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. Subjects are able to sign an informed consent form to participate in the study. 9. Have appropriate organ function, as detailed in Table 1; all screening laboratory tests should be completed within 10 days before the start of study treatment.