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, or metastatic malignant solid tumor that is refractory to or intolerable with standard treatment, 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 undergo fresh 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) or serious 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 not yet resolved to NCI-CTCAE version 5.0, Grade ≤ 1 or baseline. * A known hypersensitivity to either the drug substances or inactive ingredients in the drug product. * Part 2 (expansion) Only:Multiple primary malignancies within 3 years, except adequately resected non- melanoma skin cancer, curatively treated in-situ disease, other solid tumors curatively treated, or contralateral breast cancer.