Low-Dose Apatinib Plus Chemotherapy in ADC-Pretreated HER2-Positive and HER2-Low Advanced Breast Cancer

This is a single-arm, open-label, phase 2 clinical trial using a stratified Simon minimax two-stage design. The study aims to evaluate the efficacy and safety of low-dose apatinib (250 mg daily) combined with physician-selected single-agent chemotherapy (TPC, capecitabine preferred) in patients with HER2-positive or HER2-low advanced/metastatic breast cancer who progressed after prior ADC therapy (T-DXd or SHR-A1811). Patients are stratified by HER2 status, HR status, lines of prior ADC therapy, and metastasis sites. Approximately 56 patients will be enrolled. The primary endpoint is objective response rate (ORR). Secondary endpoints include PFS1, PFS2, TTP, safety, and OS.

Inclusion Criteria: 1. Written informed consent obtained before any study procedures. 2. Age ≥ 18 years. 3. Histologically confirmed diagnosis of advanced breast cancer (ABC) or metastatic breast cancer (MBC). 4. HER2 status confirmed by local or central laboratory: either HER2-positive (IHC 3+ or ISH+) OR HER2-low (IHC 1+ or IHC 2+/ISH-). 5. Disease progression after at least one cycle of prior antibody-drug conjugate (ADC) therapy (e.g., trastuzumab deruxtecan \[T-DXd\], SHR-A1811, or other HER2-directed ADC). 6. For hormone receptor-positive (HR+) patients, prior treatment with CDK4/6 inhibitor is required. 7. At least one measurable lesion as defined by RECIST 1.1. 8. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 (or 0-2 per protocol). 9. Adequate organ function including: absolute neutrophil count ≥ 1.5 × 10\^9/L, platelets ≥ 100 × 10\^9/L, hemoglobin ≥ 90 g/L; total bilirubin ≤ 1.5 × ULN; ALT/AST ≤ 2.5 × ULN (≤ 5 × ULN if liver metastases); serum creatinine ≤ 1.5 × ULN or creatinine clearance ≥ 50 mL/min. 10. Expected survival ≥ 3 months. 11. Female patients of childbearing potential must have a negative pregnancy test and agree to use effective contraception; male patients with partners of childbearing potential must agree to use effective contraception. Exclusion Criteria: 1. Known hypersensitivity to apatinib mesylate, capecitabine, or other study chemotherapy agents. 2. Uncontrolled hypertension defined as systolic blood pressure \> 140 mmHg or diastolic blood pressure \> 90 mmHg despite optimal antihypertensive therapy. 3. History of severe cardiovascular disease within 6 months prior to enrollment, including myocardial infarction, unstable angina, coronary artery bypass graft, stroke, or New York Heart Association (NYHA) Class III/IV heart failure. 4. Active bleeding, coagulation disorders, or requirement for therapeutic anticoagulation with warfarin or direct oral anticoagulants (low-dose prophylactic anticoagulation allowed). 5. Unhealed surgical wounds, active peptic ulcer, or gastrointestinal perforation/fistula within 6 months. 6. Untreated or progressive central nervous system (CNS) metastases (patients with stable, treated brain metastases who have discontinued corticosteroids for ≥ 2 weeks may be eligible per investigator discretion). 7. History of interstitial lung disease (ILD), current ILD, or pneumonitis requiring steroids. 8. Pregnant or breastfeeding women. 9. Other active malignancies within 3 years prior to enrollment (except cured basal cell carcinoma of skin, squamous cell carcinoma of skin, or carcinoma in situ of cervix). 10. Concomitant use of strong CYP3A4 inhibitors or inducers that cannot be safely interrupted. 11. Any other condition that, in the investigator's judgment, would make the patient inappropriate for study participation or unable to comply with study procedures.