Holleran, John P.

3109 Background: Poly(ADP-ribose) glycohydrolase (PARG) is an enzyme that catalyzes the removal of poly-ADP-ribose (PAR) chains from proteins during DNA damage repair. PARG inhibition leads to selective cell death in tumors with underlying replication fork defects, including BRCAm tumors, through a mechanism distinct from PARP inhibition. ETX-19477 is an oral, potent, and selective PARG inhibitor that shows robust preclinical activity in mouse models of ovarian, breast, and gastric cancers. Methods: ERADIC8 is an open-label, multicenter, Phase 1/2 study, consisting of two parts, dose escalation and dose expansion. A Bayesian Optimal Interval Design was used to enroll patients (pts) into dose escalation cohorts, with enrichment for pts with BRCAm high grade serous ovarian (HGSOC) and breast cancers. The primary objective is to assess safety/tolerability of ETX-19477 and define the maximum tolerated dose (MTD) and recommended Phase 2 dose(s). Secondary and exploratory objectives include pharmacokinetics, pharmacodynamics, and preliminary anti-tumor activity per RECIST v1.1. ETX-19477 is given orally once or twice daily in 21-day cycles. Results: As of Jan 6 2026, 45 pts were enrolled into the study across 11 dose levels (range: 80-750 mg QD, 190-350 mg BID). Tumor types included ovarian (n=24, BRCAm: 18), breast (n=8, BRCAm: 7), endometrial (n=6, BRCAm: 0), colorectal (CRC, n=3, BRCAm: 1), and other (n=4, BRCAm: 0) cancers. Treatment-related AEs (TRAEs; any grade) occurring in >20% of pts were nausea (53%), vomiting (38%), and fatigue (24%). The only Gr3 TRAE occurring in >5% of pts was neutropenia (16%). There was one Gr4 TRAE of neutropenia in 1 pt (2%) and no Gr5 TRAEs. ETX-19477 exposure is dose proportional, and the BID regimen achieves >85% PARG inhibition throughout the dosing interval in a blood-based PAR accumulation pharmacodynamic assay. As of the data cutoff, dose escalation is ongoing at 350 mg BID (MTD has not been defined) and dose expansion is ongoing at 200-300 mg BID. Among BRCAm, HGSOC pts enrolled at BID dose levels, there were 7 evaluable pts with a median of 4 prior lines of therapy (range 2–11): 2 pts achieved a partial response (PR, 29%) and 3 pts achieved stable disease (SD, 43%). Both pts with PRs were platinum-resistant and received prior PARP inhibitor treatment. 1 pt with BRCAm CRC remains on active therapy (250 mg BID) with SD for 7 months. Conclusions: ETX-19477 was well tolerated and showed anti-tumor activity in heavily pretreated pts with BRCAm, platinum-resistant HGSOC during dose escalation. These findings provide the first clinical proof-of-concept of PARG inhibition in HGSOC. ETX-19477 is being further evaluated in the dose expansion phase of the study. Clinical trial information: NCT06395519 .

The Secretin receptor (SCTR) has been cloned as the first member of class B G protein‐coupled receptors (GPCRs) , giving rise to this highly important group of therapeutic targets, which are crucial for hormonal homeostasis . Despite the wide recognition of their functional importance, the development of potent small molecule ligands for Class B GPCRs remained challenging . To date, no ligands other than secretin peptide and its close analogs are known to interact with SCTRs, despite its therapeutic relevance in multiple diseases, such as heart failure, obesity, diabetes and cancer . In order to enhance our chances for discovering orally bioavailable drug candidates targeting an ‚undruggable’ class B GPCR, we focused our efforts on the identification of positive allosteric modulators (PAMs), which are likely to interact via structurally distinct binding sites and potentiate effects of native hormones and neurotransmitters . We developed a comprehensive tool box with robust high‐throughput screening and lead characterization/optimization assays capable of identifying and characterizing biologically relevant SCTR small molecule modulators. The dominant physiological coupling of SCTRs occurs via Gs proteins. Hence, we utilized this signaling mechanism to establish diverse cAMP accumulation assays suitable for simultaneous detection of PAMs modulating secretin and its potential metabolites. We also established a TR‐FRET based binding assay to further characterize potential PAMs with respect to their kinetic properties. Since functional selectivity has been proven to be therapeutically relevant for this family of membrane proteins, we additionally designed assays to investigate signaling mechanisms beyond Gs protein coupling, such as β‐arrestin recruitment, receptor internalization and calcium flux. High‐throughput screening (HTS) of a 360,000 compound library provided promising sets of hits, which have been further investigated regarding their allosteric activity and mechanism of action. We identified three related scaffolds with significant PAM activity at SCTRs. Selectivity studies revealed that this specific set of compounds demonstrated positive modulation not only on SCTRs but also on GLP‐1Rs (glucagon‐like peptide 1 receptors). Interestingly, dual activation of SCTRs and GLP‐1Rs has been shown to be beneficial for appetite regulation and glucose homeostasis without having proliferative effects on exocrine or total pancreas mass . To conclude, HTS and the development of a diverse panel of binding and functional assays has led to the discovery of small molecules with a unique dual‐acting pharmacological profile and potential to serve as starting points for developing first‐in‐class drug candidates for the treatment of comorbid conditions obesity and diabetes. Support or Funding Information Funding: NHLBI award 5R01HL133501