Optimal Application Timing of ADC Drugs for Advanced Breast Cancer

Inclusion Criteria: 1. Female patients aged 18-75 years; 2. Histologically confirmed advanced HER2-negative breast cancer, including IHC 2+/ISH-, IHC 1+/ISH-, and IHC 0/ISH- subtypes; 3. Completed first-line combination chemotherapy for advanced/metastatic disease (specific regimen not restricted), with disease progression (PD) evaluated per RECIST criteria (HR-positive patients must have received at least one line of endocrine therapy); 4. Electrocorticography (ECOG) performance status \< 2; 5. Estimated life expectancy ≥ 12 weeks; 6. Adequate bone marrow function, defined as: * ANC ≥ 1.5 × 10⁹/L * Platelets ≥ 90 × 10⁹/L * Hemoglobin ≥ 90 g/L 7. Adequate hepatic and renal function, defined as: * Total bilirubin ≤ 1.5 × upper limit of normal (ULN) * AST or ALT ≤ 2.5 × ULN (≤ 5 × ULN for patients with liver metastases) * Creatinine clearance ≥ 60 mL/min 8. Signed informed consent obtained prior to any study-related procedures or treatments, confirming the patient's willingness to participate and comply with study requirements. Exclusion Criteria: 1. Prior treatment with an ADC after disease recurrence or metastasis; 2. Pregnant or breastfeeding women; 3. No evaluable recurrent or metastatic lesions as defined by RECIST 1.1 criteria; 4. Symptomatic brain parenchymal and/or leptomeningeal metastases with symptoms not adequately controlled by treatment; 5. History of other malignancies within the past 5 years, except for adequately treated carcinoma in situ of the cervix, cutaneous squamous cell carcinoma, or well-controlled localized basal cell carcinoma of the skin; 6. Psychiatric disorders or other conditions that may interfere with patient compliance; 7. Recent history of serious and uncontrolled systemic diseases, such as clinically significant cardiovascular disease, pulmonary disease, metabolic disorders, or arterial/venous thromboembolic events; 8. Concurrent use of other investigational drugs, or participation in another clinical trial within 30 days prior to enrollment; 9. Known or suspected allergy to any study drug or its excipients; 10. Any other condition that, in the opinion of the investigator, renders the patient unsuitable for participation in this trial.