Inclusion Criteria:

          1. Age ≥18 years.

          2. ECOG performance status of 0-2.

          3. HER2+ (as defined by ASCO/CAP clinical practice guideline) metastatic breast cancer.

          4. Untreated or previously treated and progressing CNS disease.

          5. Measurable CNS metastases.

          6. Must be able to undergo MRI of the brain.

          7. Adequate organ function.

        Exclusion Criteria:

          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.