Safety and Efficacy of T-DXd vs. CDK4/6i-based ET as First-line Therapy of HR-positive and HER2-low/Ultralow Advanced Breast Cancer Patients Classified as Non-luminal Subtype

This trial studies a type of advanced breast cancer defined as hormone receptor HR-positive/HER2-negative and classified as non-luminal by gene expression profiling (PAM50). Patients will be treated with trastuzumab deruxtecan (T-DXd) or with physician's choice of CDK4/6 inhibitor (CDK4/6i) plus endocrine therapy (ET). The main purpose of the study is to analyze the efficacy of T-DXd in patients who have HR-positive and HER2-low/ultralow advanced breast cancer classified as non-luminal subtype.