Cholera is an epidemic infectious diarrhoeal disease that for centuries has proven its frightening ability to cause rapid and widespread loss of human life. All symptoms associated with cholera are a result of rapid dehydration due to infection by pathogenic strains of the bacterium Vibrio cholerae. The damaging effects associated with cholera are mainly attributed to the toxin, which is secreted by the bacterium and infects cells lining the gastrointestinal tract. A sialidase, also secreted by the bacterium, is believed to facilitate toxin uptake by the gastrointestinal epithelium. V. cholerae sialidase is therefore a potential target for therapeutic intervention. A survey of the literature reveals that sialidases from different species share common features with respect to their structure, substrate specificity and catalytic mechanism. The unsaturated sialic acid, Neu5Ac2en, inhibits most exosialidases with a dissociation constant of inhibitor of -10-4 to-10-6 M and has frequently been used as a template in the design of more potent sialidase inhibitors. In the case of V. cholerae sialidase, there have been no inhibitors reported to date that are significantly more potent than Neu5Ac2en itself The present research aimed to develop a range of mimics of Neu5Ac2en, which contain various substituents to replace the C-6 glycerol side chain, as potential inhibitors of V cholerae sialidase. The x-ray crystal structure of V cholerae sialidase was used to explore potential interactions between active site residues and C-6 modified Neu5Ac2en mimetics of known inhibitory potency. Opportunities for interactions within the glycerol side chain pocket in the active site of V cholerae sialidase are discussed. A novel synthetic strategy was developed for the synthesis of a series of glucuronidebased Neu5Ac2en mimetics starting from readily available GIcNAc. This approach was employed for the preparation of Neu5Ac2en mimetics that contained an ether or thioether substituent as replacement of the glycerol side chain of Neu5Ac2en. Progress was also made towards the synthesis of a series of C-6 acylamino Neu5Ac2en mimetics. Analysis by 1H NMR spectroscopy showed that the acylamino derivatives adopted a half-chair conformation that was similar to the conformation of Neu5Ac2en but different to the conformation adopted by the ether and thioether derivatives prepared. The inhibitory activity of the C-6 ether and thioether Neu5Ac2en mimetics prepared was evaluated in vitro using an enzyme assay. It was found that most of the derivatives inhibited V. cholerae sialidase with a K1 of approximately 1O-4 M. The derivatives containing a hydrophobic side chain were found to be slightly more potent compared to derivatives with more hydrophilic side chains. A more detailed study of binding interactions between the C-6 thioether Neu5Ac2en mimetics and V cholerae sialdiase was carried out using STD 1H NMR spectroscopy and computational molecular modelling.

The combat against viral diseases has been, and still is, a major challenge in the field of drug development. Viruses are intracellular parasites that use the host cell ma-chinery for their replication and release. Therefore it is difficult to target and destroy the viral particle without disturbing the essential functions of the host cell. This thesis describes studies towards antiviral agents targeting adenovirus type 37 (Ad37), which causes the severe ocular infection epidemic keratoconjunctivitis (EKC). Cell surface oligosaccharides serve as cellular receptors for many pathogens, including viruses and bacteria. For EKC-causing adenoviruses, cell surface oligo-saccharides with terminal sialic acid have recently been shown to be critical for their attachment to and infection of host cells. The work in this thesis support these re-sults and identifies the minimal binding epitope for viral recognition. As carbo-hydrate–protein interactions in general, the sialic acid–Ad37 interaction is very weak. Nature overcomes this problem and vastly improves the binding affinity by presenting the carbohydrates in a multivalent fashion. Adenoviruses interact with their cellular receptors via multiple fiber proteins, whereby it is likely that the ideal inhibitor of adenoviral infections should be multivalent. This thesis includes design and synthesis of multivalent sialic acid glycoconjugates that mimic the structure of the cellular receptor in order to inhibit adenoviral attachment to and infection of human corneal epithelial (HCE) cells. Synthetic routes to three different classes of sialic acid conjugates, i.e. derivatives of sialic acid, 3’-sialyllactose and N-acyl modified sialic acids, and their multivalent counterparts on human serum albumine (HSA) have been developed. Evaluation of these conjugates in cell binding and cell infectivity assays revealed that they are effective as inhibitors. Moreover the results verify the hypothesis of the multivalency effect and clearly shows that the power of inhibition is significantly increased with higher orders of valency. Potential inhibi-tors could easily be transferred to the eye using a salve or eye drops, and thereby they would escape the metabolic processes of the body, a major drawback of using carbohydrates as drugs. The results herein could therefore be useful in efforts to develop an antiviral drug for treatment of EKC.