The structure-activity relationship of the salicylimide derived inhibitors of UDP-sugar producing pyrophosphorylases.

UDP-sugars are key precursors for biomass production in nature (synthesis of cellulose, hemicellulose, etc.). They are produced de novo by distinct UDP-sugar producing pyrophosphorylases. Studies on the roles of these enzymes using genetic knockouts were hampered by sterility of the mutants and by functional-complementation from related enzyme(s), hindering clear interpretation of the results. In an attempt to override these difficulties, we turned to the reverse chemical genetics approaches to identify compounds which interfere with the activity of those enzymes in vivo. Hit expansion on one of such compounds, a salicylimide derivative, allowed us to identify several inhibitors with a range of activities. The present study provides a structure-activity relationship for these compounds.

Identification and characterization of inhibitors of UDP-glucose and UDP-sugar pyrophosphorylases for in vivo studies.

UDP-sugars serve as ultimate precursors in hundreds of glycosylation reactions (e.g. for protein and lipid glycosylation, synthesis of sucrose, cell wall polysaccharides, etc.), underlying an important role of UDP-sugar-producing enzymes in cellular metabolism. However, genetic studies on mechanisms of UDP-sugar formation were frequently hampered by reproductive impairment of the resulting mutants, making it difficult to assess an in vivo role of a given enzyme. Here, a chemical library containing 17 500 compounds was separately screened against purified UDP-glucose pyrophosphorylase (UGPase) and UDP-sugar pyrophosphorylase (USPase), both enzymes representing the primary mechanisms of UDP-sugar formation. Several compounds have been identified which, at 50 µm, exerted at least 50% inhibition of the pyrophosphorylase activity. In all cases, both UGPase and USPase activities were inhibited, probably reflecting common structural features of active sites of these enzymes. One of these compounds (cmp #6), a salicylamide derivative, was found as effective inhibitor of Arabidopsis pollen germination and Arabidopsis cell culture growth. Hit optimization on cmp #6 yielded two analogs (cmp #6D and cmp #6D2), which acted as uncompetitive inhibitors against both UGPase and USPase, and were strong inhibitors in the pollen test, with apparent inhibition constants of less than 1 µm. Their effects on pollen germination were relieved by addition of UDP-glucose and UDP-galactose, suggesting that the inhibitors targeted UDP-sugar formation. The results suggest that cmp #6 and its analogs may represent useful tools to study in vivo roles of the pyrophosphorylases, helping to overcome the limitations of genetic approaches.

High-Throughput Screening Using a Whole-Cell Virus Replication Reporter Gene Assay to Identify Inhibitory Compounds against Rift Valley Fever Virus Infection.

Rift Valley fever virus (RVFV) is an emerging virus that causes serious illness in humans and livestock. There are no approved vaccines or treatments for humans. The purpose of the study was to identify inhibitory compounds of RVFV infection without any preconceived idea of the mechanism of action. A whole-cell-based high-throughput drug screening assay was developed to screen 28,437 small chemical compounds targeting RVFV infection. To accomplish both speed and robustness, a replication-competent NSs-deleted RVFV expressing a fluorescent reporter gene was developed. Inhibition of fluorescence intensity was quantified by spectrophotometry and related to virus infection in human lung epithelial cells (A549). Cell toxicity was assessed by the Resazurin cell viability assay. After primary screening, 641 compounds were identified that inhibited RVFV infection by ≥80%, with ≥50% cell viability at 50 µM concentration. These compounds were subjected to a second screening regarding dose-response profiles, and 63 compounds with ≥60% inhibition of RVFV infection at 3.12 µM compound concentration and ≥50% cell viability at 25 µM were considered hits. Of these, six compounds with high inhibitory activity were identified. In conclusion, the high-throughput assay could efficiently and safely identify several promising compounds that inhibited RVFV infection.

2-[4,5-Difluoro-2-(2-fluorobenzoylamino)-benzoylamino]benzoic acid, an antiviral compound with activity against acyclovir-resistant isolates of herpes simplex virus types 1 and 2.

Herpes simplex viruses 1 and 2 (HSV-1 and HSV-2) are responsible for lifelong latent infections in humans, with periods of viral reactivation associated with recurring ulcerations in the orofacial and genital tracts. In immunosuppressed patients and neonates, HSV infections are associated with severe morbidity and, in some cases, even mortality. Today, acyclovir is the standard therapy for the management of HSV infections. However, the need for novel antiviral agents is apparent, since HSV isolates resistant to acyclovir therapy are frequently isolated in immunosuppressed patients. In this study, we assessed the anti-HSV activity of the antiadenoviral compounds 2-[2-(2-benzoylamino)-benzoylamino]benzoic acid (benzavir-1) and 2-[4,5-difluoro-2-(2-fluorobenzoylamino)-benzoylamino]benzoic acid (benzavir-2) on HSV-1 and HSV-2. Both compounds were active against both viruses. Importantly, benzavir-2 had potency similar to that of acyclovir against both HSV types, and it was active against clinical acyclovir-resistant HSV isolates.

Synthesis, biological evaluation, and structure-activity relationships of 2-[2-(benzoylamino)benzoylamino]benzoic acid analogues as inhibitors of adenovirus replication.

2-[2-Benzoylamino)benzoylamino]benzoic acid (1) was previously identified as a potent and nontoxic antiadenoviral compound (Antimicrob. Agents Chemother. 2010, 54, 3871). Here, the potency of 1 was improved over three generations of compounds. We found that the ortho, ortho substituent pattern and the presence of the carboxylic acid of 1 are favorable for this class of compounds and that the direction of the amide bonds (as in 1) is obligatory. Some variability in the N-terminal moiety was tolerated, but benzamides appear to be preferred. The substituents on the middle and C-terminal rings were varied, resulting in two potent inhibitors, 35g and 35j, with EC(50) = 0.6 µM and low cell toxicity.

Taloside inhibitors of galectin-1 and galectin-3.

Galectin-1 and galectin-3 have roles in cancer and inflammation. Galectin-1 has recently emerged as a significant protein produced by tumour cells to promote tumour development, angiogenesis and metastasis and consequently represents an important target to inhibit. The design of inhibitors targeting the carbohydrate recognition domain that is known to recognize galactose is an important approach in the fight against cancer. Based on the analysis of crystal structures, we pursued the concept that if the galactose was replaced with talose (the C2 epimer of galactose) as a scaffold, then O2 substituents would be directed closer to the protein surface and provide opportunity to design inhibitors that are more specific towards particular galectins. Our elucidation of X-ray crystal structures of two of our synthesized talosides in complex with galectin-1 and galectin-3 provides the first atomic information on the interactions of galectins, and indeed any protein, with talosides. These results have enabled a structure-based rationale for the specificity differences shown by galectin-1 and galectin-3 towards these talosides and demonstrate new opportunities for further exploitation as specific inhibitors of galectins.

Inhibition mechanism of human galectin-7 by a novel galactose-benzylphosphate inhibitor.

Galectins are involved in many cellular processes due to their ability to bind carbohydrates. Understanding their functions has shown the necessity for potent and specific galectin inhibitors. Human galectin-7 (hGal-7), in particular, has been highlighted as an important marker in many types of cancer by either inhibiting or promoting tumour growth. Producing ligands able to selectively target hGal-7 will offer promising tools for deciphering cancer processes in which hGal-7 is involved as well as present potential solutions for future therapeutics. Here we report the high resolution crystal structure of hGal-7 in complex with a synthetic 2-O-benzylphosphate-galactoside inhibitor (which is > 60-fold more potent than its parent galactoside). The high resolution crystallographic analysis highlights the validity of using saccharide derivatives, conserving properties of the galactose binding, while enhanced affinity and specificity is provided by the added phosphate group. This structural information will allow the design of further inhibitors with improved potency and specificity.

Syntheses of pseudoceramines A-D and a new synthesis of spermatinamine, bromotyrosine natural products from marine sponges.

Herein we report the total syntheses of pseudoceramine A-D (2-5) and spermatinamine (1) isolated from the marine sponge Pseudoceratina sp. Direct acyl substitution of α-hydroxyiminoesters with amine nucleophiles was developed as a key transformation. The synthetic compounds confirm the reported structures and importantly gives access to non-symmetrical spermine based natural products carrying two different bromotyrosine building blocks. Our new synthesis of spermatinamine is two steps shorter and more efficient than the previously reported sequence.

Arene-anion based arginine-binding motif on a galactose scaffold: structure-activity relationships of interactions with arginine-rich galectins.

Two series of C3-benzamido and O2-anion-substituted galactopyranosides were synthesized and studied as binders to arginine-rich proteins galectin-1, -3, -7, -8N (N-terminal domain), and -9N (N-terminal domain). The first series had a 4-methylbenzamide at C3 and the anionic O2-substituent was varied. The second series varied the 4-substituent of the C3-benzamide, whereas the anionic O2 substituent was kept as a sulfate. The influence of the O2-anion substituent correlated negatively with the oxygen charge density in case of galectin-1, -3, and -9N. In the second series, the electron-donating capacity of the 4-substituent of the C3-benzamides correlated positively with the magnitude of the affinity enhancement by the 2O-sulfate.

Inhibition of galectins with small molecules.

Evidence that the galectin family of proteins plays crucial roles in cancer, inflammation, and immunity has accumulated over the last decade. The galectins have consequently emerged as interesting drug targets. A majority of galectin functions occurs by means of cross-linking glycoproteins and by doing so controlling glycoprotein cellular localization and residence times. The glycoprotein cross-linking occurs when galectin dimers or multimers, or galectins with two binding sites, bind galactose-containing glycans of the glycoproteins. Such galectin-glycan interactions have been successfully blocked with compounds having multivalent presentation of galactose, lactose, or N-acetyllactosamine, with peptides, and with small carbohydrate (galactose) derivatives. This review summarizes and analyzes attempts to develop efficient and selective small-molecule galectin inhibitors through derivatization of monosaccharides, mainly galactosides, with non-carbohydrate structures that protrude into subsites adjacent to the core-conserved galactose-recognizing site of the galectins.

Mutational tuning of galectin-3 specificity and biological function.

Galectins are defined by a conserved ß-galactoside binding site that has been linked to many of their important functions in e.g. cell adhesion, signaling, and intracellular trafficking. Weak adjacent sites may enhance or decrease affinity for natural ß-galactoside-containing glycoconjugates, but little is known about the biological role of this modulation of affinity (fine specificity). We have now produced 10 mutants of human galectin-3, with changes in these adjacent sites that have altered carbohydrate-binding fine specificity but that retain the basic ß-galactoside binding activity as shown by glycan-array binding and a solution-based fluorescence anisotropy assay. Each mutant was also tested in two biological assays to provide a correlation between fine specificity and function. Galectin-3 R186S, which has selectively lost affinity for LacNAc, a disaccharide moiety commonly found on glycoprotein glycans, has lost the ability to activate neutrophil leukocytes and intracellular targeting into vesicles. K176L has increased affinity for ß-galactosides substituted with GlcNAcß1-3, as found in poly-N-acetyllactosaminoglycans, and increased potency to activate neutrophil leukocytes even though it has lost other aspects of galectin-3 fine specificity. G182A has altered carbohydrate-binding fine specificity and altered intracellular targeting into vesicles, a possible link to the intracellular galectin-3-mediated anti-apoptotic effect known to be lost by this mutant. Finally, the mutants have helped to define the differences in fine specificity shown by Xenopus, mouse, and human galectin-3 and, as such, the evidence for adaptive change during evolution.

Synthesis of 2-(2-aminopyrimidine)-2,2-difluoroethanols as potential bioisosters of salicylidene acylhydrazides.

Salicylidene acylhydrazides are inhibitors of type III secretion in several gram-negative pathogens. To further develop the salicylidene acylhydrazides, scaffold hopping was applied to replace the core fragment of the compounds. The novel 2-(2-amino-pyrimidine)-2,2-difluoroethanol scaffold was identified as a possible analog to the salicylidene acylhydrazide core structure. The synthesis of a library of 2-(2-amino-pyrimidine)-2,2-difluoro-ethanols is described in this paper.

Synthesis of 3-azido-3-deoxy-beta-D-galactopyranosides.

Three efficient routes to 3-azido-3-deoxy-beta-D-galactopyranosides were developed relying on a double inversion protocol at C3. Two of the routes were demonstrated to work with both O- and S-glycosides. In all three routes, the 2-O-acetyl-3-azido-4,6-O-benzylidene-3-deoxy-beta-D-galactopyranosides were obtained by an azide inversion of the key intermediates 2-O-acetyl-4,6-O-benzylidene-3-O-trifluoromethanesulfonyl-beta-D-gulopyranosides. The intermediate gulopyranosides were in turn obtained from 2-O-acetyl-4,6-O-benzylidene-3-O-trifluoromethanesulfonyl-beta-D-galactopyranosides, installed in one pot from the 4,6-O-benzylidene-beta-D-galactopyranosides, by inversion with nitrite or acetate. For O-glycosides, the gulopyranoside configuration could alternatively be obtained from the 4,6-O-benzylidene-beta-D-galactopyranoside by elimination to give the 2,3-dianhydro derivative followed by a highly stereoselective cis-dihydroxylation.

Protein subtype-targeting through ligand epimerization: talose-selectivity of galectin-4 and galectin-8.

A series of O2 and O3-derivatized methyl beta-d-talopyranosides were synthesized and evaluated in vitro as inhibitors of the galactose-binding galectin-1, -2, -3, -4 (N- and C-terminal domains), 8 (N-terminal domain), and 9 (N-terminal domain). Galectin-4C and 8N were found to prefer the d-talopyranose configuration to the natural ligand d-galactopyranose configuration. Derivatization at talose O2 and/or O3 provided selective submillimolar inhibitors for these two galectins.

Arginine binding motifs: design and synthesis of galactose-derived arginine tweezers as galectin-3 inhibitors.

Anionic O2 derivatives of methyl 3-deoxy-3-(4-methylbenzamido)-1-thio-beta-D-galactopyranoside have been synthesized as inhibitors against galectin-3. The sulfate, H-phosphonate, and benzyl phosphate derivatives showed an increased affinity as compared to the parent unsubstituted galactopyranoside. Modeling revealed arginine-144 being pinched by the C3 benzamide and O2 anionic substituents in that the benzamide stacked face-to-face and the anionic O2 substituent ion-paired with the guanidinium moiety.

Different affinity of galectins for human serum glycoproteins: galectin-3 binds many protease inhibitors and acute phase proteins.

Here we report the first survey of galectins binding to glycoproteins of human serum. Serum was subjected to affinity chromatography using immobilized galectins, and the bound glycoproteins were analyzed by electrophoresis, Western blotting, and mass spectrometry. Galectins-3, -8, and -9 bound a much broader range of ligands in serum than previously known, galectin-1 bound less, and galectins-2, -4, and -7 bound only traces or no serum ligands. Galectin-3 bound most major glycoproteins, including alpha-2-macroglobulin and acute phase proteins such as haptoglobin. It bound only a selected minor fraction of transferrin, and bound none or little of IgG. Galectins-8 and -9 bound a similar range of glycoproteins as galectin-3, but in lower amounts, and galectin-8 had a relative preference for IgA. Galectin-1 bound mainly a fraction of alpha-2-macroglobulin and only traces of other glycoproteins. The binding of galectin-3 to serum glycoproteins requires affinity for LacNAc, since a mutant (R186S), which has lost this affinity, did not bind any serum glycoproteins. The average affinity of galectin-3 for serum glycoproteins was estimated to correspond to K(d) approximately 1-5 muM by modeling of the affinity chromatography and a fluorescence anisotropy assay. Since galectins are expressed on endothelial cells and other cells exposed to serum components, this report gives new insight into function of galectins and the role of their different fine specificity giving differential binding to the serum glycoproteins.

Affinity of galectin-8 and its carbohydrate recognition domains for ligands in solution and at the cell surface.

Galectin-8 has two different carbohydrate recognition domains (CRDs), the N-terminal Gal-8N and the C-terminal Gal-8C linked by a peptide, and has various effects on cell adhesion and signaling. To understand the mechanism for these effects further, we compared the binding activities of galectin-8 in solution with its binding and activation of cells. We used glycan array analysis to broaden the specificity profile of the two galectin-8 CRDs, as well as intact galectin-8s (short and long linker), confirming the unique preference for sulfated and sialylated glycans of Gal-8N. Using a fluorescence anisotropy assay, we examined the solution affinities for a subset of these glycans, the highest being 50 nM for NeuAcalpha2,3Lac by Gal-8N. Thus, carbohydrate-protein interactions can be of high affinity without requiring multivalency. More importantly, using fluorescence polarization, we also gained information on how the affinity is built by multiple weak interactions between different fragments of the glycan and its carrier molecule and the galectin CRD subsites (A-E). In intact galectin-8 proteins, the two domains act independently of each other in solution, whereas at a surface they act together. Ligands with moderate or weak affinity for the isolated CRDs on the array are bound strongly by intact galectin-8s. Also galectin-8 binding and signaling at cell surfaces can be explained by combined binding of the two CRDs to low or medium affinity ligands, and their highest affinity ligands, such as sialylated galactosides, are not required.

Efficient and expedient two-step pyranose-retaining fluorescein conjugation of complex reducing oligosaccharides: galectin oligosaccharide specificity studies in a fluorescence polarization assay.

Fluorescence labeling of naturally occurring saccharides provides a tool for studying lectins. A practical and efficient two-step protocol for fluorescence labeling of reducing sugars without disrupting their pyranose structure has been developed, consisting of generation of the amino sugar using NH(4)HCO(3)(s)/NH(3)(aq, concentrated) followed by BOP-mediated acylation with derivatives of 5- or 6-carboxyfluorescein. The acylated conjugates were subsequently run against galectins-1, -3, and -8, beta-galactoside recognizing lectins of current interest, in a fluorescence polarization binding assay. Upon analyzing a collection of isomerically pure 5- and 6-carboxyfluorescein derivatives with different tether lengths, we found that conjugates based on 5-carboxyfluorescein gave significantly better results than the ones based on 6-carboxyfluorescein and that galectins-1 and -8 favored conjugates with different tether lengths than did galectin-3. The results show that fluorescence labeling can be chemically tuned to find optimal probes for individual galectins but also probes interacting well with many galectins.