Metabolic oligosaccharide engineering with N-Acyl functionalized ManNAc analogs: cytotoxicity, metabolic flux, and glycan-display considerations.

Metabolic oligosaccharide engineering (MOE) is a maturing technology capable of modifying cell surface sugars in living cells and animals through the biosynthetic installation of non-natural monosaccharides into the glycocalyx. A particularly robust area of investigation involves the incorporation of azide functional groups onto the cell surface, which can then be further derivatized using "click chemistry." While considerable effort has gone into optimizing the reagents used for the azide ligation reactions, less optimization of the monosaccharide analogs used in the preceding metabolic incorporation steps has been done. This study fills this void by reporting novel butanoylated ManNAc analogs that are used by cells with greater efficiency and less cytotoxicity than the current "gold standard," which are peracetylated compounds such as Ac₄ ManNAz. In particular, tributanoylated, N-acetyl, N-azido, and N-levulinoyl ManNAc analogs with the high flux 1,3,4-O-hydroxyl pattern of butanoylation were compared with their counterparts having the pro-apoptotic 3,4,6-O-butanoylation pattern. The results reveal that the ketone-bearing N-levulinoyl analog 3,4,6-O-Bu₃ ManNLev is highly apoptotic, and thus is a promising anti-cancer drug candidate. By contrast, the azide-bearing analog 1,3,4-O-Bu₃ ManNAz effectively labeled cellular sialoglycans at concentrations ∼3- to 5-fold lower (e.g., at 12.5-25 µM) than Ac₄ ManNAz (50-150 µM) and exhibited no indications of apoptosis even at concentrations up to 400 µM. In summary, this work extends emerging structure activity relationships that predict the effects of short chain fatty acid modified monosaccharides on mammalian cells and also provides a tangible advance in efforts to make MOE a practical technology for the medical and biotechnology communities.