Synthesis of delta4-beta-D-glucopyranosiduronic acids as mimetics of 2,3-unsaturated sialic acids for sialidase inhibition

Uronosyl phosphonate-based sialidase inhibitor synthesis and conformational analysis

With a view to development of novel sialidase inhibitors, mimetics of the natural inhibitor Neu5Ac2en have been prepared in which a phosphonate group replaces the sialic acid glycerol side chain. Different hex-4-en derivatives adopt half-chair conformations that place the glycosyl phosphonate in an equatorial position. For the α-L-threo-hex-4-en derivative this conformation is equivalent to that of Neu5Ac2en, and opposite to that seen for alkyl O-glycosides with the same overall stereochemistry.

Synthesis of some novel D-glucuronic acid acetylated derivatives as potential anti-tumor agents

A structurally diverse series of Δ(4,5) -uronamide derivatives have been chemically synthesized starting from D-glucuronic acid itself by means of acetylation, activation, amide bond formation and base-catalyzed elimination protocols. Structure elucidation for all products along with optimization of the synthetic steps is described. The synthesized compounds were evaluated for their in-vitro anti-tumor activity against MCF-7, TK-10 and UACC-62 cell lines. The compounds 5, 11, 13, 15 and 16 were the most active against TK-10 cell line. On the other hand, the most active compounds against the MCF-7 cell line were 11 and 15. However, compounds 5, 7, 11, 13, 15 and 16 were the most active against the UACC-62 cell line.

Neuraminidase-1, a subunit of the cell surface elastin receptor, desialylates and functionally inactivates adjacent receptors interacting with the mitogenic growth factors PDGF-BB and IGF-2

We recently established that the elastin-binding protein, which is identical to the spliced variant of beta-galactosidase, forms a cell surface-targeted complex with two proteins considered "classic lysosomal enzymes": protective protein/cathepsin A and neuraminidase-1 (Neu1). We also found that cell surface-residing Neu1 can desialylate neighboring microfibrillar glycoproteins and facilitate the deposition of insoluble elastin, which contributes to the maintenance of cellular quiescence. Here we provide evidence that cell surface-residing Neu1 contributes to a novel mechanism that limits cellular proliferation by desialylating cell membrane-residing sialoglycoproteins that directly propagate mitogenic signals. We demonstrated that treatment of cultured human aortic smooth muscle cells (SMCs) with either a sialidase inhibitor or an antibody that blocks Neu1 activity induced significant up-regulation in SMC proliferation in response to fetal bovine serum. Conversely, treatment with Clostridium perfringens neuraminidase (which is highly homologous to Neu1) decreased SMC proliferation, even in cultures that did not deposit elastin. Further, we found that pretreatment of aortic SMCs with exogenous neuraminidase abolished their mitogenic responses to recombinant platelet-derived growth factor (PDGF)-BB and insulin-like growth factor (IGF)-2 and that sialidosis fibroblasts (which are exclusively deficient in Neu1) were more responsive to PDGF-BB and IGF-2 compared with normal fibroblasts. Furthermore, we provide direct evidence that neuraminidase caused the desialylation of both PDGF and IGF-1 receptors and diminished the intracellular signals induced by the mitogenic ligands PDGF-BB and IGF-2.

Modelling, synthesis and biological evaluation of novel glucuronide-based probes of Vibrio cholerae sialidase

The development of sialidase inhibitors is an area of continuing interest due to their potential use as therapeutic agents to combat viral and bacterial infections. Herein, we report our studies involving the sialidase from the pathogen Vibrio cholerae, through the modelling, synthesis and biological evaluation of mimetics of 5-acetamido-2,6-anhydro-3,5-dideoxy-D-glycero-D-galacto-non-2-enonic acid (Neu5Ac2en, 1), a naturally occurring sialidase inhibitor. These mimetics are O- and S-glycosides of N-acetyl-D-glucosaminuronic acid in which the aglycone portion effectively replaces the C-6 glycerol side chain of Neu5Ac2en (1). The choice of aglycones was aided by use of the X-ray crystal structure of V. cholerae sialidase complexed with Neu5Ac2en (1). All Neu5Ac2en mimetics tested were found to inhibit V. cholerae sialidase as determined using a standard fluorometric assay.

Lysosomal Sialidase (Neuraminidase-1) Is Targeted to the Cell Surface in a Multiprotein Complex That Facilitates Elastic Fiber Assembly

We have established previously that the 67-kDa elastin-binding protein (EBP), identical to the spliced variant of beta-galactosidase, acts as a recyclable chaperone that facilitates secretion of tropoelastin. (Hinek, A., Keeley, F. W., and Callahan, J. W. (1995) Exp. Cell Res. 220, 312-324). We now demonstrate that EBP also forms a cell surface-targeted molecular complex with protective protein/cathepsin A and sialidase (neuraminidase-1), and provide evidence that this sialidase activity is a prerequisite for the subsequent release of tropoelastin. We found that treatment with sialidase inhibitors repressed assembly of elastic fibers in cultures of human skin fibroblasts, aortic smooth muscle cells, and ear cartilage chondrocytes and caused impaired elastogenesis in developing chick embryos. Fibroblasts derived from patients with congenital sialidosis (primary deficiency of neuraminidase-1) and galactosialidosis (secondary deficiency of neuraminidase-1) demonstrated impaired elastogenesis, which could be reversed after their transduction with neuraminidase-1 cDNA or after treatment with bacterial sialidase, which has a similar substrate specificity to human neuraminidase-1. We postulate that neuraminidase-1 catalyzes removal of the terminal sialic acids from carbohydrate chains of microfibrillar glycoproteins and other adjacent matrix glycoconjugates, unmasking their penultimate galactosugars. In turn, the exposed galactosugars interact with the galectin domain of EBP, thereby inducing the release of transported tropoelastin molecules and facilitating their subsequent assembly into elastic fibers.

Unsaturated N-acetyl- D-glucosaminuronic acid glycosides as inhibitors of influenza virus sialidase

The threat of pandemic influenza is a significant concern of governments worldwide. There is a very limited and relatively expensive armament to tackle such a pandemic should it occur. This fact provides much impetus to the scientific community for the discovery of new and less expensive anti-influenza drugs. Our longstanding interest in the inhibition of influenza virus sialidase, coupled with the development of simple carbohydrates that mimic an unsaturated derivative of the enzyme's naturally-occurring ligand, N-acetylneuraminic acid, has led us to investigate the development of influenza virus sialidase inhibitors based on these mimetics. We have successfully prepared a range of these compounds, in good yield, from the relatively inexpensive carbohydrate N-acetylglucosamine utilising a short synthetic procedure. We have employed a sialidase inhibition assay for biological evaluation of the target compounds and to our delight these mimetics have displayed significant inhibition of influenza virus sialidase.

An investigation of the activity of recombinant rat skeletal muscle cytosolic sialidase

Rat cytosolic sialidase is expressed at elevated levels in skeletal muscle and is believed to play a role in the myogenic differentiation of muscle cells. Here, we observed varying levels of enhancement of sialidase activity in the presence a range of divalent cations. In particular, a significant enhancement of activity was observed in the presence of Ca2+. Conversely, inhibition of the sialidase activity was found when the enzyme was incubated in the presence of Cu2+, EDTA, and a range of carbohydrate-based inhibitors. Finally, an investigation of the enzymatic hydrolysis of a synthetic substrate, 4-methylumbelliferyl N-acetyl-alpha-D-neuraminide, by 1H NMR spectroscopy revealed that the reaction catalysed by rat skeletal muscle cytosolic sialidase proceeds with overall retention of anomeric configuration. This result further supports the notion that all sialidases appear to be retaining enzymes.

An efficient approach to N-acetyl-d-glucosaminuronic acid-based sialylmimetics as potential sialidase inhibitors

A novel approach to the synthesis of beta-glycosides of N-acetyl-D-glucosaminuronic acid, in six steps and good overall yield from N-acetyl-d-glucosamine, has been developed. The key synthetic step was the Lewis acid mediated O-glycosidation of methyl 1,3,4-tri-O-pivaloyl-N-acetyl-D-glucosaminuronate (11). Elaboration of glucosaminuronides 15 and 18 provided novel sialylmimetics 21 and 22, which showed inhibition of Vibrio cholerae sialidase.

Syntheses and neuraminidase inhibitory activity of multisubstituted cyclopentane amide derivatives

In further studies aimed toward identifying effective and safe inhibitors of influenza neuraminidases, we synthesized a series of multisubstituted cyclopentane amide derivatives. Amides prepared were 14 examples of N-substituted alkyl or aralkyl types from primary amines, 13 examples of the N,N-disubstituted alkyl, aralkyl, or substituted-alkyl type from secondary amines, and 12 examples from cycloaliphatic or substituted cycloaliphatic secondary amines. These compounds bearing two chiral centers, at position-1 in the ring and position-1' in the side chain attached at position 3, were tested for their ability to inhibit A and B forms of influenza neuraminidase. The 1-ethylpropylamide, diethylamide, dipropylamide, and 4-morpholinylamide showed very good inhibitory activity (IC(50) = 0.015-0.080 microM) vs the neuraminidase A form, but modest activity (IC(50) = 3.0-9.2 microM) vs the neuraminidase B form. Since the parent amides bear two chiral centers (C-1 and C-1'), three of the better inhibitors were tested at higher levels of diastereomeric purity. The diastereomers corresponding to the active forms of the 1-(ethyl)propylamide, the diethylamide, and the dipropylamide (all of the same configuration at the C-1' chiral center), and the diastereomer of the diethylamide representing the active form at both C-1' and C-1 were isolated or synthesized from precursors that were isolated as diastereomers. These diastereomers showed some improvement in neuraminidase inhibition over the parent diastereomeric mixtures. 1-Carboxy-1-hydroxy derivatives of the best active compounds, the diethylamide and the dipropylamide, were also prepared. These compounds were not as active as the compounds without the 1-hydroxy group. In an in vivo study, the C-1' active isomer of the diethylamide from the 1-carboxy series was tested in influenza-infected mice by oral and intranasal administration and found to be very effective only intranasally in preventing weight loss at doses as low as 0.1 (mg/kg)/day.

Saturation transfer difference (STD) 1H-NMR experiments and in silico docking experiments to probe the binding of N-acetylneuraminic acid and derivatives to Vibrio cholerae sialidase

Saturation transfer difference (STD) (1)H NMR experiments were used to probe the epitope binding characteristics of the sialidase [EC 3.2.1.18] from the bacterium Vibrio cholerae, the causative agent of cholera. Binding preferences were investigated for N-acetylneuraminic acid (Neu5Ac, 1), the product of the sialidase catalytic reaction, for the known sialidase inhibitor 5-acetamido-2,6-anhydro-3,5-dideoxy-D-glycero-D-galacto-non-2-enoic acid (Neu5Ac2en, 2), and for the uronic acid-based Neu5Ac2en mimetic iso-propyl 2-acetamido-2,4-dideoxy-alpha-L-threo-hex-4-enopyranosiduronic acid (3), in which the native glycerol side-chain of Neu5Ac2en is replaced with an O-iso-propyl ether. The STD experiments provided evidence, supporting previous studies, that Neu5Ac (1) binds to the sialidase as the alpha-anomer. Docking experiments using DOCK (version 4.0.1) revealed further information regarding the binding characteristics of the enzyme active site in complex with Neu5Ac2en (2) and the Neu5Ac2en mimetic (3), indicating an expected dominant interaction of the acetamide moiety with the protein.

Synthesis and biological evaluation of sialylmimetics as rotavirus inhibitors

Rotaviruses cause severe gastroenteritis in infants and are estimated to be responsible for over 600 000 deaths annually, primarily in developing countries. The development of potential inhibitors of this virus is therefore of great interest, particularly since the safety and efficacy of rotaviral vaccines has recently been questioned. This study describes the synthesis of a variety of compounds that can be considered as mimetics of N-acetylneuraminic acid thioglycosides and the subsequent in vitro biological evaluation of these sialylmimetics as inhibitors of rotaviral infection. Our results show that readily accessible carbohydrate-based compounds have the potential to act as inhibitors of rotaviral replication in vitro, presumably through inhibition of the rotaviral adhesion process.

Synthesis of linear-B saccharopeptides via enzymatic galactosylation of non-natural glucosamide acceptors

A series of D- and L-glycopyranuronic acids are coupled to glucosamines to give saccharopeptides. These 'disaccharides', in which the acetyl moiety of the natural N-acetyl-glucosamine is replaced by various sugar acids, turned out to be surprisingly good substrates for beta(1-4)-galactosyl-transferase and alpha(1-3)-galactosyl-transferase. The enzymes transfer successively two galactose units from the donor UDP-galactose onto these acceptor substrates, despite the far reaching alterations, regio- and stereospecifically in the expected manner to give linear-B saccharopeptides.

Synthesis of novel sialylmimetics as biological probes

Glycomimetics are increasingly being recognised as powerful tools in the search for novel compounds that possess useful biological properties. This paper describes our preliminary efforts towards the development of novel mimetics of sialic acid thioglycosides. These sialylmimetics are readily prepared and have been shown, in some instances, to have biological properties similar to sialic acid thioglycosides.

Rapid access to uronic acid-based mimetics of Kdn2en from D-glucurono-6,3-lactone

A concise route to novel mimetics of Kdn2en, based on delta4-uronic acids, from D-glucurono-6,3-lactone is presented. Uronic acid-based mimetics in which an aliphatic ether (O-glycoside), a thioether (S-glycoside), or acetamide takes the place of the natural C-6 glycerol sidechain of the sialic acid were synthesized from the key intermediate, methyl 2,3,4-tri-O-acetyl-alpha-D-glucopyranosyluronate bromide.