Biotheryx has announced that the first patient has been dosed in the dose expansion phase of its clinical trial evaluating BTX-9341, a novel CDK4/6 degrader, in combination with fulvestrant for the treatment of advanced Breast Cancer.

The study targets patients with HR-positive, HER2-negative breast cancer who have previously received CDK4/6 inhibitor therapy in advanced or metastatic settings. The dose expansion phase is a randomised trial designed to assess the efficacy and safety of the combination therapy.

The trial is being conducted across multiple sites in the United States and is expected to enroll approximately 80 patients across two treatment arms. The study builds on results from the earlier dose escalation phase, which evaluated safety, tolerability, pharmacokinetics and pharmacodynamic activity of BTX-9341.

The primary endpoint of the trial is Overall Response Rate (ORR), while key secondary endpoints include Clinical Benefit Rate (CBR) and Progression-Free Survival (PFS), as assessed by investigators.

Commenting on the development, Leah Fung, Chief Executive Officer, Biotheryx, said the transition to the dose expansion phase is supported by encouraging early-stage data. She noted that the company aims to further evaluate the therapy’s potential to improve outcomes for patients who have developed resistance to existing CDK4/6 inhibitors.

BTX-9341 is a first-in-class, oral protein degrader targeting CDK4 and CDK6, proteins that play a key role in cancer cell proliferation. In preclinical studies, the therapy demonstrated strong anti-tumour activity by selectively degrading CDK4/6, inhibiting CDK2 and Cyclin E transcription and inducing cell cycle arrest.

Unlike conventional CDK4/6 inhibitors, BTX-9341 is designed to overcome resistance mechanisms that often limit treatment effectiveness in second-line metastatic breast cancer, positioning it as a promising next-generation therapeutic option.