This clinical trial is trying to determine if the administration of a novel antibody-drug conjugate, that selectively delivers a highly potent chemotherapy to tumor cells, will result in meaningful clinical activity for treating breast cancer that has spread to the brain or to the leptomeninges. The trial enrolls patients that have HER2-negative breast cancer (the most common subtype), which includes both triple-negative tumors and hormone receptor positive tumors. It is a single-arm trial, meaning that all patients on the trial will receive the study drug, named daptopotamab deruxtecan (or Dato-Dxd) across the U.S. in the summer of 2024. | 
