Exploiting glycans for a new approach to treat cancers

Sialic acids are nine-carbon carbohydrates that play essential roles in human health and disease. In mammals, the two predominant forms are N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), with CMP-Neu5Gc generated from CMP-Neu5Ac through the action of the cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) enzyme. In humans, Neu5Gc is absent due to species-specific pseudogenisation of the CMAH gene. Emerging evidence indicates that interactions between Neu5Ac-containing glycans in the tumour microenvironment and Sialic acid-binding immunoglobulin-like lectin (Siglec) receptors expressed on tumour-infiltrating immune cells function as a glyco-immune checkpoint, contributing to tumour-associated immunosuppression and representing a promising target for cancer immunotherapy.

The primary objective of this study was to investigate glycan-based therapeutic strategies for prostate cancer, including an immunotherapy approach to genetically re-engineer tumour cell surface glycans through restoration of CMAH activity and conversion of Neu5Ac to Neu5Gc, thereby enhancing selective immune recognition of cancer cells. LNCaP prostate cancer cells were transfected with the rat CMAH gene, resulting in preferential incorporation of Neu5Gc into mucin-associated O-glycans, which are key mediators of immune suppression. Enzymatic removal of sialic acids using sialidase led to a marked reduction in Neu5Gc levels, while cell-tracing experiments demonstrated the transfer of Neu5Gc to neighbouring cells, supporting the existence of a glycan amplification loop within the tumour microenvironment. In parallel, HEK-293 cells transfected with rat CMAH exhibited reduced binding to Siglec-2 through Siglec-15, with the exception of Siglec-10, indicating the potential for disruption of Siglec-mediated immune inhibition. In addition, the prodrug glyco-gemcitabine was investigated for selective cytotoxicity in prostate-specific membrane antigen (PSMA) presenting prostate cancer cells following cleavage by a bacterial glycosidase AvpGS, targeted to cancer cell surfaces by conjugation to PSMA-targeting peptides.

Overall, this study identifies tumour-associated glycans as compelling immunotherapeutic targets and supports a novel glyco-immunomodulatory strategy aimed at restoring immune recognition at the cancer cell surface.