This is an open-label, Single-arm, Phase I clinical study to evaluate the pharmacokinetics, tolerability and safety of margetuximab plus chemotherapy in Chinese patients with advanced HER2+ breast cancer who have received standard anti-HER2 directed therapy in the metastatic setting (mandatory including trastuzumab).

The primary endpoint of this study is PK parameters of margetuximab.

Approximately 16~20 Chinese subjects will be enrolled. Eligible subjects are HER2 positive, metastatic breast cancer who has received standard anti-HER2 directed therapy in the metastatic setting in Chinese patients. Subjects should have received treatment with at least one, and no more than four lines of therapy overall in the metastatic setting.

Eligible subjects will receive margetuximab plus chemotherapy. The dosage and administering of margetuximab is 15 mg/kg IV Q3W. Investigators selected one of three backbone chemotherapy regimens given at standard doses: capecitabine, vinorelbine or gemcitabine. Subject will receive the treatment until disease progression, unacceptable toxicity, withdrawal of consent, initiation of subsequent anti-tumor treatment therapy, or death (whichever occurs first).

Inclusion Criteria:

• Written informed consent obtained prior to performing any protocol-related procedures
• Male or female, age ≥ 18 years old at the time of screening.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• Subject has histologically confirmed HER2 positive metastatic breast cancer.
• Have received standard anti-HER2 directed therapy in the metastatic setting (mandatory to have trastuzumab), regardless of having received (neo)adjuvant anti-HER2 therapy or not.
• Have received treatment with no more than four lines of therapy overall in the metastatic setting (including anti-HER2 targeted therapy or chemotherapy) and must have disease progressed on or after, the most recent line of therapy. per RECIST 1.1.
• Previous adverse events associated with anti-tumor therapy have been recovered to NCI-CTCAE v4.03 Grade ≤1 (except NCI-CTCAE v4.03 Grade ≤2 alopecia, stable sensory neuropathy, or stabilized electrolyte disturbance after fluid transfusion).
• Subject has life expectancy ≥12 weeks.
• Subject has no supportive therapy of blood transfusion or growth factor within 4 weeks before dosed and has adequate organ functions
• Subject has a negative test result of pregnancy test at screening.

Exclusion Criteria:

• Subject has symptomatic, uncontrolled brain or pia mater metastasis.
• Subject has third interstitial effusion that cannot be controlled by drainage or other means.
• Subject has local or systemic anti-tumor treatment within 2 weeks prior to enrollment
• Subject has any investigational treatment within 4 weeks prior to enrollment (including margetuximab)
• Subject has history of major surgery with unrecovered surgical effect within 4 weeks prior to enrollment
• Subject has other malignant tumor (complete cured in situ cervical cancer, cutaneous basal cell carcinoma or cutaneous squamous cell carcinoma are not included) within 5 years prior to enrollment
• Subject has severe and uncontrolled disease