Tetra- versus Pentavalent Inhibitors of Cholera Toxin

Fighting Shigella by Blocking Its Disease-Causing Toxin

Shiga toxin is an AB₅ toxin produced by Shigella species, while related toxins are produced by Shiga toxin-producing Escherichia coli (STEC). Infection by Shigella can lead to bloody diarrhea followed by the often fatal hemolytic uremic syndrome (HUS). In the present paper, we aimed for a simple and effective toxin inhibitor by comparing three classes of carbohydrate-based inhibitors: glycodendrimers, glycopolymers, and oligosaccharides. We observed a clear enhancement in potency for multivalent inhibitors, with the divalent and tetravalent compounds inhibiting in the millimolar and micromolar range, respectively. However, the polymeric inhibitor based on galabiose was the most potent in the series exhibiting nanomolar inhibition. Alginate and chitosan oligosaccharides also inhibit Shiga toxin and may be used as a prophylactic drug during shigella outbreaks.

Multivalent Inhibitors of Channel-Forming Bacterial Toxins

Rational design of multivalent molecules represents a remarkable modern tool to transform weak non-covalent interactions into strong binding by creating multiple finely-tuned points of contact between multivalent ligands and their supposed multivalent targets. Here, we describe several prominent examples where the multivalent blockers were investigated for their ability to directly obstruct oligomeric channel-forming bacterial exotoxins, such as the pore-forming bacterial toxins and B component of the binary bacterial toxins. We address problems related to the blocker/target symmetry match and nature of the functional groups, as well as chemistry and length of the linkers connecting the functional groups to their multivalent scaffolds. Using the anthrax toxin and AB5 toxin case studies, we briefly review how the oligomeric toxin components can be successfully disabled by the multivalent non-channel-blocking inhibitors, which are based on a variety of multivalent scaffolds.

Strong Inhibition of Cholera Toxin B Subunit by Affordable, Polymer-Based Multivalent Inhibitors

Cholera is a potentially fatal bacterial infection that affects a large number of people in developing countries. It is caused by the cholera toxin (CT), an AB₅ toxin secreted by Vibrio cholera. The toxin comprises a toxic A-subunit and a pentameric B-subunit that bind to the intestinal cell surface. Several monovalent and multivalent inhibitors of the toxin have been synthesized but are too complicated and expensive for practical use in developing countries. Meta-nitrophenyl α-galactoside (MNPG) is a known promising ligand for CT, and here mono- and multivalent compounds based on MNPG were synthesized. We present the synthesis of MNPG in greatly improved yields and its use while linked to a multivalent scaffold. We used economical polymers as multivalent scaffolds, namely, polyacrylamide, dextran, and hyperbranched polyglycerols (hPGs). Copper-catalyzed alkyne azide cycloaddition reaction (CuAAC) produced the inhibitors that were tested in an ELISA-type assay and an intestinal organoid swelling inhibition assay. The inhibitory properties varied widely depending on the type of polymer, and the most potent conjugates showed IC₅₀ values in the nanomolar range.

Carbohydrate inhibitors of cholera toxin

Cholera is a diarrheal disease caused by a protein toxin released by Vibrio cholera in the host's intestine. The toxin enters intestinal epithelial cells after binding to specific carbohydrates on the cell surface. Over recent years, considerable effort has been invested in developing inhibitors of toxin adhesion that mimic the carbohydrate ligand, with particular emphasis on exploiting the multivalency of the toxin to enhance activity. In this review we introduce the structural features of the toxin that have guided the design of diverse inhibitors and summarise recent developments in the field.

Towards new cholera prophylactics and treatment: Crystal structures of bacterial enterotoxins in complex with GM1 mimics

Cholera is a life-threatening disease in many countries, and new drugs are clearly needed. C-glycosidic antagonists may serve such a purpose. Here we report atomic-resolution crystal structures of three such compounds in complexes with the cholera toxin. The structures give unprecedented atomic details of the molecular interactions and show how the inhibitors efficiently block the GM1 binding site. These molecules are well suited for development into low-cost prophylactic drugs, due to their relatively easy synthesis and their resistance to glycolytic enzymes. One of the compounds links two toxin B-pentamers in the crystal structure, which may yield improved inhibition through the formation of toxin aggregates. These structures can spark the improved design of GM1 mimics, either alone or as multivalent inhibitors connecting multiple GM1-binding sites. Future developments may further include compounds that link the primary and secondary binding sites. Serving as decoys, receptor mimics may lessen symptoms while avoiding the use of antibiotics.

Synthetic multivalent ligands for cholera & cholera-like toxins: Protected cyclic neoglycopeptides

Synthesis of a set of novel glycopeptide analogues as potential cholera/cholera-like toxin inhibitors in their protected form is described. They include di-, tri-, tetra- and pentavalent scaffolds. The synthetic steps were achieved using a combination of solvent-free mechanochemical as well as the conventional solution-phase reactions. During the conventional DIC-HOBt-mediated peptide coupling followed for the preparation of certain glycopeptide analogues an interesting in situ Fmoc deprotection was observed which has been demonstrated to hold potential for synthesiszing glycopeptides/neoglycopeptides with extended polyamide chains.

Fighting Cholera One-on-One: The Development and Efficacy of Multivalent Cholera-Toxin-Binding Molecules

A series of diseases, ranging from cholera via travelers' diarrhea to hamburger disease, are caused by bacterially produced toxic proteins. In particular, a toxic protein unit is brought into the host cell upon binding to specific membrane-bound oligosaccharides on the host cell membrane. For example, the protein that causes cholera, cholera toxin (CT), has five identical, symmetrically placed binding pockets (B proteins), on top of which the toxic A protein resides. A promising strategy to counteract the devastating biological effects of this AB5 protein involves the development of inhibitors that can act as mimics of membrane-bound GM1 molecules, i.e., that can bind CT strongly and selectively. To reach this goal, two features are essential: First of all, the inhibitor should display oligosaccharides that resemble as much as possible the naturally occurring cell-surface pentasaccharide onto which CT normally binds, the so-called GM1 sugar (the oligosaccharide part of which is then labeled GM1os). Second, the inhibitor should be able to bind CT via multivalent interactions so as to bind CT as strongly as possible to allow for a real competition with the cell-membrane-bound GM1 molecules. In this Account, we present elements of the path that leads to strong CT inhibition by outlining the roles of multivalency and the development and use of GM1 mimics. First, multivalency effects were investigated using "sugar-coated" platforms, ranging from dendritic structures with up to eight oligosaccharides to platforms that mimicked the fivefold symmetry of CT itself. The latter goal was reached either via synthetic scaffolds like corannulene or calix[5]arene or via the development of a neolectin CT mimic that itself carries five GM1os groups. Second, the effect of the nature of the oligosaccharide appended to this platform was investigated via the use of oligosaccharides of increasing complexity, from galactose and lactose to the tetrasaccharide GM2os and eventually to GM1os itself. The combination of these threads gives rise to a series of inhibitors that can strongly bind CT, with IC50 values below 100 pM, and in some cases can even bind one-on-one.