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— ETX-19477 demonstrated robust efficacy, with a 57% objective response rate in BRCA-mutated platinum-resistant ovarian cancer and earlier-line HR+/HER2- breast cancer (n=7) —
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— Favorable safety profile, including low rates of hematologic toxicity, supports further monotherapy development and future combination strategies —
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— These data provide the first clinical proof-of-concept for PARG inhibition in ovarian and breast cancer, including durable RECIST responses in both tumor types —
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SAN DIEGO — 858 Therapeutics, a clinical-stage biotechnology company, today announced preliminary safety and efficacy data from its ongoing, first-in-human Phase 1/2 trial of PARG inhibitor ETX-19477. These data will be presented in a poster session at the 2026 American Society of Clinical Oncology (ASCO) Annual Meeting on Saturday, May 30.
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The ASCO abstract released today includes clinical data from 45 patients enrolled in the Phase 1a/b dose escalation and expansion portions of the study. The poster presentation will feature an expanded dataset of 53 patients, including seven patients who meet the eligibility criteria for the ongoing Phase 2 expansion cohorts in BRCA-mutated ovarian cancer and HR+/HER2- breast cancer. These criteria limit prior therapy to no more than five lines for ovarian cancer and no more than two lines for breast cancer. In this Phase 2-eligible subset, ETX-19477 demonstrated robust antitumor activity, with a 57% objective response rate (n=7). Durable RECIST responses were observed in both tumor types, providing the first clinical proof-of-concept for PARG inhibition in ovarian and breast cancer patients. ETX-19477 was generally well tolerated, with low rates of hematologic toxicity observed to date.
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“These data represent an important clinical milestone for ETX-19477 and for the emerging field of PARG inhibition,” said Jeffrey A. Stafford, CEO of 858 Therapeutics. “We are encouraged by the clinical efficacy and RECIST responses observed to date, in both ovarian and breast cancer patients. These treatment outcomes, together with the robust PK-PD relationship established during dose escalation, support the continued development of ETX-19477, including combination approaches that can meaningfully expand the patient populations treatable with ETX-19477.”
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858 Therapeutics is actively enrolling patients in Phase 2 monotherapy expansion cohorts, with a focus on BRCA-mutated ovarian and breast cancer. Additional details on the ETX-19477 clinical program can be found on clinicaltrials.gov ( NCT06395519).
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ASCO Presentation Details
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Title
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: First-in-human phase 1/2 study of ETX-19477, an oral, potent, and selective PARG inhibitor, in patients with advanced solid tumors (ERADIC8)
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Abstract Number
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: 3109
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Poster Session
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: Developmental Therapeutics—Molecularly Targeted Agents and Tumor Biology
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Date and Time
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: Saturday, May 30, 2026, 1:30 PM-4:30 PM CDT
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Presenter
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: Ezra Rosen, M.D., Ph.D., Memorial Sloan Kettering Cancer Center, New York, NY
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About ETX-19477
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Poly(ADP-ribose) glycohydrolase (PARG) is an enzyme that catalyzes the removal of poly-ADP-ribose (PAR) chains from proteins during the DNA damage response. PARG inhibition leads to selective cell death in tumors with underlying replication fork defects, including BRCA-mutated tumors, through a mechanism distinct from PARP inhibition. ETX-19477 is an oral, potent, and selective PARG inhibitor that is efficacious in preclinical mouse models of ovarian, breast, and gastric cancers. 858 Therapeutics is evaluating ETX-19477 in a Phase 1/2 study in patients with advanced solid tumors at multiple sites in the U.S. For more information on the Phase 1/2 study, please visit: https://clinicaltrials.gov/study/NCT06395519.
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About 858 Therapeutics
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858 Therapeutics is a biotechnology and drug discovery company developing a portfolio of small molecule therapeutics directed against novel oncology and immunology targets. Its lead programs focus on important nodes in cancer biology, including DNA damage repair, innate immunity, and RNA epigenetics. 858 is headquartered in the biotech hub of San Diego, CA. For more information, please visit www.8five8tx.com.
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Contacts
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Media Contacts
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For 858 Therapeutics:
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Sanjay Trehan
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English (US)