Clinical Research Directory

Inclusion Criteria: 1. Adult female patients aged 18 to 80 years with metastatic breast cancer confirmed by pathology or imaging, not amenable to curative surgery or radiotherapy. 2. Pathologically confirmed estrogen receptor (ER) and/or progesterone receptor (PR) positive (≥10% tumor cells by immunohistochemistry) and HER2-positive (IHC 3+ or FISH-positive). 3. Disease progression during or within 1 year after neoadjuvant/adjuvant trastuzumab, or prior trastuzumab-based therapy for metastatic disease. 4. Eastern Cooperative Oncology Group (ECOG) performance status 0-2 and estimated life expectancy ≥3 months. 5. Prior treatment-related toxicity resolved to Grade ≤1 per NCI CTCAE v4.03 (except alopecia or other toxicity deemed by the investigator not to pose a safety risk). 6. Adequate bone marrow function within 1 week before enrollment: white blood cell count ≥3.0×10⁹/L, absolute neutrophil count ≥1.5×10⁹/L, platelet count ≥100×10⁹/L. 7. Adequate hepatic, renal, and cardiac function within 1 week before enrollment: total bilirubin ≤1.5×ULN; ALT/AST ≤2.5×ULN (≤5×ULN in patients with liver metastases); serum creatinine ≤1.5×ULN or creatinine clearance ≥60 mL/min; left ventricular ejection fraction ≥55%; QTcF ≤470 ms. 8. Patient understands the study procedures, voluntarily agrees to participate, and provides written informed consent. Exclusion Criteria: 1. No prior trastuzumab, chemotherapy, or endocrine therapy. 2. Central nervous system metastases. 3. Visceral crisis. 4. Considered by the investigator to be a candidate for chemotherapy. 5. Inability to swallow, chronic diarrhea, or bowel obstruction that may affect drug administration or absorption. 6. More than one prior line of endocrine therapy, chemotherapy, or targeted therapy for metastatic disease. 7. Other malignancy within the past 5 years, except curatively treated cervical carcinoma in situ, basal cell carcinoma, or squamous cell carcinoma of the skin. 8. Concurrent other anticancer therapy. 9. Known allergy to any component of the study drugs; history of immunodeficiency, including HIV positivity, HCV, active hepatitis B, or other acquired/congenital immunodeficiency; history of organ transplantation. 10. Severe cardiac disease or condition, including but not limited to: heart failure or systolic dysfunction (LVEF ≤50%), uncontrolled hypertension, significant ventricular arrhythmia, high-grade atrioventricular block, angina requiring anti-anginal therapy, clinically significant valvular disease, or ECG evidence of acute myocardial infarction. 11. Pregnancy or lactation, or a positive pregnancy test at baseline in women of childbearing potential. 12. Any severe concomitant disease that, in the investigator's judgment, would compromise patient safety or study completion. 13. History of neurological or psychiatric disorders, including epilepsy or dementia. 14. Any other condition that, in the investigator's judgment, makes the patient unsuitable for the study.