1. Women age ≥18 years.
2. ECOG performance status of 0-2.
3. HER2+ metastatic breast cancer as defined byASCO/CAP clinical practice guideline.
4. New untreated CNS disease or CNS disease progression.
5. Measurable CNS metastases.
6. Must be able to undergo MRI of the brain.
7. Adequate organ function.
1. Any indication for immediate CNS-directed therapy.
2. History of generalized or complex partial seizures.
3. Any other manifestation of neurologic progression that in the opinion of the treating
physician is due to brain metastases.
4. Leptomeningeal disease.
5. Prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent or with an agent
directed to another stimulatory or co-inhibitory T-cell receptor.
6. Prior therapy with tucatinib.
7. Active autoimmune disease that has required systemic treatment in excess of prednisone
10mg daily or equivalent in the past 2 years.
Complete inclusion/exclusion criteria are detailed in the protocol.