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Development of a minimal saponin vaccine adjuvant based on QS-21

Abstract

Adjuvants are materials added to vaccines to enhance the immunological response to an antigen. QS-21 is a natural product adjuvant under investigation in numerous vaccine clinical trials, but its use is constrained by scarcity, toxicity, instability and an enigmatic molecular mechanism of action. Herein we describe the development of a minimal QS-21 analogue that decouples adjuvant activity from toxicity and provides a powerful platform for mechanistic investigations. We found that the entire branched trisaccharide domain of QS-21 is dispensable for adjuvant activity and that the C4-aldehyde substituent, previously proposed to bind covalently to an unknown cellular target, is also not required. Biodistribution studies revealed that active adjuvants were retained preferentially at the injection site and the nearest draining lymph nodes compared with the attenuated variants. Overall, these studies have yielded critical insights into saponin structure–function relationships, provided practical synthetic access to non-toxic adjuvants, and established a platform for detailed mechanistic studies.

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Figure 1: Aryl iodide saponin 6 exhibits potent adjuvant activity and low toxicity in a preclinical mouse-vaccination model.
Figure 2: Radioiodinated saponin [131I]-6 and fluorescent saponin 3 localize to lymph nodes and injection site in mice.
Figure 3: Truncated saponin 16 lacks the entire branched trisaccharide domain of QS-21 but retains potent adjuvant activity and low toxicity in a preclinical mouse-vaccination model.
Figure 4: Oleanolic acid derivative 18, which lacks both the C4-aldehyde substituent and the C16-alcohol in the triterpene domain of QS-21, exhibits poor adjuvant activity in a preclinical mouse-vaccination model.
Figure 5: Caulophyllogenin derivative 19 and echinocystic acid derivative 20, which lack the C4-aldehyde substituent but retain the C16-alcohol in the triterpene domain of QS-21, exhibit potent adjuvant activity and no toxicity in a preclinical mouse-vaccination model.
Figure 6: Adjuvant-active quillaic acid derivative 16 localizes to the injection site and lymph nodes in mice, but adjuvant-attenuated oleanolic acid derivative 18 does not.

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Acknowledgements

Dedicated to the memory of our mentor and colleague, David Y. Gin (1967–2011). We thank S. J. Danishefsky, M. M. Adams and W. E. Walkowicz for helpful discussions, G. Sukenick, H. Liu, H. Fang and S. Rusli for expert NMR and mass spectral support and K. K. Sevak, N. Ramos and C. B. Davis for technical support with biodistribution and radiometric studies. Generous financial support was provided by the European Commission (Marie Curie International Outgoing Fellowship to A.F-T.), the National Institutes of Health (R01 AI085622 to J.S.L. and D.Y.G., R01 GM058833 to D.S.T. and D.Y.G., CCSG P30 CA008748 in support of the Center of Comparative Medicine and Pathology and the Radiochemistry and Molecular Imaging Probe Core), William and Alice Goodwin and the Commonwealth Foundation for Cancer Research and the Experimental Therapeutics Center of MSKCC.

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Contributions

A.F-T., E.K.C., N.P., J.R.G., G.R., J.S.L., D.S.T. and D.Y.G. conceived and designed the experiments. A.F-T. and E.K.C. performed the syntheses. C.G. performed the preclinical mouse-vaccination experiments. C.G. and N.P. performed the biodistribution experiments. J.R.G. performed the fluorescence imaging experiments. A.F-T., E.K.C., N.P., J.R.G., P.O.L., G.R., J.S.L., D.S.T. and D.Y.G. analysed the data. A.F-T. and D.S.T. wrote the manuscript.

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Correspondence to Govind Ragupathi, Jason S. Lewis or Derek S. Tan.

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Competing interests

J.R.G., P.O.L., G.R. and D.Y.G are founders of Adjuvance Technologies Inc. and have financial interests in the company.

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Fernández-Tejada, A., Chea, E., George, C. et al. Development of a minimal saponin vaccine adjuvant based on QS-21. Nature Chem 6, 635–643 (2014). https://doi.org/10.1038/nchem.1963

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