Sialic acid derivatives and mimetics: tools for the investigation of sialic acid processing enzymes

I. Synthesis and inhibitory activity of sialic acid derivatives targeting viral sialate-O-acetylesterases and sialate-O-acetyltransferases Sialate-O-acetylation is a common structural modification of sialic acid, which has been associated with many human disease states (including cancer and autoimmune disease). This highly regulated and tissue-specific modification is carried out by sialate- O-acetylesterase (SOAE) and sialate-O-acetyltransferase (SOAT) enzymes. The availability of these enzymes make inhibition studies a viable endeavour, considering that SOAT/SOAE inhibitors may provide interesting tools/drug leads for the development of antiviral compounds or treatments for various disease states. A synthesis of suitable 4-O- and 9-O-functionalised sialic acid derivatives has been established which enabled the investigation of 4- and 9-sialate-O-acetylesterase enzymes. Sialic acid derivatives were screened for the inhibition of a set of viral SOAEs and while no inhibition of 4-SOAE could be detected, a 9-O-methyl derivative showed inhibition of the recombinant influenza C virus SOAE. The functionalised sialic acid motif thus serves as an initial template for the design and synthesis of future sialic acid derivatives towards SOAT/SOAT inhibition. II. New tools for the characterization and investigation of influenza virus neuraminidases: towards novel influenza virus sensors Tamiflu™ (Oseltamivir), has been employed as a mimetic of the sialic acid “oxocarbenium” intermediate formed during enzymatic hydrolysis, leading to inhibition of virus-bound neuraminidase (NA) enzyme. Phospha-isosteres of oseltamivir provide access to monoesters which retain the efficacy of the pharmacophore and allow the synthesis of novel influenza neuraminidase-specific materials. Phospha-oseltamivir-stabilised gold nanoparticles (“TamiGold”) have been synthesised and NA inhibition studies with “small TamiGold” show activity against influenza virus strains investigated compared to control gold nanoparticles. The binding interactions displayed by “large TamiGold” may provide the basis for a colorimetric method of influenza detection and as such a novel prototype influenza sensor. To the best of our knowledge this is the first example of a multivalent approach to influenza virus binding utilising sialylmimetic scaffolds immobilised on a nanoparticle platform which specifically target the NA (instead of the hemagglutinin, HA). The synthesis of phospha-oseltamivir conjugates and their ligation to biological reporter groups afford small molecule tools with high affinity and selectivity towards influenza NA. These derivatives can be applied towards novel multivalent phospha-oseltamivir materials and used as novel diagnostics, independent of existing methods.