SKB264 +/- KL-A167 in Recurrent or Metastatic HER2-negative Breast Cancer

Inclusion Criteria:

1. Males or females aged ≥ 18 and ≤ 75 years at the time of signing the informed consent form (ICF);
2. Histological and/or cytological diagnosis of TNBC or HR+/HER2- BC based on pathology reports on recent biopsy samples or other pathological samples (central laboratory confirmation is not required)；
3. Patients have not received prior systemic chemotherapy for locally advanced, recurrent and metastatic disease；
4. Ability to provide fresh or archival tumor tissue for biomarker testing and analysis;
5. Patients with at least one measurable lesion per RECIST v1.1 criteria, and patients with only skin or bone lesions cannot be enrolled;
6. Patients with an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 with an expected survival of ≥ 12 weeks;
7. Adequate organ and bone marrow function;
8. Patients must recover from all toxicities due to prior treatment (recovery to ≤ Grade 1 based on CTCAE v5.0 assessment, or meeting the inclusion criteria in the protocol) with the exception of alopecia and vitiligo;
9. Female subjects of childbearing potential and male patients with partners of childbearing potential who use effective medical contraception during the study treatment period and for 6 months after the end of dosing (see Appendix for specific contraceptive measures);
10. Patients voluntarily participate in the study, sign the ICF, and will be able to comply with the protocol-specified visits and relevant procedures.

Exclusion Criteria:

1. History of other malignancies;
2. Patients with a history of central nervous system (CNS) metastases or current CNS metastases.
3. Imaging (CT or MRI) shows that the tumor has invaded large blood vessels or the investigator judges that the tumor is likely to invade important blood vessels and cause fatal hemorrhage during the follow-up study；
4. Received any systemic immune-stimulatory agents within 4 weeks prior to the first dose of study; Received any traditional Chinese medicine for approved anti-tumor indications within 2 weeks prior to the first dose of study;
5. Received other clinical investigational drugs within 4 weeks or major surgery within 4 weeks prior to the first dose of the study treatment;
6. Patients who required systemic corticosteroids (\> 10 mg/day prednisone or equivalent; low-dose corticosteroids are allowed, such as ≤10 mg/day prednisone or equivalent, if the dose is stable for 4 weeks), or other immunosuppressive therapy within 2 weeks prior to the first dose. Steroids are allowed as prophylaxis for hypersensitivity reactions;
7. Patients who occurred arteriovenous thrombosis within 6 months prior to the first dose of study treatment，Such as cerebrovascular accidents, deep vein thrombosis, and pulmonary embolism, etc.
8. Prior treatment with a TROP2-targeted drug or checkpoint inhibitor ;
9. Serious or uncontrolled cardiac disease or clinical symptoms requiring treatment；
10. Patients with (noninfectious) interstitial lung disease (ILD) or history of pneumonia requiring steroid therapy; patients with serious pulmonary function impairment due to lung disease;
11. Uncontrolled systemic disease as judged by the investigator, included uncontrolled hypertension, uncontrolled diabetes, pesence of pleural effusion, pericardial effusion, or ascites that is clinically symptomatic or requires repeated drainage;
12. Active autoimmune disease requiring systemic treatment within the past 2 years;
13. Active hepatitis B or hepatitis C; known history of positive human immunodeficiency virus (HIV) test or known acquired immunodeficiency syndrome (AIDS); positive syphilis antibody test;
14. Known hypersensitivity to the study drug or any of its components, or severe allergic reactions to other monoclonal antibodies;
15. Pregnant or lactating women;
16. Any patient whose condition deteriorates rapidly during the screening process prior to the first dose, such as severe changes in performance status, unstable pain requiring adjustment of analgesic therapy, etc;
17. Other circumstances that, in the opinion of the investigator, are not appropriate for participation in this study.