This is an open-label, three-cohort phase 2 study of KN026 in subjects with advanced breast Cancer.

KN026 is an anti-HER2 bispecific antibody that can simultaneously bind two non-overlapping epitopes of HER2, leading to a dual HER2 signal blockade.The HER2- positive mBC subjects without systemic treatment will be enrolled in cohort 1 and received 30 mg/kg KN026 and docetaxol. The HER2 low expression and hormone-receptor positive mBC subjects failed standard chemotherapy and hormone therapy will be enrolled in cohort 2 and received 30 mg/kg KN026 monotherapy. The HER2 low expression and hormone-receptor negative/weak positive mBC subjects failed standard chemotherapy will be enrolled in cohort 3 and received 30 mg/kg KN026 and 5 mg/kg KN046

Inclusion Criteria:

• Male or female subject >= 18 years
• Histologically/cytologically confirmed, locally advanced unresectable or metastatic breast cancer
• ECOG score 0 or 1
• Life expectancy >3 months
• According to the definition of RECIST1.1, the patient has at least one measurable lesion
• Adequate organ function prior to start treatment with KN026
• Able to understand, voluntarily participate and willing to sign the ICF

Exclusion Criteria:

• Accepted any other anti-tumor drug therapies within 4 weeks before fist dose
• Accepted radiotherapy within 4 weeks before enrollment
• Subjects are eligible with clinically controlled and stable neurologic function >= 4 weeks, which is no evidence of CNS disease progression; Subjects with spinal cord compression and cancerous meningitis are not eligible
• Pregnant or nursing females；or intend pregnancy within this study period or within 6 monthes after the end of this study
• History of immunodeficiency, including HIV positive or other acquired, congenital immunodeficiency disease, or a history of organ transplantation
• History of immunodeficiency, including HIV positive or other acquired, congenital immunodeficiency disease, or a history of organ transplantation
• Cavity effusion (pleural effusion, ascites, pericardial effusion, etc.) are not well controlled, and need locally treatment or repeated drainage