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

Inclusion Criteria: Patients who meet all of the following criteria are eligible for inclusion in this study: * Male or female, age ≥18 years. * ECOG performance status ≤2, with an estimated life expectancy of \>3 months. * Histologically confirmed unresectable or metastatic breast cancer that is:HER2 IHC 1+ triple-negative breast cancer, or HR-positive/HER2-ultralow or HER2 IHC 1+ breast cancer. * Planned to receive monotherapy with Sacituzumab Govitecan and Trastuzumab Deruxtecan. * For triple-negative breast cancer: prior systemic therapy lines ≥1. * For HR-positive/HER2-negative metastatic breast cancer: prior endocrine therapy is required. Prior chemotherapy, immunotherapy, or targeted therapy is allowed; 0-2 lines of chemotherapy in the advanced/metastatic setting are permitted. * Prior treatment-related adverse events have recovered to ≤Grade 1 per NCI CTCAE v5.0 (except for alopecia, non-clinically significant or asymptomatic laboratory abnormalities). * Presence of at least one measurable or evaluable lesion. * Willingness to provide archival or fresh tumor tissue samples for multi-omics analysis, including: * Baseline prior to ADC1 treatment, * After 2 cycles of ADC1 therapy, * At progression on ADC1 therapy or baseline prior to ADC2 therapy, * At progression on ADC2 therapy. * FFPE tissue blocks are preferred over unstained slides. Patients must consent to tumor biopsy; if tissue is unavailable, biopsy is not feasible, or the patient declines biopsy, eligibility may be discussed and approved by the investigator. * No severe cardiac dysfunction, with left ventricular ejection fraction (LVEF) ≥50% within 28 days before treatment. * Adequate organ function within 7 days prior to treatment initiation: * Bone marrow: absolute neutrophil count (ANC) ≥1.5 × 10⁹/L; platelet count ≥100 × 10⁹/L; hemoglobin ≥90 g/L (no transfusion or blood component use within 14 days before first dose; no growth factor support within 7 days before first dose). * Liver: total bilirubin ≤1.5 × ULN (or \<3 × ULN if Gilbert's syndrome or documented liver metastases present); ALT and AST ≤2.5 × ULN (≤5 × ULN if liver metastases present). * Renal: serum creatinine ≤1.5 × ULN OR creatinine clearance ≥50 mL/min (calculated by Cockcroft-Gault formula). * Coagulation: INR ≤1.5 × ULN and aPTT ≤1.5 × ULN (unless therapeutically anticoagulated). * Voluntarily signed informed consent form and willingness to comply with the study protocol. * Willingness to provide complete clinical medical record data. Exclusion Criteria: Patients meeting any of the following criteria will be excluded from the study: * Contraindication or known hypersensitivity to sacituzumab govitecan, trastuzumab deruxtecan, or any of their components (including topoisomerase I inhibitors). * Prior treatment with sacituzumab govitecan, trastuzumab deruxtecan, or any other drug targeting the same molecular pathway. * History of another primary malignancy, except for: * Malignancy treated with curative intent with no evidence of active disease for ≥2 years prior to study intervention and with low risk of recurrence, or * Adequately treated carcinoma in situ of the cervix, stage I endometrioid carcinoma of the uterus, or non-melanoma skin cancer (e.g., basal cell carcinoma or squamous cell carcinoma). * Active infection or uncontrolled systemic disease, including but not limited to: * Active HIV, HBV, or HCV infection, * Active autoimmune disease, * Symptomatic pleural effusion, ascites, or pericardial effusion requiring drainage, * Severe or uncontrolled cardiac disease requiring treatment, * Poorly controlled diabetes or hypertension despite medical therapy. * Participation in another investigational drug trial within 4 weeks prior to the first study treatment (observational studies are permitted) or major surgery within 4 weeks prior to the first study treatment. * Prior anticancer therapy within specified timeframes: * Chemotherapy, radiotherapy, targeted therapy, or immunotherapy within 4 weeks before the first study treatment, * Anticancer endocrine therapy or traditional Chinese medicine with antitumor intent within 2 weeks before the first study treatment. * History of or current interstitial lung disease, severe chronic obstructive pulmonary disease, severe pulmonary insufficiency, symptomatic bronchospasm, or other clinically significant pulmonary disorders. * Pregnant or breastfeeding women, or women of childbearing potential with a positive pregnancy test at baseline; patients (male or female) who are unwilling to use effective contraception throughout the study period. * Known neurological or psychiatric disorders (e.g., epilepsy, dementia) or cognitive impairment that would compromise understanding of the study objectives, compliance, or ability to provide informed consent; or significant peripheral neuropathy. * Primary central nervous system (CNS) tumor or untreated/symptomatic CNS metastases. Patients with asymptomatic, stable CNS metastases not requiring steroids or specific CNS-directed therapy for at least 4 weeks prior to screening may be eligible. * Any other condition deemed unsuitable for participation by the investigator.