ZL-1211

Primary Objectives Phase I: Determine the maximum tolerated dose (MTD) or maximum administered dose (MAD) of ZL-1211 (if MTD is not determined), and determine the recommended Phase II dose (RP2D) of ZL-1211 Phase I and Phase II: Evaluate the safety and tolerability of ZL-1211 in patients with unresectable or metastatic solid tumors Phase II: Evaluate the preliminary anti-tumor activity of ZL-1211 Secondary Objectives Phase I and Phase II: Evaluate the pharmacokinetics (PK) of ZL-1211 Phase I and Phase II: Evaluate the correlation between CLDN18.2 expression level and ZL-1211 treatment response Phase I and Phase II: Evaluate the immunogenicity of ZL-1211 Phase II: Evaluate the preliminary anti-tumor activity of ZL-1211 Exploratory Objectives Phase II: Further evaluate the preliminary anti-tumor activity of ZL-1211 Evaluate the expression of CLDN18.2 for the future development of a companion diagnostic for CLDN18.2 immunohistochemistry (IHC). This will be assessed in all patient samples included in the study. Assess the tumor immune microenvironment (as a potential predictive biomarker for ZL-1211 effectiveness). This objective will not be included for patients enrolled at the mainland China study centers.