Inetetamab Plus Cyclophosphamide Metronomic Chemotherapy Plus Aromatase Inhibitor in Metastatic HER2+/HR+ Breast Cancer

Inclusion Criteria: 1. Voluntarily sign the informed consent form; 2. 18-75 years old; 3. The expected survival period is ≥12 weeks; 4. Eastern Cooperative Oncology Group (ECOG) score \[0-2\] points; 5. The diagnosis of invasive carcinoma by histology or cytology; Estrogen receptor (ER) positive (defined as \>1% nuclear ER staining); HER2 negative (defined as IHC 0 or 1+, or HER2(2+) with HER2 FISH detection no amplification); 6. Inoperable or recurrent/metastatic breast cancer patients with aromatase inhibitor treatment failure; 7. In the state of disease progression before enrollment; 8. Measurable disease according to RECIST version 1.1 or only bone metastasis; 9. Adequate hematological, hepatic and renal function; 10. NYHA class I or II and Left ventricular ejection fraction (LVEF) ≥50%. 11. The diagnosis of invasive carcinoma by histology or cytology: Hormone receptor (HR) positive (defined as \>1% nuclear estrogen receptor staining); HER2 positive (defined as IHC 3+, or HER2 FISH detection amplification); 12. In the state of disease progression before enrollment; 13. Have lesions able to and agree to perform tissue biopsy at the time requested in the study; 14. Treatment ≥1 line after recurrence/metastasis, or relapse within 12 months after completing trastuzumab-based adjuvant therapy or during trastuzumab adjuvant therapy; 15. Previously received trastuzumab for anti-HER2 therapy; 16. Measurable disease according to RECIST version 1.1. Exclusion Criteria: 1. Allergic to the ingredients of Inetetamab, cyclophosphamide or similar drugs; 2. Concomitant diseases/conditions that is not controllable, and any other major illness that, in the investigator's judgment, will substantially increase the risk associated with the patient's participation in this study; 3. Patients who cannot accept drugs orally; 4. Women who are pregnant or breastfeeding or planning to give birth; 5. Patients with currently symptomatic brain or meningeal metastasis; 6. History of other primary malignancy; 7. Resistant to steroidal or nonsteroidal aromatase Inhibitor; 8. Have used Inetetamab; 9. Patients with life-threatening, symptomatic, metastatic visceral disease.