Synthesis of QS‐21‐Xylose: Establishment of the Immunopotentiating Activity of Synthetic QS‐21 Adjuvant with a Melanoma Vaccine

Abstract

The success of antitumor and antiviral vaccines often requires the use of an adjuvant, a substance that is itself not necessarily immunogenic but significantly enhances the immune response of a patient to a coadministered antigen.[1] The adjuvant of choice in many recent immunotherapeutic advances against cancer, [2] human immunodeficiency virus, [3,4] and malaria [5,6] is the natural product saponin QS-21. [7] Extensive studies in the development of conjugate cancer vaccines demonstrated the superiority of QS-21 over 18 competing adjuvants in preclinical models. [8] Isolated in minute quantities from the cortex of the Quillaja saponaria (QS) tree, QS-21 is not a homogeneous substance; rather, it is the 21st of 22 fractions in an early reversed-phase (RP) HPLC trace of semipurified QS-bark extracts. [7,9] This adjuvant-active fraction comprises at least two principal constituents (Scheme 1), QS-21-Xyl (1) and QS-21-Api (2), which differ in the terminal sugar residue within the linear tetrasaccharide segment. [10,11] Although the multi-component QS-21 fraction has fueled numerous previous and ongoing vaccine clinical trials, the challenge to procure samples of consistent composition from natural sources is formidable. Indeed, high variability in saponin composition is seen even among QS trees of similar age and local environment (e.g., soil, season).[12] Furthermore, recent metabolomic analyses of semipurified QS-bark extracts revealed a complex mixture of over 100 distinct saponins, [13] considerably more than what the original 22-fraction RP HPLC profile might imply. [7,9] Because of the variability and heterogeneity of QS extracts, it is difficult to establish the adjuvant activity and toxicity of the component saponins. The possibility that trace quantities of additional saponins may be present in the QS-21 fraction impacts not only on efficacy and formulation aspects, but also on regulatory hurdles in clinical vaccine development in humans.Efforts to further advance QS-21 in the clinic, as well as to illuminate its unknown mechanism of action, require access to adjuvant-active samples of known composition. The recent chemical synthesis of the isomer QS-21-Api (2) enabled independent access to homogenous samples of this particular saponin. [14] We now report the chemical synthesis of the second

Selection of Adjuvants for Enhanced Vaccine Potency

“…Therefore, their adjuvant activity can be further modified by structural modifications such as GPI-0100 [237]. The widely used saponin-based adjuvants are Quil A, isolated from the bark of Quillaja saponaria (QS) Molina, and QS21, a more purified form, is the 21st of 22 fractions in RP-HPLC trace of semipurified QS bark extracts [238].…”

“…They have been proven to be very effective in enhancing the immunogenicity of various vaccines, including Aβ(1-15) in mice [240]; foot-and -mouth disease virus (FMDV) antigen in mice [241]; L1 and A33 proteins in mice [242]; vaccinia virus proteins in monkeys [242]; parasitic antigens in sheeps [243]; keyhole limpet hemocyanin (KLH) conjugate antigens [238,244,245]. The effect is often toward a Th1-type response in comparison with aluminum-based vaccine such as Aβ (1-15), PADRE-Aβ(1-15)-MAP) [240], L1 and A33 proteins in mice [242], parasitic antigen (Fasciola hepatica) in sheeps [243].…”

“…Yet, use of KLH conjugate alone may or may not initiate adequate immune response and another adjuvant often has to be used. Such combinations include KLH conjugates with QS21 for cancer antigen (GD3; ganglioside) [238], with Freund's adjuvant for influenza virus A M2 peptide [269], and with other adjuvants for cancer antigens (MUC1 and GD3) [237,244]. The structure of linkers between hapten/antigen and KLH carrier protein may have a significant effect on the immunogenicity and selectivity of the conjugated vaccine [270].…”

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

“…To address these challenges, our group previously achieved the first chemical synthesis of QS-21 14-15 . Subsequent studies revealed that simplified acyl chain domain variants having stable amide linkages retain adjuvant activity and exhibit decreased toxicity 16 .…”

Carbohydrate antigen delivery by water soluble copolymers as potential anti-cancer vaccines

“…47,50 The strong anti-carrier responses can potentially interfere with the generation of glycan specific antibodies due to antigen competition. 51–53 The antibodies generated against the polymer backbone were analyzed.…”