This is a randomized, controlled, open-label, multicenter, phase Ш clinical study designed to compare the efficacy and safety of KN026 in combination with HB1801 to trastuzumab in combination with pertuzumab and docetaxel in the first-line treatment of subjects with HER2-positive recurrent or metastatic breast cancer. The statistical assumption for this study is superiority. The primary study endpoint was PFS as assessed by Blinded Independ Review Committee (BIRC).

The purpose of this study is to assess the efficacy and safety of KN026 combined with HB1801 versus trastuzumab combined with pertuzumab and docetaxel as first-line treatment for HER2-positive recurrent or metastatic breast cancer. This study will establish an independent data monitoring committee (IDMC) to conduct safety assessments after approximately 20 subjects in the treatment group complete the first cycle of treatment. During the safety assessment period, the study will continue to enroll subjects, and safety review meetings will be at 1 year of randomization in the first subject. In addition, the IDMC plans to conduct one interim analysis of efficacy during the study period.

The primary study hypotheses are that the combination of KN026 combined with HB1801 is superior to trastuzumab combined with pertuzumab and docetaxel with respect to: Progression-free Survival (PFS) as assessed by the BIRC per Response Evaluation Criteria in Solid Tumors (RECIST 1.1).

Key Inclusion Criteria:

• Voluntarily enrolled in this study and signed an informed consent form (ICF).
• Age ≥ 18 years.
• Recurrent or metastatic breast cancer confirmed by histology and/or cytology.
• Latest tumor tissue sample confirmed as HER2 positive by central laboratory testing.
• No prior systemic chemotherapy and/or HER2-targeted therapy for recurrent or metastatic breast cancer.
• Eastern Cooperative Oncology Group (ECOG) physical status score of 0 - 1.
• Presence of lesion (RECIST 1.1).
• Adequate organ and bone marrow function

Key Exclusion Criteria:

• Ineligible for any of the agents on the study
• Untreated or unstable parenchymal metastases, spinal cord metastases or compression, or carcinomatous encephalitis.
• Pregnant or lactating women.
• Presence of other circumstances that may interfere with the subject's participation in the study procedures or are inconsistent with the maximum benefit of the subject's participation in the study or affect the results of the study: e.g., history of mental illness, drug or substance abuse, any other disease or condition of clinical significance, etc.