This clinical trial is focused on studying a type of breast cancer known as HER2-positive breast cancer, which is a form of cancer that has high levels of a protein called HER2. The study is investigating the effectiveness of a treatment combination that includes the medications Tucatinib, Trastuzumab, and Pertuzumab. These medications are used as maintenance therapy, which means they are given to help keep the cancer from coming back or getting worse after initial treatment. The trial will compare the effects of this combination with a placebo, which is a substance with no active medication.

The purpose of the study is to see how well the treatment works in preventing the cancer from progressing, which means getting worse or spreading. Participants in the study will receive either the combination of medications or a placebo, and their health will be monitored over time. The study will involve regular check-ups and assessments to track the progress of the disease and any side effects of the treatment. The trial aims to provide more information on how effective this treatment is in managing advanced or metastatic HER2-positive breast cancer.

Throughout the study, participants will undergo various tests and evaluations, including imaging tests like MRI to check for any changes in the brain, as some patients may have brain metastases, which are cancer cells that have spread to the brain. The study will also look at the overall survival of participants, which is the length of time they live after starting the study, and their quality of life during the treatment. The trial is expected to continue for several years to gather enough data to determine the effectiveness of the treatment.