Study of MT-5111 in HER2-positive Solid Tumors

Inclusion Criteria: 1. Histologically confirmed, unresectable, locally advanced or metastatic solid cancers: * Part A (Dose-Escalation): All HER2-positive solid cancers are eligible * Part B (Dose-Expansion): Any type of HER2-positive solid cancer, including breast cancer, and gastric or gastroesophageal adenocarcinomas (GEA). 2. HER2-positive in the latest tumor sample tested for HER2 (testing to be done on a metastatic lesion in cases of metastatic cancers). 3. Relapsed or refractory to or intolerant of existing therapy(ies) 4. At least 1 measurable or evaluable lesion according to RECIST 1.1 (Subjects with evaluable disease only may be included in the dose escalation phase) 5. ECOG performance score of ≤ 1 6. Adequate Bone marrow function as determined by: * Absolute neutrophil count (ANC) ≥ 1,000/mm3 * Platelet count ≥ 75,000 mm³ and * Hemoglobin ≥ 8.0 g/dL * Red blood cell transfusion within 2 weeks of study treatment start is allowed if hemoglobin levels remain stable 7. Kidney function: * Creatinine clearance (CLcr) ≥ 50 mL/min either measured or estimated using the Cockcroft-Gault formula 8. Cardiac Function: * Left ventricular ejection fraction (LVEF) ≥ 55% on the echocardiogram (ECHO) assessment (preferred), or multigated acquisition (MUGA) scan, and QTcF ≤ 480 ms for women and QTcF ≤ 450 ms for men \[average from three QTcF values on the triplicate 12-lead electrocardiogram (ECG)\] at baseline 9. Hepatic function: * Total bilirubin ≤ 1.5 x ULN, or ≤ 3 x ULN for subjects with Gilbert's Syndrome and * AST ≤ 3 x ULN (or ≤ 5 x ULN if liver metastasis) and ALT ≤ 3 x ULN (or ≤ 5 x ULN if liver metastasis) Exclusion Criteria: 1. History or current evidence of another tumor that is histologically distinct from the tumor under study 2. Current evidence of new or growing CNS metastases during screening * Subjects with known CNS metastases will be eligible if they meet protocol specified criteria 3. Evidence of CTCAE Grade \>1 toxicity before the start of treatment, except for hair loss and those Grade 2 toxicities listed as permitted in other eligibility criteria 4. History or evidence of significant cardiovascular disease 5. Current evidence of active, uncontrolled hepatitis B virus, hepatitis C virus, human immunodeficiency virus (HIV) (evidenced by detectable viral load by PCR) or acquired immunodeficiency syndrome (AIDS) related illness 6. Current evidence of ≥ grade 2 underlying pulmonary disease 7. Certain exclusionary prior treatments