QS21

QS-21 is a purified saponin adjuvant derived from the bark of Quillaja saponaria Molina (soap bark tree). It is used to enhance the immune response to vaccines, particularly by promoting both humoral and cellular immunity. As a component of several vaccine adjuvant systems, QS-21 is typically combined with other immunostimulants and delivery vehicles to improve antigen presentation and T-cell activation.

In mechanism, QS-21 is thought to interact with cell membranes to form immunostimulating complexes that facilitate antigen delivery and dendritic cell activation. It can help drive a balanced Th1-type response and augment antibody production. QS-21 is often incorporated into liposome- or emulsion-based adjuvant formulations, which helps modulate its potency and tolerability.

Applications and development have focused on vaccines for infectious diseases and cancer. One prominent example is the malaria vaccine RTS,S, which uses a formulation that includes QS-21 as part of an adjuvant system to boost efficacy. QS-21 has also been evaluated in various experimental vaccines and clinical trials, sometimes in combination with other adjuvants such as MPL (monophosphoryl lipid A).

Safety and tolerability are dose-dependent considerations. Local reactogenicity and systemic symptoms can occur, and QS-21 has a history of hemolytic potential at high concentrations. In approved or advanced-stage adjuvant systems, formulations are carefully designed to optimize efficacy while maintaining an acceptable safety profile.

See also: Quillaja saponaria, ISCOM, adjuvant systems.