A Trial Comparing GQ1005 and T-DM1 in Patients With HER2-Positive Unresectable or Metastatic Breast Cancer

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.