Metabolizable oil-in-water (o/w) emulsions have been widely used as effective vaccine adjuvants. While squalene appears to be the preferred metabolizable oil,1 various emulsifier compositions exist in human and veterinary products. For example, MF59® employs polysorbate 80 (Tween® 80) and sorbitan trioleate (Span® 85), while AS03® utilizes only polysorbate 80. Other commonly used pharmaceutical emulsifiers include poloxamers (Pluronics®) and phospholipids. Emulsifier selection is based on emulsion stabilizing capacity and/or biological activity. Some surfactants, including polysorbate 80 and poloxamer 401, have been shown to induce apoptosis and necrosis, which in turn cause immunostimulation.2
We recently demonstrated that substitution of squalene with other metabolizable oils such as long- or medium-chain triglycerides (MCT) in o/w emulsions dramatically affects adjuvant activity when combined with an inactivated trivalent split-virus influenza vaccine.3 In contrast, substitution of egg phosphatidylcholine with different synthetic phosphatidylcholine emulsifiers in a squalene emulsion resulted in similar biological activity.4 In the present work, we expand upon these findings to evaluate the effects on emulsion stability, antigen-adjuvant compatibility, and biological activity of (i) different emulsifier classes (i.e. poloxamer versus polysorbate versus phosphatidylcholine), (ii) different oil classes (i.e. squalene versus MCT), and (iii) common excipients (i.e. glycerol, antioxidant, phosphate buffer), when formulated with a recombinant pandemic influenza vaccine.