This is a Phase 1, multi-center, open-label, multiple dose, dose escalation study to evaluate the safety, tolerability, dose limiting toxicities (DLTs), maximum tolerated dose (MTD) and/or Recommended Phase 2 Dose (RP2D), pharmacokinetic (PK), pharmacodynamics, and preliminary anti-tumor activity of AV-203, an ERBB3 inhibitory antibody, administered once every 2 weeks via intravenous (IV) infusion in subjects with metastatic or advanced solid tumors. Once the RP2D is determined, patients with tumor types of interest will be evaluated in an expansion cohort at the RP2D for safety and anti-tumor activity.

Inclusion Criteria:

• ≥ 18 years of age
• Histologically and/or cytologically confirmed primary diagnosis
• Metastatic or advanced solid tumor, that has recurred or progressed following standard therapies, or for which no standard therapy exists
• Must have available tumor tissue or be willing to undergo biopsy prior to enrollment
• Eastern Cooperative Oncology Group (ECOG) Performance status of 0 or 1
• Blood Chemistry and Hematology results within defined limits

Exclusion Criteria:

• History of severe allergic or anaphylactic reactions or hypersensitivity to recombinant proteins or excipients in the investigational agent
• Current central nervous system (CNS) or leptomeningeal metastases, or history of CNS or leptomeningeal metastases.
• Significant conduction disturbance, history of a severe arrhythmia, or history of a familial arrhythmia
• Significant cardiovascular disease
• Significant thromboembolic or vascular disorders within prior 3 months
• Any other medical condition or psychiatric condition that, in the opinion of the Investigator, might interfere with the subject's participation in the trial or interfere with the interpretation of trial results
• Known history of positive results for hepatitis C, hepatitis B, or human immunodeficiency virus.
• For female subjects, pregnancy or lactation.