Tucatinib is primarily used in the treatment of certain types of breast cancer, specifically HER2-positive breast cancer. It is often prescribed for patients who have already undergone other treatments but have seen their cancer progress. Tucatinib works by inhibiting the HER2 protein, which is overexpressed in these cancer cells, thereby slowing down or stopping the growth of the tumor. It is commonly used in combination with other medications like trastuzumab and capecitabine to enhance its effectiveness. This targeted therapy is particularly beneficial for patients with metastatic breast cancer, offering a more personalized approach to treatment.