Design and Synthesis of Neutralizable Fondaparinux

Rationally designed chemoenzymatic synthesis of heparan sulfate oligosaccharides with neutralizable anticoagulant activity and low severe complications

The identification of enzymes involved in biosynthesis of heparan sulfate (HS) and heparin and their successful application in chemoenzymatic synthesis have provided great impetus to rationally design well-defined oligosaccharides as ideal alternatives to animal-derived heterogeneous heparin anticoagulants clinically to treat clotting disorders. Herein, we revisited the substrate specificity of recombinant 2-O-sulfotransferases produced in different expression systems for the highly efficient chemoenzymatic synthesis of HS oligosaccharides containing the rare 2-O-sulfated GlcA (GlcA2S) residues, followed by further assembly into the highly sulfated HS dodecasaccharide (12-mer) and decasaccharide (10-mer) containing the antithrombin-binding domain and the trisulfated disaccharide (GlcA2S-GlcNS6S) units rarely found in natural heparin. The GlcA2S-containing HS 10-mer demonstrated both the effectively reversible anticoagulant activity similar to that of unfractionated heparin and the lower potential risk for life-threatening heparin-induced thrombocytopenia compared with enoxaparin, indicating its promising prospect as the next-generation HS/heparin-like anticoagulant therapeutics.

Efficient Small-Molecule Reversal Agents for Anticoagulant Fondaparinux

Fondaparinux is a highly anionic synthetic heparinoid pentasaccharide used as an anticoagulant for specific clinical conditions and surgeries. As a non-natural small-molecule drug, it presents pharmacokinetic and pharmacodynamic advantages, as well as high stability and low immunogenicity, when compared with different forms of heparin. However, its broader usage is hampered by different factors like price, existence of alternative anticoagulants, or, specifically in this case, the lack of an effective antidote that is highly recommendable for avoiding uncontrolled bleeding. In this work, we describe two synthetic small molecules derived from spermine (3AC and 3FF) that efficiently revert the anticoagulant activity of fondaparinux. In an in vitro enzymatic assay related to blood coagulation, the spermine derivatives show potent activity as fondaparinux antidotes, with higher activity than ciraparantag (a small molecule in the clinical phase as an anticoagulant antidote) and much higher activity than protamine, the only approved antidote for unfractioned heparin but inefficient against fondaparinux. Remarkably, naked-eye ex vivo tests demonstrated their efficacy in freshly extracted mice blood. Mechanistic studies show that both small molecules strongly bind fondaparinux in buffered water, as detected by fluorescence and NMR spectroscopy and confirmed by molecular dynamics simulations. Thus, these spermine derivatives are promising reversal agents against heparinoid anticoagulants with a wide range of molecular weights, overcoming the drawbacks of those antidotes based on biomacromolecules.

Precision Synthesis and Antiliver Fibrosis Activity of a Highly Branched Acidic 63-Mer Pectin Polysaccharide

Natural polysaccharides possess various biological functions and have become increasingly important as drug candidates for biomedical development. However, the accessibility to multiple-branched and large-sized acidic polysaccharides with well-defined structures and the identification of related active glycan domains remain challenging. Here, we report the precision synthesis of a highly branched acidic pectin polysaccharide up to a 63-mer containing 10 different glycosidic linkages from Lycium barbarum. The synthetic strategy relies on highly stereoselective modular assembly of an orthogonally protected decasaccharide backbone, efficient synthesis of three side chain glycans by the integration of stereocontrolled one-pot chemoselective glycosylations and a hydrogen-bond-mediated aglycone delivery approach, and convergent assembly of the target polysaccharide in a branched site-specific glycosylation manner via flexible orthogonal protecting group manipulations. Structure-activity relationship studies of synthetic polysaccharide 63-mer and its short fragments (9-mer, 10-mer, 11-mer, and 33-mer) suggest that the decasaccharide as an active glycan domain exhibits better antiliver fibrosis activity.

Synergistic immune augmentation enabled by covalently conjugating TLR4 and NOD2 agonists

Enhancing the efficacy of subunit vaccines relies significantly on the utilization of potent adjuvants, particularly those capable of triggering multiple immune pathways. To achieve synergistic immune augmentation by Toll-like receptor 4 agonist (TLR4a) and nucleotide-binding oligomerization-domain-containing protein 2 agonist (NOD2a), in this work, we conjugated RC529 (TLR4a) and MDP (NOD2a) to give RC529-MDP, and evaluated its adjuvanticity for OVA antigen. Compared to the unconjugated RC529+MDP, RC529-MDP remarkably enhanced innate immune responses with 6.8-fold increase in IL-6 cytokine, and promoted the maturation of antigen-presenting cells (APCs), possibly because of the conjugation of multiple agonists ensuring their delivery to the same cell and activation of various signaling pathways within that cell. Furthermore, RC529-MDP improved OVA-specific antibody response, T cells response and the memory T cells ratio relative to the unconjugated mixture. Therefore, covalently conjugating TLR4 agonist and NOD2 agonist was an effective strategy to enhance immune responses, providing the potential to design and develop more effective vaccines.

Overview of the current procedures in synthesis of heparin saccharides

Natural heparin, a glycosaminoglycan consisting of repeating hexuronic acid and glucosamine linked by 1 → 4 glycosidic bonds, is the most widely used anticoagulant. To subvert the dependence on animal sourced heparin, alternative methods to produce heparin saccharides, i.e., either heterogenous sugar chains similar to natural heparin, or structurally defined oligosaccharides, are becoming hot subjects. Although the success by chemical synthesis of the pentasaccharide, fondaparinux, encourages to proceed through a chemical approach generating homogenous product, synthesizing larger oligos is still cumbersome and beyond reach so far. Alternatively, the chemoenzymatic pathway exhibited exquisite stereoselectivity of glycosylation and regioselectivity of modification, with the advantage to skip the tedious protection steps unavoidable in chemical synthesis. However, to a scale of drug production needed today is still not in sight. In comparison, a procedure of de novo biosynthesis in an organism could be an ultimate goal. The main purpose of this review is to summarize the current available/developing strategies and techniques, which is expected to provide a comprehensive picture for production of heparin saccharides to replenish or eventually to replace the animal derived products. In chemical and chemoenzymatic approaches, the methodologies are discussed according to the synthesis procedures: building block preparation, chain elongation, and backbone modification.

Acceptor-Reactivity-Controlled Stereoconvergent Synthesis and Immunological Activity of a Unique Pentasaccharide from the Cell Wall Polysaccharide of Cutibacterium acnes C7

Bacterial cell-surface polysaccharides are involved in various biological processes and have attracted widespread attention as potential targets for developing carbohydrate-based drugs. However, the accessibility to structurally well-defined polysaccharide or related active oligosaccharide domains remains challenging. Herein, we describe an efficiently stereocontrolled approach for the first total synthesis of a unique pentasaccharide repeating unit containing four difficult-to-construct 1,2-cis-glycosidic linkages from the cell wall polysaccharide of Cutibacterium acnes C7. The features of our approach include: 1) acceptor-reactivity-controlled glycosylation to stereoselectively construct two challenging rare 1,2-cis-ManA2,3(NAc)2 (β-2,3-diacetamido-2,3-dideoxymannuronic acid) linkages, 2) combination use of 6-O-tert-butyldiphenylsilyl (6-O-TBDPS)-mediated steric shielding effect and ether solvent effect to stereoselectively install a 1,2-cis-glucosidic linkage, 3) bulky 4,6-di-O-tert-butylsilylene (DTBS)-directed glycosylation to stereospecifically construct a 1,2-cis-galactosidic linkage, 4) stereoconvergent [2+2+1] and one-pot chemoselective glycosylation to rapidly assemble the target pentasaccharide. Immunological activity tests suggest that the pentasaccharide can induce the production of proinflammatory cytokine TNF-α in a dose-dependent manner.

Integrated chemoenzymatic synthesis of a comprehensive sulfated ganglioside glycan library to decipher functional sulfoglycomics and sialoglycomics

Ganglioside glycans are ubiquitous and complex biomolecules that are involved in a wide range of biological functions and disease processes. Variations in sialylation and sulfation render the structural complexity and diversity of ganglioside glycans, and influence protein-carbohydrate interactions. Structural and functional insights into the biological roles of these glycans are impeded due to the limited accessibility of well-defined structures. Here we report an integrated chemoenzymatic strategy for expeditious and systematic synthesis of a comprehensive 65-membered ganglioside glycan library covering all possible patterns of sulfation and sialylation. This strategy relies on the streamlined modular assembly of three common sialylated precursors by highly stereoselective iterative sialylation, modular site-specific sulfation through flexible orthogonal protecting-group manipulations and enzymatic-catalysed diversification using three sialyltransferase modules and a galactosidase module. These diverse ganglioside glycans enable exploration into their structure-function relationships using high-throughput glycan microarray technology, which reveals that different patterns of sulfation and sialylation on these glycans mediate their unique binding specificities.

Stereoconvergent and Chemoenzymatic Synthesis of Tumor-Associated Glycolipid Disialosyl Globopentaosylceramide for Probing the Binding Affinity of Siglec-7

Disialosyl globopentaosylceramide (DSGb5) is a tumor-associated complex glycosphingolipid. However, the accessibility of structurally well-defined DSGb5 for precise biological functional studies remains challenging. Herein, we describe the first total synthesis of DSGb5 glycolipid by an efficient chemoenzymatic approach. A Gb5 pentasaccharide-sphingosine was chemically synthesized by a convergent and stereocontrolled [2 + 3] method using an oxazoline disaccharide donor to exclusively form β-anomeric linkage. After investigating the substrate specificity of different sialyltransferases, regio- and stereoselective installment of two sialic acids was achieved by two sequential enzyme-catalyzed reactions using α2,3-sialyltransferase Cst-I and α2,6-sialyltransferase ST6GalNAc5. A unique aspect of the approach is that methyl-β-cyclodextrin-assisted enzymatic α2,6-sialylation of glycolipid substrate enables installment of the challenging internal α2,6-linked sialoside to synthesize DSGb5 glycosphingolipid. Surface plasmon resonance studies indicate that DSGb5 glycolipid exhibits better binding affinity for Siglec-7 than the oligosaccharide moiety of DSGb5. The binding results suggest that the ceramide moiety of DSGb5 facilitates its binding by presenting multivalent interactions of glycan epitope for the recognition of Siglec-7.

Synthesis of Anticoagulant Pentasaccharide Fondaparinux via 3,5-Dimethyl-4-(2'-phenylethynylphenyl)phenyl Glycosides

Here, we disclosed a convenient procedure for the preparation of EPP [3,5-dimethyl-4-(2'-phenylethynylphenyl)phenyl] glycosides and their application to an effective synthesis of fondaparinux, the clinically approved anticoagulant heparin pentasaccharide. The use of EPP glycosides in the one-pot orthogonal glycosylation for the synthesis of heparin-like tetrasaccharides has also been achieved.

Rational Design and Expedient Synthesis of Heparan Sulfate Mimetics from Natural Aminoglycosides for Structure and Activity Relationship Studies

Heparan sulfate (HS) contains variably repeating disaccharide units organized into high- and low-sulfated domains. This rich structural diversity enables HS to interact with many proteins and regulate key signaling pathways. Efforts to understand structure-function relationships and harness the therapeutic potential of HS are hindered by the inability to synthesize an extensive library of well-defined HS structures. We herein report a rational and expedient approach to access a library of 27 oligosaccharides from natural aminoglycosides as HS mimetics in 7-12 steps. This strategy significantly reduces the number of steps as compared to the traditional synthesis of HS oligosaccharides from monosaccharide building blocks. Combined with computational insight, we identify a new class of four trisaccharide compounds derived from the aminoglycoside tobramycin that mimic natural HS and have a strong binding to heparanase but a low affinity for off-target platelet factor-4 protein.

Chemoenzymatic Total Synthesis of Haemophilus ducreyi Lipooligosaccharide Core Octasaccharides Containing Natural and Unnatural Sialic Acids

The first total synthesis of Haemophilus ducreyi lipooligosaccharide core octasaccharides containing natural and unnatural sialic acids has been achieved by an efficient chemoenzymatic approach. A highly convergent [3 + 3] coupling strategy was developed to chemically assemble a unique hexasaccharide bearing multiple rare higher-carbon sugars d-glycero-d-manno-heptose (d,d-Hep), l-glycero-d-manno-heptose (l,d-Hep), and 3-deoxy-α-d-manno-oct-2-ulosonic acid (Kdo). Key features include sequential one-pot glycosylations for oligosaccharide assembly and the construction of the challenging α-(1 → 5)-linked Hep-Kdo glycosidic bond by gold-catalyzed glycosylation with a glycosyl ortho-alkynylbenzoate donor. Furthermore, the sequential enzyme-catalyzed regio- and stereoselective introduction of a galactose residue using β-1,4-galactosyltransferase and different sialic acids using a one-pot multienzyme sialylation system was efficiently accomplished to provide the target octasaccharides.