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Selective galectin inhibitors are valuable research tools and could also be used as drug candidates. In that context, TD139, a thiodigalactoside galectin-3 inhibitor, is currently being evaluated clinically for the treatment of idiopathic pulmonary fibrosis. Herein, we describe a new strategy for the preparation of TD139. Starting from inexpensive levoglucosan, we used a rarely employed reaction cascade: Payne rearrangement/azidation process leading to 3-azido-galactopyranose. The latter intermediate was efficiently converted into TD139 in a few simple and practical steps.

Emerging insights into the functional spectrum of tissue lectins leads to identification of new targets for the custom-made design of potent inhibitors, providing a challenge for synthetic chemistry. The affinity and selectivity of a carbohydrate ligand for a lectin may immensely be increased by a number of approaches, which includes varying geometrical or topological features. This perspective leads to the design and synthesis of glycoclusters and their testing using assays of physiological relevance. Herein, hydroquinone, resorcinol, benzene-1,3,5-triol and tetra(4-hydroxyphenyl)ethene have been employed as scaffolds and propargyl derivatives obtained. The triazole-containing linker to the α/β-O/S-glycosides of GlcNAc/GalNAc presented on these scaffolds was generated by copper-catalysed azide-alkyne cycloaddition. This strategy was used to give a panel of nine glycoclusters with bi-, tri- and tetravalency. Maintained activity for lectin binding after conjugation was ascertained for both sugars in solid-phase assays with the plant agglutinins WGA (GlcNAc) and DBA (GalNAc). Absence of cross-reactivity excluded any carbohydrate-independent reactivity of the bivalent compounds, allowing us to proceed to further testing with a biomedically relevant lectin specific for GalNAc. Macrophage galactose(-binding C)-type lectin, involved in immune defence by dendritic cells and in virus uptake, was produced as a soluble protein without/with its α-helical coiled-coil stalk region. Binding to ligands presented on a matrix and on cell surfaces was highly susceptible to the presence of the tetravalent inhibitor derived from the tetraphenylethene-containing scaffold, and presentation of GalNAc with an α-thioglycosidic linkage proved favorable. Cross-reactivity of this glycocluster to human galectins-3 and -4, which interact with Tn-antigen-presenting mucins, was rather small. Evidently, the valency and spatial display of α-GalNAc residues is a key factor to design potent and selective inhibitors for this lectin.

The emerging functional versatility of cellular glycans makes research on the design of synthetic inhibitors a timely topic. In detail, the combination of ligand (or headgroup or contact site) structure with spatial parameters that depend on topological and geometrical factors underlies the physiological selectivity of glycan-protein (lectin) recognition. We herein tested a panel of bi-, tri- and tetravalent compounds against two plant agglutinins and adhesion/growth-regulatory lectins (galectins). In addition, we examined the impact of headgroup tailoring (converting lactose to 2'-fucosyllactose) in combination with valency increase in two assay types of increasing biorelevance (from solid-phase binding to cell binding). Compounds were prepared using copper-catalysed azide alkyne cycloaddition from peracetylated lactosyl or 2'-fucosyllactosyl azides. Significant inhibition was achieved for the plant toxin with a tetravalent compound. Different levels of sensitivity were noted for the three groups of the galectin family. The headgroup extension to 2'-fucosyllactose led to a selectivity gain, especially for the chimera-type galectin-3. Valency increase established discrimination against the homodimeric proteins, whereas the combination of valency with the headgroup extension led to discrimination against the tandem-repeat-type galectin-8 for chicken galectins but not human galectins-3 and -4. Thus, detailed structure-activity profiling of glycoclusters combined with suitably modifying the contact site for the targeted lectin will help minimize cross-reactivity among this class of closely related proteins.

The galectins are a family of [small beta]-galactoside-binding proteins that have been implicated in cancer and inflammation processes. Herein, we report the synthesis of a library of 28 compounds that was tested for binding to galectins-1, -3, -7, -8N and -9N. An aromatic nucleophilic substitution reaction between 1,5-difluoro-2,4-dinitrobenzene and a galacto thiol gave 5-fluoro-2,4-dinitrophenyl 2,3,4,6-tetra-O-acetyl-1-thio-beta-D-galactopyranoside. This versatile intermediate was then modified in a two dimensional manner: either by further substitution of the second fluoride by amines or thiols, or by reduction of the nitro groups and acylation of the resulting amines, or both. Deacetylation then gave a library of aromatic beta-galactosides that showed variable inhibitory activity against the different galectins, as shown by screening with a fluorescence-polarisation assay. Particularly efficient inhibitors were found against galectin-7, while less impressive enhancements of inhibitor affinity over methyl beta-D-galactopyranoside were found for galectin-1, -3, -8N and -9N. The best inhibitors against galectin-7 showed significantly higher affinity (K(d) as low as 140 microM) than both beta-methyl galactoside (K(d) 4.8 mM) and the unsubstituted beta-phenyl thiogalactoside (non-inhibitory). The best inhibitors against galectin-7 were poor against the other galectins and thus have potential as structurally simple and selective tools for dissecting biological functions of galectin-7.