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 25mug of rPA adsorbed to 500mug of aluminum in aluminum hydroxide gel Alhydrogel had a significantly higher quantitative anti-rPA IgG ELISA titers p0.0001 and toxin neutralizing antibody TNA assay titers p0.0001 than rabbits tested at the next lowest concentration of aluminum 158mug. Rabbits injected with two doses of 50mug of rPA formulated with 500mug of aluminum also had significantly higher serological responses, as measured by a quantitative anti-rPA IgG ELISA p0.0001 and TNA assay p0.0001, 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 1212 and 1112, respectively. Rabbits injected with a single dose of 50mug of rPA formulated with 500mug of aluminum and 0.2 formaldehyde had significantly higher ELISA p0.0001 and TNA assay p0.0001 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 1424 and 924, respectively p0.2476. 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.