Development of semisynthetic saponin immunostimulants

Abstract

Many natural saponins demonstrate immunostimulatory adjuvant activities, but they also have some inherent drawbacks that limit their clinical use. To overcome these limitations, extensive structure-activity-relationship (SAR) studies have been conducted. The SAR studies of QS-21 and related saponins reveal that their respective fatty side chains are crucial for potentiating a strong cellular immune response. Replacing the hydrolytically unstable ester side chain in the C28 oligosaccharide domain with an amide side chain in the same domain or in the C3 branched trisaccharide domain is a viable approach for generating robust semisynthetic saponin immunostimulants. Given the striking resemblance of natural momordica saponins (MS) I and II to the deacylated Quillaja Saponaria (QS) saponins (e.g., QS-17, QS-18, and QS-21), incorporating an amide side chain into the more sustainable MS, instead of deacylated QS saponins, led to the discovery of MS-derived semisynthetic immunostimulatory adjuvants VSA-1 and VSA-2. This review focuses on the authors' previous work on SAR studies of QS and MS saponins.

Keywords: Immunostimulant; Momordica cochinchinensis; Saponin; VSA-1; VSA-2; Vaccine adjuvant.

Fig. 1

Structure of three pairs of saponin isomers with significantly different adjuvant activities

Fig. 2

Gin’s SAR studies toward hydrolytically stable QS-21 analogs [73]

Fig. 3

Structure of synthetic QS analogs. api β-D-apiofuranosyl, xyl β-D-xylopyranosyl, glc β-D-glucopyranosyl, Ac acetyl

Fig. 4

Structure of natural QS saponins and their deacylated counterparts

Fig. 5

Serum anti-OVA antibody responses in mice immunized with OVA alone or with GPI-0100 (100 μg) or VSA-1 (100 μg) via the s.c. route [74]

Fig. 6

Serum antibody activity to seven serotypes on Day 42. BALB/c mice (8–10 weeks of age, six per group) were immunized via the subcutaneous route (s.c.) on days 0, 14 and 28. Serum samples were collected prior to the first immunization and 2 weeks following the last immunization. The pooled serum samples of each group were analyzed by ELISA

Fig. 7

Structure of MS II derivatives 16 and VSA-2

Fig. 8

Serum IgG, IgG1, and IgG2a anti-OVA responses in mice immunized by the s.c. route with OVA (20.0 µg) alone or with OVA (20.0 µg) in combination with QS-21 (20.0 µg) or VSA-1 (50.0 µg) on days 0, 14 and 28 [96]

Fig. 9

Analysis of mouse spleen cells: CD8⁺ T-cell surface expression of PD-1 (A), intracellular levels of IFNγ in CD8⁺ T-cells (B), expression of granzyme B in CD8⁺ T-cells (C), CD4⁺ T-cell surface expression of PD-1 (D), intracellular levels of IFN-γ in CD4⁺ T-cells (E), expression of IL-4 in CD4⁺ T-cells (F), and expression of IL-21 CD4⁺ T-cells (G) [96]

Fig. 10

Analysis of mouse spleen cells: T_FH cells (A), T_FR cells (B), ratio of T_FH to T_RF (C), CD19⁺ B-cells (D), and dark zone B-cells (E) [96]