Potent Synergistic Tumor Cell Suppression from Combination of ONC201 and Epigenetic Modulators EZH2 or HDAC Inhibitors Provides a Novel Treatment Strategy for Solid Tumors

Abstract ONC201 is a promising anti-cancer agent that upregulates cytotoxic TRAIL pathway signaling in cancer cells. It has broad spectrum anti-tumor activity and achieved tumor regression and prolonged disease stabilization in clinical trials, especially in patients with H3K27M mutant gliomas. Based on the fact that the H3K27M mutation reduces the level of H3K27 dimethylation (H3K27me2) and trimethylation (H3K27me3), we hypothesized that cancer cells could be sensitized to ONC201 by epigenetic modulators that impact on histone methylation or acetylation. We tested the synergy between epigenetic modulators and ONC201 in tumor cell lines. 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. We also treated cancer cell lines originating from colorectal cancer, breast cancer, gastric cancer, prostate cancer, GBM and DIPG with combination of histone deacetylase inhibitor (HADCi), vorinostat, and ONC201. Cell viability was determined with the Cell Titer-Glo assay. Apoptosis was evaluated through flow cytometry analysis of cell distribution. 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. Cytotoxic TRAIL pathway signaling was evaluated using Western blot analysis of DR5. Our results demonstrate that ONC201 synergistically reduced cell viability, induced integrated stress response, cytotoxic TRAIL pathway signaling and apoptosis in combination with epigenetic modulators. There was greater induction of ATF4 and DR5 in combination therapy-treated tumor cells versus monotherapy. Immunoblotting analysis showed that combination of ONC201 and EZH2 inhibitor EPZ-6438 reduced H3K27me3 and EZH2. ONC201 is an antagonist of dopamine receptor D2 (DRD2). Dopamine receptor D5 (DRD5) is a dopamine receptor family member that opposes DRD2 signaling. The DRD2+DRD5- biomarker signature is associated with enhanced ONC201 tumor cell sensitivity. In order to investigate the role of dopamine receptors in anti-tumor effect of ONC201, we also tested the mRNA level of dopamine receptors by RT-PCR and observed that both vorinostat and EPZ-6438 upregulated DRD5 more than DRD2. Thus, we suspect other target genes may be involved in sensitization to ONC201 due to epigenetic modulation. We are currently investigating the interplay between H3K27 methylation versus acetylation in ONC201 sensitivity with or without additional therapeutic modulation by epigenetic drugs. Our results unravel potent synergy between ONC201 and EZH2 inhibitors or HDAC inhibitors and provide further insights into the role of H3K27me3 in ONC201 drug sensitivity. Citation Format: Yiqun Zhang, Lanlan Zhou, Michael Glantz, Howard Safran, Attila Seyhan, Wafik S. El-Deiry. Potent synergistic tumor cell suppression from combination of ONC201 and epigenetic modulators EZH2 or HDAC inhibitors provides a novel treatment strategy for solid tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4042.