AS01

AS01 is a proprietary liposome-based adjuvant system developed by GlaxoSmithKline for use in vaccines. It is part of the company’s Adjuvant System family and is designed to enhance both antibody and cellular immune responses by delivering immunostimulatory molecules with an antigen in a liposomal carrier. The AS01 formulation combines 3-O-desacyl-4'-monophosphoryl lipid A (MPLA), a Toll-like receptor 4 agonist, with QS-21, a saponin, all within small unilamellar liposomes. Variants of AS01 exist, including AS01_B and AS01_E, which differ in the relative amounts of MPLA and QS-21 to tune immunogenicity and reactogenicity for different vaccines.

AS01 is used to boost immune responses in certain vaccines. The best-known application is RTS,S/AS01, a malaria vaccine in which the circumsporozoite protein antigen is formulated with the AS01 adjuvant. Another licensed vaccine, Shingrix, uses AS01_B as its adjuvant system to support immune responses to a recombinant herpes zoster protein. The adjuvant system is intended to promote a Th1-biased response and robust antigen presentation by activating antigen-presenting cells at the injection site and in draining lymph nodes.

Safety and regulatory status: AS01-containing vaccines undergo standard safety assessment for reactogenicity and adverse events; in general, AS01 can increase local and systemic reactions but contributes to established vaccine efficacy in approved contexts. Regulatory experience with RTS,S/AS01 includes World Health Organization recommendations for use in malaria-endemic regions and evaluation in clinical trials worldwide.