Diverse monoclonal antibodies against the CA 19‐9 antigen show variation in binding specificity with consequences for clinical interpretation

The CA 19‐9 antigen is currently the best individual marker for the detection of pancreatic cancer. In order to optimize the CA 19‐9 assay and to develop approaches to further improve cancer detection, it is important to understand the specificity differences between CA 19‐9 antibodies and the consequential affect on biomarker performance. Antibody arrays enabled multiplexed comparisons between five different CA 19‐9 antibodies used in the analysis of plasma samples from pancreatic cancer patients and controls. Major differences were observed between antibodies in their detection of particular patient samples. Glycan array analysis revealed that certain antibodies were highly specific for the canonical CA 19‐9 epitope, sialyl‐Lewis A, while others bound sialyl‐Lewis A in addition to a related structure called sialyl‐Lewis C and modification with Nue5Gc. In a much larger patient cohort we confirmed the binding of Sialyl‐Lewis C glycan by one of the antibodies and showed that the broader specificity led to the detection of an increased number of cancer patients without increasing detection of pancreatitis patient samples. This work demonstrates that variation between antibody specificity for cancer‐associated glycans can have significant implications for biomarker performance and highlights the value of characterizing and detecting the range of glycan structures that are elevated in cancer.