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

Abstract

AbstractThe 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.