Osemitamab (also known as TST001) is a humanized IgG1-kappa monoclonal antibody that specifically targets claudin-18.2 (CLDN18.2), a tight-junction protein that is normally confined to gastric mucosal epithelial cells but becomes aberrantly exposed and overexpressed in a variety of solid tumors, including gastric, pancreatic, esophageal, and ovarian cancers. As an IgG1 isotype, it is engineered to engage immune effector functions such as antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). By binding to CLDN18.2, osemitamab aims to induce tumor cell killing through these immune mechanisms and potentially disrupt tumor-specific signaling. It is being developed as a potential therapeutic for CLDN18.2-positive advanced solid tumors, with a focus on gastric/gastroesophageal junction adenocarcinoma.