Biotheryx Inc, a biopharmaceutical company focused on the discovery and development of first-in-class protein degraders for cancer and inflammatory diseases, on Monday announced the completion of enrolment in the ongoing Phase 1a clinical trial of BTX-9341, a potent and selective CDK4/6 degrader, for the treatment of advanced and/or metastatic HR+/HER2- breast cancer in patients who have previously received CDK4/6 inhibitor therapy either in the adjuvant or metastatic setting.

Dose escalation of BTX-9341 as a monotherapy was followed by a combination with fulvestrant. The primary objective of the Phase 1a trial is to assess safety, tolerability, pharmacokinetic and pharmacodynamic activity of BTX-9341 as a monotherapy and in combination with fulvestrant.

Based on the recommended dose from Phase 1a, there will be a formal evaluation of efficacy in the dose expansion phase of the trial.

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 Cyclin E and CDK2 transcription, cell cycle arrest and ultimately superior in vivo efficacy in breast cancer xenografts. Additionally, 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- breast cancer, Biotheryx said.