Oral Metronomic Capecitabine Combined With Pyrotinib in ADC-treated HER2-positive Metastatic Breast Cancer

Inclusion Criteria: 1. Aged ≥18 and ≤75 years. 2. Histologically or cytologically confirmed HER2-positive metastatic breast cancer. 3. Patients must have either experienced disease progression following anti-HER2 antibody-drug conjugate (ADC) therapy in the advanced/metastatic setting (including regimens containing SHR-A1811, T-DXd, T-DM1, or other approved ADCs) or discontinued prior anti-HER2 ADC treatment due to intolerable toxicity, financial constraints, or patient preference without evidence of disease progression. 4. ECOG performance status of 0 to 2. 5. The functions of the main organs are basically normal 6. Signed informed consent Exclusion Criteria: 1. Prior treatment with a TKI or capecitabine (or other fluoropyrimidine-based chemotherapy) in the advanced or metastatic setting. 2. Known dihydropyrimidine dehydrogenase (DPD) deficiency. 3. Pregnant or lactating patients; 4. Malignancy (except basal cell carcinoma of the skin, which has been cured, and carcinoma in situ of the cervix) in the past 5 years; 5. History of allergic reactions attributed to compounds of similar chemical or biologic composition to the study agent or accompanying supportive medications; 6. Serious physical or mental illnesses or laboratory abnormalities that may increase the risk of participating in the study or interfere with the study results 7. Deemed by the investigator to be ineligible for participation in the study.