A Study to Compare DB-1303/BNT323 Versus T-DM1 in 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. 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. NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.