Asciminib in HER2+ Breast Cancer Brain Metastases

Inclusion Criteria: * Age ≥18 years at the time of consent * Patients with HER2+ metastatic breast cancer with at least one progressive or new brain metastasis measuring \>5mm; prior local therapy to other intracranial lesions allowed * At least 1 prior standard of care therapy for metastatic disease * Must have previously received T-DXd and Tucatinib. If a patient has not received T-DXd or Tucatinib as part of standard early line therapy due to allergies or intolerability, the requirement of prior T-DXd and Tucatinib treatment is waived. * Participants must have adequate treatment washout period when applicable before enrollment, defined as: * \>4 weeks from any major surgery * \>1 week from any cranial radiation treatment * For cytotoxic containing agents, 5 half-lives or at least 21 days (whichever is shorter) * For weekly chemotherapy regimens, \>2 weeks from chemotherapy; for every 3 weekly regimens, \>3 weeks from chemotherapy. At least 2 weeks from other systemic or targeted or investigational therapies (other than endocrine therapy) for breast cancer. No washout is required for endocrine therapy (e.g. aromatase inhibitors, tamoxifen, fulvestrant) but patients should discontinue prior to start of protocol therapy. * Patients on ovarian suppression are allowed (but not required) to continue ovarian suppression at the discretion of their treating provider. * Adequate organ function and bone marrow reserve as determined by the investigator * Adequate hepatic a…