Carbohydrate scaffolds in chemical genetic studies

Targeting GRP receptor: Design, synthesis and preliminary biological characterization of new non-peptide antagonists of bombesin

We report the rational design, synthesis, and in vitro preliminary evaluation of a new small library of non-peptide ligands of Gastrin Releasing Peptide Receptor (GRP-R), able to antagonize its natural ligand bombesin (BN) in the nanomolar range of concentration. GRP-R is a transmembrane G-protein coupled receptor promoting the stimulation of cancer cell proliferation. Being overexpressed on the surface of different human cancer cell lines, GRP-R is ideal for the selective delivery to tumor cells of both anticancer drug and diagnostic devices. What makes very challenging the design of non-peptide BN analogues is that the 3D structure of the GRP-R is not available, which is the case for many membrane-bound receptors. Thus, the design of GRP-R ligands has to be based on the structure of its natural ligands, BN and GRP. We recently mapped the BN binding epitope by NMR and here we exploited the same spectroscopy, combined with MD, to define BN conformation in proximity of biological membranes, where the interaction with GRP-R takes place. The gained structural information was used to identify a rigid C-galactosidic scaffold able to support pharmacophore groups mimicking the BN key residues' side chains in a suitable manner for binding to GRP-R. Our BN antagonists represent hit compounds for the rational design and synthesis of new ligands and modulators of GRP-R. The further optimization of the pharmacophore groups will allow to increase the biological activity. Due to their favorable chemical properties and stability, they could be employed for the active receptor-mediated targeting of GRP-R positive tumors.

Design and efficient synthesis of pyrazoline and isoxazole bridged indole C-glycoside hybrids as potential anticancer agents

C-glycosides are important class of molecules exhibit diverse biological activities and present as structural motif in many natural products. Two series of new pyrazoline and isoxazole bridged indole C-glycoside molecular hybrids (n = 36) were efficiently synthesized starting from diverse indole 3-carboxaldehydes derived α, β-unsaturated ketone derivatives of β-D-glucosyl-propan-2-one, β-D-galactosyl-propan-2-one and β-D-mannosyl-propan-2-one, reacting with hydrazine hydrate and hydroxyl amine hydrochloride in shorter reaction time (15 min) under microwave assisted condition. Anticancer activity of these newly synthesized pyrazoline and isoxazole bridged indoles C-glycoside hybrids were determined in details through cellular assays against MCF-7, MDA-MB-453 and MDA-MB-231 cancer cell lines. The selected library members displayed low micromolar (IC₅₀ = 0.67–4.67 µM) and selective toxicity against breast cancer cell line (MCF-7). Whereas these compounds were nontoxic towards normal cell line (MCF-10A). Mechanistic studies showed that, active compounds inhibit COX-2 enzyme, which was also supported by molecular docking studies. These findings are expected to provide new leads towards anticancer drug discovery.

Discovery of small molecule inhibitors of xyloglucan endotransglucosylase (XET) activity by high-throughput screening

Small molecules (xenobiotics) that inhibit cell-wall-localised enzymes are valuable for elucidating the enzymes' biological roles. We applied a high-throughput fluorescent dot-blot screen to search for inhibitors of Petroselinum xyloglucan endotransglucosylase (XET) activity in vitro. Of 4216 xenobiotics tested, with cellulose-bound xyloglucan as donor-substrate, 18 inhibited XET activity and 18 promoted it (especially anthraquinones and flavonoids). No compounds promoted XET in quantitative assays with (cellulose-free) soluble xyloglucan as substrate, suggesting that promotion was dependent on enzyme-cellulose interactions. With cellulose-free xyloglucan as substrate, we found 22 XET-inhibitors - especially compounds that generate singlet oxygen ((1)O2) e.g., riboflavin (IC50 29 μM), retinoic acid, eosin (IC50 27 μM) and erythrosin (IC50 36 μM). The riboflavin effect was light-dependent, supporting (1)O2 involvement. Other inhibitors included tannins, sulphydryl reagents and triphenylmethanes. Some inhibitors (vulpinic acid and brilliant blue G) were relatively specific to XET, affecting only two or three, respectively, of nine other wall-enzyme activities tested; others [e.g. (-)-epigallocatechin gallate and riboflavin] were non-specific. In vivo, out of eight XET-inhibitors bioassayed, erythrosin (1 μM) inhibited cell expansion in Rosa and Zea cell-suspension cultures, and 40 μM mycophenolic acid and (-)-epigallocatechin gallate inhibited Zea culture growth. Our work showcases a general high-throughput strategy for discovering wall-enzyme inhibitors, some being plant growth inhibitors potentially valuable as physiological tools or herbicide leads.

Deoxysugars as antituberculars and alpha-mannosidase inhibitors

A promising modified sugar molecule was identified which was active against multidrug-resistant (MDR) strains of Mycobacterium tuberculosis, suggesting involvement of a new target. The compound was demonstrated to be bactericidal, inhibited the growth of M. tuberculosis in mice, and targeted alpha-mannosidase as a competitive inhibitor with a Ki value of 353.9 μM.

The structural biology of enzymes involved in natural product glycosylation

The glycosylation of microbial natural products often dramatically influences the biological and/or pharmacological activities of the parental metabolite. Over the past decade, crystal structures of several enzymes involved in the biosynthesis and attachment of novel sugars found appended to natural products have emerged. In many cases, these studies have paved the way to a better understanding of the corresponding enzyme mechanism of action and have served as a starting point for engineering variant enzymes to facilitate to production of differentially-glycosylated natural products. This review specifically summarizes the structural studies of bacterial enzymes involved in biosynthesis of novel sugar nucleotides.