Efficacy and Safety of PD-L1 Monoclonal Antibody Combined With Metronomic VEX in Advanced Triple-negative Breast Cancer

Inclusion Criteria: 1. Female participants aged ≥18 years who have signed informed consent and have an expected survival of ≥3 months. 2. Eastern Cooperative Oncology Group (ECOG) performance status score ≤2 within 21 days prior to the first dose of medication. 3. Participants with clear clinical records of metastatic triple-negative breast cancer, as specified in the American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines. 4. Participants with metastatic breast cancer who have received no more than first-line chemotherapy. 5. Participants with at least one measurable lesion, as defined by the Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1). 6. Participants with a history of receiving anthracycline and/or taxane-based treatments, including: 1. Use of anthracyclines and/or taxanes during adjuvant or neoadjuvant therapy before breast cancer recurrence. 2. Ineffectiveness observed during or after chemotherapy based on anthracyclines and/or taxanes. 3. Participants deemed unsuitable for first-line treatment with anthracycline and/or taxane-based chemotherapy according to the investigator's judgment. 7. Completion of radiotherapy before the first dose of study medication, with a minimum interval of 4 weeks since the end of radiotherapy. 8. For participants with a history of surgery, a minimum interval of 30 days between surgery and the first dose of medication, with complete recovery from the surgical procedure. 9. Normal bone marrow function, evidenced by: 1. Absolute neutrophil count (ANC) ≥ 1500/mm². 2. Platelets ≥ 100,000/mm². 3. Hemoglobin (Hb) ≥ 10 g/dL. 10. Normal liver function, evidenced by: 1. Total bilirubin ≤ 1.5 times the upper limit of normal (ULN). 2. Aspartate transaminase (AST)/alanine transaminase (ALT) ≤ 2.5 ULN (≤ 5.0 times ULN in the presence of liver metastasis). 3. Alkaline phosphatase \< 5 x ULN. 11. Normal renal function, evidenced by a creatinine clearance rate \> 40 mL/min (Cockcroft-Gault formula). 12. Negative pregnancy test results within 7 days prior to the first dose of medication for women of childbearing potential, who also agree to employ necessary contraceptive measures. 13. Ability to understand and voluntarily sign the informed consent form before any study-related assessments/procedures. 14. Willingness and ability to comply with the study visit schedule and other protocol requirements. Exclusion Criteria: 1. Current or past history of malignancies other than breast cancer, excluding: cured non-melanoma skin cancer, cured cervical carcinoma in situ, or cured other primary solid tumors with no evidence of disease activity and no curative treatment within the last 3 years. 2. Solely having pleural effusion, ascites, bone metastases, or other unmeasurable lesions. 3. Malabsorption syndrome or diseases significantly affecting gastrointestinal function, prior gastrectomy, or resection of the proximal small intestine that may affect the absorption of oral chemotherapeutic agents. 4. Swallowing difficulties or inability to swallow tablets. 5. Symptomatic brain or leptomeningeal metastasis; suspected signs or symptoms of central nervous system (CNS) involvement should be excluded by CT or MRI scans. 6. Other severe diseases or medical conditions discovered by the investigator during screening, including: 1. Clinically significant heart diseases. 2. Unstable diabetes. 3. Uncontrolled hypercalcemia. 4. Clinically significant active infections within the last 2 weeks. 7. History of organ transplantation. 8. Peripheral neuropathy of grade ≥2 according to NCI version 5.0. 9. Concurrent use of any other antitumor therapy for metastatic breast cancer. 10. Requirement for concurrent anticoagulant therapy. 11. Status of pregnancy, lactation, or unwillingness to use effective contraception for at least one month during the entire study period and for at least one month after the last dose of the study drug. 12. Known history of neurologic or psychiatric disorders or diseases that could compromise treatment compliance. 13. Less than a 3-week interval between the end of previous chemotherapy and the first dose of study medication. 14. Progressive disease during or after oral investigational drug treatment. 15. Use of any investigational drug within 30 days before starting study treatment. Less than 30 days since the use of any other investigational drug or device. 16. Known allergy to any component of the study drug.