Tislelizumab Combined with Nab-Paclitaxel for First-Line Treatment of Advanced Triple-Negative Breast Cancer with Bone Metastasis，an Exploratory, Single-Arm, Multicenter Phase II Clinical Study

Inclusion Criteria:

1. Female, aged ≥18 years and <70 years;

2. Metastatic triple-negative breast cancer (TNBC) confirmed by histologicalexamination, i.e., negative for human epidermal growth factor receptor 2 (HER2),estrogen receptor (ER), and progesterone receptor (PR);

3. Eastern Cooperative Oncology Group (ECOG) performance status score: 0-1;

4. Expected survival time ≥3 months;

5. ≥1 bone metastasis confirmed by histology or imaging. There must be at least onemeasurable lesion defined according to RECIST 1.1 criteria. For lesions that haveundergone previous radiotherapy, they can only be considered measurable if there isdefinite disease progression after radiotherapy;

6. Patients with initially diagnosed stage IV (staging based on AJCC 8th edition) orrecurrent/metastatic TNBC who are not suitable for surgical treatment, and have notreceived prior systemic therapy for advanced disease. Prior neoadjuvant and/oradjuvant therapy with taxanes and other antitumor treatments is allowed, but it mustmeet the criteria of no progression after neoadjuvant therapy, and the time intervalbetween the end of taxane (neo)adjuvant therapy and recurrence/metastasis must be ≥12 months, and the time interval between the end of capecitabine (neo)adjuvanttherapy and recurrence/metastasis must be ≥6 months. If immunotherapy was conductedduring neoadjuvant phase, the time interval between the last immunotherapy and thescreening enrollment for this study must be ≥24 months;

7. Prior local radiotherapy for metastatic sites is allowed, with no limit on the timeof completion of radiotherapy, but the patient must have recovered from the effectsof radiotherapy before enrolled;

8. Function of vital organs meets the protocol requirements within 1 week before thefirst administration of the study drug;

9. Patients voluntarily participate in this study and sign the informed consent form.

Exclusion Criteria:

1. Currently participating in other clinical studies;

2. Untreated central nervous system disease (patients with asymptomatic central nervoussystem metastases are allowed);

3. Carcinomatous meningitis;

4. Uncontrolled pleural effusion, ascites, or pericardial effusion (patients withindwelling drainage tubes are allowed to enroll);

5. Existing neuropathy ≥ Grade 2 according to the Common Terminology Criteria forAdverse Events (CTCAE v5.0);

6. Uncontrolled tumor-related pain; patients requiring analgesia are allowed to enrollif they have a stable and effective analgesia regimen at the time of enrollment;

7. Patients expected to use any other systemic or local anti-tumor therapy during thestudy treatment; local radiotherapy for symptomatic isolated lesions or isolatedbrain metastases is allowed after evaluation and approval by the investigator; localradiotherapy is not allowed for all target lesions during the study treatment;

8. Diagnosis of other malignancies within 5 years. Exceptions include: radicallytreated early-stage malignancies (carcinoma in situ or stage I tumors), such asadequately treated cervical carcinoma in situ, basal cell or squamous cell skincancer;

9. Currently pregnant/lactating women;

10. Evidence of significant comorbidities that may affect the patients' compliance withthe study protocol or interpretation of the study results;

11. Severe cardiovascular disease;

12. Known allergy to any drug used in the study;

13. Patients with any active autoimmune disease requiring systemic treatment (i.e., useof disease-modulating drugs, corticosteroids, or immunosuppressive drugs) within thepast 2 years (including but not limited to: interstitial pneumonia, uveitis,enteritis, hepatitis, hypophysitis, nephritis, hyperthyroidism, hypothyroidism;patients with autoimmune-related hypothyroidism receiving stable doses of thyroxinereplacement therapy and patients with controllable type 1 diabetes receiving stabledoses of insulin are allowed; patients with vitiligo or childhood asthma that hascompletely resolved and requires no intervention in adulthood are allowed; patientswith asthma requiring medical intervention with bronchodilators are not allowed).Replacement therapy (such as thyroxine, insulin, or physiological corticosteroidreplacement therapy for adrenal or pituitary insufficiency) is not consideredsystemic treatment;

14. Active hepatitis B patients (including chronic or acute, defined as HBsAg positiveat baseline). Previously active HBV infection or cured hepatitis B patients areallowed to enroll if they meet the following conditions: viral hepatitis patientswith HBcAb positive and HBsAg negative, with HBV DNA copy number less than the upperlimit of normal value of the testing laboratory at the research center beforeenrollment;

15. Active hepatitis C patients. HCV antibody-positive patients are allowed to enrollonly if HCV RNA test results are negative;

16. Other liver diseases with known clinically significance, including alcoholichepatitis or other hepatitis, cirrhosis, hereditary liver diseases;

17. Known history of HIV positivity or acquired immune deficiency syndrome (AIDS);

18. Prior receipt of allogeneic stem cell or solid organ transplantation;

19. Patients with active tuberculosis (TB) who are receiving anti-tuberculosis treatmentor have received anti-tuberculosis treatment within 1 year before screening;

20. Received systemic immunosuppressive drug treatment within 4 weeks before the firstadministration of the study drug (including but not limited to corticosteroids,cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor necrosisfactor [TNF]-α drugs). Patients receiving short-term, low-dose systemicimmunosuppressive drug treatment or a single bolus of systemic immunosuppressivedrugs (e.g., glucocorticoids for 48 hours to treat contrast allergy) may be enrolledin the study after approval by the medical monitor. Patients receivingmineralocorticoids (such as fludrocortisone), corticosteroids for chronicobstructive pulmonary disease or asthma, or low-dose corticosteroids for orthostatichypotension or adrenal insufficiency are eligible for the study;

21. Any other severe medical condition or abnormal clinical laboratory results that mayhinder the patient's safe participation and completion of the study，based on theinvestigator's judgment