Fulvestrant+Abemaciclib With Run-In of Fulvestrant in Er-Positive, Her2-Negative Metastatic Breast Cancer

This trial tests whether giving fulvestrant (a hormone-blocking drug) as a lead-in treatment before adding abemaciclib (a CDK4/6 inhibitor) can improve outcomes in postmenopausal women with estrogen receptor-positive (ER+), HER2-negative metastatic breast cancer that has already progressed on another CDK4/6 inhibitor plus aromatase inhibitor. **You may be eligible if...** - You are over 18 with confirmed ER+, HER2-negative metastatic breast cancer - Your cancer progressed while on a CDK4/6 inhibitor combined with an aromatase inhibitor as your most recent treatment - You have measurable disease on scans **You may NOT be eligible if...** - You have not previously been treated with a CDK4/6 inhibitor - Your cancer is HER2-positive - You have significant other health issues preventing participation Talk to your doctor to see if this trial is right for you.