Summary
This trial studies how well the drug tucatinib works when given with trastuzumab
deruxtecan (T-DXd). It will also look at what side effects happen when these drugs are
given together. A side effect is anything a drug does besides treating cancer.
Participants in this trial have HER2-positive (HER2+) breast cancer that has either
spread to other parts of the body (metastatic) or cannot be removed completely with
surgery (unresectable). All participants will get both tucatinib and T-DXd.