Phase II Clinical Trial of PD-L1 in Combination With Vinorelbine + Cyclophosphamide + Capecitabine (VEX) Metronomic Chemotherapy With or Without Radiotherapy for Advanced Triple-negative Breast Cancer.

Subjects are eligible to be included in the study only if all of the following criteria apply: 1. Male or female who is 18 years of age or older on the day of signing the informed consent form. 2. Subjects with metastatic triple-negative breast cancer, with clear clinical data records; specific reference to the "American Society of Clinical Oncology - College of American Pathologists (ASCO/CAP) guidelines." 3. Have received less than or equal to 1 line of chemotherapy for metastatic breast cancer; 4. Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 as assessed within 10 days before the start of study treatment; 5. Subjects must have at least one measurable lesion present; specific reference to the "Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST v1.1)" guidelines; 6. Subjects may have previously received treatment with anthracyclines (e.g., doxorubicin, epirubicin) and/or taxanes (e.g., paclitaxel, docetaxel) as follows: a) The subject has used anthracyclines and/or taxanes during adjuvant and/or neoadjuvant therapy before breast cancer recurrence. b) The subject has experienced treatment failure during or after chemotherapy based on anthracyclines and/or taxanes. c) According to the investigator's judgment, the Subject is not suitable for anthracyclines and/or taxanes-based chemotherapy as a first-line treatment option. 7. Female subjects are not pregnant, not lactating, and agree to use necessary contraceptive measures. 8. Sub…