Studies on selection blockers. 5. Design, synthesis, and biological profile of sialyl Lewis x mimetics based on modified serine-glutamic acid dipeptides

Selectins-The Two Dr. Jekyll and Mr. Hyde Faces of Adhesion Molecules-A Review

Selectins belong to a group of adhesion molecules that fulfill an essential role in immune and inflammatory responses and tissue healing. Selectins are glycoproteins that decode the information carried by glycan structures, and non-covalent interactions of selectins with these glycan structures mediate biological processes. The sialylated and fucosylated tetrasaccharide sLe^x is an essential glycan recognized by selectins. Several glycosyltransferases are responsible for the biosynthesis of the sLe^x tetrasaccharide. Selectins are involved in a sequence of interactions of circulated leukocytes with endothelial cells in the blood called the adhesion cascade. Recently, it has become evident that cancer cells utilize a similar adhesion cascade to promote metastases. However, like Dr. Jekyll and Mr. Hyde’s two faces, selectins also contribute to tissue destruction during some infections and inflammatory diseases. The most prominent function of selectins is associated with the initial stage of the leukocyte adhesion cascade, in which selectin binding enables tethering and rolling. The first adhesive event occurs through specific non-covalent interactions between selectins and their ligands, with glycans functioning as an interface between leukocytes or cancer cells and the endothelium. Targeting these interactions remains a principal strategy aimed at developing new therapies for the treatment of immune and inflammatory disorders and cancer. In this review, we will survey the significant contributions to and the current status of the understanding of the structure of selectins and the role of selectins in various biological processes. The potential of selectins and their ligands as therapeutic targets in chronic and acute inflammatory diseases and cancer will also be discussed. We will emphasize the structural characteristic of selectins and the catalytic mechanisms of glycosyltransferases involved in the biosynthesis of glycan recognition determinants. Furthermore, recent achievements in the synthesis of selectin inhibitors will be reviewed with a focus on the various strategies used for the development of glycosyltransferase inhibitors, including substrate analog inhibitors and transition state analog inhibitors, which are based on knowledge of the catalytic mechanism.

Rational Design of Novel, Potent Small Molecule Pan-Selectin Antagonists

This report describes the first results of a rational hit-finding strategy to design novel small molecule antiinflammatory drugs targeting selectins, a family of three cellular adhesion molecules. Based on recent progress in understanding of molecular interaction between selectins and their natural ligands as well as progress in clinical development of synthetic antagonists like 1 (bimosiamose, TBC1269), this study was initiated to discover small molecule selectin antagonists with improved pharmacological properties. Considering 1 as template structure, a ligand-based approach followed by focused chemical synthesis has been applied to yield novel synthetic small molecules (MWr < 500) with a trihydroxybenzene motif, bearing neither peptidic nor glycosidic components, with nanomolar in vitro activity. Biological evaluation involves two kinds of in vitro assays, a static molecular binding assay, and a dynamic HL-60 cell attachment assay. As compared to controls, the novel compounds showed improved biological in vitro activity both under static and dynamic conditions.

Sialyl Lewis(x) analogs based on a quinic acid scaffold as the fucose mimic

(-)-Quinic acid was used as a starting material for the preparation of sialyl Lewis(x) mimetics in order to target E-selectin. Spatial orientation of the hydroxyl groups of quinic acid could mimic the l-fucose ones. Introduction of a side chain ending with a carboxylic acid was effected to replace the sialic acid interaction at the carbohydrate recognition domain. A first series of derivatives, incorporating amino acids linked to quinic acid, were tested for their affinity and found to interact with E-selectin with IC(50) within the millimolar range.

Synthesis and neuroprotective activity of analogues of glycyl-l-prolyl-l-glutamic acid (GPE) modified at the α-carboxylic acid

The synthesis of nine GPE* analogues, wherein the alpha-carboxylic acid group of glutamic acid has been modified, is described by coupling readily accessible N-benzyloxycarbonyl-glycyl-L-proline 2 with various analogues of glutamic acid. Pharmacological evaluation of the novel compounds was undertaken to further understand the role of the glutamate residue on the observed neuroprotective properties of the endogenous tripeptide GPE.

Highly sensitive cell-based assay system to monitor the sialyl Lewis X biosynthesis mediated by alpha1-3 fucosyltransferase-VII

The sialyl Lewis X (sLe(x)) determinant on leukocytes serves as a ligand for selectin family cell adhesion molecules, and selectin-carbohydrate interaction is considered to play an important role in the process of leukocyte extravasation during inflammation. Among several alpha1-3 fucosyltransferases (FucTs), FucT-VII plays a critical role in the biosynthesis of sLe(x)-epitopes. Therefore, small molecules specifically designed to inhibit the FucT-VII enzyme may have potential as anti-inflammatory agents. Here, we have developed a versatile cell-based assay system to monitor sLe(x) biosynthesis using the GeneSwitch System. This system is a mifepristone (MFP)-inducible mammalian expression system, and human transfectant T lymphoblasts expressed the mRNA of FucT-VII and the sLe(x)-epitopes on the cell surface in a time-dependent manner in the presence of MFP, with very low background transcription. Furthermore, when the transfectants were treated with the FucT-VII inhibitor panosialin, sLe(x) expression on the induced cells was inhibited dose dependently without alteration at the mRNA level of FucT-VII. These results suggest that the FucT-VII may be a major regulator of the biosynthesis of the sLe(x)-epitopes on T lymphoblasts, and this cell-based assay may be utilized for a screening system of FucT-VII inhibitors.

Design and synthesis of sialyl Lewis(x) mimics as E- and P-selectin inhibitors

The selectins are a family of cell-adhesion proteins that mediate the rolling of leukocytes on activated endothelial cells through the recognition of the carbohydrate epitope sialyl Lewis(x) (sLe(x)). Control of the leukocyte-endothelial cell adhesion process may prove useful in cases where excess recruitment of leukocytes can contribute to acute diseases such as stroke and reperfusion injury and chronic diseases such as psoriasis and rheumatoid arthritis. The development of molecules that block the interactions between sLe(x) and the selectins has become an active area of research. In this review, we will highlight the various approaches taken toward the development of sLe(x) mimetics as antagonists of E- and P-selectin, including the use of structural information about the selectins and their interactions with sLe(x) that have been revealed through the use of NMR, protein crystallography and molecular modeling.

Asthma: future directions

Asthma continues to be a significant health care problem, as reflected by the increasing rise in disease morbidity and mortality. Because steroids are relatively safe, clinically effective, and easy to administer, they remain the gold standard of treatment. After many decades of use, however, it is apparent that inhaled corticosteroids have failed to halt the progression of the asthma epidemic. Newer, more effective drugs are being developed to combat this disease, and the interest in developing new medications to treat allergic disease and asthma has increased exponentially. The financial burden of asthma has also been a significant motivating factor in the development of new medications. It is estimated that in 1998 the total cost of asthma on society was $11 billion [175]. This consideration has further intensified the quest to develop more effective asthma medications. Table 1 reviews the wide array of drugs currently being investigated. With the development and approval of novel asthma treatments, millions of asthma sufferers will undoubtedly have increased therapeutic options for control of their disease in the near future.

Role of SA-Le(a) and E-selectin in metastasis assessed with peptide antagonist

E-selectin ligand Sialyl-Lewis a (SA-Le(a)) carbohydrate is expressed on many carcinomas. Peptide mimicking SA-Le(a) (DLWDWVVGKPAG) was previously selected from a recombinant library by screening with monoclonal antibody (MAb) NS19-9. In this study, the residues critical for interaction with the NS19-9 were mapped using peptide array generated by substitution of various amino acid residues. The replacement of Trp 5 with Phe resulted in a change of peptide's secondary structure and increased binding with MAb and E-selectin, suggesting improved carbohydrate mimicry. Colonization of tumor cells expressing SA-Le(a) was blocked by the peptide and was completely abolished in E-selectin knock out mice. The data suggest the critical role of carbohydrate antigens and E-selectin in metastasis and that peptides mimicking carbohydrate antigens can function as antagonists of this process.

Cell surface fucose ablation as a therapeutic strategy for malignant neoplasms

The sugar alpha-L-fucose is overexpressed in many human malignancies, especially on specific glycoproteins, glycolipids, certain mucins, and putative cell adhesion ligands found on cancer cell surfaces. Many of these molecules are known or suspected mediators of cell-cell adhesion, cell signaling, motility, or invasion. As knowledge of fucose metabolism evolves and specific mechanisms of its distribution and incorporation are more exactly documented, modulation of fucose expression in cancer is becoming increasingly more feasible. The authors propose that cancer cell surface alpha-L-fucose is a logical target for selective therapeutic ablation. Reduction of fucose content on the surfaces of malignant cells should effectively cripple the cells' physiologic functions by altering or dysregulating cell-cell or cell-matrix interactions, critical for maintaining the malignant phenotype. Significant therapeutic benefits might include modulation of adhesion abnormalities in the cancer cells, reduction of cancer cell motility or invasiveness, reexposure to immune surveillance, or a combination of these events.

Drug design, synthesis, and evaluation of a non-sugar-based selectin antagonist

We have designed a series of simple rigid compounds (2) having a phenyl ring attached to three essential groups necessary for selectin binding, i.e., a fucose unit, a carboxylic acid, and the hydrophobic part. In this series of compound 2, 2a exhibited strong inhibitory activity in in vitro P-selectin mediated cell adhesion assay. The novel type of compound 2a would be a potential lead compound for selectin antagonist.

Studies on selectin blocker. 9. SARs of non-sugar selectin blocker against E-, P-, L-selectin bindings

As a part of study of selectin blockers, we have already reported that a non-sugar selectin antagonist (3) was successfully discovered using a computational screening (Hiramatsu, Y.; Tsukida, T.; Nakai, Y.; Inoue, Y.; Kondo, H. J. Med. Chem. 2000, 43, 1476). To investigate the SARs of compound 3 against E-, P-, and L-selectins, we synthesized the derivatives of compound 3 and evaluated their inhibitory activities toward selectin bindings. The structural diversity of compound 3 contained the following: (1) a modification of the spacer unit (4--7), (2) a modification of the tail unit (8--11), (3) a modification of the head unit (12--18). As a result, it was found that a non-sugar based selectin blocker (3) could be a potential lead compound for E-, P-, and L-selectin blockers and some of the derivatives showed broad and/or selective inhibitory activities toward the E-, P-, and L-selectins. In addition, it was found that the experimental evidence well supported that the computational screening using 3D-pharmacophore model could be useful methodology to find out a new lead for the several type of selectin blockers, which included a broad and/or a selective inhibitor.

A solid-phase synthesis for beta-turn mimetics of sialyl Lewis X

A solid-phase synthesis of heterocyclic beta-turn mimetics of sialyl Lewis X, which is a natural carbohydrate ligand of selectins, was established. This synthetic method could be very useful for drug discovery of selectin antagonists using combinatorial chemistry techniques.

Synthesis of 1,1-linked galactosyl mannosides carrying a thiazine ring as mimetics of sialyl Lewis X antigen: investigation of the effect of carboxyl group orientation on P-selectin inhibition

This paper describes the synthesis of 1,1-linked galactosyl mannosides as sialyl Lewis X mimetics that contain a spiro-ring to position the carboxylate group in a well-defined orientation. It was found that compound 4 is more active as a P-selectin inhibitor (IC50 = 19 microM) than the parent disaccharide 2, which contains a flexible carboxyl group (IC50 = 193 microM). This result is consistent with that observed in the previous NMR study of sialyl Lewis X bound to P-selectin. The chemistry described here should be useful for the development of selective inhibitors of E-, P-, and L-selectins.

Study on selectin blocker. 8. Lead discovery of a non-sugar antagonist using a 3D-pharmacophore model

We have developed a pharmacophore model of a ligand/E-selectin complex to screen drug candidates for selectin blockers. In a series of sugar mimetic studies of the E-selectin ligand, sialyl Lewis X (sLe(x)), we have already found a potent compound, a sulfated Le(x) analogue (1), and also have proposed how compound 1 binds to E-selectin (Tsujishita, H.; Hiramatsu, Y.; Kondo, N.; Ohmoto, H.; Kondo, H.; Kiso, M.; Hasegawa, A. J. Med. Chem. 1997, 40, 362-369). To find drug candidates that fit into the binding pocket of E-selectin, we constructed an original 3D-pharmacophore model from structural information of a compound 1/E-selectin complex model and screened lead compounds for selectin blockers using a commercially available database ACD-3D. As a result, we discovered a lead compound (2) containing good selectin inhibitory activity, and in addition, we succeeded to preliminarily optimize it to a more active lead compound (3) with micromolar IC(50) values, based on the 3D-pharmacophore model investigation. This methodology using the 3D-pharmacophore model could be applicable as a pre-screen system for selectin blockers.

The design, synthesis, and evaluation of novel conformationally rigid analogues of sialyl Lewis(x)

The design and synthesis of a series of analogues of sialyl Lewis(x)(1) which incorporate conformationally rigid tetralin and naphthalene ring systems(2-4) has led to novel compounds which have similar potency to 1 as inhibitors of cell adhesion.

A stereoselective alpha-fucosylation reaction using 1-hydroxy 2,3,4-tri-O-benzyl-L-fucose donor for the practical synthesis of selectin blocker

A 1-hydroxy 2,3,4-tri-O-benzyl-L-fucose donor affords a high stereoselectivity of glycosylation in the presence of TMSOTf and is a very useful substrate for the preparation of an alpha-L-fucosyl dipeptide in a stereoselective manner. The donor will be a key component in the preparation of an attractively biological selectin blocker 1.

Exploration of beta-turn scaffolding motifs as components of sialyl Le(X) mimetics and their relevance to P-selectin

Monocyclic and bicyclic lactam units representing beta-turn surrogates were incorporated into a sialyl Le(X) structure by replacement of the natural sugar components. Low micromolar activity was found in a new P-selectin binding assay.

Synthesis of sialyl Lewis X mimetics using the Ugi four-component reaction

Application of the Ugi four-component condensation to rapidly synthesize a library of glycopeptide mimics of the tetrasaccharide SLe(x) as inhibitors of E- and P-selectin, and to study the effect of varied functionality in mimics on the inhibition is described.

Studies on selectin blockers. 6. Discovery of homologous fucose sugar unit necessary for E-selectin binding

We describe a mimic of the sugar unit of the E-selectin ligand, sialyl Lewis X (sLeX). Carbohydrates are entering the realm of rational drug design, aided by the growing understanding of the structure-function relationships. We investigated a new methodology of preparing sLeX mimetics and developed a potent E-selectin blocker characterized by beta-turn dipeptides. Another characteristic point of this E-selectin blocker is that the six-membered fucose ring was replaced with a five-membered fucose ring. Interestingly, it was found that the five-membered fucose ring could also bind to a calcium ion on the E-selectin, which could be an important role of the six-membered fucose ring. Especially, the L-Ser-D-Glu and D-Ser-L-Glu derivatives 3a,b showed 65-90-fold more potent inhibitory activities than the sulfated LeX analogue 1. In addition, molecular dynamics (MD) studies indicated that the 2- and 3-OH groups of the six-membered fucose ring, which were necessary for the calcium binding, overlapped well with the 2- and 3-OH groups of the five-membered fucose ring. These new findings could be useful for the design of new types of selectin blockers.

Synthesis of sialyl Lewis X pentasaccharide analogue for high-throughput screening of selectin blockers

We have developed an effective synthesis of sLe(x) pentasaccharide glycolipid analogue 2. As a part of application of sLe(x) pentasaccharide glycolipid 2 synthesized here, we have investigated the construction of a high-through-put screening system for discovery of selectin blockers. As a result, it was found that compound 2 was a useful ligand for in vitro ELISA assay and could be an important material for high-throughput screening of selectin blockers.