Abstract 5436: HH2853 is a selective small molecular dual inhibitor of EZH1/2 with potent anti-tumor activities

Abstract Polycomb Repressive Complex 2 (PRC2) is a multiprotein complex that catalyzes the methylation of lysine 27 on histone H3 (H3K27), and the histone methyltransferases EZH1/2 are the catalytic subunit of PRC2. Aberrant activation of EZH2 drives H3K27 methylation and plays a critical role in cancer initiation and progression. Meanwhile, loss-of-function mutation in the major components of SWI/SNF complex leads to a loss of its suppression ability against PRC2 and subsequently activates EZH2. Targeting EZH2 has been validated as a promising therapeutic strategy for cancer treatment, especially for those with EZH2 gain-of-function (GOF) mutations or alterations of SWI/SNF complex proteins that act as oncogenic drivers. Although when PRC2 was suppressed by EZH2 inhibition, the activity of EZH1 complementarily increased to replace the role of EZH2 and maintained the function of PRC2. In addition, EZH2 inhibition in hematopoietic stem cells of mice has been shown to facilitate the development of heterogeneous hematologic malignancies in an EZH1-dependent manner. As a result, dual inhibition of EZH1 and EZH2 may be more effective than EZH2 inhibition alone in suppressing PRC2 function, as well as overcoming the issues related to the development of secondary malignancies, which has been observed in the clinical development of EZH2 selective inhibitor. Thus, we discovered and developed HH2853,a dual inhibitor of EZH1/2. The potency and selectivity of HH2853 against EZH1/2 were assessed at biochemical and cellular levels, and its anti-tumor activities were evaluated in multiple tumor models in vitro and in vivo in comparison with FDA-approved EZH2 selective inhibitor tazemetostat. In addition, the pharmacokinetic (PK) profile of HH2853 was characterized. HH2853 inhibited the enzymatic activities of wild-type and mutant EZH2 with IC50 values of 2.21-5.36 nM, which was similar to that of tazemetostat. In addition, HH2853 inhibited EZH1 enzymatic activity with an IC50 of 9.26 nM, which was stronger than tazemetostat (IC50: 58.43 nM). In contrast, HH2853 up to 10 μM exhibited only marginal or minor inhibitory activity against 36 histone modification enzymes. At cellular level, HH2853 potently inhibited H3K27 mono-, di- and tri-methylation in multiple cancer cell lines with wild-type or mutant EZH2. In contrast, HH2853 up to 10 μM had no effect on the other types of methylation modifications on histone H3. Moreover, HH2853 potently inhibited the cell viability of multiple cancer cell lines with EZH2 GOF mutation or alterations in SWI/SNF complex, and exhibited superior anti-tumor efficacy in several tumor xenograft models than tazemetostat at a comparable dose level. Furthermore, HH2853 exhibited superior PK property than tazemetostat. Taken together, HH2853 is a selective EZH1/2 dual inhibitor with potent anti-tumor activities and favorable PK properties. HH2853 is currently in phase I clinical development. Citation Format: Xu-Xing Chen, Qian-Qian Shen, Zhao Zhao, Yan-Fen Fang, Jun-Yu Yang, Ying-Lei Gao, Lei Liu, Yixiang Zhang, Yi Chen, Leping Li. HH2853 is a selective small molecular dual inhibitor of EZH1/2 with potent anti-tumor activities [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5436.