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

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2021-2022

The use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates is a well-established technique and this review is the 12th update of the original article published in 1999 and brings coverage of the literature to the end of 2022. As with previous review, this review also includes a few papers that describe methods appropriate to analysis by MALDI, such as sample preparation, even though the ionization method is not MALDI. The review follows the same format as previous reviews. It is divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of computer software for structural identification. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other general areas such as medicine, industrial processes, natural products and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. MALDI is still an ideal technique for carbohydrate analysis, particularly in its ability to produce single ions from each analyte and advancements in the technique and range of applications show little sign of diminishing.

Monophosphoryl Lipid A-Rhamnose Conjugates as a New Class of Vaccine Adjuvants

Adjuvant is an integral part of all vaccine formulations but only a few adjuvants with limited efficacies or application scopes are available. Thus, developing more robust and diverse adjuvants is necessary. To this end, a new class of adjuvants having α- and β-rhamnose (Rha) attached to the 1- and 6'-positions of monophosphoryl lipid A (MPLA) was designed, synthesized, and immunologically evaluated in mice. The results indicated a synergistic effect of MPLA and Rha, two immunostimulators that function via interacting with toll-like receptor 4 and recruiting endogenous anti-Rha antibodies, respectively. All the tested MPLA-Rha conjugates exhibited potent adjuvant activities to promote antibody production against both protein and carbohydrate antigens. Overall, MPLA-α-Rha exhibited better activities than MPLA-β-Rha, and 6'-linked conjugates were slightly better than 1-linked ones. Particularly, MPLA-1-α-Rha and MPLA-6'-α-Rha were the most effective adjuvants in promoting IgG antibody responses against protein antigen keyhole limpet hemocyanin and carbohydrate antigen sTn, respectively.

Design and Synthesis of Immunoadjuvant QS-21 Analogs and Their Biological Evaluation

A series of novel immunoadjuvant QS-21 analogs were synthesized, and their effects on the in vitro hemolysis of red blood cells were evaluated using QS-21 as a control and hemolytic properties as an index. Our results show that all the QS-21 analogs had lower hemolytic effects than QS-21, and their concentrations exhibited a certain quantitative effect relationship with the hemolysis rate. Notably, saponin compounds L1-L8 produced minimal hemolysis and showed lower hemolytic effects, warranting further investigation.

A newly identified β-amyrin synthase gene hypothetically involved in oleanane-saponin biosynthesis from Talinum paniculatum (Jacq.) Gaertn

Talinum paniculatum or Javanese ginseng in Indonesia is a plant widely used as a traditional medicine. The genus Talinum produces oleanane-type saponins, such as talinumoside I. The first aim of this study was to isolate the probable gene encoding β-amyrin synthase (bAS), a key enzyme involved in the cyclization of 2,3-oxidosqualene producing the backbone of the oleanane-type saponin β-amyrin and characterize the gene sequence and the predicted protein sequence using in silico approach. The second aim was to analyze the correlation between the TpbAS gene expression level and saponin production in various plant organs. Thus, TpbAS was isolated using degenerate primers and PCR 5'/3'-Rapid Amplification of cDNA Ends (RACE), then the gene sequence and the predicted protein were in silico analyzed using various programs. TpbAS expression level was analyzed using reverse transcriptase PCR (RT-PCR), and saponin content was measured using a spectrophotometer. The results showed that the full-length TpbAS gene consists of 2298 base pairs encoding for a 765-amino acid protein. From in silico study, the (GA)n sequence was identified in the 5'-untranslated regions and predicted to be a candidate of the gene expression modulator. In addition, functional RNA motifs and sites analysis predicted the presence of exon splicing enhancers and silencers within the coding sequence and miRNA target sites candidate. Amino acid sequence analysis showed DCTAE, QW, and WCYCR motifs that were conserved in all classes of oxidosqualene cyclase enzymes. Phylogenetic tree analysis showed that TpbAS is closely related to other plant oxidosqualene cyclase groups. Analysis of TpbAS expression and saponin content indicated that saponin is mainly synthesized and accumulated in the leaves. Taken together, these findings will assist in increasing the saponin content through a metabolic engineering approach.

A Liquid Chromatography High-Resolution Tandem Mass Spectrometry Method to Quantify QS-21 Adjuvant and Its Degradation Products in Liposomal Drug Formulations

Identification and quantification of an active adjuvant and its degradation product/s in drug formulations are important to ensure drug product safety and efficacy. QS-21 is a potent adjuvant that is currently involved in several clinical vaccine trials and a constituent of licensed vaccines against malaria and shingles. In an aqueous milieu, QS-21 undergoes pH- and temperature-dependent hydrolytic degradation to form a QS-21 HP derivative that may occur during manufacturing and/or long-term storage. Intact QS-21 and deacylated QS-21 HP elicit different immune response profiles; thus, it is imperative to monitor QS-21 degradation in vaccine adjuvant formulation. To date, a suitable quantitative analytical method for QS-21 and its degradation product in drug formulations is not available in the literature. In view of this, a new liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and qualified to accurately quantify the active adjuvant QS-21 and its degradation product (QS-21 HP) in liposomal drug formulations. The method was qualified according to the FDA Guidance for Industry: Q2(R1). Study results showed that the described method presents good specificity for QS-21 and QS-21 HP detection in a liposomal matrix, good sensitivity characterized by the limit of detection (LOD)/limit of quantitation (LOQ) in the nanomolar range, linear regressions with correlation coefficients, R² > 0.999, recoveries in the range of 80-120%, and precise detection and quantification with % relative standard deviation (RSD) < 6% for QS-21 and < 9% for the QS-21 HP impurity assay. The described method was successfully used to accurately evaluate in-process and product release samples of the Army Liposome Formulation containing QS-21 (ALFQ).

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.

Development of synthetic, self-adjuvanting, and self-assembling anticancer vaccines based on a minimal saponin adjuvant and the tumor-associated MUC1 antigen

The overexpression of aberrantly glycosylated tumor-associated mucin-1 (TA-MUC1) in human cancers makes it a major target for the development of anticancer vaccines derived from synthetic MUC1-(glyco)peptide antigens. However, glycopeptide-based subunit vaccines are weakly immunogenic, requiring adjuvants and/or additional immunopotentiating approaches to generate optimal immune responses. Among these strategies, unimolecular self-adjuvanting vaccine constructs that do not need coadministration of adjuvants or conjugation to carrier proteins emerge as a promising but still underexploited approach. Herein, we report the design, synthesis, immune-evaluation in mice, and NMR studies of new, self-adjuvanting and self-assembling vaccines based on our QS-21-derived minimal adjuvant platform covalently linked to TA-MUC1-(glyco)peptide antigens and a peptide helper T-cell epitope. We have developed a modular, chemoselective strategy that harnesses two distal attachment points on the saponin adjuvant to conjugate the respective components in unprotected form and high yields via orthogonal ligations. In mice, only tri-component candidates but not unconjugated or di-component combinations induced significant TA-MUC1-specific IgG antibodies able to recognize the TA-MUC1 on cancer cells. NMR studies revealed the formation of self-assembled aggregates, in which the more hydrophilic TA-MUC1 moiety gets exposed to the solvent, favoring B-cell recognition. While dilution of the di-component saponin-(Tn)MUC1 constructs resulted in partial aggregate disruption, this was not observed for the more stably-organized tri-component candidates. This higher structural stability in solution correlates with their increased immunogenicity and suggests a longer half-life of the construct in physiological media, which together with the enhanced antigen multivalent presentation enabled by the particulate self-assembly, points to this self-adjuvanting tri-component vaccine as a promising synthetic candidate for further development.

Novel Oxime-Derivatized Synthetic Triterpene Glycosides as Potent Saponin Vaccine Adjuvants

Vaccine adjuvants are key for optimal vaccine efficacy, increasing the immunogenicity of the antigen and potentiating the immune response. Saponin adjuvants such as the carbohydrate-based QS-21 natural product are among the most promising candidates in vaccine formulations, but suffer from inherent drawbacks that have hampered their use and approval as stand-alone adjuvants. Despite the recent development of synthetic derivatives with improved properties, their full potential has not yet been reached, allowing the prospect of discovering further optimized saponin variants with higher potency. Herein, we have designed, chemically synthesized, and immunologically evaluated novel oxime-derivatized saponin adjuvants with targeted structural modifications at key triterpene functionalities. The resulting analogues have revealed important findings into saponin structure-activity relationships, including adjuvant mechanistic insights, and have shown superior adjuvant activity in terms of significantly increased antibody response augmentation compared to our previous saponin leads. These newly identified saponin oximes emerge as highly promising synthetic adjuvants for further preclinical development towards potential next generation immunotherapeutics for future vaccine applications.

Design, synthesis, and initial immunological evaluation of glycoconjugates based on saponin adjuvants and the Tn antigen

We report the first synthesis and immunological evaluation of a new glycoconjugate design based on streamlined saponin adjuvants and the Tn carbohydrate antigen. While the novel synthetic constructs induced moderate antibody responses in mice, the versatile chemical platform is amenable to further structure-activity optimizations for the development of self-adjuvanting glycoconjugate cancer vaccines.

Rational Vaccine Design in Times of Emerging Diseases: The Critical Choices of Immunological Correlates of Protection, Vaccine Antigen and Immunomodulation

Vaccines are the most effective medical intervention due to their continual success in preventing infections and improving mortality worldwide. Early vaccines were developed empirically however, rational design of vaccines can allow us to optimise their efficacy, by tailoring the immune response. Establishing the immune correlates of protection greatly informs the rational design of vaccines. This facilitates the selection of the best vaccine antigens and the most appropriate vaccine adjuvant to generate optimal memory immune T cell and B cell responses. This review outlines the range of vaccine types that are currently authorised and those under development. We outline the optimal immunological correlates of protection that can be targeted. Finally we review approaches to rational antigen selection and rational vaccine adjuvant design. Harnessing current knowledge on protective immune responses in combination with critical vaccine components is imperative to the prevention of future life-threatening diseases.

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.