Expanding the Scope of Ugi Multicomponent Bioconjugation to Produce Pneumococcal Multivalent Glycoconjugates as Vaccine Candidates

Residue-specific protein-glycan conjugation strategies for the development of pharmaceutically promising glycoconjugate vaccines: A recent update

Covalent coupling between a carbohydrate antigen and a protein carrier leads to the formation of pharmaceutically promising glycoconjugate vaccines. Most licensed glycoconjugate vaccines are acquired by random bioconjugation of native or sized glycans with the surface-exposed amino acid residues of proteins, such as lysine, cysteine, aspartic acid, glutamic amino acid, etc. In the last two decades, considerable momentum has been gained in the glycoconjugate vaccine development by discovering several residue-specific bioconjugation strategies. As a result, glycoconjugate chemistry reaches the verge of discovering well-defined and "real" homogeneous vaccines, which may be more potent to generate antimicrobial resistance against "bad-bugs". Through this literature survey, we intend to highlight the state of the art of residue-specific bioconjugation of proteins with glycans to obtain glycoconjugate vaccines. The review will also identify a potential roadmap to address the gap and the prospects in the medicinal domain.

Effect of O-Acetylation on the Antigenicity and Glycoconjugate Immunogenicity of the Streptococcus Pneumoniae Serotype 7F Capsular Polysaccharide

Streptococcus pneumoniae is a bacterial pathogen causing diseases as severe as pneumonia, sepsis and meningitis. Most commercial pneumococcal conjugate vaccines contain the 7F serotype, which is epidemiologically relevant and highly invasive. This serotype contains an O-acetyl group at the internal L-rhamnose of its polysaccharide repeating unit. Herein we report on the role of the O-acetyl moiety of 7F polysaccharide in both antigen recognition and the induction of a protective antibody response against 7F. Fully and partially de-O-acetylated 7F polysaccharides were chemically prepared and compared with the O-acetylated counterpart in their antigenicity and immunogenicity of their tetanus toxoid glycoconjugates. These comparative studies showed a slight but consistent decrease in the antigenicity for the fully de-O-acetylated polysaccharide, but not for the partly de-O-acetylated variant. The glycoconjugates derived from the O-acetylated and the fully de-O-acetylated polysaccharides had similar sizes and polysaccharide-to-protein ratio, and all proved both to be immunogenic and induce opsonophagocytic responses in mice. Nevertheless, the immune response elicited by the O-acetylated glycoconjugate was better in both quantity and quality, proving that the O-acetyl group is not strictly necessary but also not irrelevant for the antigenicity and immunogenicity of the 7F serotype polysaccharide and its glycoconjugates.

Diversification of a Novel α-Galactosyl Ceramide Hotspot Boosts the Adjuvant Properties in Parenteral and Mucosal Vaccines

The development of potent adjuvants is an important step for improving the performance of subunit vaccines. CD1d agonists, such as the prototypical α-galactosyl ceramide (α-GalCer), are of special interest due to their ability to activate iNKT cells and trigger rapid dendritic cell maturation and B-cell activation. Herein, we introduce a novel derivatization hotspot at the α-GalCer skeleton, namely the N-substituent at the amide bond. The multicomponent diversification of this previously unexplored glycolipid chemotype space permitted the introduction of a variety of extra functionalities that can either potentiate the adjuvant properties or serve as handles for further conjugation to antigens toward the development of self-adjuvanting vaccines. This strategy led to the discovery of compounds eliciting enhanced antigen-specific T cell stimulation and a higher antibody response when delivered by either the parenteral or the mucosal route, as compared to a known potent CD1d agonist. Notably, various functionalized α-GalCer analogues showed a more potent adjuvant effect after intranasal immunization than a PEGylated α-GalCer analogue previously optimized for this purpose. Ultimately, this work could open multiple avenues of opportunity for the use of mucosal vaccines against microbial infections.

Multicomponent Reactions Using C,N-Binucleophilic Nature of Aminopyrazoles: Construction of Pyrazole-Fused Heterocycles

Synthesis of pyrazole-fused heterocycles has gained considerable attention in recent years due to their wide applications in medicinal chemistry. Aminopyrazoles are versatile building blocks for the synthesis of pyrazole-fused heterocycles by multicomponent reactions. Due to the presence of multiple reaction sites, they have fascinating chemical reactivity. Thus, they have been extensively used in multicomponent reactions for the construction of pyrazole-fused heterocycles. Although few review articles on the preparation and applications of aminopyrazoles are known in the literature, to date there is no dedicated review article on the construction of pyrazole-fused heterocycles exploring the reactivity of amino pyrazoles as C,N-binucleophiles in multicomponent reactions. Considering this, herein the multicomponent reactions for the construction of pyrazole-fused heterocycles exploring C,N-binucleophilic nature of amino pyrazoles have been reported.

Ugi Four-Component Reactions Using Alternative Reactants

The Ugi four-component reaction (Ugi-4CR) undoubtedly is the most prominent multicomponent reaction (MCRs) that has sparked organic chemists' interest in the field. It has been widely used in the synthesis of diverse heterocycle molecules such as potential drugs, natural product analogs, pseudo peptides, macrocycles, and functional materials. The Ugi-4CRs involve the use of an amine, an aldehyde or ketone, an isocyanide, and a carboxylic acid to produce an α-acetamido carboxamide derivative, which has significantly advanced the field of isocyanide-based MCRs. The so-called intermediate nitrilium ion could be trapped by a nucleophile such as azide, N-hydroxyphthalimide, thiol, saccharin, phenol, water, and hydrogen sulfide instead of the original carboxylic acid to allow for a wide variety of Ugi-type reactions to occur.β In addition to isocyanide, there are alternative reagents for the other three components: amine, isocyanide, and aldehyde or ketone. All these alternative components render the Ugi reaction an aptly diversity-oriented synthesis of a myriad of biologically active molecules and complex scaffolds. Consequently, this review will delve deeper into alternative components used in the Ugi MCRs, particularly over the past ten years.

Optimization of the Process for Preparing Bivalent Polysaccharide Conjugates to Develop Multivalent Conjugate Vaccines against Streptococcus pneumoniae or Neisseria meningitidis and Comparison with the Corresponding Licensed Vaccines in Animal Models

OBJECTIVE

This study aimed to describe, optimize and evaluate a method for preparing multivalent conjugate vaccines by simultaneous conjugation of two different bacterial capsular polysaccharides (CPs) with tetanus toxoid (TT) as bivalent conjugates.

METHODS

Different molecular weights (MWs) of polysaccharides, activating agents and capsular polysaccharide/protein (CP/Pro) ratio that may influence conjugation and immunogenicity were investigated and optimized to prepare the bivalent conjugate bulk. Using the described method and optimized parameters, a 20-valent pneumococcal conjugate vaccine and a bivalent meningococcal vaccine were developed and their effectiveness was compared to that of corresponding licensed vaccines in rabbit or mouse models.

RESULTS

The immunogenicity test revealed that polysaccharides with lower MWs were better for Pn1-TT-Pn3 and MenA-TT-MenC, while higher MWs were superior for Pn4-TT-Pn14, Pn6A-TT-Pn6B, Pn7F-TT-Pn23F and Pn8-TT-Pn11A. For activating polysaccharides, 1-cyano-4-dimethylaminopyridinium tetrafluoroborate (CDAP) was superior to cyanogen bromide (CNBr), but for Pn1, Pn3 and MenC, N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDAC) was the most suitable option. For Pn6A-TT-Pn6B and Pn8-TT-Pn11A, rabbits immunized with bivalent conjugates with lower CP/Pro ratios showed significantly stronger CP-specific antibody responses, while for Pn4-TT-Pn14, higher CP/Pro ratio was better. Instead of interfering with the respective immunological activity, our bivalent conjugates usually induced higher IgG titers than their monovalent counterparts.

CONCLUSION

The result indicated that the described conjugation technique was feasible and efficacious to prepare glycoconjugate vaccines, laying a solid foundation for developing extended-valent multivalent or combined conjugate vaccines without potentially decreased immune function.

Innovations and Inventions: Why Was the Ugi Reaction Discovered Only 37 Years after the Passerini Reaction?

This year represents the 100th anniversary of the discovery of the Passerini three-component reaction. The related Ugi four-compound reaction was discovered 37 years after the Passerini reaction. Undoubtedly, both reactions are very important multicomponent reactions but the Ugi reactions outperform the Passerini reactions in terms of combinatorial space according to the equation x^y [x is the number of building blocks per component, and y is the order of the multicomponent reaction (for Passerini, y = 3; for Ugi, y = 4)]. In this work, a historical but contemporary perspective of the discoveries and innovations of the two reactions is given. From a bird's eye view and in a more general sense, the discovery of novel reactions is discussed and how it relates to inventions and innovations.

Structure-Acid Lability Relationship of N-Alkylated α,α-Dialkylglycine Obtained via a Ugi Multicomponent Reaction

Using the classical Ugi four-component reaction to fuse an amine, ketone, carboxylic acid, and isocyanide, here we prepared a short library of N-alkylated α,α-dialkylglycine derivatives. Due to the polyfunctionality of the dipeptidic scaffold, this highly steric hindered system shows an interesting acidolytic cleavage of the C-terminal amide. In this regard, we studied the structure-acid lability relationship of the C-terminal amide bond (cyclohexylamide) of N-alkylated α,α-dialkylglycine amides 1a–n in acidic media and, afterward, it was established that the most important structural features related to its cleavage. Then, it was demonstrated that electron-donating effects in the aromatic amines, flexible acyl chains (Gly) at the N-terminal and the introduction of cyclic compounds into dipeptide scaffolds, increased the rate of acidolysis. All these effects are related to the ease with which the oxazolonium ion intermediate forms and they promote the proximity of the central carbonyl group to the C-terminal amide, resulting in C-terminal amide cleavage. Consequently, these findings could be applied for the design of new protecting groups, handles for solid-phase synthesis, and linkers for conjugation, due to its easily modulable and the fact that it allows to fine tune its acid-lability.