Abstract 1131: ORIC-944, a potent and selective allosteric PRC2 inhibitor, demonstrates robust in vivo activity in prostate cancer models

The polycomb repressive complex 2 (PRC2) is responsible for the methylation of histone 3 at lysine 27 (H3K27) which leads to long-term transcriptional silencing. Through this epigenetic chromatin silencing mechanism, PRC2 plays a key role in regulating cellular functions such as cell growth and differentiation. PRC2 comprises three core subunits: the catalytic subunit enhancer of zeste homolog 2 (EZH2), embryonic ectoderm development (EED) and suppressor of zeste 12 (SUZ12). EED directly interacts with histone H3K27me3 and is essential for the histone methyltransferase activity of PRC2. PRC2 dysregulation occurs in multiple solid tumors and hematological malignancies, resulting in elevated levels of PRC2 activity and H3K27 trimethylation, and has been linked to poor prognosis in patients with metastatic prostate cancer. First-generation PRC2 inhibitors which target EZH2 have demonstrated clinical activity in several cancers, yet the pharmacological and ADME properties of these compounds require high doses that only achieve partial target inhibition at clinically active levels and exhibit drug-drug interaction (DDI) liabilities. As an alternative strategy to fully inhibit the PRC2 complex, we developed ORIC-944, a potent and highly selective allosteric inhibitor of PRC2 via binding the EED subunit. This unique EED targeting strategy can more completely inhibit PRC2, for example, in the presence of innate or acquired resistance mutations in EZH2, and by addressing the potential compensatory escape mechanism of EZH1-driven tumor growth. ORIC-944 has potential best-in-class drug properties compared to first generation PRC2 inhibitors, including superior potency and improved DDI liabilities. In diffuse large B-cell lymphoma (DLBCL) xenografts in vivo, ORIC-944 significantly depleted H3K27 trimethylation and induced tumor regressions in a dose-dependent manner. ORIC-944 demonstrated strong tumor growth inhibition as a single agent with once daily oral dosing in both enzalutamide-responsive and enzalutamide-resistant prostate cancer models. ORIC-944 caused a significant reduction in tumor cell proliferation and anti-apoptotic signaling, as measured by Ki67 and survivin, respectively. Moreover, in PK/PD assessments, ORIC-944 strongly depleted H3K27me3 in treated tumors. These in vivo studies established that effective single agent inhibition of PRC2 via EED results in decreased tumor cell growth in PRC2-dependent prostate cancer models. In summary, ORIC-944 is a potent, highly selective, allosteric, orally bioavailable PRC2 inhibitor via the EED subunit that represents a differentiated strategy to block PRC2 activity in selected cancers. We are developing ORIC-944 as a best-in-class PRC2 inhibitor for the treatment of patients with advanced prostate cancer and expect to file an IND in the second half of 2021. Citation Format: Anneleen Daemen, Jessica D. Sun, Aleksandr Pankov, Frank L. Duong, Natalie Yuen, Shravani Barkund, Shelly Kaushik, Jae H. Chang, David M. Briere, Niranjan Sudhakar, Andrew Calinisan, Aaron Burns, John M. Ketcham, Matthew A. Marx, Peter Olson, James G. Christensen, Melissa R. Junttila, Lori S. Friedman. ORIC-944, a potent and selective allosteric PRC2 inhibitor, demonstrates robust in vivo activity in prostate cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1131.