E6020: a synthetic Toll-like receptor 4 agonist as a vaccine adjuvant

Safe and cost-effective adjuvants are a critical component to enhance the efficacy of subunit vaccines. Studies have demonstrated that modified natural lipid As derived from enterobacterial lipopolysaccharides, which are agonists of Toll-like receptor 4, are beneficial to vaccine performance. The synthetic phospholipid dimer, E6020, mimics the physicochemical and biological properties of many of the natural lipid As derived from Gram-negative bacteria. Similar to its natural counterparts, E6020, which was discovered and developed by Eisai, agonizes Toll-like receptor 4, albeit in an attenuated fashion, eliciting an immunostimulatory response that is conducive to use as a vaccine adjuvant. The derivation of E6020, along with physicochemical properties and in vitro and in vivo studies of immunostimulation and adjuvant activity, are reviewed as a background to its imminent assessment in the clinic.