You will be shown a question, followed by excerpts from biomedical research papers. Please answer the question based on the provided context. Do not include any text in your response other than the answer.

Question: What is the mechanism of action of tucatinib?

Context: In this article we review the role of currently available or investigational HER2 tyrosine kinase inhibitors: lapatinib, neratinib, afatinib and tucatinib in the treatment of brain metastases in HER2-positive breast cancer patients.
BACKGROUND: Tucatinib is a potent and selective oral HER2 tyrosine kinase inhibitor, with the potential to provide a well tolerated new treatment option for patients whose disease has progressed on currently available therapies.
The utility of human epidermal growth factor 2 (HER2)-directed therapies-lapatinib, ado-trastuzumab emtansine (T-DM1), neratinib and tucatinib-is also being studied in this setting.
Tucatinib is an oral, potent, human epidermal growth factor receptor 2 (HER2)-specific tyrosine kinase inhibitor (TKI) being developed as a novel treatment for ERBB2/HER2-positive breast cancer.
BACKGROUND Tucatinib is a potent and selective oral HER2 tyrosine kinase inhibitor, with the potential to provide a well tolerated new treatment option for patients whose disease has progressed on currently available therapies.
Tucatinib is an oral, potent, human epidermal growth factor receptor 2 (HER2)-specific tyrosine kinase inhibitor (TKI) being developed as a novel treatment for ERBB2/HER2-positive breast cancer.
BACKGROUND Tucatinib is a potent and selective oral HER2 tyrosine kinase inhibitor, with the potential to provide a well tolerated new treatment option for patients whose disease has progressed on currently available therapies.
The utility of human epidermal growth factor 2 (HER2)-directed therapies-lapatinib, ado-trastuzumab emtansine (T-DM1), neratinib and tucatinib-is also being studied in this setting.
Tucatinib is an oral, potent, human epidermal growth factor receptor 2 (HER2)-specific tyrosine kinase inhibitor (TKI) being developed as a novel treatment for ERBB2/HER2-positive breast cancer.<br><b>Objective</b>: To determine the maximum tolerated dosage of tucatinib in combination with T-DM1 in the treatment of patients with ERBB2/HER2-positive metastatic breast cancer with and without brain metastases.<br><b>Design, Setting, and Participants</b>: In this phase 1b open-label, multicenter, clinical trial, 57 participants enrolled between January 22, 2014, and June 22, 2015, were 18 years of age or older with ERBB2/HER2-positive metastatic breast cancer previously treated with trastuzumab and a taxane.
Tucatinib is an oral, potent, human epidermal growth factor receptor 2 (HER2)-specific tyrosine kinase inhibitor (TKI) being developed as a novel treatment for ERBB2/HER2-positive breast cancer.