Effect of aluminum hydroxide adjuvant and formaldehyde in the formulation of rPA anthrax vaccine

The serological response and efficacy of Bacillus anthracis recombinant protective antigen (rPA) vaccines formulated with aluminum hydroxide adjuvant, either with or without formaldehyde, were evaluated in rabbits. Rabbits that had been injected with a single dose of 25 μg of rPA adsorbed to 500 μg of aluminum in aluminum hydroxide gel (Alhydrogel) had a significantly higher quantitative anti-rPA IgG ELISA titers (p < 0.0001) and toxin neutralizing antibody (TNA) assay titers (p < 0.0001) than rabbits tested at the next lowest concentration of aluminum (158 μg). Rabbits injected with two doses of 50 μg of rPA formulated with 500 μg of aluminum also had significantly higher serological responses, as measured by a quantitative anti-rPA IgG ELISA (p < 0.0001) and TNA assay (p < 0.0001), than sera from rabbits injected with a rPA vaccine formulated without adjuvant. Short-term protection against an aerosol spore challenge (448 LD50), however, was not significantly different between the two groups (12/12 and 11/12, respectively). Rabbits injected with a single dose of 50 μg of rPA formulated with 500 μg of aluminum and 0.2% formaldehyde had significantly higher ELISA (p < 0.0001) and TNA assay (p < 0.0001) titers than rabbits that had been injected with a rPA vaccine formulated with adjuvant but without formaldehyde. Short-term protection against a 125 LD50 parenteral spore challenge, however, was not significantly different between the two groups (14/24 and 9/24, respectively; p = 0.2476). Under the conditions tested in the rabbit animal model, significantly higher serological responses were observed in rabbits that had been injected with rPA formulated with aluminum hydroxide gel adjuvant and formaldehyde. However, differences in short-term efficacy were not observed.