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 informedconsent.

• Pathologically confirmed unresectable or metastatic HER2 positive breast cancerpreviously 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 writteninformed consent form prior to the study.

Exclusion Criteria:

• Prior anti-HER2 ADC therapy.

• Previous history of interstitial lung disease/noninfectious pneumonitis/radiationpneumonitis requiring steroid therapy.

• Known serious hypersensitivity to the active ingredients of the study drug, inactiveingredients in the formulation, or other antibody drugs.

• Multiple primary malignancies within 3 years, except for adequately resectednon-melanoma skin cancer, curatively treated in situ tumor, or contralateral breastcancer

• Uncontrolled infection requiring intravenous antibiotics, antiviral or antifungalagents, 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 foralopecia) not recovered to ≤Grade 1 (NCI-CTCAE v5.0) or baseline.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.