Researchers are looking for other ways to treat breast cancer (BC) that is hormone receptor-positive and human epidermal growth factor receptor 2-negative (HR+/HER2-) and either unresectable locally advanced or metastatic.

* HR positive (HR+) means the cancer cells have proteins that attach to estrogen or progesterone (hormones) which help the cancer to grow and spread
* HER2 negative (HER2-) means the cancer cells have a low amount of a protein called HER2
* Unresectable locally advanced means the cancer cannot be completely removed by surgery and has spread into nearby tissue or muscles
* Metastatic means the cancer has spread to other parts of the body

Treatment for this type of breast cancer usually includes endocrine therapy (ET) and sometimes a second treatment. The main goal of this study is to learn if people who receive patritumab deruxtecan (also known as HER3-DXd and MK-1022) live longer overall or without the cancer growing/spreading, compared to people who receive chemotherapy or a different drug called trastuzumab deruxtecan.