• Has a pathologically documented HER2-positive or 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.

• 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

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