FIH Study of RGT-419B Alone and With Endocrine Therapy in HR-Positive, HER2-Negative Advanced/Metastatic Breast Cancer

This first-in-human study tests a new drug called RGT-419B, alone or combined with hormone-blocking therapy, in women and men with hormone receptor-positive (HR+), HER2-negative advanced or metastatic breast cancer that has already progressed on standard CDK4/6 inhibitor treatment. **You may be eligible if...** - You are 18 or older with confirmed ER+, HER2-negative advanced or metastatic breast cancer - Your cancer progressed after treatment with a CDK4/6 inhibitor (such as palbociclib, ribociclib, or abemaciclib) combined with hormone therapy - You have measurable disease on scans - You have good overall health (ECOG 0–1) **You may NOT be eligible if...** - You have not yet tried a CDK4/6 inhibitor - Your cancer is HER2-positive or hormone receptor-negative - Your health or organ function is not adequate for study treatment Talk to your doctor to see if this trial is right for you.