Abstract A026: Combination strategies with BCL-XL inhibition or degradation in ovarian clear cell carcinoma

Abstract Ovarian clear cell carcinoma (OCCC) is a rare subtype of ovarian cancer with resistance to chemotherapy and poor prognosis in advanced-stage disease. OCCC has frequent alterations in the SWI/SNF chromatin remodeling complex and dysregulation of the PI3K and MAPK and HIF/VEGF signaling pathways. We showed that many OCCC cell lines are sensitive to inhibition of BCL-XL, a pro-survival protein in the intrinsic pathway of apoptosis. A BCL-XL inhibitor (A1331852) or a PROTAC degrader of BCL-XL (DT2216) promoted cell death in OCCC cell lines in combination with cisplatin or paclitaxel. Using Annexin V/PI staining and BH3 profiling (a functional assay for apoptotic priming) we found that BCL-XL inhibition or degradation combined with paclitaxel increased apoptotic cell death beyond the effect of either agent alone. We are currently analyzing reverse phase protein array data to identify upregulated and downregulated proteins in OCCC cells treated with A1331852, paclitaxel, or both. The combination of DT2216 and weekly paclitaxel is being evaluated in a phase 1b clinical trial in recurrent platinum-resistant ovarian cancer (NCT06964009, Dialectic Therapeutics). In addition, we performed chemical screens in OCCC cell lines (OVISE, OVTOKO, JHOC5, TOV21G) to identify compounds that can promote apoptotic cell death in OCCC as single agents and/or combined with BCL-XL inhibition. We applied a Selleck bioactive compound library (1,902 compounds, emphasizing FDA-approved agents) either alone or combined with BCL-XL inhibitor A1331852 at a fixed concentration (IC25 for each cell line). We used a microscopy-based assay to quantitate surviving cells after 72 hours of treatment. We focused on the set of compounds with enhanced activity when combined with A1331852 compared to monotherapy. We performed a 6-dose validation experiment with 53 compounds predicted to synergize with A1331852, from which we selected ten compounds with strong activity combined with A1331852 and validated their activity in 9 OCCC cell lines. We selected three compounds showing consistent synergy with A1331852 for further investigation: RAF265 (RAF/VEGFR inhibitor), PTC209 (BMI1 inhibitor), and PIK-75 (PI3K alpha inhibitor with other targets including CDK9 and DNA-PK). All three compounds induce cell death in multiple OCCC cell lines, which is enhanced by BCL-XL inhibition. Annexin V/PI flow cytometry demonstrated induction of apoptosis with single agent and A1331852 combination treatment. BH3 profiling in OCCC cell lines revealed that treatment with each of the novel agents promoted apoptotic priming, and this was further increased by combined treatment with A1331852. We are analyzing changes in apoptotic proteins and drug target proteins to identify potential mechanisms of synergy of the combinations. In summary, targeting BCL-XL by small molecule inhibitors or PROTAC degrader DT2216 may promote apoptotic cell death in OCCC in combination with paclitaxel chemotherapy or targeted agents. Citation Format: Brendan Shay, Abram Handly-Santana, Jaeyoung Kang, Lissah Johnson, Xingping Qin, Daohong Zhou, Stover Elizabeth. Combination strategies with BCL-XL inhibition or degradation in ovarian clear cell carcinoma [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 A026.