Adjuvant Activity of QS-21 Isomers

QS-21, a reverse-phase purified triterpene glycoside from the South American tree Quillaja saponaria, can be further resolved into two peaks when chromatographed by high performance hydrophilic interaction chromatography. These two peaks demonstrated identical pseudomolecular ion weights and fragmentation patterns when analyzed by fast atom bombardment-mass spectroscopy. Carbohydrate analysis by monosaccharide composition and linkage analysis showed that a terminal apiose in the major peak was replaced by another pentose residue in the minor peak. Hence, the two peaks were structural isomers. Dose response curves for each isomer and reverse-phase purified QS-21 were similar, with a minimum effective dose of 5 micrograms in mice. This result indicates that the portion of QS-21 containing this variation is not critical for adjuvant activity.