Inclusion Criteria:

• Histologically confirmed, unresectable, locally advanced or metastatic solid cancers: 
  • Part 1 (Dose-Escalation):  All HER2-positive solid cancers are eligible;
  • Part 2 (Dose-Expansion):  Any type of HER2-positive solid cancer, including breast cancer, gastric or gastroesophageal adenocarcinomas (GEA). 
• HER2-positive in the latest tumor sample tested for HER2 (testing to be done on a metastatic lesion in cases of metastatic cancers). 
• Relapsed or refractory to or intolerant of existing therapy(ies).
• At least 1 measurable or evaluable lesion according to RECIST 1.1.
• ECOG performance score of ≤ 1.
• Bone marrow function: 
  • Absolute neutrophil count (ANC) ≥ 1,000/mm³;
  • Platelet count ≥ 75,000 mm³;
  • Hemoglobin ≥ 8.0 g/dL; and
  • No red blood cell transfusion within 4 weeks of study treatment start is allowed.
• Kidney function: 
  • (eGFR) ≥ 50 mL/min calculated by the Cockcroft-Gault formula; and
  • Subjects with CLcr ≥ 50 mL/min will be eligible irrespective of the eGFR result.
• Cardiac Function: 
  • Left ventricular ejection fraction (LVEF) ≥ 55% on the multigated acquisition (MUGA) scan (preferred) or echocardiogram (ECHO) assessment; 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.
• Hepatic function: 
  • Total bilirubin ≤ 1.5 x ULN;, and
  • AST ≤ 3 x ULN and ALT ≤ 3 x ULN - < 5 x ULN (if hepatic metastases).

Exclusion Criteria: 

• History or current evidence of another tumor that is histologically distinct from the tumor under study.
• Current evidence of new or growing CNS metastases during screening.
• Subjects with known CNS metastases will be eligible if they meet specified criteria.
• 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.
• History or evidence of significant cardiovascular disease.
• 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.
• Current evidence of ≥ Grade 2 underlying pulmonary disease.
• Certain exclusionary prior treatments