George W. J. Fleet

Sugar-mimic glycosidase inhibitors: natural occurrence, biological activity and prospects for therapeutic application

Abstract Alkaloids mimicking the structures of monosaccharides are now believed to be widespread in plants and microorganisms, and these sugar mimics inhibit glycosidases because of a structural resemblance to the sugar moiety of the natural substrate. Naturally occurring sugar mimics with a nitrogen in the ring are classified into five structural classes: polyhydroxylated piperidines, pyrrolidines, indolizidines, pyrrolizidines and nortropanes. Glycosidases are involved in a wide range of important biological processes, such as intestinal digestion, post-translational processing of glycoproteins and the lysosomal catabolism of glycoconjugates. The realization that alkaloidal sugar mimics might have enormous therapeutic potential in many diseases such as viral infection, cancer and diabetes has led to increasing interest and demand for these compounds. Most of these effects can be shown to result from the direct or indirect inhibition of glycosidases. The glycosphingolipid (GSL) storage diseases are relatively rare hereditary disorders that are severe in nature and frequently fatal. Possible strategies for the treatment of these lysosomal storage diseases include enzyme replacement therapy, gene therapy and substrate deprivation. Recently, quite a new therapy for lysosomal storage diseases has been reported, namely a ‘chemical chaperone therapy’ for Fabry disease. In this report, the structural basis for the specificity of inhibition of alkaloidal sugar mimics and their current and potential application to biomedical problems will be reviewed.

Polyhydroxylated alkaloids - natural occurrence and therapeutic applications

Over one hundred polyhydroxylated alkaloids have been isolated from plants and micro-organisms. These alkaloids can be potent and highly selective glycosidase inhibitors and are arousing great interest as tools to study cellular recognition and as potential therapeutic agents. However, only three of the natural products so far have been widely studied for therapeutic potential due largely to the limited commercial availability of the other compounds.

Inhibition of HIV replication by amino-sugar derivatives

The plant alkaloids castanospermine, dihydroxymethyldihydroxypyrrolidine and deoxynojirimycin have recently been shown to have potential anti‐HIV activity [(1987) Proc. Natl. Acad. Sci. USA 84, 8120–8124; (1987) Nature 330, 74–77; (1987) Lancet i, 1025–1026]. They are thought to act by inhibiting α‐glucosidase I, an enzyme involved in the processing of N‐linked oligosaccharides on glycoproteins. We report here the relative efficacy of a spectrum of amino‐sugar derivatives as inhibition of HIV cytopathicity. Several α‐glucosidase inhibitors and α‐fucosidase inhibitors were found to be active at concentrations which were non‐cytotoxic.

Potent competitive inhibition of α-galactosidase and α-glucosidase activity by 1,4-dideoxy-1,4-iminopentitols: syntheses of 1,4-dideoxy-1,4-imino-d-lyxitol and of both enantiomers of 1,4-dideoxy-1,4-iminoarabinitol

Abstract The syntheses of 1,4-dideoxy-1,4-imino-D-lyxitol (3), 1,4-dideoxy-1,4-imino- d -arabinitol (4) and 1,4-dideoxy-1,4-imino- l -arabinitol (5) are reported; (3) is a potent competitive inhibitor of α-galactosidase (green coffee beans) and (4) a competitive inhibitor of α-glucosidase (Brewers yeast) suggesting that iminopentitols have considerable potential as glycosidase inhibitors. (4) was found to be identical to an alkaloid recently isolated from Angylocalyx boutiqueanus .

Glycosidase inhibition by plant alkaloids which are structural analogues of monosaccharides

Abstract ]The inhibitory activities of three plant alkaloids, deoxynojirimycin, 2,5-dihydroxymethyl-3,4-dihydroxypyrrolidine and 1,5 dideoxy-1,5-imino- D -mannitol towards glycosidases from several sources have been compared. These are structural analogues of D -glucose, D -fructose and D -mannose respectively. The occurrence of 2,5-dihydroxymethyl-3,4-dihydroxypyrrolidine in Lonchocarpus sericeus seed is confirmed and has been shown to be responsible for the glucosidase inhibition wrongly attributed to 1,5-dideoxy-1,5-imino- D -mannitol in a previous report.

Iminosugars as therapeutic agents: recent advances and promising trends

For the purpose of this article, iminosugars are polyhydroxylated secondary and tertiary amines in which the molecules resemble monosaccharide sugars in which the ring oxygen is replaced by the nitrogen. The bicyclic structures may biologically resemble disaccharides. Very few iminosugars have been available up to now for evaluation of their pharmaceutical applications. The early compounds were discovered and selected for study due to glycosidase inhibition, which is now known to not be necessary for pharmacological activity and may cause off-target effects. Glyset® and Zavesca®, derived from the glucosidase-inhibiting natural product 1-deoxynojirimycin, are the first two examples of iminosugar drugs. Since the discovery of this first generation, many new natural products have been identified with a wide range of biological activities but few are widely available. Among the biological properties of these compounds are good oral bioavailability and very specific immune modulatory and chaperoning activity. Although the natural products from plants and microorganisms can have good specificity, modifications of the template natural products have been very successful recently in producing bioactive compounds with good profiles. The field of iminosugars continues to open up exciting new opportunities for therapeutic agent discovery and offers many new tools for precisely modifying carbohydrate structures and modulating glycosidase activity in vivo. Current efforts are directed towards a greater range of structures and a wider range of biochemical targets.

Inhibition of mammalian digestive disaccharidases by polyhydroxy alkaloids.

Several polyhydroxy alkaloids, including the eight presently known to occur in plants, have been compared as inhibitors of mouse gut digestive disaccharidases. The indolizidine castanospermine inhibited all activities tested, but others showed a selectivity which could be of value in studies of carbohydrate digestion and errors of metabolism.

Potent inhibition of glycogen phosphorylase by a spirohydantoin of glucopyranose: First pyranose analogues of hydantocidin

Abstract The synthesis of two epimeric spirohydantoins of glucopyranose provides the first examples of pyranose analogues of hydantocidin: molecular modelling correctly predicted that one of the epimers would be a potent inhibitor of glycogen phosphorylase. This is the first example of specific enzyme inhibition by a spirohydantoin at the anomeric position of a sugar.

Molecular requirements of imino sugars for the selective control of N-linked glycosylation and glycosphingolipid biosynthesis

Abstract N -Butyl-deoxynojirimycin (NB-DNJ) has been approved for clinical trials as a potential therapy for Gaucher disease, a glycolipid lysosomal storage disorder. As this compound has both glycoprotein processing α-glucosidase and ceramide glucosyltransferase inhibitory activity, we have sought to determine the molecular basis for these two activities. NB-DNJ is known to resemble the positively charged oxocarbonium-like transition state for α-glucosidase I and the structure–function relationships we present now help to define the recognition epitope for the enzyme. Inhibition of ceramide glucosyltransferase by NB-DNJ was competitive for ceramide ( K i =7.4 μM) and non-competitive for UDP-glucose, indicating inhibitory activity is by ceramide mimicry. The presence of an N -alkyl chain was obligatory for transferase inhibition and increases in alkyl chain length provided a modest increase in inhibitory potency. By contrast, α-glucosidase inhibition was independent of the N -alkyl chain and changes in chain length. The effects of ring substitutions identified the C 3 hydroxyl group as being critical for both enzymes but C 1 and C 6 modifications led to a loss of transferase inhibition only. Attempts to rationalise these data for transferase inhibition using an energy minimised molecular model of NB-DNJ and ceramide predicted structural homology of three stereogenic centres and the N -alkyl chain of NB-DNJ, with the trans -alkenyl and N -acyl chain of ceramide. On the basis of these studies, modifications to imino sugar inhibitors can be suggested that allow a more selective approach for molecular inhibition of both ceramide glucosyltransferase and α-glucosidase I, leading to improved compounds for the potential treatment of lysosomal glycosphingolipid storage disorders and viral infections, respectively.

Polyhydroxylated pyrrolidines from sugar lactomes: Synthesis of 1,4-dideoxy-1,4-imino-d-glucitol from d-galactonolactone and syntheses of 1,4-dideoxy-1,4-imino-d-allitol, 1,4-dideoxy-1,4-imino-d-ribitol, and (2s,3r,4s)-3,4-dihydroxyproline from d-gulonolactone

Abstract The use of readily available sugar lactones in the synthesis of polyhydroxylated pyrrolidines is illustrated by the preparation of the glucosidase inhibitor 1,4-dideoxy-1, 4-imino-D-glucitol from D-galactonolactone and by the conversion of D-gulonolactone into 1,4-dideoxy-l,4-imino-D-allitol, 1,4-dideoxy-l,4-imino-D-ribitol, and (2S,3R,4S)-3,4-dihydroxyproline.