Abstract LB-C10: Preclinical synergistic anti-tumor effect against multiple cancer types by EZH2 inhibitor EPZ-6438 plus imipridone ONC201 to mimic H3K27M mutation observed in DIPG tumors that respond to ONC201 in the clinic

Abstract

Abstract ONC201, the founding member of the imipridone class of small molecules, has broad spectrum anti-tumor activity and achieved tumor regressions and prolonged disease stability in clinical trials, especially in patients with histone H3 K27M mutated glioma. Levels of H3K27 dimethylation (H3K27me2) and trimethylation (H3K27me3) are reduced globally in H3 K27M patient samples. EZH2 is the catalytic component of the Polycomb Repressor Complex 2 (PRC2), a conserved multi-subunit that represses gene transcription by methylating lysine 27 on Histone H3 (H3K27). EZH2 is frequently overexpressed or mutated (activating) in numerous human malignancies and EZH2 inhibitors represent a promising therapeutic strategy. We hypothesized that cancer cells could be sensitized to ONC201 by EZH2 inhibition through suppression of H3K27 methylation that may mimic tumors such as DIPG with H3K27M mutations which are known to respond to ONC201 in the clinic and are defective in H3K27 methylation. We treated cancer cells from different tissue origins including breast cancer, pancreatic cancer, colorectal cancer, GBM, and DIPG with single agent ONC201, EZH2 inhibitor EPZ-6438 or the combination of ONC201 plus EPZ-6438. Cell viability was determined with the Cell Titer-Glo assay. Integrated stress response (ISR) activity widely observed in ONC201-treated tumor cells was evaluated using Western blot analysis of ATF4 and apoptosis by analysis of PARP cleavage. Our results demonstrate that ONC201 synergistically reduced cell viability, induced integrated stress response and apoptosis in combination with EZH2 inhibitor EPZ-6438. There was greater induction of ATF4 in combination therapy-treated tumor cells versus monotherapy. Intriguingly, U-251 GBM cells that were resistant to ONC201 under hypoxia could be sensitized to ONC201 by EZH2 inhibitor EPZ-6438. Our findings provide a preclinical rationale for developing ONC201 and EZH2 inhibitors as a novel synergistic combinatorial regimen with the potential for clinical translation in the treatment of several human malignancies including GBM, pancreatic cancer, colorectal cancer, and breast cancer. Citation Format: Yiqun Zhang, Lanlan Zhou, Shengliang Zhang, Robyn Borsuk, Howard P. Safran, Attila A. Seyhan, Wafik S. El-Deiry. Preclinical synergistic anti-tumor effect against multiple cancer types by EZH2 inhibitor EPZ-6438 plus imipridone ONC201 to mimic H3K27M mutation observed in DIPG tumors that respond to ONC201 in the clinic [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr LB-C10. doi:10.1158/1535-7163.TARG-19-LB-C10