Which drug first? new study aims to improve breast cancer treatment order

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

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.