The Development of Novel Tools for the Metabolic Labelling of Glycans in Cancer

Abstract

Aberrant cell surface glycosylation is key in tumour proliferation, metastasis, and immune evasion, with hypersialylation one of the alterations most observed in tumour cells. Changes in the glycome can be monitored by incorporating unnatural sugars bearing bioorthogonal chemical reporters. Methylcyclopropenes are one such reporter with good incorporation and kinetics of the subsequentinverse-electron-demand Diels-Alder reaction (IED-DA) with tetrazines. The methylcyclopropene tagged monosaccharide most often used in vitro (Ac4ManCyoc) has yet to be used for in vivo applications. This is believed to be due to its lack of aqueous solubility. Herein a method is described for variable acetylation of tagged monosaccharides. Enhanced in vitro glycanlabelling was observed using Ac3ManCyoc or Ac2ManCyoc relative to Ac4ManCyoc. This enhancement was consistent across multiple cell lines. Other tagged sugars showed reduced labelling with decreased acetylation, presumably due to decreased cell permeability. Initial attempts at in vivo labelling of hypersialylation using Ac3ManCyoc are also described. Ac3ManCyoc and another tagged sugar Ac4GalNAz were then used for dual-sugar labelling. The ratio of incorporation of the different monosaccharides was compared across multiple cell lines. In general, a higher Ac3ManCyoc:Ac4GalNAz ratio was observed in faster growing cell lines. Thus, this could be a useful tool in assessing tumour aggressiveness. ODIBO, a strained dibenzocyclooctyne with an enhanced rate of cycloaddition with azides was synthesised, but attempts to reduce its hydrophobicity had limited success; reduced in vitro glycan labelling relative to TMDIBO (another dibenzocyclooctyne) was observed. The synthesis is reported of the first (E,Z)-cyclooctadiene. Its rate of IED-DA with tetrazines proved to be amongst the fastest reported under the same conditions. Finally, synthesis of some novel tagged fucose analogues is described. Glycan-labelling using these analogues is studied, targeting the well-known enhanced fucosylation in cancer.