LAUSR

Imipridones are a family of small molecule compounds including ONC201, ONC206 and ONC212 with anti-cancer activity mediated in part through activation of the integrated stress response, induction of TRAIL and its receptor, DR5, and activation of mitochondrial caseinolytic protease ClpP with consequent impairment of oxidative phosphorylation. Early clinical data have indicated that ONC201 provides clinical benefit in a subset of patients with histone H3K27M-mutated diffuse midline glioma (DMG). The H3K27M mutation prevents the function of EZH2 in methylating H3K27 on the mutated protein. This phenomenon implicates H3K27M and EZH2 in the mechanism of the anti-cancer effect of ONC201. EZH1 is a homolog of EZH2 and forms an alternative for EZH2 in assembling the PRC2 complex. We investigated the effects of ONC201, ONC206 or ONC212 on EZH1/2 by treating DMG cells and a panel of other solid tumor cells including GBM, DMG, CRC, PDAC, SCLC, prostate cancer, HCC and breast cancer cells with ONC201, ONC206 or ONC212. We observed that imipridones inhibit expression of both EZH1 and EZH2 in these tumors. RNA-seq analysis and RT-PCR showed no regulation of EZH1/2 at the level of transcription after ONC201 treatment. Linear regression revealed a correlation between extent of EZH1/2 inhibition and extent of cell viability suppression by ONC201 thereby providing further rationale for the combination of ONC201 and EZH1/2 inhibitors or HDAC inhibitors. We combined ONC201 or ONC206 or ONC212 plus a dual EZH1/2 inhibitor or the triple combination of ONC201, EZH2i and HDACi in DMG, GBM, prostate cancer and SCLC cells for 72 hours, performed a CellTiterGlo assay and observed synergies. The effective combination index (CI) of ONC201 plus tazemetostat ranges 0.04-0.64 in SU-DIPG-25 and 0.55-0.84 in SU-DIPG-13 DMG cells. The effective CI of ONC206 plus tazemetostat and panobinostat ranges 0.27-0.77 in U251 GBM and 0.11-0.71 in SU-DIPG-13 DMG cells. The effective CI of ONC201 plus dual EZH1/2 inhibitor, valemetostat, ranges 0.44-0.89 in H1048 SCLC cells, 0.14-0.90 in LNCaP prostate cancer cells, and 0.13-0.79 in SNB19 GBM cells. The effective CI of ONC212 plus valemetostat ranges 0.33-0.83 in 22Rv1. The synergy in inducing apoptosis was demonstrated by immunoblotting of cleaved-PARP. We also observed that ONC201 or ONC206 inhibit neogenin which is associated with invasion of DMG in SU-DIPG-13 cells. We conclude that ONC201 inhibits EZH1/2 in tumor cells and also neogenin in DMG cells. Inhibition of these proteins may play roles in the anti-cancer effect of ONC201. The synergies between ONC201 and EZH1/2 or HDAC inhibitors provide clues for developing novel therapy for the mentioned tumors. Overexpression of EZH2 is in process to elucidate the role of EZH2 in the mechanism of the anti-cancer effect of ONC201. Citation Format: Yiqun Zhang, Wafik S. El-Deiry. Imipridones ONC201 and ONC206 reduce expression of neogenin and EZH1/2 which correlate with synergy following their combination with EZH1/2 or HDAC inhibitors in treatment of DMG and other tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2671.