Studies on a Sulfoxide-Bridged Tn Antigen Mimetic: Interaction with Macrophage Galactose Lectin and Inhibition of Sialyltransferase ST6GALNAC1.

Aberrant O-glycosylation is a defining hallmark of epithelial cancers, where truncated mucin-type glycans such as Tn and sialyl-Tn (sTn) are prominently displayed. Despite their tumor specificity, these tumor-associated carbohydrate antigens (TACAs) elicit only weak immune responses, limiting their impact in vaccine-based immunotherapy. Growing evidence implicates two major factors in this poor immunogenicity: the intrinsic engagement of Tn/sTn with the immunosuppressive macrophage galactose-type lectin (MGL) on antigen-presenting cells and the "self" nature of Tn/sTn mucin carriers such as MUC1. Both processes critically depend on the N-acetyl functionalities of the Tn determinant. We previously developed a stable Tn mimetic, 2-deoxy-2-thio-α-O-galactoside (compound 1), which lacks the NHAc group and exhibits notable immunostimulatory properties in vivo. In this study, we provide new structural insights into the role of the NHAc moiety in the mimetic 1 presentation and its interaction with MGL, thereby advancing the design principles for next-generation Tn analogues with improved immunological behavior. An additional focus is on the pathogenic upregulation of sTn in tumors, primarily driven by overexpression of the sialyltransferase ST6GALNAC1. We demonstrate that mimetic 1 and its oxidized analogue 2 act as inhibitors of ST6GALNAC1representing, to the best of our knowledge, the first reported monosaccharide inhibitors of this enzyme. Although the inhibitory potency is modest, these compounds establish a valuable chemical starting point for targeting cancer-associated sialylation, an effort currently constrained by the lack of structural information for ST6GALNAC1.