e15022 Background: The human neuronal pentraxin receptor (NPTXR) gene encodes a type II transmembrane protein that functions as a trans-synaptic organizer and anchors neuronal pentraxin complexes to plasma membranes. Beside a subset of neuronal cells (Hippocampus and Cerebral Cortex), expression of NPTXR is not detected in all other tissues [ The Human Protein Atlas database]. In recent studies [Kanda et al. (2020) Mol. Cancer. doi: 10.1186/s12943-020-01251-0], transcriptome and bioinformatics analysis of gastric cancer (GC) tissues from patients with or without metastasis have revealed that NPTXR expression was associated with disease progression. NPTXR expression was also correlated negatively with survival in GC patients. It appears that NPTXR does not have a major physiological function; e.g., NPTXR−/− mice showed no abnormalities in reproduction, development, metabolism, or motor function. NPTXR is expressed in a number of GI cancers, i.e., gastric, esophageal, colorectal, pancreatic, bladder, liver, as well as breast and thyroid cancer. YB-800 is a fully humanized monoclonal antibody against NPTXR (identified by in silico analysis). Methods: The present studies were conducted to provide a rationale for the design of a YB-800-based antibody drug conjugate (ADC) for therapeutic intervention in oncology. Hence, YB-800 binding to and internalization in selected tumor cells, as well as immunohistochemistry (IHC) in tumor biopsies, were evaluated. Results: Taken together experimental data confirm that: (i) YB-800 binds to a new target (NPTXR) on selected tumor cells; confirmed by FACS analysis; (ii) IHC analysis in tumor samples demonstrates specific immunoreaction in bladder, gastric, prostate, hepatocellular carcinoma, colorectal cancer and cervical cancer, but not in adjacent healthy tissues; and (iii) once YB-800 binds to NPTXR, the conjugate is internalized in selected tumor cells in a time-dependent manner. Conclusions: In conclusion, YB-800 represents a first in class ADC candidate that could be used as novel treatment for gastro-intestinal cancers. Hence, as initial proof of concept, tesirine (DNA intercalating agent) and deruxtecan (topoisomerase I inhibitor) containing protease-sensitive linkers were coupled to YB-800 by stochastic maleimide conjugations to interchain cysteines. Initial results with YB-800-based ADCs will be discussed.