QS21

QS-21 is a purified plant extract that enhances the ability of the immune system to respond to vaccine antigens. It is derived from the soap bark tree (Quillaja saponaria), which is native to the country of Chile. QS-21 is a purified plant extract that enhances the ability of the immune system to respond to vaccine antigens. It is derived from the soap bark tree (Quillaja saponaria), which is native to the country of Chile. The extract contains water soluble triterpene glycoside compounds, which are members of a family of plant-based compounds called saponins. It is currently being tested as an immunologic adjuvant in various vaccines in attempts to improve their efficacy. It is believed to enhance both humoral and cell-mediated immunity. Isolation of QS-21 destroys the soap bark tree, which has resulted in regulation of the tree by the governments where it is grown. A semi-synthesis strategy relies on purifying the prosapogenin (triterpene and branched trisaccharide) part of the molecule and adding the reset of QS-21 synthetically; it is reported to increase the yield by 2 orders of magnitude. This approach has led to experiments probing at alternative acyl chain compositions.   QS-21 is currently under clinical evaluation as an additive for various trial vaccines, including those for HIV, malaria and cancer. As of 2002, it had been tested in more than 3000 patients in 60 clinical trials. It is a component of the FDA-approved Shingrix shingles vaccine.