This study is an open-label, dose-escalation and cohort expansion Phase I study, aiming to evaluate the safety, tolerability, PK characteristics and preliminary efficacy of ESG406, and determine the MTD, RP2D and administration regimens of ESG406. The study includes the dose escalation study in Phase Ia and the cohort expansion study in Phase Ib.

Key Inclusion Criteria:

• Males and females aged 18 to 80 years.
• Histologically or cytologically confirmed advanced or metastatic solid tumor(s) for which no effective standard therapy is available or tolerable.
• At least one measurable lesion per RECIST v1.1.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
• Life expectancy ≥12 weeks.
• Adequate organ and bone marrow function.
• Fertile men and women of childbearing potential must agree to use an effective method of birth control from providing signed consent and for 180 days after last investigational product administration. Women of childbearing potential include pre-menopausal women and women within the first 2 years of the onset of menopause.

Key Exclusion Criteria:

• Use of any cancer therapy (chemotherapy or other systemic anti-cancer therapies, immunotherapy) within 4 weeks before the first investigational product administration.
• Toxicities from prior anti-tumor therapy not recovering to ≤ Grade 1.
• Had major surgery within 4 weeks before dosing, or will not have fully recovered from surgery; or has surgery planned during the time the subject is expected to participate in the study or within 4 weeks after the last dose of study drug administration.
• Use of any investigational anti-cancer drug within 4 weeks or 5 half-lives before the first investigational product administration.
• A history of thromboembolic or cerebrovascular events within 6 months prior to the first dose of the investigational drug.
• History of (noninfectious) interstitial pneumonia (ILD)/noninfectious pneumonitis requiring steroid therapy and current ILD/noninfectious pneumonitis, or suspected ILD/noninfectious pneumonitis at screening.
• Subjects with symptomatic or untreated CNS metastases, or those requiring ongoing treatment for CNS metastases.
• Patients with Primary CNS malignancy, or patients with other malignancies within 3 years prior to the first dose.
• Patients with uncontrollable systemic diseases.
• Subjects with clinically significant cardiovascular disease.
• Human Immunodeficiency Virus (HIV) infection.
• Active hepatitis B or hepatitis C.
• Have an allergic constitution, or to be allergic to any investigational drug or excipient ingredient.
• Pregnant or lactating women.