Tucatinib is an oral, potent, selective and ATP-competitive small-molecule inhibitor of Receptor tyrosine-protein kinase ErbB2 (also called HER2 or human epidermal growth factor receptor 2) with an IC₅₀ of 14 nM against ErbB2 enzyme. It inhibits the phosphorylation of ErbB2 in BT474 cells with an IC₅₀ of 21 nM. It shows excellent anti-tumor activity in numerous mouse tumor models including breast (BT-474, MDA-MB-453), ovarian (SKOV-3) and gastric (N87) carcinoma models. In the BT-474 model, it showed significant dose-related tumor growth inhibition (TGI, 50% at 50 mg/kg/d and 96% at 100 mg/kg/d). It is approved to be used in combination chemotherapy for advanced HER2-positive breast cancer.