Accent Therapeutics Announces Trial in Progress Poster for First-in-Human Study of ATX-559 at 2025 ASCO Annual Meeting

ATX-559, a first-in-class oral DHX9 inhibitor, is being evaluated in a Phase 1/2 study with a focus
on advanced or metastatic patients with BRCA-1 and/or BRCA-2-deficient breast cancer or
MSI-H and/or dMMR solid tumors

LEXINGTON, Mass., May 22, 2025 /PRNewswire/ -- Accent Therapeutics, a clinical-stage biopharmaceutical company pioneering novel, targeted, small molecule cancer therapeutics, today announced that a Trial in Progress poster for its first-in-human, Phase 1/2 study of its first-in-class oral DHX9 inhibitor, ATX-559, will be presented at the 2025 American Society of Clinical Oncology Annual Meeting, held May 30-June 3 in Chicago, Illinois.

"As a novel inhibitor of DHX9, a previously undrugged RNA and DNA/RNA helicase, ATX-559 offers potential as a unique therapeutic approach for addressing cancers characterized by high replication stress," said Jason Sager, M.D., Chief Medical Officer of Accent Therapeutics. "We are encouraged by the robust suite of data that continue to support the clinical evaluation of ATX-559 in our ongoing Phase 1/2 study."

Initiated in late 2024, the first-in-human, Phase 1/2, open-label, dose-escalation and expansion study is designed to evaluate multiple doses of ATX-559 in solid tumor patients, with expansion cohorts of patients with advanced or metastatic BRCA-1 and/or BRCA-2-deficient breast cancer and microsatellite instability-high (MSI-H) and/or deficient mismatch repair (dMMR) solid tumors (NCT06625515). Primary endpoints include identification of the recommended Phase 2 dose (RP2D) and assessment of the safety and tolerability of ATX-559. The study additionally aims to characterize the pharmacokinetics (PK), pharmacodynamics (PD), and preliminary anti-tumor activity (as assessed per RECIST v1.1 criteria) of orally administered ATX-559. The study is currently open and enrolling. Additional undisclosed solid tumor indications undergoing replicative stress and representing significant patient populations may be explored either in parallel or in subsequent studies.

Details for the presentation are as follows:

Presentation Title: Trial in Progress: First-in-Human Study of ATX-559, an Oral Inhibitor of DHX9, in Patients with Advanced or Metastatic Solid Tumors, and Molecularly Defined Cancers

• Abstract Number: TPS3181
• Session Type: Poster Session
• Session Title: Developmental Therapeutics—Molecularly Targeted Agents and Tumor Biology
• Session Date and Time: Monday, June 2, 1:30 pm – 4:30 pm CDT
• Location: Hall A – Posters and Exhibits | On Demand
• Presenter: Meredith Pelster, M.D., M.Sc.

The poster will be available on the Accent Therapeutics website following the meeting.

About ATX-559
ATX-559 is a first-in-class potent and selective inhibitor of DHX9, a novel and previously undrugged RNA and DNA/RNA helicase, shown to play a critical role in tumors with high levels of replication stress (including breast, ovarian, colorectal, endometrial, gastric, and others), representing large patient populations with significant unmet medical need. DHX9 has been reported to play important roles in replication, transcription, translation, RNA splicing, RNA processing, and maintenance of genomic stability, making it a compelling novel oncology target. In addition to exploiting key tumor vulnerabilities in DNA repair deficient backgrounds (e.g., BRCA) and hyper-mutated states (e.g., MSI-H/dMMR), Accent is exploring the sensitivity of other tumor types to DHX9 inhibition, and the potential to combine DHX9 inhibitors with other cancer treatments to maximize its full potential for helping patients. Accent retains full worldwide rights to ATX-559, currently being evaluated in a Phase 1/2 clinical trial (NCT06625515), and the DHX9 program.

About Accent Therapeutics
Accent Therapeutics is a clinical-stage biopharmaceutical company pioneering a new class of small molecule precision cancer therapies targeting critical intracellular dependencies that span multiple types of cancer. Building upon industry-leading expertise in RNA-modifying proteins (RMPs) and the systematic mapping of both the RMP space and adjacent high-value areas for drug discovery, the company employs a flexible model that allows for a diversity of approaches to developing potentially transformative biomarker-driven cancer medicines. Accent's therapies are designed for both novel and known, but suboptimally addressed, high-impact oncology targets with the potential to benefit large patient populations with significant unmet need. The company's two lead programs—ATX-559, targeting DHX9, and ATX-295, targeting KIF18A—are currently being evaluated in Phase 1/2 clinical studies. For more information on Accent's mission to translate extraordinary science into life-changing therapeutics for patients living with cancer, visit www.accenttx.com or follow us on LinkedIn.

Media Contact
Amanda Sellers, Deerfield Group
amanda.sellers@deerfieldgroup.com

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SOURCE Accent Therapeutics