Abstract 1145: PH020-803: an MTA-cooperative and brain-penetrable PRMT5 inhibitor that selectively targets MTAP-deleted tumors
Authors: Gao Feng , Liu Bin , Jing Liandong , Wu Yongyong , Zhang Pengzhi , Yuan Shuai , Zhao Hui , Gao Yu , Li Zhizhong , Wang Xiaofan , Guo Yongqi 
Published: June 2023

Abstract
Background: Methylthioadenosine phosphorylase (MTAP) gene deletion occurs in ~10% of human cancers and is enriched in non-small cell lung cancer, pancreatic cancer and brain cancer. Since MTAP is the only enzyme responsible for methylthioadenosine (MTA) degradation, homozygous deletion of MTAP gene leads to intracellular accumulation of MTA. MTA competes with S-adenosylmethionine (SAM, the methyl donor) and results in partial inhibition of protein arginine methyltransferase 5 (PRMT5) activity. Recent studies reported that tumor cells with MTAP loss are vulnerable to PRMT5 inhibition. Based on the synthetic lethality interaction, an MTA cooperative PRMT5 inhibitor has the potential to selectively target MTAP-deleted tumors and avoid hematologic toxicity of substrate-competitive (such as GSK3326595) or SAM-competitive (such as JNJ64619178) PRMT5 inhibitors.
Methods: A pair of HCT116 isogenic cell lines (MTAP-/- and MTAP+/+) were used to determine the effects of PH020-803 on cell proliferation and intracellular symmetric dimethylarginine (SDMA) content. Human CD34+ hematopoietic stem cells (HSCs) were collected to evaluate hematological toxicity. Intravenous injection of PH020-803 in rats were performed to assess the brain-penetration capability. The in vivo efficacy was tested in cell derived xenograft (CDX) mouse models with tumor harboring MTAP gene deletion. The in vitro and in vivo pharmacokinetic and safety properties were assessed with corresponding assay methods.
Results: PH020-803 dramatically decreased SDMA content and inhibited proliferation in HCT116 MTAP-/- cells (SDMA, IC50=1.04 nM; proliferation, IC50=19 nM), but had very weak effect on HCT116 MTAP+/+ cells (SDMA, IC50=535.5 nM; proliferation, IC50=1620 nM). In contrast, both GSK3326595 and JNJ64619178 had no obvious selectivity (proliferation: 595, IC50=8 nM and 35 nM in MTAP-/- cells and MTAP+/+ cells, respectively; 178, IC50=1 nM and 3 nM, respectively). PH020-803 mildly inhibited CD34+ HSC (IC50=900.2 nM), compared to potent cytotoxicity of GSK3326595 (IC50=15.8 nM). PH020-803 showed excellent brain penetration with a Kp value of 0.16. In two CDX models (HCT116 MTAP-/- and LU99), PH020-803 at a dose of 100 mg/kg QD potently inhibited tumor growth without hematologic toxicity observed. A 10-day repeated dosing study in mice (300 mg/kg QD) suggested that red blood cell and reticulocyte counts significantly decreased in GSK3326595 group, but not in PH020-803 group. Caco-2 and MDCK-MDR1 assays demonstrated excellent membrane permeability of PH020-803 without obvious efflux. Accordingly, PH020-803 possessed good pharmacokinetic profile in mice and rats with absolute bioavailability at 74% and 58%, respectively.
Conclusion: The present data suggest that PH020-803 is an MTA-cooperative and brain-penetrable PRMT5 inhibitor that selectively targets MTAP-deleted tumors.
Citation Format: Feng Gao, Bin Liu, Liandong Jing, Yongyong Wu, Pengzhi Zhang, Shuai Yuan, Hui Zhao, Yu Gao, Zhizhong Li, Xiaofan Wang, Yongqi Guo. PH020-803: an MTA-cooperative and brain-penetrable PRMT5 inhibitor that selectively targets MTAP-deleted 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 1145.