This study is designed to compare efficacy and safety of GQ1005 versus T-DM1 in HER2-positive, unresectable and/or metastatic breast cancer patients previously treated with trastuzumab and taxane.

This is a phase III, multicenter, open-label, randomized controlled study comparing GQ1005 with Trastuzumab Emtansine (T-DM1) in patients with HER2-positive unresectable or metastatic breast cancer previously treated with trastuzumab and taxane. Approximately 228 patients with unresectable or metastatic HER2-positive breast cancer will be randomized 1:1 to receive GQ1005 or T-DM1, respectively.

Inclusion Criteria:

• Male or female adults ≥ 18 years at the time of voluntary signing of informed consent.
• Pathologically confirmed unresectable or metastatic HER2 positive breast cancer previously treated with trastuzumab and taxane
• Eastern Cooperative Oncology Group (ECOG) performance status score is 0 or 1.
• Presence of at least one measurable lesion according to RECIST v1.1
• Expected survival time ≥ 12 weeks.
• Patients must give informed consent to this study and voluntarily sign written informed consent form prior to the study NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.

Exclusion Criteria:

• Prior anti-HER2 ADC therapy.
• Previous history of interstitial lung disease/noninfectious pneumonitis/radiation pneumonitis requiring steroid therapy.
• Known serious hypersensitivity to the active ingredients of the study drug, inactive ingredients in the formulation, or other antibody drugs.
• Multiple primary malignancies within 3 years, except for adequately resected non-melanoma skin cancer, curatively treated in situ tumor, or contralateral breast cancer
• Uncontrolled infection requiring intravenous antibiotics, antiviral or antifungal agents, autoimmune disease requiring treatment, uncontrolled diabetes, hypertension, or other systemic disease that makes compliance with study procedures difficult
• Unrecovered toxicity from prior anticancer therapy, defined as toxicity (except for alopecia) not recovered to ≤Grade 1 (NCI-CTCAE v5.0) or baseline.