Inclusion Criteria:  Has a pathologically documented HER2-expressing 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.  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. 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. Part 2 only: Prior treatment with HER2 ADC agents except T-DM1, RC48-ADC