In this study, people are followed while they receive their regular cancer treatment with ADC medicines that their doctor chooses. The study does not give the medicines. It only watches how people do during their treatment. People can join before starting their first ADC or before starting their second ADC. The study follows them through each ADC treatment and then continues to check on them for two years after the second ADC. During treatment, people will answer questions about how they feel, the medicines they take, and any side effects. They will also fill out questionnaires about symptoms every other cycle. They will give small blood samples at certain times. If a biopsy is already being done as part of their normal care, extra tissue may be collected for research, but this is optional.

This study focuses on patients with hormone receptor-positive / HER2 negative (HR+/HER2-) metastatic breast cancer or triple negative breast cancer (TNBC).

People can join this study if they are getting ready to start an antibody drug conjugate medicine or are already receiving one as part of their regular cancer care. Their medical history and information about their cancer must be available for the study team to look at. They must also provide basic information such as their age, race, and ethnicity. They need to be able to report how they feel each day and any side effects they experience. They must also be able to give the study team access to tissue samples from past biopsies.

This study wants to learn how antibody drug conjugates, also called ADCs, work when they are given one after another to treat metastatic breast cancer. The study team will watch how patients do during their regular care and collect blood samples and leftover tissue to understand how safe and helpful these medicines are. The goal is to gather information that may help improve treatment for people with metastatic breast cancer in the future.