Tucatinib an orally bioavailable, small molecule tyrosine kinase inhibitor (TKI) that is highly selective for HER2, a growth factor receptor that is over-expressed in multiple cancers, including breast, colorectal, esophageal, gastric, lung and ovarian cancers. It is over-expressed in approximately 20% of breast cancers. It has shown efficacy and safety in nonclinical and clinical studies. Tucatinib may therefore be a beneficial therapeutic option for patients with solid malignancies including breast cancer that are associated with HER2 overexpression or mutation. As tucatinib in combination has shown early evidence of activity against brain metastases from HER2+ breast cancer, tucatinib also has the potential to benefit early-stage patients with occult micrometastatic disease in the brain.