Current Topics in Medicinal Chemistry - Volume 8, Issue 2, 2008

The idea for this special issue of Current Topics in Medicinal Chemistry was generated during a Gordon Research Conference on Carbohydrates that I attended in 2005. Having attended several of these conferences over the past 25 years, I was struck by the growth in our understanding of the chemistry and biology of carbohydrates, and the possibilities that now exist for the application of that understanding to the development of new therapies in medicine. It seemed an appropriate time to bring wider exposure of the advances in carbohydrate science to the medicinal chemistry community. The important role that cell-surface carbohydrates play in biological interactions has come to be widely appreciated as tools have been developed for the chemical synthesis and study of carbohydrate-protein and carbohydrate-lipid conjugates. Glycoconjugates on the surface of a cell provide receptors for cell-cell recognition at the molecular level, and are involved in such processes as the regulation of cell growth and repair, cell adhesion and migration, and in pathological conditions such as tumor metastasis in cancer. Our recognition of the roles that carbohydrates play in these processes will continue to inspire research in the field of synthetic vaccines, a topic that is featured in several of the articles in this issue. The use of synthetic vaccines incorporating oligosaccharides holds promise in the treatment of bacterial infections, and also in the treatment of AIDS and cancer. Understanding the biosynthesis of cell-surface oligosaccharides is also important to the identification of potential targets for pathogenic organisms, and the use of vaccine adjuvants has been expanded to include carbohydrate mimetics of lipid A that interact with toll-like receptors. The anticoagulant heparin, a glycosaminoglycan, is one of the most widely prescribed medications in the world. The pharmacokinetics and pharmacodynamics of heparin may be improved through the use of immobilized heparins, which also have potential for applications in regulating other biological pathways. Carbohydrate-based drugs are finding new applications in the treatment of CNS disorders, and pyranose N-glycosyl amines have been found to have a range of interesting biological activities and potential in the treatment of infection and inflammation. New synthetic methodology will always find important applications in the development of carbohydrate-based drugs of the complexity found in this issue. A review on the use of ultrasound and microwaves in carbohydrate chemistry and one on palladium-catalyzed glycosylation describe advances in carbohydrate synthesis by these methods. Our ability to synthesis complex, carbohydrate-derived therapeutic agents, coupled with our increased understanding of the roles of glycoconjugates in biology will hopefully lead to promising treatments for a wide range of diseases.

The design of vaccine adjuvants and stand-alone immunotherapeutics has historically been a mix of alchemy and accident partly because of the complex nature of the molecular mechanisms involved in immune system function. The recent discovery of pattern recognition receptors and toll-like receptors (TLRs) in particular on cells of the immune system has shown the important role that stimulation of these cell receptors by microbial products plays in both innate and adaptive immune responses. Considerable effort has been directed at developing pharmaceutically acceptable mimetics of many TLR-active natural products, including the main cell-surface component of Gram-negative bacteria: lipopolysaccharide (LPS). LPS and its active principle, lipid A, are potent stimulators of host defense systems via their interaction with TLR4. However, the profound pyrogenicity and lethal toxicity of LPS and lipid A have precluded their medicinal use. Structure/activity investigations on natural S. minnesota R595 lipid A and its derivatives have led to the development of a novel class of synthetic lipid A mimetics known as aminoalkyl glucosaminide phosphates (AGPs). This review discusses the evolution of the AGPs and related TLR4-active glycolipids with emphasis on structure/activity relationships in the AGP series and pre-clinical/clinical development of selected AGPs, including the potent vaccine adjuvant RC-529.

Heparin, an anticoagulant, has been used in many forms to treat various diseases. These forms include soluble heparin and heparin immobilized to supporting matrices by physical adsorption, by covalent chemical methods and by photochemical attachment. These immobilization methods often require the use of spacers or linkers. This review examines and compares various techniques that have been used for the immobilization of heparin as well as applications of these immobilized heparins. In the applications reviewed, immobilized heparin is compared with soluble heparin for efficient and versatile use in each of the various applications.

Interest in the chemistry and biological properties of non-nucleoside N-glycosidic compounds has gathered pace over the past several years; the occurrence of the N-glycoside moiety in glycoproteins and a range of active natural products has prompted the synthesis of a diverse spectrum of related materials with promising potential in medicinal chemistry. Particularly prominent has been the synthesis of novel N-glycosyl amides, 1,2,3-triazoles, and progress in the construction and diversification of natural products such as the mannopeptimycins and indolocarbazoles, each of which are discussed in this article.

To address the medicinal chemist's need for new synthetic methods for the preparation of unnatural carbohydrates, a new de novo method for carbohydrate synthesis has been developed. These routes use a palladium catalyzed glycosylation reaction to stereoselectively control the anomeric center and subsequent diastereoselective post glycosylation to install the remaining sugar stereocenters. The utility of this method was demonstrated by the syntheses and biological evaluation of several digitoxin oligosaccharide analogues.

Synthetic advances made possible chemical assembly of complex oligosaccharide fragments of polysaccharide domains on the surface of human pathogenic bacteria. These oligosaccharides may be recognized by antibodies raised against high molecular weight, native, polysaccharides. In addition to their antigenicity, synthetic oligosaccharides can also function as haptens in their protein conjugates that can elicit not only oligo- but also polysaccharide-specific IgG antibodies in animal models and in humans. A major milestone in the development of new generation vaccines was the demonstration that protein conjugates of synthetic fragments of the capsular polysaccharide of Haemophilus influenzae type b are as efficacious in preventing childhood meningitis and other diseases as is the corresponding licensed commercial vaccine containing the bacterial polysaccharide. The lessons learnt in this and other endeavors described herein are manifold. For example, they teach us about the significance of the oligosaccharide epitope size, the number of their copies per protein in the conjugate, the possible effect of the spacer on anti-saccharide immune response, and the proper choice of the carrier protein combined with the selection of the animal model. The H. influenzae b story also teaches us that that the synthetic approach can be commercially viable.

The surfaces of almost all microbes are decorated with remarkable variations of polysaccharides such as Oantigen, capsular polysaccharides (CPS), and exopolysaccharides (EPS) in bacteria, lipoarabinomannans (LAM) in mycobacteria and lipophosphoglycan (LPG) in Leishmania. These polysaccharides play important roles in many biological processes, and they can function as the virulence determinants in the pathogens. The basic structures of these polysaccharides are known, but they show species-specificity or stage-specificity. For example, there are 186 O-serotypes and 80 capsular serotypes in E. coli. Despite the variation, the range of strategies used for the biosynthesis and assembly of these microbial polysaccharides is limited. Depending on the assembly and translocation mechanisms, O-antigen biosynthesis is subdivided into three pathways, of which the Wzy-dependent pathway is widely used not only in Oantigen, but also in CPS and EPS.

Methods to transform carbohydrates are often complex and tedious, both due to the vast array of naturally occurring and synthetically designed scaffolds which may manifest meager to drastic reactivity, dependent upon the transformation sought and the stereogenic site chosen. In order to facilitate and expedite desired synthetic transformation, many researchers are utililizing microwave and ultrasonic irradiation to achieve their goals, in generally high yields within a shorter period of time, and often without undesirous byproducts. The basic physical principles underlying the energy regimes are qualitatively discussed prior to review of the applications in carbohydrate syntheses and transformation. This literature review looks at research involving glycosylations, -OH group conversions, isotopic incorporation, and C-N bond formation. Instances of improved yields and selectivities resultant from the use of these high-energy sources will be highlighted.

Mental illness affects a quarter of the US population. Recently, it has been shown that new, carbohydrate-based drugs hold promise in the treatment of central nervous system (CNS) disorders. A variety of ways in which drugs of this sort may reduce the symptoms of epilepsy, depression and other affective disorders have been proposed, including: targeting the immune system, disrupting glycolysis, acting at different sites in the hypothalamic-pituitary-adrenal (HPA) axis, and targeting specific biochemical pathways such as the inositol pathway. In the present review, the structureactivity relationships (SARs) of a wide variety of CNS-active carbohydrates are presented.

CHF5074 - A novel NSAID-derived γ-secretase modulator that reduces brain β-amyloid pathology without peripheral toxicity. Some nonsteroidal ant-inflamatory drugs (NSAIDs) have been shown to decrease Aβ42 production, and upon long-term use, may delay or prevent the onset of Alzheimer's disease (AD) in both transgenic mice and humans [1-3]. The mechanism proposed to account for this effect on Aβ42 is allosteric modulation of presenilin- 1, the major component of the γ-secretase complex responsible for the formation of Aβ [4,5]. Importantly, the inhibition of Aβ42 is independent of the anti-COX activity, and the γ-secretase modulator activity depends on the NSAID chemotype, with some NSAIDs (ibuprofen, indomethacin, flurbiprofen and sulindac) displaying activity and other not (naproxen, aspirin and celecoxib)[1,4,6]. Moreover, the Aβ42-lowering effects of NSAIDs differ from classical γ-secretase inhibitors as they do not inhibit γ-secretasemediate cleavage of APP at the η site or Notch-1; however, the application of NSAIDs for the treatment of AD has been hindered by the gastrointestinal toxicity of COX inhibition [1,7]. A recent manuscript from Imbimido, et.al. (J. Pharm Exp. Ther. 2007, 323, 822-830) presented the first demonstration that chronic administration of an NSAID-derived γ-secretase modulator (CHF 5074), devoid of both anticyclooxygenase (COX) and Notch-inter-fering activities, can significantly reduce the deposition of Aβ in the brain [8]. CHF5074 in human neuroglioma cells preferentially lowers Aβ42 (IC50 = 40 μM) without effect on COX-1 or COX-2 enzymes up to 300 μM, and also had no effect on the expression profile of several Notch intracellular domain-responsive genes when dosed at 100 μM. In addition, CHF5074 possessed good oral bioavailability in rats (%F = 50) and a long half-life (t1/2 = 20.7 h). In the study, aged Tg2576 mice were chronically dosed with CHF5074 (in diet 61 mg/kg/day) for 4 months, a well tolerated and safe dose [8]. Compared with controls, the area occupied by plaques and the number of plaques in cortex (-52% and -48%, respectively) and hippocampus (- 76% and -66%, respectively) were significantly reduced in CHF5074 treated animals. Furthermore, biochemical analysis demonstrated that CHF5074-tretaed animals displayed reduced total brain Aβ40 (-49%) and Aβ42 (-43%) levels. In a human neuroglioma cell line expressing the Swedish mutated form of APP (H4swe), CHF5074 reduced Aβ40 and Aβ42 secretion with IC50 values of 3.6 μM and 18.4 μM, respectively [8]. Histopathological examination of the GItract of the chronically treated Tg2567 mice produced no abnormal findings, further indicating the lack of COXrelated toxicity [8]. Based on these data, CHF5074 represents a promising new lead for the development of a potential therapeutic agent for the treatment of Alzheimer's disease. REFERENCES [1] Weggen, S.; Eriksein, J.L.; Das, P.; Sagi, S.A.; Wang, R.; Pietrzik, C.U.; Findlay, K.A.; Smith, T.E.; Murphy, M.P.; Butler, T. Nature 2001, 414, 212-216. [2] Lim, G.P.; Yang, F.; Chu, T.; Chen, P.; Beech, W.; Teter, B.; Tran, T.; Ubeda, O.; Ashe, K.H.; Frautschy, S.A.; Cole, G.M. Neurobiol. Aging 2001, 22, 983-991. [3] Wilcock, D.M.; Jantzen, P.T.; Li, Q.; Morgan, D.; Gordon, M.N. Neuroscience 2007, 144, 9