This study is looking at how effective a new treatment called **DB-1303/BNT323** is compared to regular chemotherapy for a type of breast cancer known as *HER2-low, hormone receptor positive metastatic breast cancer*. HER2-low means the cancer cells have lower levels of a protein called HER2, which can affect how the cancer grows. Metastatic means the cancer has spread to other parts of the body. The study involves people who have already tried at least two different kinds of treatments. The study will compare **DB-1303** to usual chemotherapy drugs like *capecitabine*, *paclitaxel*, or *nab-paclitaxel*. These drugs are given to help stop the cancer from growing. People will be randomly chosen to get either the new treatment or one of the usual drugs. The goal is to see if **DB-1303** helps people live longer without the cancer getting worse. - **Key Points to Consider:** - The study is conducted at about 230 centers globally. - Participants will continue treatment until the cancer progresses, or side effects are too much. - No previous chemotherapy for advanced breast cancer is allowed. This study is for adults 18 and older and requires specific eligibility criteria to be met.