Abstract 3264: Ex vivo treatment in high grade serous ovarian cancer demonstrates the benefit of EZH2 inhibition in combination with standard therapy

Abstract While high-grade serous ovarian cancers (HGSOC) are initially sensitive to platinum-based therapy, resistance to chemotherapy remains a major challenge. Recently, the role of EZH2 in establishing a chemo-resistant state has been described in small cell lung cancer with a potential mechanism thought to involve the silencing of SLFN111. Preclinical studies showed that EZH2 inhibition results in re-expression of SLFN11 protein and restores sensitivity to DNA damaging agents. A study in ovarian cancer2 emphasized the need to investigate the effect further. Here we describe the response of platinum-resistant HGSOC patient-derived xenograft (PDX) tumors to CPI-0209, an investigational new drug that is currently being explored for its potential as a reversible second generation EZH2 inhibitor as monotherapy and in combination with carboplatin or PARP inhibition. Four luciferized PDXs (DF101, DF149, DF181, and DF216) were selected based on known platinum sensitivity and BRCA mutation status. PDX tumor ascites were harvested from mice at the IVIS signal intensity of 1011. Xenograft-derived organotypic tumor spheroids (xDOTS3) were loaded into 3D devices (AIM Biotech), treated with either CPI-0209, carboplatin, or olaparib alone, or with CPI-0209 for 7 days prior to adding carboplatin or olaparib for an additional 5-11 days. H3K27me3 was assessed by immunofluorescence. Expression of SLFN11 following treatment was assessed by western blot. Activity was measured at endpoint by dual-labeling fluorescence microscopy. EZH2-dependent target engagement via demethylation of H3K27me3 was seen in all models. While DF181 (BRIP1m) showed resistance to all treatments, DF216 (BRCAwt) showed a consistent trend in reduction in live area in the combination of carboplatin or olaparib with CPI-0209 vs single agent, and significant reduction (>20%; P<0.05) vs control. DF101 (BRCAm) and DF149 (BRCAwt), which showed no evidence of sensitivity to carboplatin and CPI-0209 monotherapy, showed sensitivity to olaparib which was not enhanced by the addition of CPI-0209. Further assessment of DF216 via western blot analysis showed elevated expression of SLFN11 after 7 days of EZH2i treatment. Since PDX in vivo studies are time-consuming and costly, it is impractical to use them for large drug screens. Here we show that our short-term and efficient xDOTS platform allows for rapid evaluation of multiple targeted therapies or therapeutic combinations. The results suggested that CPI-0209 directed combination therapy could enhance activity over standard chemotherapy alone in a platinum-resistant model of ovarian cancer ex vivo. Increased SLFN11 protein level supported the importance of EZH2-SLFN11 axis. Further exploration of CPI-0209 combination in ovarian cancer in known genomic contexts is warranted. Citation Format: Ha V. Vo, Qing Zeng, David A. Barbie, Prafulla C. Gokhale, Elizabeth Adams, Cloud P. Paweletz, Elena Ivanova. Ex vivo treatment in high grade serous ovarian cancer demonstrates the benefit of EZH2 inhibition in combination with standard therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3264.