Reduced EZH1/2 expression in imipridone-treated cells correlates with synergy following combinations with EZH1/2 or HDAC inhibitors in diffuse glioma and other tumors

Small molecule imipridones including ONC201, ONC206 and ONC212 have anti-cancer activity mediated in part through the integrated stress response, induction of TRAIL and its receptor DR5, and activation of mitochondrial caseinolytic protease ClpP with impaired oxidative phosphorylation. ONC201 provides clinical benefit in a subset of patients with histone H3K27M-mutated diffuse glioma (DG). We hypothesized that EZH2 inhibitors (EZH2i) may sensitize tumors to imipridones by mimicking H3K27M mutation. EZH1 is a homolog and alternative for EZH2 in assembling PRC2 complex. We combined ONC201, ONC206 or ONC212 plus dual EZH1/2i in tumors and observed synergy. We observed synergies with imipridones combined with HDACi or triple combination of ONC201/ONC206, EZH2i and HDACi in DG, GBM, prostate cancer and SCLC cells. Our observations implicate EZH1/2 suppression in mechanism of anti-cancer effect of imipridones. We investigated effects of imipridones on EZH1/2 in DG cells and solid tumor cells including GBM, CRC, PDAC, SCLC, prostate cancer, gastric cancer, HCC and breast cancer cells and found inhibition of EZH1/EZH2 expression across tumor types and cell viability suppression by imipridones is correlated with EZH1/2 reduction. Imipridone or EZH2i-treated tumor cells showed similar cytokine profile changes. RNA-seq showed ONC201 and EHZ2i tazemetostat-treated cells have similar transcriptional profiles and share overlap of top regulated genes. Thus, imipridones inhibit EZH1/2 in tumor cells in a manner that mimics H3K27M mutation supporting their role in anti-cancer efficacy. ONC201 and EZH2i share similar targets and actions on tumors. Synergistic combinations of imipridones plus EZH1/2i or imipridones, EZH2i and HDACi merit further investigation.