An antibody-drug conjugate (ADC) composed of a humanized monoclonal antibody directed against the tumor-associated antigen (TAA) claudin 18.2 (CLDN18.2; A2 isoform of claudin-18) conjugated via a cleavable linker, to an undisclosed cytotoxic agent, with potential antineoplastic activity. Upon administration, anti-CLDN18.2 ADC TQB2103 specifically targets and binds to CLDN18.2 expressed on tumor cells. Upon endocytosis and the transfer to lysosomes, the payload is released via enzymatic cleavage of the linker. The cytotoxic agent causes DNA damage in and kills CLDN18.2-expressing tumor cells. Through bystander effect, TQB2103 is also able to kill adjacent tumor cells. CLDN18.2, a tight junction protein and stomach-specific isoform of claudin-18, is expressed on a variety of tumor cells. Its expression in healthy tissues is strictly confined to short-lived differentiated epithelial cells of the gastric mucosa.