Sequencing SG vs. T-DXd in HER2-Low/TROP2-High Metastatic Breast Cancer

This investigator-initiated study aims to evaluate the efficacy of sequential treatment with sacituzumab govitecan (SG) and trastuzumab deruxtecan (T-DXd) in patients with HER2 IHC 1+ /Trop-2-high metastatic breast cancer. It will also explore the correlation between changes in tumor-associated protein expression levels and ADC efficacy, and investigate mechanisms of ADC resistance. The study will establish a multicenter cohort of patients treated with SG/T-DXd: Cohort A (Triple-Negative Breast Cancer, TNBC): Includes patients with unresectable locally advanced or metastatic triple-negative breast cancer (HER2 IHC 1+) who have received ≥1 prior line of systemic therapy for metastatic disease. Prior to enrollment, all patients will undergo Trop-2 expression testing. Patients identified as Trop-2-high will be randomized 1:1 to either:SG Treatment (Trop-2 Group), orT-DXd Treatment (HER2 Group). The initially administered ADC is designated as ADC1. Upon disease progression, patients may cross over to receive the other ADC as sequential therapy (designated as ADC2). One intervening non-ADC therapy (e.g., targeted therapy, chemotherapy) is permitted prior to cross-over. Randomization will maintain a 1:1 allocation between the Trop-2 and HER2 groups. Cohort B (HR+/HER2-ultralow or HER2 IHC 1+ Breast Cancer): Includes patients with unresectable locally advanced or metastatic HR+/HER2-ultralow or HER2 IHC 1+ breast cancer who have progressed on prior endocrine therapy. Prior to enrollment, all patients will undergo Trop-2 expression testing. Patients identified as Trop-2-high will be randomized 1:1 to either:SG Treatment (Trop-2 Group), orT-DXd Treatment (HER2 Group). Upon progression, patients may cross over to receive the other ADC as sequential therapy. One intervening non-ADC therapy (e.g., targeted therapy, chemotherapy) is permitted prior to cross-over. Randomization will maintain a 1:1 allocation between the Trop-2 and HER2 groups.