This is an open-label, single arm phase II trial to evaluate the efficacy, safety and tolerability of KN035 in combination with trastuzumab and docetaxel. Eligible patient will be enrolled and receive protocol defined therapies until progressive disease, unacceptable toxicity or withdrawal of informed consent. Tumor assessment will be performed according to RECIST 1.1 criteria.

Inclusion Criteria:

• Written informed consent
• Female subjects with age 18 to 70 (inclusive) years at the time of consent
• LVEF ≥50% at baseline
• ECOG performance status of 0-1
• Have not received 1L treatment for recurrent or metastatic breast cancer
• Adequate organ function

Exclusion Criteria:

• History of exposure to the cumulative doses of doxorubicin > 400 mg / m2 or equivalent
• History of autoimmune diseases
• Active brain metastasis
• Concurrent diseases that compromise patient's safety