Biotheryx Announces First Patient Dosed in the Dose Expansion Phase of BTX-9341-101 for the Treatment of HR+/HER2- Breast Cancer

SAN DIEGO, March 18, 2026 /PRNewswire/ -- Biotheryx, Inc., a biopharmaceutical company focused on the discovery and development of first-in-class protein degraders for cancer and inflammatory diseases, today announced that the first patient has been dosed in the dose expansion phase of its clinical trial evaluating BTX-9341, a potent and selective CDK4/6 degrader, in combination with fulvestrant for the treatment of HR+/HER2- breast cancer in patients who have previously received CDK4/6 inhibitor therapy in the advanced and/or metastatic setting.

The dose expansion portion of the trial is a randomized study designed to evaluate the efficacy and safety of BTX-9341 in combination with fulvestrant. The trial is being conducted at multiple sites in the United States and is expected to enroll approximately 80 patients across two treatment arms based on the recommended dose identified in the earlier dose escalation phase. This dose expansion study builds on the recently completed dose escalation phase that evaluated the safety, tolerability, pharmacokinetics and pharmacodynamic activity of BTX-9341. The primary endpoint of this study is the Overall Response Rate (ORR), with key secondary endpoints including the measurement of investigator-assessed Clinical Benefit Rate (CBR) and Progression Free Survival (PFS).

"We are pleased to have dosed the first patient in the dose expansion portion of our clinical trial evaluating BTX-9341 in combination with fulvestrant." said Dr. Leah Fung, Chief Executive Officer of Biotheryx. "The encouraging safety, pharmacokinetics, and preliminary activity observed in the earlier dose escalation portion of the study support advancing BTX-9341 into this next stage of clinical evaluation. We believe the differentiated mechanism of targeted CDK4/6 degradation has the potential to address resistance seen with currently available CDK4/6 inhibitors, and we look forward to further assessing BTX-9341 in combination with fulvestrant to improve outcomes for patients with HR+/HER2- breast cancer who have previously received CDK4/6 inhibitor therapy."  

About BTX-9341

BTX-9341 is a first-in-class, oral degrader of CDK4/6, important targets for a range of cancers and clinically validated in HR+/HER2- breast cancer. In preclinical breast cancer models, BTX-9341 demonstrated superiority to CDK4/6 inhibitors through potent and highly selective catalytic degradation of CDK4 and CDK6, robust inhibition of CDK2 and Cyclin E transcription, cell cycle arrest, and ultimately superior in vivo efficacy in breast cancer xenografts. Beyond this increased efficacy potential, BTX-9341 is differentiated from CDK4/6 inhibitor approaches through the ability to overcome key resistance mechanisms that limit the impact of inhibitors in second line HR+/HER2- metastatic breast cancer.

About Biotheryx, Inc.

Biotheryx is a clinical-stage, biopharmaceutical company discovering and developing a portfolio of first-in-class protein degraders, including molecular glues and bifunctional degraders. Members of our founding and scientific teams previously developed the first U.S. Food and Drug Administration (FDA) approved modulators of Cereblon, the most widely validated E3 ligase involved in protein degradation. We have applied our expertise in Cereblon modulation to build our proprietary PRODEGY platform and pipeline of protein degraders for oncology and inflammatory diseases. Our approach deploys molecular glues to target undruggable proteins and bifunctional degraders to target validated proteins that conventional strategies, like protein inhibition, have insufficiently addressed. The Biotheryx pipeline includes BTX-9341, a first-in-class, oral, CDK4/6 bifunctional degrader that inhibits the transcription of CDK2 and Cyclin E, in first-in-human dose escalation and optimization clinical study in HR+/HER2- breast cancer patients who have progressed on CDK4/6 inhibitor therapy. Our pre-clinical pipeline advances undisclosed bifunctional degraders and molecular glues, including degraders as payloads for antibody drug conjugation. For more information, please visit www.biotheryx.com and engage with us on LinkedIn.

SOURCE Biotheryx, Inc.