Synthesis of a spacer-containing repeating unit of the capsular polysaccharide of Streptococcus pneumoniae type 23F

Simplified process for preparing native and depolymerized capsular polysaccharides of Streptococcus pneumoniae

Streptococcus pneumoniae is a major pathogen of bacterial pneumonia, meningitis, sepsis, and otitis media. The pathogenicity of this bacterium is largely attributed to its polysaccharide capsule, a protective layer around bacterial cell that enables bacteria to resist against host defense. Capsular polysaccharides (CPSs) of S. pneumoniae have been used as antigens to develop a variety of pneumococcal vaccines against invasive pneumococcal disease (IPD). These vaccines have been proven to be effective in reducing the incidence of IPD cases that are caused by vaccine-covered serotypes at the global scale. A crucial step in the manufacture of pneumococcal polysaccharide and conjugate vaccines is to purify native and depolymerized CPSs to meet strict quality standards in purity and structural integrity. The major impurities comprise proteins, nucleic acids and cell wall polysaccharides (CWPS). Traditionally, the removal of impurities to obtain purified native CPSs involves a complex process of purification, after which purified CPSs need to be further size-reduced to obtain depolymerized CPSs by multi-step approaches. In this study, we streamlined the process of CPS purification, which involves firstly ultrafiltration, followed by one-step acid precipitation, and finally diafiltration to obtain pure native CPSs. Furthermore, hydrolysis using trifluoroacetic acid (TFA) was integrated into the process to obtain purified depolymerized CPSs. The native and depolymerized CPSs produced by this optimized process were comparable to the materials obtained by the traditional approaches in purity and structural integrity, which would meet the quality standards of CPSs for vaccine production in the current edition of the European Pharmacopeia.

Synthesis of the pentasaccharide repeating unit with a conjugation-ready linker corresponding to the O-antigenic polysaccharide of Acinetobacter junii strain 65

A straightforward synthesis of the pentasaccharide with a readily available linker arm corresponding to the O-antigenic polysaccharide of Acinetobacter junii strain 65 has been achieved in good yield. The synthesis has been carried out using thioglycosides as glycosyl donor in the presence of a combination of N-iodosuccinimide (NIS) and trifluoromethanesulfonic acid (TfOH) as thiophilic activator. The yields of the glycosylation steps were very good with satisfactory stereochemistry at the glycosidic linkages. The pentasaccharide derivative has also been obtained using a one-pot iterative glycosylation strategy.

Chemical synthesis of natural and azido-modified UDP-rhamnose and -arabinofuranose for glycan array-based characterization of plant glycosyltransferases

Chemical syntheses of UDP-rhamnose and UDP-arabinofuranose and respective azido-modified analogues are reported. The prepared substrates are useful for the glycan array-based analysis of glycosyltransferases, as exemplified with the plant cell wall-biosynthetic enzymes PvXAT3, AtRRT4 and PtRRT5.

Glycoconjugates: Synthesis, Functional Studies, and Therapeutic Developments

Glycoconjugates are major constituents of mammalian cells that are formed via covalent conjugation of carbohydrates to other biomolecules like proteins and lipids and often expressed on the cell surfaces. Among the three major classes of glycoconjugates, proteoglycans and glycoproteins contain glycans linked to the protein backbone via amino acid residues such as Asn for N-linked glycans and Ser/Thr for O-linked glycans. In glycolipids, glycans are linked to a lipid component such as glycerol, polyisoprenyl pyrophosphate, fatty acid ester, or sphingolipid. Recently, glycoconjugates have become better structurally defined and biosynthetically understood, especially those associated with human diseases, and are accessible to new drug, diagnostic, and therapeutic developments. This review describes the status and new advances in the biological study and therapeutic applications of natural and synthetic glycoconjugates, including proteoglycans, glycoproteins, and glycolipids. The scope, limitations, and novel methodologies in the synthesis and clinical development of glycoconjugates including vaccines, glyco-remodeled antibodies, glycan-based adjuvants, glycan-specific receptor-mediated drug delivery platforms, etc., and their future prospectus are discussed.

Discovery of Semi- and Fully-Synthetic Carbohydrate Vaccines Against Bacterial Infections Using a Medicinal Chemistry Approach

The glycocalyx, a thick layer of carbohydrates, surrounds the cell wall of most bacterial and parasitic pathogens. Recognition of these unique glycans by the human immune system results in destruction of the invaders. To elicit a protective immune response, polysaccharides either isolated from the bacterial cell surface or conjugated with a carrier protein, for T-cell help, are administered. Conjugate vaccines based on isolated carbohydrates currently protect millions of people against Streptococcus pneumoniae, Haemophilus influenzae type b, and Neisseria meningitides infections. Active pharmaceutical ingredients (APIs) are increasingly discovered by medicinal chemistry and synthetic in origin, rather than isolated from natural sources. Converting vaccines from biologicals to pharmaceuticals requires a fundamental understanding of how the human immune system recognizes carbohydrates and could now be realized. To illustrate the chemistry-based approach to vaccine discovery, I summarize efforts focusing on synthetic glycan-based medicinal chemistry to understand the mammalian antiglycan immune response and define glycan epitopes for novel synthetic glycoconjugate vaccines against Streptococcus pneumoniae, Clostridium difficile, Klebsiella pneumoniae, and other bacteria. The chemical tools described here help us gain fundamental insights into how the human system recognizes carbohydrates and drive the discovery of carbohydrate vaccines.

Synthetic Analogs of Streptococcus pneumoniae Capsular Polysaccharides and Immunogenic Activities of Glycoconjugates

Streptococcus pneumoniae is a Gram-positive bacterium (pneumococcus) that causes severe diseases in adults and children. It was established that some capsular polysaccharides of the clinically significant serotypes of S. pneumoniae in the composition of commercial pneumococcal polysaccharide or conjugate vaccines exhibit low immunogenicity. The review considers production methods and structural features of the synthetic oligosaccharides from the problematic pneumococcal serotypes that are characterized with low immunogenicity due to destruction or detrimental modification occurring in the process of their preparation and purification. Bacterial serotypes that cause severe pneumococcal diseases as well as serotypes not included in the composition of the pneumococcal conjugate vaccines are also discussed. It is demonstrated that the synthetic oligosaccharides corresponding to protective glycotopes of the capsular polysaccharides of various pneumococcal serotypes are capable of inducing formation of the protective opsonizing antibodies and immunological memory. Optimal constructs of oligosaccharides from the epidemiologically significant pneumococcal serotypes are presented that can be used for designing synthetic pneumococcal vaccines, as well as test systems for diagnosis of S. pneumoniae infections and monitoring of vaccination efficiency .

A set of rhamnosylation-specific antibodies enables detection of novel protein glycosylations in bacteria

Despite its potential importance for bacterial virulence, protein rhamnosylation has not yet been sufficiently studied. Specific anti-SerRha, anti-ThrRha and anti-AsnRha antibodies allowed the identification of previously unknown monorhamnosylated proteins in cytosol and membrane fractions of bacterial cell lysates. Mapping of the complete rhamnoproteome in pathogens should facilitate development of targeted therapies against bacterial infections.

Synthesis of the biological repeating unit of Streptococcus pneumoniae serotype 23F capsular polysaccharide

The 3-aminopropyl glycoside of the biological repeating unit ofStreptococcus pneumoniaeserotype 23F capsular polysaccharide was efficiently synthesized by a linear assembly strategy.

Investigation on the Synthesis of Shigella flexneri Specific Oligosaccharides Using Disaccharides as Potential Transglucosylase Acceptor Substrates

Chemo-enzymatic strategies hold great potential for the development of stereo- and regioselective syntheses of structurally defined bioactive oligosaccharides. Herein, we illustrate the potential of the appropriate combination of a planned chemo-enzymatic pathway and an engineered biocatalyst for the multistep synthesis of an important decasaccharide for vaccine development. We report the stepwise investigation, which led to an efficient chemical conversion of allyl α-d-glucopyranosyl-(1→4)-α-l-rhamnopyranosyl-(1→3)-2-deoxy-2-trichloroacetamido-β-d-glucopyranoside, the product of site-specific enzymatic α-d-glucosylation of a lightly protected non-natural disaccharide acceptor, into a pentasaccharide building block suitable for chain elongation at both ends. Successful differentiation between hydroxyl groups features the selective acylation of primary alcohols and acetalation of a cis-vicinal diol, followed by a controlled per-O-benzylation step. Moreover, we describe the successful use of the pentasaccharide intermediate in the [5 + 5] synthesis of an aminoethyl aglycon-equipped decasaccharide, corresponding to a dimer of the basic repeating unit from the O-specific polysaccharide of Shigella flexneri 2a, a major cause of bacillary dysentery. Four analogues of the disaccharide acceptor were synthesized and evaluated to reach a larger repertoire of O-glucosylation patterns encountered among S. flexneri type-specific polysaccharides. New insights on the potential and limitations of planned chemo-enzymatic pathways in oligosaccharide synthesis are provided.

Synthesis of α-L-rhamnosyl ceramide and evaluation of its binding with anti-rhamnose antibodies

An α-L-rhamnosyl ceramide (1, α-L-RhaCer) has been prepared that was recognized by anti-L-rhamnose (anti-Rha) antibodies. During these studies we explored the use of an α-L-rhamnosyl thioglycoside and a trichloroacetimidate as a glycosyl donors. Subsequently, the acceptors desired for glycosylation, 3-O-benzoylazidosphingosine or 3-O-alloxycarbonylsphingosine, were prepared from D-xylose. The thioglycoside donor, 2,3,4-tri-O-acetyl-1-(4-tolyl)thio-α-L-rhamnopyranoside, and the trichloroacetimidate donor, 2,3,4-tri-O-acetyl-1-(2,2,2-trichloroethanimidate)-α-L-rhamnopyranoside, were synthesized in 50% and 78% yield overall, respectively. The synthesis of the glycosylation acceptor employed an addition-fragmentation olefination that was successfully carried out in 53% yield. With the successful synthesis of key intermediates, α-L-RhaCer (1) was prepared without any insurmountable obstacles. Anti-Rha antibodies were prepared in BALB/c mice by immunizing them with rhamnose-ovalbumin (Rha-Ova) with Sigma Adjuvant System (SAS) and the anti-L-Rha antibodies were isolated from the blood sera. Liposomes and EL4 tumor cells were used as model systems to demonstrate the ability of 1 to insert into a lipid bilayer. The interaction of the liposomes or the EL4 cells with α-L-RhaCer (1) and anti-Rha antibodies were investigated by fluorescence microscopy and flow cytometry, respectively, to confirm the ability of glycolipid 1 to be displayed on the tumor cell surface as well as the ability to be recognized by anti-Rha antibodies.

Short synthesis of the common trisaccharide core of kankanose and kankanoside isolated from Cistanche tubulosa

A short synthetic approach was developed for the synthesis of a common trisaccharide core found in kankanose, kankanoside F, H₁, H₂, and I isolated from the medicinally active plant Cistanche tubulosa. All glycosylations were carried out under nonmetallic reaction conditions. Yields were very good in all intermediate steps.

The cephalostatins. 21. Synthesis of bis-steroidal pyrazine rhamnosides (1)

The synthesis of bis-steroidal pyrazines derived from 3-oxo-11,21-dihydroxypregna-4,17(20)-diene (4) and glycosylation of a D-ring side chain with α-L-rhamnose have been summarized. Rearrangement of steroidal pyrazine 10 to 14 was found to occur with boron triflouride etherate. Glycosylation of pyrazine 10 using 2,3,4-tri-O-acetyl-α-L-rhamnose iodide led to 1,2-orthoester-α-L-rhamnose pyrazine 17b. By use of a persilylated α-L-rhamnose iodide as donor, formation of the orthoester was avoided. Bis-steroidal pyrazine 10 and rhamnosides 17b and 21c were found to significantly inhibit cancer cell growth in a murine and human cancer cell line panel. Pyrazine 9 inhibited growth of the nosocomial pathogen Enterococcus faecalis.

First total synthesis of rhodexin A

An efficient total synthesis of rhodexin A (1) is reported. An initial inverse-electron-demand Diels-Alder reaction of the acyldiene 6 with the silyl enol ether 7 gave the cycloadduct 8 with the required 4 contiguous stereocenters in a single step. This compound was then transformed into the tetracyclic enone 16, which was converted to rhodexin A (1).

Expeditious and convenient synthesis of pregnanes and its glycosides as potential anti-dyslipidemic and anti-oxidant agents

A series of new pregnane derivatives and its glycosides were synthesized in order to find new 'leads' against some important targets. The 3beta-hydroxy-16alpha-(2-hydroxy ethoxy) pregn-5-en-20-one (5) was synthesized from 3beta-hydroxy-5,16-pregnadiene-20-one (2) by adopting general modified procedure using BF(3):Et(2)O as a catalyst. Reduction of 5, with sodium borohydride yielded 3beta,20beta-dihydroxy-16alpha-(2-hydroxy ethoxy) pregn-5-en (7) as the major isolable product. O-alkylation of the C-20-oxime-pregnadiene (9) with 1,5-dibromopentane yielded 20-(O-5-bromopentyl)-oximino-3beta-hydroxy-pregn-5,16-diene (11). Synthesis of C-16 substituted pregnane glycosides (20) and (21) were accomplished with the imidate method using BF(3):Et(2)O. The synthesis of 4-chlorobenzoate (3) and 2-chlorobenzoate (4), derivatives of 2 were also accomplished. These compounds were evaluated for their anti-dyslipidemic and anti-oxidant activity and amongst them compounds 3 and 7 showed more lipid lowering and anti-oxidant activity.

Synthesis of Lewis A trisaccharide analogues in which D-glucose and L-rhamnose replace D-galactose and L-fucose, respectively

In our effort to design a safe anti-cancer vaccine based on the tumor associated carbohydrate antigen Le(a)Le(x), we are studying the cross-reactivity between the Le(a) natural trisaccharide antigen and analogues in which the L-fucose, D-galactose, and/or D-glucosamine residues are replaced by L-rhamnose or D-glucose, respectively. We describe here the chemical synthesis of two such Le(a) trisaccharide analogues. In one trisaccharide, D-glucose replaces D-galactose and in the second analogue L-rhamnose and D-glucose replace L-fucose and D-galactose, respectively. Introduction of the rhamnose and fucose moiety onto the poorly reactive 4-OH group of the N-acetylglucosamine residue in a disaccharide acceptor was successful after bis-N-acetylation of the amine group. These analogues will be used in competitive binding experiments with anti-Le(a) antibodies and their solution conformations will be studied.

Capsular polysaccharide-protein conjugate vaccines and intravenous immunoglobulins

Capsular polysaccharides (CPs), present on the surface of most pathogenic bacteria, have been recognised as virulence factors. Antibodies specific to these polysaccharides can mediate the killing of these bacteria by phagocytes in the presence of complement. The conjugation of polysaccharides to carrier proteins enhances their immunogenicity and renders the immune response T-cell dependent. The currently licensed capsular polysaccharide vaccines and polysaccharide-protein conjugate vaccines under development for the prevention of bacterial infections will be discussed in this review. Use of these vaccines for active vaccination and for the vaccination of healthy plasma donors to produce hyperimmune iv. immunoglobulins for the passive immunisation of appropriate patient populations is also discussed.

Synthesis of bidesmosidic dihydrodiosgenin saponins bearing a 3-O-β-chacotriosyl moiety

3-O-beta-Chacotriosyl-26-O-beta-D-glucopyranosyl-(25R)-furost-5-en (1), a mimic of the antitumor active proto-dioscin, was concisely synthesized from diosgenin in a linear nine steps and in 17% overall yield. Its congeners with a alpha-l-rhamnopyranosyl, beta-lactosyl, or without a substituent at the 26-OH (13-15) were also prepared. Compound 1, as well as 13-15, did not show any inhibition against tumor cells, implying that proto-dioscin might be also inactive, but readily converted into the antitumor active dioscin.

Synthesis of a model compound related to an anti-ulcer pectic polysaccharide

A stereocontrolled synthesis of the model compound for an anti-ulcer active polysaccharide (Bupleuran 2IIc) is described. Glycosidation of the disaccharide acceptor, 2-O-acetyl-3-O-benzyl-4-O-(p-methoxybenzyl)-alpha-L-rhamnopyranosyl-(1-- >4)-2,3,6-tri-O-benzyl-alpha-D-galactopyranosyl trichloroacetimidate, with the disaccharide receptor, allyl 3,4-di-O-benzyl-alpha-L-rhamnopyranosyl-(1-->4)-2,3,6-tri-O-benzyl-beta- D-galactopyranoside, using silver triflate (AgOTf) as a promoter gave the desired tetrasaccharide derivative, which was transformed into the acidic tetrasaccharide, corresponding to a segment of the rhamnogalacturonan (Bupleuran 2IIc) polysaccharide, propyl alpha-L-Rha-(1-->4)-alpha-D-GalA-(1-->2)-alpha-L-Rha-(1-->4)-beta-D-GalA , via removal of the corresponding ether and ester protecting groups, followed by oxidation.

beta-1,4-Galactosyltransferase-catalyzed synthesis of the branched tetrasaccharide repeating unit of Streptococcus pneumoniae type 14

A chemoenzymatic approach is described towards the branched tetrasaccharide repeating unit, beta-D-Galp- (1-->4)-beta-D-Glcp-(1-->6)-[beta-D-Galp-(1-->4)]-beta-D-GlcpNac, of Streptococcus pneumoniae type 14 in a form suitable for conjugation. The linear trisaccharide acceptor, beta-D-Galp-(1-->4)-beta-D-Glcp-(1-->6)-beta-D-GlcpNAc-(1-->O)CH2CH++ + = CH2, was synthesized by coupling of peracetylated lactosyl trichloroacetimidate to a suitably protected glucosamine building block and subsequent deprotection steps. The obtained derivative was found to be a good acceptor for bovine milk beta-1,4-galactosyltransferase, and the resulting branched tetrasaccharide beta-allyl glycoside was isolated and characterized by NMR spectroscopy and FAB mass spectrometry. Reaction of the anomeric allyl function with cysteamine under UV-irradiation gave the beta-aminoethylthio-extended glycoside suitable for further coupling of the tetrasaccharide to protein carriers.

Synthesis and two-dimensional nuclear magnetic resonance analysis of a tetra- and a hexa-saccharide fragment of the O-specific polysaccharide of Shigella dysenteriae type 1

The synthesis of the tetra- and hexa-saccharide methyl glycosides alpha-D-Galp-(1-->3)-alpha-D-GlcpNAc-(1-->3)-alpha-L-Rhap-(1-->3)- alpha-L-Rhap- OMe (1), and alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1-->2)-alpha-D-Galp-(1--> 3)-alpha-D-GlcpNAc- (1-->3)-alpha-L-Rhap-(1-->3)-alpha-L-Rhap-OMe (3) is described, which represent various epitopes of the O-specific polysaccharide of Shigella dysenteriae type 1. The following monosaccharide intermediates were used: 1,3-di-O-acetyl-2-O-benzoyl-4-O-benzyl-alpha-L-rhamnopyranose (6 alpha), methyl 2,4-di-O-benzyl-alpha-L-rhamnopyranoside (7), methyl 2,4-di-O-benzoyl-1-thio-alpha-L-rhamnopyranoside (8), 2,3,4-tri-O-benzoyl-alpha-L-rhamnopyranosyl bromide (9), methyl 3,4,6-tri-O-benzyl-2-O-(4-methoxybenzyl)-1-thio-beta-D- galactopyranoside (13), methyl 2,3,4,6-tetra-O-benzyl-1-thio-beta-D- galactopyranoside (16), and 2-azido-4,6-O-benzylidene-3-O-bromoacetyl-2-deoxy-beta-D- glucopyranosyl chloride (19). A detailed analysis of the 1H and 13C NMR spectra of oligosaccharides 1 and 3 confirmed that the hexasaccharide 3 better approaches the conformation of the native polysaccharide, than either 1 or the homologous pentasaccharide 41.

Epitope specificity of rabbit immunoglobulin G (IgG) elicited by pneumococcal type 23F synthetic oligosaccharide- and native polysaccharide-protein conjugate vaccines: comparison with human anti-polysaccharide 23F IgG

Streptococcus pneumoniae type 23F capsular polysaccharide (PS23F) consitss of a repeating glycerol-phosphorylated branched tetrasaccharide. The immunogenicities of the following related antigens were investigated: (i) a synthetic trisaccharide comprising the backbone of one repeating unit, (ii) a synthetic tetrasaccharide comprising the complete repeating unit, and (iii) native PS23F (all three conjugated to keyhole limpet hemocyanin [KLH]) and (iv) formalin-killed S. pneumoniae 23F. All antigens except the trisaccharide-KLH conjugate induced relatively high anti-PS23F antibody levels in rabbits. The epitope specificity of such antibodies was then studied by means of an inhibition immunoassay. The alpha(1-->2)-linked L-rhamnose branch was shown to be immunodominant for immunoglobulin G (IgG) induced by tetrasaccharide-KLH, PS23F-KLH, and killed S. pneumoniae 23F: in most sera L-rhamnose totally inhibited the binding of IgG to PS23F. Thus, there appears to be no major difference in epitope specificity between IgG induced by tetrasaccharide-KLH and that induced by antigens containing the polymeric form of PS23F. Human anti-PS23F IgG (either vaccine induced or naturally acquired) had a different epitope specificity: none of the inhibitors used, including L-rhamnose and tetrasaccharide-KLH, exhibited substantial inhibition. These observations suggest that the epitope recognized by human IgG on PS23F is larger than the epitope recognized by rabbit IgG. Both human and rabbit antisera efficiently opsonized type 23F pneumococci, as measured in a phagocytosis assay using human polymorphonuclear leukocytes.