A Study of Tucatinib Given Before Surgery to People With HER2+ Cancers That Have Spread to the Brain

All patients will receive tucatinib per-protocol at standard dose of 300 mg orally twice daily on days - 4, -3, -2, -1 and day 0 (in AM). The post-surgery treatment (systemic and/or local) will be decided according to treating physician discretion and is not a study intervention. Tissue samples of brain metastases along with blood/plasma and CSF samples will be analyzed to evaluate brain tumor penetration of tucatinib as well as biologic response to tucatinib in patients with brain metastases from HER2+/mutant breast cancer who are undergoing clinically indicated brain surgery. Patients may continue tucatinib post-operatively at the discretion of the treating oncologist with monitoring as per clinical routine; this is not a study intervention for Cohort A and Cohort B. Patients on Cohort C may receive tucatinib post-operatively through the study according to physician descretion.