This clinical trial is focused on studying a type of breast cancer known as unresectable and/or metastatic HER2-low or HER2 IHC 0 breast cancer. This means the cancer cannot be removed by surgery and has spread to other parts of the body. The study is testing a treatment called trastuzumab deruxtecan, also known by its code name DS-8201a. This treatment is a special kind of medicine called an anti-HER2-antibody drug conjugate, which is designed to target and attack cancer cells that have low levels of a protein called HER2.

The purpose of the study is to evaluate how effective this treatment is for patients with this specific type of breast cancer. Participants in the study will receive the treatment as a solution that is given through a vein, known as an intravenous infusion. The study will follow participants over a period of time to see how long it takes before they need another cancer treatment or if the cancer progresses. Some participants may receive a placebo, which is a substance with no active medication, to compare the effects of the treatment.

Throughout the study, participants will have regular visits to monitor their health and the effects of the treatment. This will include various tests and assessments to ensure the treatment is working as intended and to check for any side effects. The study aims to provide valuable information on whether trastuzumab deruxtecan can be an effective treatment option for people with this type of breast cancer.