ABemacicliB or Abemaciclib and HydroxYchloroquine to Target Minimal Residual Disease in Breast Cancer

This study is testing whether abemaciclib (a targeted cancer drug that blocks cell division) with or without hydroxychloroquine can eliminate residual cancer cells that may persist in the bone marrow after initial breast cancer treatment — potentially preventing recurrence. **You may be eligible if...** - You have been diagnosed with invasive breast cancer within the past 5 years - Your cancer had high-risk features such as positive lymph nodes, large tumor size, HER2-positive status, or triple-negative status - Sensitive bone marrow testing has detected small numbers of remaining cancer cells (disseminated tumor cells) **You may NOT be eligible if...** - Your breast cancer has already spread to distant organs (stage IV) - You have significant heart rhythm problems (QT prolongation) - You have retinal eye disease that would be worsened by hydroxychloroquine Talk to your doctor to see if this trial is right for you.