Correlation between anthrax lethal toxin neutralizing antibody levels and survival in guinea pigs and nonhuman primates vaccinated with the AV7909 anthrax vaccine candidate

Abstract

The anthrax vaccine candidate AV7909 is being developed as a next generation vaccine for a post-exposure prophylaxis (PEP) indication against anthrax. AV7909 consists of the Anthrax Vaccine Adsorbed (AVA, BioThrax®) bulk drug substance adjuvanted with the immunostimulatory oligodeoxynucleotide (ODN) compound, CPG 7909. The addition of CPG 7909 to AVA enhances both the magnitude and the kinetics of antibody responses in animals and human subjects, making AV7909 a suitable next-generation vaccine for use in a PEP setting. The studies described here provide initial information on AV7909-induced toxin-neutralizing antibody (TNA) levels associated with the protection of animals from lethal Bacillus anthracis challenge. Guinea pigs or nonhuman primates (NHPs) were immunized on Days 0 and 28 with various dilutions of AV7909, AVA or a saline or Alhydrogel+CPG 7909 control. Animals were challenged via the inhalational route with a lethal dose of aerosolized B. anthracis (Ames strain) spores and observed for clinical signs of disease and mortality. The relationship between pre-challenge serum TNA levels and survival following challenge was determined in order to calculate a threshold TNA level associated with protection. Immunisation with AV7909 induced a rapid, highly protective TNA response in guinea pigs and NHPs. Surprisingly, the TNA threshold associated with a 70% probability of survival for AV7909 immunized animals was substantially lower than the threshold which has been established for the licensed AVA vaccine. The results of this study suggest that the TNA threshold of protection against anthrax could be modified by the addition of an immune stimulant such as CPG 7909 and that the TNA levels associated with protection may be vaccine-specific.

Keywords: Animal rule; Anthrax vaccine; CPG 7909 adjuvant; Correlate of protection; Cynomolgus macaque; Guinea pig; TNA threshold.

Figure 1. Group geometric mean TNA NF₅₀ response to vaccination with AV7909

Guinea pigs (GP) or NHP were immunized with dilutions of AV7909 or AVA on days 0 and 28. TNA NF50 geometric means with 95% confidence intervals by study day are shown. In the first guinea pig study, titres were measured on day 21 instead of day 28. The number of animals in each dose group for the studies is shown in Table 1. TNA was below the lower limit of quantitation (LLOQ) at all time points in the control groups (data not shown). LLOQ is indicated by the dotted line (^.…).

Figure 2. Logistic regression analyses for AV7909 GP and NHP Studies

The logistic regression curves (solid line) with 95% CI (dashed lines) illustrate the changes in predicted survival probability with increasing immune responses. Individual animals (○) that survived or died are shown at the top and bottom of each figure, respectively. The combined data from Guinea Pig studies 1 and 2 and NHP studies 1 and 2 are shown in the two lower panels. Individual TNA NF₅₀ values below the lower limit of quantitation (LLOQ) were replaced with the LLOQ value (LLOQ = 0.063 for GP TNA and LLOQ = 0.105 for NHP TNA). GP Study 1 N=62; GP Study 2 N= 138; NHP Study 1 N=39; NHP Study 2 N=68

Figure 2. Logistic regression analyses for AV7909 GP and NHP Studies

The logistic regression curves (solid line) with 95% CI (dashed lines) illustrate the changes in predicted survival probability with increasing immune responses. Individual animals (○) that survived or died are shown at the top and bottom of each figure, respectively. The combined data from Guinea Pig studies 1 and 2 and NHP studies 1 and 2 are shown in the two lower panels. Individual TNA NF₅₀ values below the lower limit of quantitation (LLOQ) were replaced with the LLOQ value (LLOQ = 0.063 for GP TNA and LLOQ = 0.105 for NHP TNA). GP Study 1 N=62; GP Study 2 N= 138; NHP Study 1 N=39; NHP Study 2 N=68

Figure 3. Scatter plots for AV7909- and AVA-immunized survivors and non-survivors

The combined data from Guinea Pig studies 1 and 2 and NHP studies 1 and 2 are shown. Circles, ○, represent individual TNA NF₅₀ pre-challenge titres (day 69 for guinea pigs (GP) and day 70 for NHP) of non-survivors; squares, □, represent the individual TNA pre-challenge titres of survivors. Groups of animals were immunized with dilutions of AV7909 on day 0 and 28 and were challenged on day 70 post first-immunisation. The dotted horizontal line represents the limit of quantitation (LLOQ) of the assay.

Figure 3. Scatter plots for AV7909- and AVA-immunized survivors and non-survivors

The combined data from Guinea Pig studies 1 and 2 and NHP studies 1 and 2 are shown. Circles, ○, represent individual TNA NF₅₀ pre-challenge titres (day 69 for guinea pigs (GP) and day 70 for NHP) of non-survivors; squares, □, represent the individual TNA pre-challenge titres of survivors. Groups of animals were immunized with dilutions of AV7909 on day 0 and 28 and were challenged on day 70 post first-immunisation. The dotted horizontal line represents the limit of quantitation (LLOQ) of the assay.