Safety, Pharmacokinetics, Pharmacodynamics and Preliminary Efficacy of MT-304 in Adults With Advanced HER2-Expressing Solid Tumors

Inclusion Criteria: * Aged 18 years or above * Histologically confirmed diagnosis of metastatic or advanced epithelial cancer expressing HER2 (Note: Participants with other tumor types expressing HER2 may be considered pending discussion with the Medical Monitor). * Measurable lesion per RECIST 1.1 criteria. * Eastern Cooperative Oncology Group (ECOG) performance status Grade of 0 or 1. * Adequate Organ function Exclusion Criteria: * Known active CNS metastasis and/or carcinomatous meningitis. * Any acute illness including fever. * History of symptomatic congestive heart failure * History of (noninfectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease. * Uncontrolled pleural effusion, pericardial effusion, or ascites * Active autoimmune disease not related to prior therapy for primary malignancy that has required systemic therapy in the last 1 year.