Tucatinib Continuation Study

The study is seeking participants who * continue to benefit without disease progression from tucatinib as judged by the treating physician. * are willing to follow protocol requirements. All participants in this study will receive tucatinib. Tucatinib will be taken by mouth 2 times a day. We will examine the experiences of people receiving the study medicine. This will help us assess if tucatinib is safe. Participants will stay in the study for up to 5 years or until tucatinib is permanently discontinued, whichever comes first. During this time, they will have one clinical visit every three weeks to receive the study treatment. They will also talk with the treating physician about their experiences.