Disitamab vedotin is an investigational cancer medication being studied for the treatment of advanced breast cancer. It is designed to target and attach to a protein called HER2 that is found on the surface of some cancer cells. Once attached, the medication delivers a powerful drug directly into the cancer cells to help destroy them. This medication is being tested in patients whose cancer shows different levels of HER2 protein expression and who may have received other treatments in the past.

Trastuzumab deruxtecan (also known as Enhertu) is an approved cancer medication that is mentioned in this trial as a treatment that some patients may have received before joining this study. Like disitamab vedotin, it also targets the HER2 protein on cancer cells and delivers medication directly to them. This drug is used as a comparison point to understand how patients respond to the new medication being tested.

HER2 Positive Advanced Breast Cancer – This is a type of breast cancer that has spread beyond the original tumor site to other parts of the body and tests positive for a protein called HER2. HER2 is a growth-promoting protein found on the outside of all breast cells. In this form of cancer, the cancer cells have higher than normal levels of HER2, which causes the cells to grow and divide more quickly. The disease progresses as cancer cells continue to multiply and spread to distant organs and tissues. This condition represents an advanced stage where the cancer is no longer confined to the breast or nearby lymph nodes.

Hormone Receptor Positive HER2-Low Advanced Breast Cancer – This is an advanced form of breast cancer where the tumor cells have receptors for hormones such as estrogen or progesterone and show low levels of HER2 protein expression. The presence of hormone receptors means the cancer cells can use hormones to fuel their growth. The HER2-low classification indicates that while some HER2 protein is present, it is not at high enough levels to be considered HER2 positive. As the disease progresses, cancer cells spread beyond the breast to other parts of the body. The combination of hormone receptor positivity and low HER2 expression defines the specific biological behavior of this cancer type.

Hormone Receptor Positive HER2-Ultralow Advanced Breast Cancer – This is an advanced breast cancer subtype where tumor cells contain hormone receptors but have extremely minimal levels of HER2 protein expression, even lower than HER2-low cancers. The cancer cells can respond to hormones like estrogen or progesterone, which promote their growth and division. The ultralow HER2 expression represents a very specific biological characteristic that distinguishes this cancer from other breast cancer types. As with other advanced breast cancers, the disease has spread beyond its original location to distant sites in the body. This classification helps identify patients based on the very limited presence of HER2 protein on cancer cells.

HER2-Low Triple-Negative Breast Cancer – This is an advanced form of breast cancer that lacks receptors for estrogen and progesterone and does not overexpress HER2 protein, but shows low levels of HER2 expression. Triple-negative refers to the absence of the three most common receptors typically found in breast cancer cells. Despite being classified as triple-negative, these tumors have detectable low levels of HER2 protein. The disease progresses as cancer cells multiply and spread to other organs and tissues throughout the body. This subtype is particularly challenging because it does not respond to hormone therapies or standard HER2-targeted treatments.