Synthesis and structure verification of the vaccine adjuvant QS-7-Api. Synthetic access to homogeneous Quillaja saponaria immunostimulants

Chemical and biological characterization of vaccine adjuvant QS-21 produced via plant cell culture

Many vaccines, including those using recombinant antigen subunits, rely on adjuvant(s) to enhance the efficacy of the host immune responses. Among the few adjuvants clinically approved, QS-21, a saponin-based immunomodulatory molecule isolated from the tree bark of Quillaja saponaria (QS) is used in complex formulations in approved effective vaccines. High demand of the QS raw material as well as manufacturing scalability limitation has been barriers here. We report for the first-time successful plant cell culture production of QS-21 having structural, chemical, and biologic, properties similar to the bark extracted product. These data ensure QS-21 and related saponins are broadly available and accessible to drug developers.

Potential Immunoregulatory Mechanism of Plant Saponins: A Review

Saponins are extracted from different parts of plants such as seeds, roots, stems, and leaves and have a variety of biological activities including immunomodulatory, anti-inflammatory effects, and hypoglycemic properties. They demonstrate inherent low immunogenicity and possess the capacity to effectively regulate both the innate and adaptive immune responses. Plant saponins can promote the growth and development of the body's immune organs through a variety of signaling pathways, regulate the activity of a variety of immune cells, and increase the secretion of immune-related cytokines and antigen-specific antibodies, thereby exerting the role of immune activity. However, the chemical structure of plant saponins determines its certain hemolytic and cytotoxicity. With the development of science and technology, these disadvantages can be avoided or reduced by certain technical means. In recent years, there has been a significant surge in interest surrounding the investigation of plant saponins as immunomodulators. Consequently, the objective of this review is to thoroughly examine the immunomodulatory properties of plant saponins and elucidate their potential mechanisms, with the intention of offering a valuable point of reference for subsequent research and advancement within this domain.

Synthesis of immunostimulatory saponins: A sweet challenge for carbohydrate chemists

Saponins are a large family of natural glycosides showing a wide range of biological activities. Current research efforts on saponins as vaccine adjuvants have been mainly focused on the development of synthetic analogs. By mimicking the immunomodulatory saponins from Quillaja saponaria (QS), less complex and readily accessible analogs have been synthesized to improve the industrial applicability and efficacy of saponins as vaccine adjuvants. Through the exploration of several structural modifications on the skeleton of QS saponins, including changes in the sugar and aglycone compositions as well as in the nature and configuration of the glycosidic bonds, structure-activity relationship (SAR) studies developed by Pr. Gin in the early 2010s were taken as a starting point for the development of a new generation of immunomodulatory candidates. In this review, the recent synthetic strategies and SAR studies of mono- and bidesmosidic QS saponins are discussed. Original concepts of vaccination including self-adjuvanticity and the development of saponin-based glycoconjugates are described. The synthesis and semi-synthesis of saponin alternatives to QS, such as Momordica saponin and onjisaponin derivatives, are also discussed in this review.

Glycoconjugates: Synthesis, Functional Studies, and Therapeutic Developments

Glycoconjugates are major constituents of mammalian cells that are formed via covalent conjugation of carbohydrates to other biomolecules like proteins and lipids and often expressed on the cell surfaces. Among the three major classes of glycoconjugates, proteoglycans and glycoproteins contain glycans linked to the protein backbone via amino acid residues such as Asn for N-linked glycans and Ser/Thr for O-linked glycans. In glycolipids, glycans are linked to a lipid component such as glycerol, polyisoprenyl pyrophosphate, fatty acid ester, or sphingolipid. Recently, glycoconjugates have become better structurally defined and biosynthetically understood, especially those associated with human diseases, and are accessible to new drug, diagnostic, and therapeutic developments. This review describes the status and new advances in the biological study and therapeutic applications of natural and synthetic glycoconjugates, including proteoglycans, glycoproteins, and glycolipids. The scope, limitations, and novel methodologies in the synthesis and clinical development of glycoconjugates including vaccines, glyco-remodeled antibodies, glycan-based adjuvants, glycan-specific receptor-mediated drug delivery platforms, etc., and their future prospectus are discussed.

Chemical synthesis of saponins: An update

Saponins, as secondary metabolites in terrestrial plants and marine invertebrate, constitute one of the largest families of natural products. The long history of folk medicinal applications of saponins makes them attractive candidates for innovative drug design and development. Chemical synthesis has become a practical alternative to the availability of the natural saponins and their modified analogs, so as to facilitate SAR studies and the discovery of optimal structures for clinical applications. The recent achievements in the synthesis of these complex saponins reflect the advancements of both steroid/triterpene chemistry and carbohydrate chemistry. This chapter provides an updated review on the chemical synthesis of natural saponins, covering the literature from 2014 to 2020.

Immunostimulatory 6/6/6/6 Tetracyclic Triterpenoid Saponins with the Methyl-30 Incorporated Cyclization from the Root of Colquhounia elegans

Two novel triterpenoid saponins, colqueleganoids A (1) and B (2), with the first methyl-30 incorporated 6/6/6/6-cyclized carbon skeleton (named colquelegane), were isolated from the root of Colquhounia elegans. Their structures including absolute configuration were determined by spectroscopic methods and X-ray crystallographic analyses. Interestingly, both compounds significantly enhanced TNF-α production and 1 also increased the IL-6 production in RAW264.7 macrophages stimulated with lipopolysaccharide (LPS), suggesting their potential application as immunostimulants in immunotherapy and vaccination.

Replacing the Rhamnose-Xylose Moiety of QS-21 with Simpler Terminal Disaccharide Units Attenuates Adjuvant Activity in Truncated Saponin Variants

Adjuvants are key immunostimulatory components in vaccine formulations, which improve the immune response to the co-administered antigen. The saponin natural product QS-21 is one of the most promising immunoadjuvants in the development of vaccines against cancer and infectious diseases but suffers from limitations that have hampered its widespread human use. Previous structure-activity relationship studies have identified simplified saponin variants with truncated carbohydrate chains, but have not focused on the influence of the linear oligosaccharide domain of QS-21 in adjuvant activity. Herein, an expeditious 15-step synthesis of new linear trisaccharide variants of simplified QS-21-derived adjuvants is reported, in which the complex terminal xylose-rhamnose moiety has been replaced with commercially available, simpler lactose and cellobiose disaccharides in a β-anomeric configuration. In vivo immunological evaluation of the synthetic saponins showed attenuated antibody responses, highlighting the negative impact of such carbohydrate modifications on adjuvant activity, which could be associated with higher saponin conformational flexibility.

Natural and Synthetic Saponins as Vaccine Adjuvants

Saponin adjuvants have been extensively studied for their use in veterinary and human vaccines. Among them, QS-21 stands out owing to its unique profile of immunostimulating activity, inducing a balanced Th1/Th2 immunity, which is valuable to a broad scope of applications in combating various microbial pathogens, cancers, and other diseases. It has recently been approved for use in human vaccines as a key component of combination adjuvants, e.g., AS01b in Shingrix^® for herpes zoster. Despite its usefulness in research and clinic, the cellular and molecular mechanisms of QS-21 and other saponin adjuvants are poorly understood. Extensive efforts have been devoted to studies for understanding the mechanisms of QS-21 in different formulations and in different combinations with other adjuvants, and to medicinal chemistry studies for gaining mechanistic insights and development of practical alternatives to QS-21 that can circumvent its inherent drawbacks. In this review, we briefly summarize the current understandings of the mechanism underlying QS-21’s adjuvanticity and the encouraging results from recent structure-activity-relationship (SAR) studies.

Natural and synthetic carbohydrate-based vaccine adjuvants and their mechanisms of action

Modern subunit vaccines based on homogeneous antigens offer more precise targeting and improved safety compared with traditional whole-pathogen vaccines. However, they are also less immunogenic and require an adjuvant to increase the immunogenicity of the antigen and potentiate the immune response. Unfortunately, few adjuvants have sufficient potency and low enough toxicity for clinical use, highlighting the urgent need for new, potent and safe adjuvants. Notably, a number of natural and synthetic carbohydrate structures have been used as adjuvants in clinical trials, and two have recently been approved in human vaccines. However, naturally derived carbohydrate adjuvants are heterogeneous, difficult to obtain and, in some cases, unstable. In addition, their molecular mechanisms of action are generally not fully understood, partly owing to the lack of tools to elucidate their immune-potentiating effects, thus hampering the rational development of optimized adjuvants. To address these challenges, modification of the natural product structure using synthetic chemistry emerges as an attractive approach to develop well-defined, improved carbohydrate-containing adjuvants and chemical probes for mechanistic investigation. This Review describes selected examples of natural and synthetic carbohydrate-based adjuvants and their application in synthetic self-adjuvanting vaccines, while also discussing current understanding of their molecular mechanisms of action.

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

Impact of C28 Oligosaccharide on Adjuvant Activity of QS-7 Analogues

We have synthesized a number of Quillaja saponaria Molina (QS) saponin analogues with a different C28 sugar unit, which features either 3,4-diacetyl groups or a 3,4-cyclic carbonate group at the reducing end fucoside to mimic the naturally occurring saponin adjuvant QS-7. Immunological evaluations of these analogues in BALB/c mice indicate that truncating the C28 oligosaccharide of the natural product to the tetrasaccharide (as in 5d (β)) could retain the adjuvant's activity in enhancing IgG1 and IgG2a productions, albeit the activity is lower than that of QS-21. Further truncation or changing stereochemistry of glycosidic linkage between the tetrasaccharide and the triterpenoid quillaic acid (QA) core or within the tetrasaccharide eliminated the saponins' adjuvant activity in terms of IgG production. On the other hand, increasing resemblance to QS-7 increased adjuvant activity and led to saponin 3's similar IgG1 and IgG2a activities to QS-21's, indicating that the unique adjuvant activities of QS saponins are determined by their specific structures.

Synthesis and Evaluation of QS-7-Based Vaccine Adjuvants

We have designed and synthesized two analogs (5 and 6) of QS-7, a natural saponin compound isolated from Quillaja saponaria (QS) Molina tree bark. The only structural difference between compound 5 and 6 is that 5 is acetylated at the 3- and 4-O positions of the quillaic acid C28 fucosyl unit while 6 is not. However, the two analogs show significantly different immunostimulant profiles. Compound 5 may potentiate a mixed Th1/Th2 (Th, T helper cells) immune response against the specific antigens while compound 6 may only induce a Th2-biased immunity. These results suggest that the 3- and/or 4-O acetyl groups of the fucosyl unit may play an important role in tuning the adjuvanticity of the QS-7 analogs, and compound 5 can serve as a structurally defined synthetic adjuvant when a mixed Th1/Th2 immune responses is desired.

Synthesis and Evaluation of a QS-17/18-Based Vaccine Adjuvant

We have synthesized a QS-17/18 analogue (7) and evaluated its adjuvant activity in the formulation with rHagB antigen. Compound 7 and QS-21 analogues 5 and 6 are presumably the major components of GPI-0100, a widely used complex mixture of semisynthetic derivatives of Quillaja saponaria (QS) Molina saponins. The QS-17/18 analogue 7 shows an adjuvant activity profile similar to that of GPI-0100, potentiating mixed Th-1/Th-2 immune responses, which is different from those of QS-21 analogues 5 and 6 that probably only induce a Th2-like immunity. The combination of QS-17/18 and QS-21 analogues does not show a synergistic effect. These results suggest that QS-17/18 analogue 7 might be the active component of GPI-0100 responsible for its immunostimulant property. Therefore, compound 7 can not only be a structurally defined alternative to GPI-0100 but also provide a valuable clue for rational design of new QS-based vaccine adjuvants with better adjuvant properties.

Direct Dehydrative Glycosylation of C1-Alcohols

Due to the central role played by carbohydrates in a multitude of biological processes, there has been a sustained interest in developing effective glycosylation methods to enable more thorough investigation of their essential functions. Among the myriad technologies available for stereoselective glycoside bond formation, dehydrative glycosylation possesses a distinct advantage given the unique properties of C1-alcohols such as straightforward preparation, stability, and a general reactivity compatible with a diverse set of reaction conditions. In this Focus Review, a survey of direct dehydrative glycosylations of C1-alcohols is provided with an emphasis on recent achievements, pervading limitations, mechanistic insights, and applications in total synthesis.

Synthesis of the hexacyclic triterpene core of the jujuboside saponins via tandem Wolff rearrangement-intramolecular ketene hetero-Diels-Alder reaction

The jujubosides are saponin natural products reported to have immunoadjuvant, anticancer, antibacterial, antifungal, and antisweet activities. The triterpene component, jujubogenin contains a unique tricyclic ketal motif comprising the DEF ring system. Herein, we describe our efforts toward the total synthesis of jujubogenin, using a sterically-demanding intermolecular Diels-Alder reaction to assemble the C-ring and a tandem Wolff rearrangement-intramolecular ketene hetero-Diels-Alder reaction to form the DF-ring system. Acid-catalyzed cyclization of the resulting bicyclic enol ether then closes the E-ring to provide the hexacyclic core of jujubogenin.

Design, synthesis and evaluation of optimized saponin variants derived from the vaccine adjuvant QS-21

The saponin natural product QS-21 is one of the most potent investigational adjuvants, which are substances added to vaccines to enhance the immunogenicity of the antigen and potentiate the immune response. While QS-21 has been coadministered with vaccines against cancers and infectious diseases in many clinical trials, its inherent liabilities (scarcity, heterogeneity, instability, and dose-limiting toxicity) have limited its widespread clinical use. Furthermore, its molecular mechanisms of action are poorly understood. Structural modification of the natural product using chemical synthesis has become an important strategy to overcome these limitations. This review focuses mainly on research efforts in the group of the late Professor David Y. Gin on the development of optimized synthetic saponin adjuvants derived from QS-21. A number of QS21 variants incorporating stable acyl chain amide linkages, truncated carbohydrate domains, and targeted modifications at the triterpene and central glycosyl ester linkage were designed, chemically synthesized, and immunologically evaluated. These studies delineated key minimal structural requirements for adjuvant activity, established correlations between saponin conformation and activity, and provided improved, synthetically accessible saponin adjuvants. Moreover, leveraging these structure–activity relationships, novel saponin probes with high potency and reduced toxicity were developed and used in biodistribution and fluorescence imaging studies, yielding early insights into their enigmatic mechanisms of action.

Rapid assembly of the doubly-branched pentasaccharide domain of the immunoadjuvant jujuboside A via convergent B(C6F5)3-catalyzed glycosylation of sterically-hindered precursors

A convergent synthesis of the complex, doubly-branched pentasaccharide domain of the natural-product immunoadjuvant jujuboside A is described. The key step is a sterically-hindered glycosylation reaction between a branched trisaccharide trichloroacetimidate glycosyl donor and a disaccharide glycosyl acceptor. Conventional Lewis acids (TMSOTf, BF3·Et2O) were ineffective in this glycosylation, but B(C6F5)3 catalyzed the reaction successfully. Inherent complete diastereoselectivity for the undesired α-anomer was overcome by rational optimization with a nitrile solvent system (1 : 5 t-BuCN/CF3Ph) to provide flexible, effective access to the β-linked pentasaccharide.

Semisynthesis of Analogues of the Saponin Immunoadjuvant QS-21

Saponins are triterpene glycoside natural products that exhibit many different biological properties, including activation and modulation of the immune system, and have therefore attracted significant interest as immunological adjuvants for use in vaccines. QS-21 is the most widely used and promising saponin adjuvant but suffers from several liabilities, such as scarcity, dose-limiting toxicity, and hydrolytic instability. Chemical synthesis has emerged as a powerful approach to obtain homogeneous, pure samples of QS-21 and to improve its properties and therapeutic profile by providing access to optimized, synthetic saponin variants. Herein, we describe a general method for the semisynthesis of these molecules from QS-21, with detailed synthetic protocols for two saponin variants developed in our recent work.

Development of Improved Vaccine Adjuvants Based on the Saponin Natural Product QS-21 through Chemical Synthesis

Vaccines based on molecular subunit antigens are increasingly being investigated due to their improved safety and more precise targeting compared to classical whole-pathogen vaccines. However, subunit vaccines are inherently less immunogenic; thus, coadministration of an adjuvant to increase the immunogenicity of the antigen is often necessary to elicit a potent immune response. QS-21, an immunostimulatory saponin natural product, has been used as an adjuvant in conjunction with various vaccines in numerous clinical trials, but suffers from several inherent liabilities, including scarcity, chemical instability, and dose-limiting toxicity. Moreover, little is known about its mechanism of action. Over a decade-long effort, beginning at the University of Illinois at Urbana-Champaign and continuing at the Memorial Sloan Kettering Cancer Center (MSKCC), the group of Prof. David Y. Gin accomplished the total synthesis of QS-21 and developed a practical semisynthetic approach to novel variants that overcome the liabilities of the natural product. First, semisynthetic QS-21 variants were designed with stable amide linkages in the acyl chain domain that exhibited comparable in vivo adjuvant activity and lower toxicity than the natural product. Further modifications in the acyl chain domain and truncation of the linear tetrasaccharide domain led to identification of a trisaccharide variant with a simple carboxylic acid side chain that retained potent adjuvant activity, albeit with reemergence of toxicity. Conversely, an acyl chain analogue terminating in a free amine was inactive but enabled chemoselective functionalization with radiolabeled and fluorescent tags, yielding adjuvant-active saponin probes that, unlike inactive congeners, accumulated in the lymph nodes in vaccinated mice and internalized into dendritic cells. Subtle variations in length, stereochemistry, and conformational flexibility around the central glycosidic linkage provided QS-21 variants with adjuvant activities that correlated with specific conformations found in molecular dynamics simulations. Notably, deletion of the entire branched trisaccharide domain afforded potent, truncated saponin variants with negligible toxicity and improved synthetic access, facilitating subsequent investigation of the triterpene core. The triterpene C4-aldehyde substituent, previously proposed to be important for QS-21 adjuvant activity, proved to be dispensable in these truncated saponin variants, while the presence of the C16 hydroxyl group enhanced activity. Novel adjuvant conjugates incorporating the small-molecule immunopotentiator tucaresol at the acyl chain terminus afforded adjuvant-active variants but without significant synergistic enhancement of activity. Finally, a new divergent synthetic approach was developed to provide versatile and streamlined access to additional linear oligosaccharide domain variants with modified sugars and regiochemistries, opening the door to the rapid generation of diverse, synthetically accessible analogues. In this Account, we summarize these multidisciplinary studies at the interface of chemistry, immunology, and medicine, which have provided critical information on the structure–activity relationships (SAR) of this Quillaja saponin class; access to novel, potent, nontoxic adjuvants for use in subunit vaccines; and a powerful platform for investigations into the mechanisms of saponin immunopotentiation.

Quillaja Saponin Variants with Central Glycosidic Linkage Modifications Exhibit Distinct Conformations and Adjuvant Activities

A mouse vaccination model and molecular dynamics studies reveal characteristic conformations of active QS-21 variants.

Immunological adjuvants such as the saponin natural product QS-21 help stimulate the immune response to co-administered antigens and have become increasingly important in the development of prophylactic and therapeutic vaccines. However, clinical use of QS-21 is encumbered by chemical instability, dose-limiting toxicity, and low-yielding purification from the natural source. Previous studies of structure–activity relationships in the four structural domains of QS-21 have led to simplified, chemically stable variants that retain potent adjuvant activity and low toxicity in mouse vaccination models. However, modification of the central glycosyl ester linkage has not yet been explored. Herein, we describe the design, synthesis, immunologic evaluation, and molecular dynamics analysis of a series of novel QS-21 variants with different linker lengths, stereochemistry, and flexibility to investigate the role of this linkage in saponin adjuvant activity and conformation. Despite relatively conservative structural modifications, these variants exhibit striking differences in in vivo adjuvant activity that correlate with specific conformational preferences. These results highlight the junction of the triterpene and linear oligosaccharide domains as playing a critical role in the immunoadjuvant activity of the Quillaja saponins and also suggest a mechanism of action involving interaction with a discrete macromolecular target, in contrast to the non-specific mechanisms of emulsion-based adjuvants.

Versatile strategy for the divergent synthesis of linear oligosaccharide domain variants of Quillaja saponin vaccine adjuvants

We describe a new, versatile synthetic approach to Quillaja saponin variants based on the natural product immunoadjuvant QS-21. This modular, divergent strategy provides efficient access to linear oligosaccharide domain variants with modified sugars and regiochemistries. This new synthetic approach opens the door to the rapid generation of diverse analogues to identify novel saponin adjuvants with improved synthetic accessibility.

Synthesis and preclinical evaluation of QS-21 variants leading to simplified vaccine adjuvants and mechanistic probes

QS-21 is a potent immunostimulatory saponin that is currently under clinical investigation as an adjuvant in various vaccines to treat infectious diseases, cancers, and cognitive disorders. Herein, we report the design, synthesis, and preclinical evaluation of simplified QS-21 congeners to define key structural features that are critical for adjuvant activity. Truncation of the linear tetrasaccharide domain revealed that a trisaccharide variant is equipotent to QS-21, while the corresponding disaccharide and monosaccharide congeners are more toxic and less potent, respectively. Modification of the acyl chain domain in the trisaccharide series revealed that a terminal carboxylic acid is well-tolerated while a terminal amine results in reduced adjuvant activity. Acylation of the terminal amine can, in some cases, restore adjuvant activity and enables the synthesis of fluorescently labeled QS-21 variants. Cellular studies with these probes revealed that, contrary to conventional wisdom, the most highly adjuvant active of these fluorescently labeled saponins does not simply associate with the plasma membrane, but rather is internalized by dendritic cells.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Direct O-glycosidation of resin bound thioglycosides

The application of the safety-catch linker concept to solid-phase glycoconjugate synthesis is described. The process allows for direct conjugation of resin bound glycans to complex aglycones during cleavage. Large excesses of either coupling partner are not required, and even very hindered alcohols serve as acceptors in the reaction.

Enhancing Immunogenicity of Cancer Vaccines: QS-21 as an Immune Adjuvant

Saponins comprise a class of plant natural products that incorporate a lipophilic terpenoid core, to which is appended one or more carbohydrate residues. They are amphiphilic molecules and often exhibit toxic biological profiles, likely as a result of their roles as vital components in protective coatings to defend against phytopathogen infection and insect predation. The most notable of adjuvant-active saponins investigated for vaccine development come from the Chilean Soapbark Tree, Quillaja saponaria (i.e., QS). More than 30 years ago, semi-purified extracts (i.e., Quil A) from the cortex of Quillaja saponaria were found to be highly effective as adjuvants in veterinary vaccines. However, due to significant and variable toxicity effects, Quil A was not deemed appropriate for human vaccines. More refined purification methods have led to multiple fractions which are derived from the original plant extract. As such, QS-21 to date appears to be one of the more scientifically interesting and robust adjuvants in use in vaccinology. The role of QS-21 as an adjuvant for use in a variety of cancer vaccine trials and its comparison to other adjuvants is discussed in this review.

The saponins: polar isoprenoids with important and diverse biological activities

Saponins are polar molecules that consist of a triterpene or steroid aglycone with one or more sugar chains. They are one of the most numerous and diverse groups of plant natural products. These molecules have important ecological and agronomic functions, contributing to pest and pathogen resistance and to food quality in crop plants. They also have a wide range of commercial applications in the food, cosmetics and pharmaceutical sectors. Although primarily found in plants, saponins are produced by certain other organisms, including starfish and sea cucumbers. The under explored biodiversity of this class of natural products is likely to prove to be a vital resource for discovery of high-value compounds. This review will focus on the biological activity of some of the best-studied examples of saponins, on the relationship between structure and function, and on prospects for synthesis of ‘‘designer’’ saponins.

Design and synthesis of potent Quillaja saponin vaccine adjuvants

The success of antitumor and antiviral vaccines often requires the use of an adjuvant, a substance that significantly enhances the immune response to a coadministered antigen. Only a handful of adjuvants have both sufficient potency and acceptable toxicity for clinical investigation. One promising adjuvant is QS-21, a saponin natural product that is the immunopotentiator of choice in many cancer and infectious disease vaccine clinical trials. However, the therapeutic promise of QS-21 adjuvant is curtailed by several factors, including its scarcity, difficulty in purification to homogeneity, dose-limiting toxicity, and chemical instability. Here, we report the design, synthesis, and evaluation of chemically stable synthetic saponins. These novel, amide-modified, non-natural substances exhibit immunopotentiating effects in vivo that rival or exceed that of QS-21 in evaluations with the GD3-KLH melanoma conjugate vaccine. The highly convergent synthetic preparation of these novel saponins establishes new avenues for discovering improved molecular adjuvants for specifically tailored vaccine therapies.

Recent progress in chemical and chemoenzymatic synthesis of carbohydrates

The important roles that carbohydrates play in biological processes and their potential application in diagnosis, therapeutics, and vaccine development have made them attractive synthetic targets. Despite ongoing challenges, tremendous progresses have been made in recent years for the synthesis of carbohydrates. The chemical glycosylation methods have become more sophisticated and the synthesis of oligosaccharides has become more predictable. Simplified one-pot glycosylation strategy and automated synthesis are increasingly used to obtain biologically important glycans. On the other hand, chemoenzymatic synthesis continues to be a powerful alternative for obtaining complex carbohydrates. This review highlights recent progress in chemical and chemoenzymatic synthesis of carbohydrates with a particular focus on the methods developed for the synthesis of oligosaccharides, polysaccharides, glycolipids, and glycosylated natural products.