Adaptive Therapy With Capecitabine for Treatment of Metastatic ER Positive, HER2 Negative Breast Cancer

This study is testing an "adaptive therapy" approach to using capecitabine (an oral chemotherapy pill) for metastatic breast cancer that is estrogen receptor-positive and HER2-negative. Instead of giving the maximum dose continuously, adaptive therapy adjusts the dose based on how the cancer is responding — the goal is to keep cancer cells in check longer rather than trying to eliminate them all at once. **You may be eligible if...** - You are 18 or older - You have confirmed ER-positive, HER2-negative metastatic breast cancer - You have not yet received chemotherapy or antibody-drug conjugate therapy for your metastatic disease - Your cancer is measurable on scans - Your blood counts, liver, and kidney function meet the required thresholds **You may NOT be eligible if...** - You have already had chemotherapy or antibody-drug conjugate therapy for metastatic breast cancer - You are pregnant, breastfeeding, or unwilling to use contraception - You have uncontrolled brain metastases or spinal cord compression - You have had a recent heart attack or uncontrolled heart arrhythmia - You have a known enzyme deficiency (DPYD) that makes capecitabine dangerous - You have had a severe allergic reaction to fluorouracil or capecitabine Talk to your doctor to see if this trial is right for you.