A Phase 1/2a Study of DB-1303/BNT323 in Advanced/Metastatic Solid Tumors

Inclusion Criteria:

• Has a pathologically documented HER2-positive or HER2-expressing (except for cohort 2h where the requirement is HER2-null), advanced/unresectable, recurrent, ormetastatic malignant solid tumor that is refractory to or intolerable with standardtreatment, or for which no standard treatment is available.

• At least 1 measurable lesion (per RECIST 1.1)

• Provide signed informed consent

• ECOG performance status (PS) of 0-1.

• LVEF ≥ 50% by ECHO or MUGA

• Adequate organ functions

• Provide pre-existing diagnosis of HER2 status or resected tumor samples or undergofresh tumor biopsy for HER2 testing.

• Life expectancy of ≥ 3 months.

Additional Inclusion Criteria for Part 2 Expansion Group 9:

1. Has pathologically documented advanced/unresectable, recurrent, or metastatic EC (including UCS and USPC) and has progressed on or after at least 1 line of systemic treatment including platinum-based therapy and exposure to ICI but no more than prior 3 lines of therapy for advanced/unresectable, or metastatic disease. Note: endocrine therapy will not qualify as a systemic therapy line.

Exclusion Criteria:

• History of symptomatic CHF (New York Heart Association [NYHA] classes II-IV) orserious cardiac arrhythmia requiring treatment.

• History of myocardial infarction or unstable angina within 6 months before Day 1.

• Average QTcF > 450 ms in males and > 470 ms in females

• History of clinically significant lung diseases

• Uncontrolled infection requiring IV antibiotics, antivirals, or antifungals.

• HIV infection with AIDS defining illness or active viral hepatitis.

• Clinically active brain metastases

• Unresolved toxicities from previous anticancer therapy, defined as toxicities notyet resolved to NCI-CTCAE version 5.0, Grade ≤ 1 or baseline.

• A known hypersensitivity to either the drug substances or inactive ingredients inthe drug product.

• Part 2 (expansion) Only:Multiple primary malignancies within 3 years, exceptadequately resected non- melanoma skin cancer, curatively treated in-situ disease,other solid tumors curatively treated, or contralateral breast cancer.