Abstract 2083: Discovery of ETX-19477, a novel and selective PARG inhibitor with high potency against tumors with underlying replication stress

Abstract DNA damage response (DDR) is an important cellular process that is regulated by poly ADP ribosylation (PARylation). PARP1 binds to DNA break sites and then catalyzes PARylation, which leads to recruitment of repair factors and initiation of base-excision and single-strand break (SSB) repair. Removal of PAR chains is performed by PARG and is also critical for successful completion of DNA repair. Suppression of PAR chain hydrolysis via PARG depletion or inhibition leads to defective single-strand break repair, reduced kinetics of repair, and hypersensitivity to certain DNA damaging agents. Cancer cells demonstrate high levels of replication stress, characterized as the slowing or stalling of replication forks during DNA replication. Replication stress generates DNA SSBs and exposes unligated Okazaki fragments which activate PARP1. PARP1 catalyzed PARylation slows replication fork progression as a protective mechanism, initiates damage repair, and promotes fork reversal. PAR chain removal, via PARG activity, enables completion of damage repair, replication fork restart, and recovery from replication stress. Under conditions of replication stress, PARP inhibition can result in unrestrained fork progression and the occurrence of DNA double strand breaks from fork runoff. In contrast, PARG inhibition causes prolonged slowing or stalling of replication fork progression, which can lead to replication catastrophe and cancer cell death. Because of these opposing roles, PARG inhibitors show distinct pharmacology as compared to PARP inhibitors in certain cancer cells including cancer cells that are BRCA wild type, homologous recombination proficient, and PARP inhibitor insensitive or resistant. ETX-19477 is a novel inhibitor of PARG that demonstrates low nM, on-target activity in cells as measured by the accumulation of PAR chains. ETX-19477 induces death and/or inhibits the proliferation of multiple cancer cell types including ER+HER2- breast, serous and mucinous ovarian, lung, and gastric cancers and induces pan-nuclear γH2AX expression, a hallmark of replication catastrophe. Preclinical studies demonstrate that ETX-19477 is orally bioavailable with favorable pharmacokinetics. Oral treatment of mice with ETX-19477 is well tolerated and shows robust dose-dependent in vivo anti-tumor activity in breast and ovarian cell-line derived and patient-derived xenograft models. Efficacy is observed with either once or twice daily dosing and with a discontinuous dosing regimen. In mouse xenograft models, induction of γH2AX expression in tumor tissue correlates with tumor growth inhibition. Preclinical data show ETX-19477 demonstrates potent and specific anti-proliferative effects by exposing underlying existing replication stress vulnerabilities in cancer cells across multiple tumor types. ETX-19477 will be advanced into clinical trials in 2024. Citation Format: John P. Holleran, Tamara S. Rodems, Shweta Sharma, Angelina M. Santini, Cindy Wuerz, Juping Liu, Lu Tian, Joseph Dosch, Joy Nader, Richard Rivera, Shyama Herath, Joseph Marakovits, Christopher McBride, Ryan Stansfield, Amy Salyards, James M. Veal, Jeffrey A. Stafford, Gretchen Bain. Discovery of ETX-19477, a novel and selective PARG inhibitor with high potency against tumors with underlying replication stress [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2083.