A Study of U3-1402 (Patritumab Deruxtecan) in Subjects With Metastatic Breast Cancer

U3-1402 (Patritumab Deruxtecan) is an antibody drug conjugate comprising a recombinant fully human anti HER3 monoclonal antibody linked to a linker containing topoisomerase I inhibitor. This is a phase II study of U3-1402 (patritumab deruxtecan) in subjects with MBC. The study will be conducted in 3 parts (Part A , Part B, and Part Z). All enrolled subjects in Part A will undergo pretreatment biopsies to determine if subjects with particular biomarker expression (ER/PR/HER2/HER3) show preliminary efficacy. Part B will enroll subgroups of participants that are metastatic, hormone receptor-positive (HR+) HER2-negative or triple-negative (mTNBC) regardless of HER3 expression that were defined from Part A analysis. Part Z will enroll participants with HER2- positive (HER2+) MBC.