Trial of Trastuzumab Deruxtecan in Previously Treated HER2

This study is testing trastuzumab deruxtecan (T-DXd), an antibody-drug conjugate that delivers chemotherapy directly to cancer cells, in people with metastatic or unresectable breast cancer that has never tested positive for the HER2 protein in any prior testing — called HER2-zero breast cancer. **You may be eligible if...** - You are 18 or older - You have metastatic or unresectable breast cancer - All prior tumor biopsies have consistently shown HER2-zero status (no detectable HER2 protein at all) - You have never had a biopsy show HER2-positive or HER2-low results - You have hormone receptor-positive (HR+) breast cancer that has progressed after hormone therapy, or hormone receptor-negative (HR-) breast cancer **You may NOT be eligible if...** - Any of your previous biopsies showed any HER2 expression (even low levels) - Your cancer is still responding to hormone therapy - You have brain metastases that are unstable - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.