Basket Study of Tucatinib and Trastuzumab in Solid Tumors With HER2 Alterations

This trial studies how well tucatinib works for solid tumors that make either more HER2 or a different type of HER2 than usual (HER2 alterations) The solid tumors studied in this trial have either spread to other parts of the body (metastatic) or cannot be removed completely with surgery (unresectable). All participants will get both tucatinib and trastuzumab. People with hormone-receptor positive breast cancer will also get a drug called fulvestrant. The trial will also look at what side effects happen. A side effect is anything a drug does besides treating cancer.