Surfactants in vaccine adjuvants: description and perspectives

Amphiphilic molecules, called surfactants, are made of a lipophilic part linked to a hydrophilic part. These substances have physicochemical properties of self-aggregation, solubilisation or emulsification. Their ability to be attracted at the same time by polar and non-polar compounds forces them to be at the interface. Surfactants most frequently used in vaccines can be of natural or synthetic origin, non-ionic, cationic or amphoteric with a formula weight between 600 and 4000 g/mol. They can be used as adjuvants, solubilisers or stabilisers of emulsions. Physicochemical properties are defined through solubility parameters or solution behaviours and hydrophilic lipophilic balance (HLB). Application properties are assessed through placebo formulations simulating the final use. Synthetic products are preferred to products of natural origin for quality reasons. Development of a new generation of surfactant with precise chemical definition and consistency may generate new candidates for potential vaccine adjuvants.