One of the most widely used and potent immunological adjuvants is a mixture of soluble triterpene glycosides purified from the soap bark tree (Molina became the major focus for saponin research focused on adjuvant activity . was found to be highly toxic in mice but saponins QS-7 and QS-21 were far less toxic. QS-21 being more abundant than QS-7 was selected and has been the most widely studied saponin adjuvant for more than 15 years. Because QS-21 was originally designated as a particular fraction on a complex RP-HPLC trace it is not surprising that it comprises several distinct saponin molecules. There are two principal isomeric molecular constituents of the QS-21 fraction (Figure 1). Both of these saponins incorporate a Triisopropylsilane central triterpene core to which a branched trisaccharide is attached Triisopropylsilane at the terpene C3 oxygen functionality and a linear tetrasaccharide is linked to the triterpene C28 carboxylate group. A fourth component within the saponin structure is a glycosylated pseudo-dimeric acyl chain attached to the fucose moiety via a hydrolytically labile ester linkage. The isomeric components differ in the constitution of the terminal sugar residue of the tetrasaccharide in which the major and minor compounds incorporate either an apiose (65%) or a xylose (35%) carbohydrate respectively. Figure 1 The principal molecular constituents of QS-21. Adjuvant active saponins from a variety of sources other than Molina have been identified and their structure determined. Based on this empiric but nevertheless relevant survey it is possible to draw some tentative conclusions concerning the contributions of different portions of the QS-21 molecule to toxicity on the one hand and adjuvant activity on the other as recently reviewed by Sun fraction QS-21 was most potent with antibody titers induced against the three antigens generally proportional to cytokine response induced against KLH and also proportional to dose. However when the dose resulted in greater than 10% weight loss increasing doses were seen to result in decreasing titers. These studies emphasized the two critical and generally proportional variables in adjuvant discovery: immunologic potency and toxicity. Clinical trials with vaccines targeting the ganglioside GM2 have come to the same conclusion: conjugation to KLH and the use of immunological adjuvant QS-21 is the optimal approach. Initially GM2 ganglioside was incorporated onto the surface of liposomes containing mutant R595 BCG proteasomes and monophosphoryl lipid A . Of these the use of BCG was found to be optimal but subsequent comparison of this GM2 adhered BCG vaccine with GM2-KLH conjugate vaccines containing no adjuvant or mixed with BCG detox (BCG cell wall skeletons plus monophosphoryl lipid A ) or QS-21 found that GM2-KLH conjugate plus QS-21 was strikingly superior inducing both IgM and IgG antibody responses . Doses of QS-21 in the 100-200 μg range were optimal and well tolerated in the cancer patient population . The toxicity in this dose range was Mouse monoclonal to MYL3 2-10 cm of erythema and induration at injection sites in most patients as well as occasional mild low-grade flu-like symptoms. At this time over 1000 patients have been vaccinated with QS-21 containing vaccines with a dose of 100 μg and no dose-limiting toxicity has been described. Although reformulations of QS-21 with certain excipients helps to reduce pain and improve acceptability of 50 μg doses  the consistent grade I or Triisopropylsilane II local toxicity and occasional grade I flu-like symptoms after vaccination will probably exclude the use of QS-21 for routine immunizations in general pediatric or adult populations at doses above 50 μg. The semisynthetic saponin adjuvant GPI-0100 which had been identified in our initial preclinical studies to exceed QS-21 in potency  was also tested in clinical trials. While a GPI-0100 dose of 2 mg was well tolerated in prostate cancer patients and antibody titers appeared to be superior to those induced by 100 μg of QS-21 four out of six women immunized with vaccines containing GPI-0100 at this dose demonstrated grade II or III hepatic toxicity (transaminitis) lasting Triisopropylsilane 3-6 months prior to normalization. Low (500 μg) doses of GPI-0100 were well tolerated but antibody responses were no longer as high as those obtained with 100 μg of QS-21 [10-14]. Current status of clinical trials utilizing QS-21 as an immunological adjuvant The majority of clinical experience with QS-21 in therapeutic vaccinology has occurred in cancer patient populations where it has been widely utilized especially in the adjuvant setting (after surgical resection of all known local or systemic metastases)..