRP) cells with alisertib re-sensitized the cells to cisplatin. Conclusions: In vivo, the combination of cisplatin and alisertib has additive effects on tumor burden, enhancing treatment responses, and mitigating therapeutic resistance. In vitro, alisertib re-sensitizes cells that are resistant to cisplatin, including both acquired and innate resistance, and it is dependent on CBX2 expression. Citation Format: Ritsuko Iwanaga, Tomomi Yamamoto, Elizabeth R. Woodruff, Alex Petkov, Alan M. Elder, Lily E. Feldman, Sandra Orsulic, Benjamin G. Bitler, Lindsay W. Brubaker. Alisertib, a newly defined CBX2 inhibitor, overcomes cisplatin resistance and tumor progression in high grade serous carcinoma (HGSC) [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Ovarian Cancer Research; 2025 Sep 19-21; Denver, CO. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl):Abstract nr A032.