Evaluation of the AV7909 Anthrax Vaccine Toxicity in Sprague Dawley Rats Following Three Intramuscular Administrations

Abstract

AV7909 is a next-generation anthrax vaccine under development for post-exposure prophylaxis following suspected or confirmed Bacillus anthracis exposure, when administered in conjunction with the recommended antibacterial regimen. AV7909 consists of the FDA-approved BioThrax^® vaccine (anthrax vaccine adsorbed) and an immunostimulatory Toll-like receptor 9 agonist oligodeoxynucleotide adjuvant, CPG 7909. The purpose of this study was to evaluate the potential systemic and local toxicity of AV7909 when administered via repeat intramuscular injection to the right thigh muscle (biceps femoris) to male and female Sprague Dawley rats. The vaccine was administered on Days 1, 15, and 29 and the animals were assessed for treatment-related effects followed by a 2-week recovery period to evaluate the persistence or reversibility of any toxic effects. The AV7909 vaccine produced no apparent systemic toxicity based on evaluation of clinical observations, body weights, body temperature, clinical pathology, and anatomic pathology. Necrosis and inflammation were observed at the injection sites as well as in regional lymph nodes and adjacent tissues and were consistent with immune stimulation. Antibodies against B. anthracis protective antigen (PA) were detected in rats treated with the AV7909 vaccine, confirming relevance of this animal model for the assessment of systemic toxicity of AV7909. In contrast, sera of rats that received saline or soluble CPG 7909 alone were negative for anti-PA antibodies. Overall, 3 intramuscular immunizations of Sprague Dawley rats with AV7909 were well tolerated, did not induce mortality or any systemic adverse effects, and did not result in any delayed toxicity.

Keywords: AV7909; CPG 7909; animal; anthrax; rats; toxicity; vaccine.

Figure 1.. Body Weight Change

Body weight change is presented as mean and standard deviation. A steady increase in body weight over the course of the study in both male and female rats that were administered with the AV7909 test article was noted except on Days 29 and 43 where there was a slight decrease in body weights as the rats were fasting. Group 1 – 0 mg (control article) Group 2 – 0.5 mg CPG 7909 Group 3 – 0.5 mL AV7909 Lot 1 (0.5 mL AVA and 0.5 mg CPG 7909) Group 4 – 0.5 mL AV7909 Lot 2 (0.5 mL AVA and 0.25 mg CPG 7909)

Figure 2.. Anti-PA IgG levels as measured by ELISA

Anti-PA IgG levels in rats are reported as mean and standard error for the group of animals that were administered with the AV7909 vaccine. No detectable levels of anti-PA IgG were observed in control group animals or the adjuvant-alone group and hence not included in the figure Group 3 – 0.5 mL AV7909 Lot 1 (0.5 mL AVA and 0.5 mg CPG 7909) Group 4 – 0.5 mL AV7909 Lot 2 (0.5 mL AVA and 0.25 mg CPG 7909)

Figure 3.. Toxin-neutralizing antibody (ED₅₀) levels as measured by TNA

TNA ED₅₀ (the dilution that neutralizes 50% of the cytotoxic effect of the anthrax lethal toxin) levels presented as mean and standard deviation. ED₅₀ values were obtained from animals that were immunized with three doses of AV7909 vaccine. In the AV7909-vaccinated group, median ED₅₀ values were in the order of 10000 that suggests strong immune response generated by the vaccine. The toxin neutralizing antibodies were not detectable (below limit of quantitation) in the control and CPG 7909 alone groups and not shown in the figure. Group 3 – 0.5 mL AV7909 Lot 1 (0.5 mL AVA and 0.5 mg CPG 7909) Group 4 – 0.5 mL AV7909 Lot 2 (0.5 mL AVA and 0.25 mg CPG 7909)