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

Prior treatment

• 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.
• 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).
• 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).
• Concomitant use of strong CYP3A4 inhibitors or inducers that cannot be safely interrupted.

Other illness

• Known hypersensitivity to apatinib mesylate, capecitabine, or other study chemotherapy agents.
• Uncontrolled hypertension defined as systolic blood pressure \> 140 mmHg or diastolic blood pressure \> 90 mmHg despite optimal antihypertensive therapy.
• Active bleeding, coagulation disorders, or requirement for therapeutic anticoagulation with warfarin or direct oral anticoagulants (low-dose prophylactic anticoagulation allowed).
• Unhealed surgical wounds, active peptic ulcer, or gastrointestinal perforation/fistula within 6 months.
• History of interstitial lung disease (ILD), current ILD, or pneumonitis requiring steroids.
• Any other condition that, in the investigator's judgment, would make the patient inappropriate for study participation or unable to comply with study procedures.

Other

• Pregnant or breastfeeding women.

• Written informed consent obtained before any study procedures.
• Age ≥ 18 years.
• Histologically confirmed diagnosis of advanced breast cancer (ABC) or metastatic breast cancer (MBC).
• HER2 status confirmed by local or central laboratory: either HER2-positive (IHC 3+ or ISH+) OR HER2-low (IHC 1+ or IHC 2+/ISH-).
• 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).
• For hormone receptor-positive (HR+) patients, prior treatment with CDK4/6 inhibitor is required.
• At least one measurable lesion as defined by RECIST 1.1.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 (or 0-2 per protocol).
• 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.
• Expected survival ≥ 3 months.
• 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.