Aromatic heterocycle galectin-1 interactions for selective single-digit nM affinity ligands

Discovery of the Selective and Orally Available Galectin-1 Inhibitor GB1908 as a Potential Treatment for Lung Cancer

We have previously described a new series of selective and orally available galectin-1 inhibitors resulting in the thiazole-containing glycomimetic GB1490. Here, we show that the introduction of polar substituents to the thiazole ring results in galectin-1-specific compounds with low nM affinities. X-ray structural analysis of a new ligand-galectin-1 complex shows changes in the binding mode and ligand-protein hydrogen bond interactions compared to the GB1490-galectin-1 complex. These new high affinity ligands were further optimized with respect to affinity and ADME properties resulting in the galectin-1-selective GB1908 (Kd galectin-1/3 0.057/6.0 μM). In vitro GB1908 inhibited galectin-1-induced apoptosis in Jurkat cells (IC50 = 850 nM). Pharmacokinetic experiments in mice revealed that a dose of 30 mg/kg b.i.d. results in free levels of GB1908 in plasma over galectin-1 Kd for 24 h. GB1908 dosed with this regimen reduced the growth of primary lung tumor LL/2 in a syngeneic mouse model.

Discovery of Selective and Orally Available Galectin-1 Inhibitors

A new series of orally available α-d-galactopyranosides with high affinity and specificity toward galectin-1 have been discovered. High affinity and specificity were achieved by changing six-membered aryl-triazolyl substituents in a series of recently published galectin-3-selective α-d-thiogalactosides (e.g., GB1107 Kd galectin-1/3 3.7/0.037 μM) for five-membered heterocycles such as thiazoles. The in vitro pharmacokinetic properties were optimized, resulting in several galectin-1 inhibitors with favorable properties. One compound, GB1490 (Kd galectin-1/3 0.4/2.7 μM), was selected for further characterization toward a panel of galectins showing a selectivity of 6- to 320-fold dependent on galectin. The X-ray structure of GB1490 bound to galectin-1 reveals the compound bound in a single conformation in the carbohydrate binding site. GB1490 was shown to reverse galectin-1-induced apoptosis of Jurkat cells at low μM concentrations. No cell cytotoxicity was observed for GB1490 up to 90 μM in the A549 cells. In pharmacokinetic studies in mice, GB1490 showed high oral bioavailability (F% > 99%).

Glycomimetics for the inhibition and modulation of lectins

Carbohydrates are essential mediators of many processes in health and disease. They regulate self-/non-self- discrimination, are key elements of cellular communication, cancer, infection and inflammation, and determine protein folding, function and life-times. Moreover, they are integral to the cellular envelope for microorganisms and participate in biofilm formation. These diverse functions of carbohydrates are mediated by carbohydrate-binding proteins, lectins, and the more the knowledge about the biology of these proteins is advancing, the more interfering with carbohydrate recognition becomes a viable option for the development of novel therapeutics. In this respect, small molecules mimicking this recognition process become more and more available either as tools for fostering our basic understanding of glycobiology or as therapeutics. In this review, we outline the general design principles of glycomimetic inhibitors (Section 2). This section is then followed by highlighting three approaches to interfere with lectin function, i.e. with carbohydrate-derived glycomimetics (Section 3.1), novel glycomimetic scaffolds (Section 3.2) and allosteric modulators (Section 3.3). We summarize recent advances in design and application of glycomimetics for various classes of lectins of mammalian, viral and bacterial origin. Besides highlighting design principles in general, we showcase defined cases in which glycomimetics have been advanced to clinical trials or marketed. Additionally, emerging applications of glycomimetics for targeted protein degradation and targeted delivery purposes are reviewed in Section 4.

Design and synthesis of novel 3-triazolyl-1-thiogalactosides as galectin-1, -3 and -8 inhibitors

Galectins are galactoside-binding proteins that play a role in various pathophysiological conditions, making them attractive targets in drug discovery. We have designed and synthesised a focused library of aromatic 3-triazolyl-1-thiogalactosides targeting their core site for binding of galactose and a subsite on its non-reducing side. Evaluation of their binding affinities for galectin-1, -3, and -8N identified acetamide-based compound 36 as a suitable compound for further affinity enhancement by adding groups at the reducing side of the galactose. Synthesis of its dichlorothiophenyl analogue 59 and the thiodigalactoside analogue 62 yielded promising pan-galectin inhibitors.

Glycopolymers Decorated with 3-O-Substituted Thiodigalactosides as Potent Multivalent Inhibitors of Galectin-3

Galectin-3 (Gal-3) participates in many cancer-related metabolic processes. The inhibition of overexpressed Gal-3 by, e.g., β-galactoside-derived inhibitors is hence promising for cancer treatment. The multivalent presentation of such inhibitors on a suitable biocompatible carrier can enhance the overall affinity to Gal-3 and favorably modify the interaction with Gal-3-overexpressing cells. We synthesized a library of C-3 aryl-substituted thiodigalactoside inhibitors and their multivalent N-(2-hydroxypropyl)methacrylamide (HPMA)-based counterparts with two different glycomimetic contents. Glycopolymers with a higher content of glycomimetic exhibited a higher affinity to Gal-3 as assessed by ELISA and biolayer interferometry. Among them, four candidates (with 4-acetophenyl, 4-cyanophenyl, 4-fluorophenyl, and thiophen-3-yl substitution) were selected for further evaluation in cancer-related experiments in cell cultures. These glycopolymers inhibited Gal-3-induced processes in cancer cells. The cyanophenyl-substituted glycopolymer exhibited the strongest antiproliferative, antimigratory, antiangiogenic, and immunoprotective properties. The prepared glycopolymers appear to be prospective modulators of the tumor microenvironment applicable in the therapy of Gal-3-associated cancers.

Engineering the ligand specificity of the human galectin-1 by incorporation of tryptophan analogs

Galectin-1 is a β-galactoside-binding lectin with manifold biological functions. A single tryptophan residue (W68) in its carbohydrate binding site plays a major role in ligand binding and is highly conserved among galectins. To fine tune galectin-1 specificity, we introduced several non-canonical tryptophan analogs at this position of human galectin-1 and analyzed the resulting variants using glycan microarrays. Two variants containing 7-azatryptophan and 7-fluorotryptophan showed a reduced affinity for 3'-sulfated oligosaccharides. Their interaction with different ligands was further analyzed by fluorescence polarization competition assay. Using molecular modeling we provide structural clues that the change in affinities comes from modulated interactions and solvation patterns. Thus, we show that the introduction of subtle atomic mutations in the ligand binding site of galectin-1 is an attractive approach for fine-tuning its interactions with different ligands.

Synthesis of fluorinated thiodigalactoside analogues

In this work, we report the first synthesis of fluorinated thiodigalactoside analogues. We used tri-isopropylsilyl thioglycosides as masked glycosyl thiol nucleophiles for the elaboration of two monofluorinated heterodimers, one difluorinated homodimer, and one difluorinated heterodimer. Moreover, we also present an alternative synthesis of 3-deoxy-3-fluorogalactose and 4-deoxy-4-fluorogalactose from a common precursor. Finally, this small set of more stable thiodigalactoside analogues could be interesting inhibitors of galactose-specific lectins.

Structure-based identification of galectin-1 selective modulators in dietary food polyphenols: a pharmacoinformatics approach

Abstract

In this study, a set of dietary polyphenols was comprehensively studied for the selective identification of the potential inhibitors/modulators for galectin-1. Galectin-1 is a potent prognostic indicator of tumor progression and a highly regarded therapeutic target for various pathological conditions. This indicator is composed of a highly conserved carbohydrate recognition domain (CRD) that accounts for the binding affinity of β-galactosides. Although some small molecules have been identified as galectin-1 inhibitors/modulators, there are limited studies on the identification of novel compounds against this attractive therapeutic target. The extensive computational techniques include potential drug binding site recognition on galectin-1, binding affinity predictions of ~ 500 polyphenols, molecular docking, and dynamic simulations of galectin-1 with selective dietary polyphenol modulators, followed by the estimation of binding free energy for the identification of dietary polyphenol-based galectin-1 modulators. Initially, a deep neural network-based algorithm was utilized for the prediction of the druggable binding site and binding affinity. Thereafter, the intermolecular interactions of the polyphenol compounds with galectin-1 were critically explored through the extra-precision docking technique. Further, the stability of the interaction was evaluated through the conventional atomistic 100 ns dynamic simulation study. The docking analyses indicated the high interaction affinity of different amino acids at the CRD region of galectin-1 with the proposed five polyphenols. Strong and consistent interaction stability was suggested from the simulation trajectories of the selected dietary polyphenol under the dynamic conditions. Also, the conserved residue (His44, Asn46, Arg48, Val59, Asn61, Trp68, Glu71, and Arg73) associations suggest high affinity and selectivity of polyphenols toward galectin-1 protein.

Graphic Abstract

Synthesis of Galectin Inhibitors by Regioselective 3'-O-Sulfation of Vanillin Lactosides Obtained under Phase Transfer Catalysis

Vanillin-based lactoside derivatives were synthetized using phase-transfer catalyzed reactions from per-O-acetylated lactosyl bromide. The aldehyde group of the vanillin moiety was then modified to generate a series of related analogs having variable functionalities in the para- position of the aromatic residue. The corresponding unprotected lactosides, obtained by Zemplén transesterification, were regioselectively 3'-O-sulfated using tin chemistry activation followed by treatment with sulfur trioxide-trimethylamine complex (Men3N-SO3). Additional derivatives were also prepared from the vanillin's aldehyde using a Knoevenagel reaction to provide extended α, β-unsaturated carboxylic acid which was next reduced to the saturated counterpart.

Stereo- and regioselective hydroboration of 1-exo-methylene pyranoses: discovery of aryltriazolylmethyl C-galactopyranosides as selective galectin-1 inhibitors

Galectins are carbohydrate recognition proteins that bind carbohydrates containing galactose and are involved in cell signaling and cellular interactions, involving them in several diseases. We present the synthesis of (aryltriazolyl)methyl galactopyranoside galectin inhibitors using a highly diastereoselective hydroboration of C1-exo-methylene pyranosides giving inhibitors with fourfold or better selectivity for galectin-1 over galectin-3, -4C (C-terminal CRD), -4N (N-terminal CRD), -7, -8C, -8N, -9C, and -9N and dissociation constants down to 170 µM.