Novel nanoemulsion adjuvant stabilized by TPGS possesses equivalent physicochemical properties, Turbiscan stability, and adjuvanticity to AS03 for eliciting robust immunogenicity of subunit vaccines in mice

Emulsion-based antigen delivery systems have emerged as a novel approach to enhance the effectiveness of subunit vaccines. This study presents the development of a newly formulated oil-in-water (o/w) nanoemulsion adjuvant (NEA) composed of squalene oil and α-tocopheryl polyethylene glycol 1000 succinate (TPGS), which serves dual roles as an emulsifier and an immunostimulator. In comparison to AS03, an FDA-approved emulsion adjuvant that includes α-tocopherol, squalene, and polysorbate 80, NEA is devoid of α-tocopherol and exhibits comparable physicochemical properties, including particle size, polydispersity index, morphology, pH, zeta potential, and viscosity. Stability assessments conducted using Turbiscan Lab indicated that NEA undergoes an uplift process without experiencing flocculation, agglomeration or delamination. Model subunit antigens of recombinant glycoprotein E (gE) targeting the varicella-zoster virus (VZV) and highly purified hemagglutinin (HA) protein against trivalent seasonal influenza viruses (TIV) were employed to assess the adjuvanticity of NEA. It was revealed that the specific anti-gE IgG titers induced by the gE/NEA were markedly higher than those generated by gE alone, with titers of 13,000 <i>vs</i> 3,000 for the primary vaccination, and 5 × 10⁶ <i>vs</i> 5 × 10⁴ for the booster vaccination. Additionally, the TIV/NEA group exhibited a significantly improved immunogenic response relative to TIV alone across all three HA antigens at six-week after immunization, as evidenced by anti-HA titers of 256 <i>vs</i> 32. Furthermore, the NEA demonstrated no significant difference in efficacy compared to AS03 in both the VZV and TIV vaccines. Consequently, NEA presents a promising alternative to AS03 for the development of effective subunit vaccines.