Anthrax vaccine-induced antibodies provide cross-species prediction of survival to aerosol challenge

Abstract

Because clinical trials to assess the efficacy of vaccines against anthrax are not ethical or feasible, licensure for new anthrax vaccines will likely involve the Food and Drug Administration's "Animal Rule," a set of regulations that allow approval of products based on efficacy data only in animals combined with immunogenicity and safety data in animals and humans. U.S. government-sponsored animal studies have shown anthrax vaccine efficacy in a variety of settings. We examined data from 21 of those studies to determine whether an immunological bridge based on lethal toxin neutralization activity assay (TNA) can predict survival against an inhalation anthrax challenge within and across species and genera. The 21 studies were classified into 11 different settings, each of which had the same animal species, vaccine type and formulation, vaccination schedule, time of TNA measurement, and challenge time. Logistic regression models determined the contribution of vaccine dilution dose and TNA on prediction of survival. For most settings, logistic models using only TNA explained more than 75% of the survival effect of the models with dose additionally included. Cross-species survival predictions using TNA were compared to the actual survival and shown to have good agreement (Cohen's κ ranged from 0.55 to 0.78). In one study design, cynomolgus macaque data predicted 78.6% survival in rhesus macaques (actual survival, 83.0%) and 72.6% in rabbits (actual survival, 64.6%). These data add support for the use of TNA as an immunological bridge between species to extrapolate data in animals to predict anthrax vaccine effectiveness in humans.

Figure 1

Plots of the predicted probability of survival for the 11 settings as a function of antibody (black curve) along with 95% confidence bands (dashed grey curves). The estimated antibody that provides 50% protection is given in black on the x-axis, along with a 95% confidence interval. Each point is an animal (rabbits=orange, cynos=sky blue, rhesus=blue-green).

Figure 2

Estimates of PA50, the TNA value that provides 50% protection, together with 95% confidence intervals. Each point is either a setting or the overall meta-analysis estimate (double size point) for that species (rabbits=orange, cynos=sky blue, rhesus=blue-green). Meta-analytic overall estimates use random effects models (rabbits: 100 [95% CI 46, 218], cynos: 30 [95% CI 19,46], rhesus: 122 [95% CI 21, 701]). Note the confidence interval for the overall cynos may be too small since there are only 2 settings used to estimate the random effects and it was estimated as 0.

Figure 3

Vaccine Efficacy in the 11 experimental settings by dose group. Each point is VE estimated from one dose group compared to all control animals in that setting, vertical lines are 95% confidence intervals on the VE. The value of the point on the horizontal axis is the geometric mean of the TNA measured about 4 weeks after the last vaccination for the animals from that dose group. Orange is rabbits, sky blue is cynos and blue-green is rhesus. Logistic lines are from linear regression of empirical logits on log10(GMT) and only extend over the range for which there are geometric mean TNA values. Orange dashed is rabbits, sky blue double-dashed is cynos, and blue-green solid is rhesus.

Figure 4

The effect of TNA on survival for the 130 rabbits of setting 3. Each point is an animal and random noise was added to the points to avoid overlap. Colored curves are from logistic model with a continuous effect for log10dose from Model 2, where log10(0) is set to log10(0.005). The curves only cover the range for which there are observed TNA values in that dose group, so no lines are drawn for dose=0 and dose=0.01 since all TNA values are below the limit of detection. The black line is Model 1 where dose is not included.

Figure 5

Graphical representation of first row in Table 3. Orange logistic line is the predicted survival based on Setting 3 (rabbits). The lines going from the horizontal axis to the logistic curve, then to the vertical axis represent the TNA values for the 29 cyno monkeys in setting 6. Random noise was added to the lines close to TNA=11.5 (half the limit of detection), and those lines represent 8 monkeys, 5 that died and 3 that survived. The sky blue tick on the vertical axis represents the mean predicted survival (70.1) for cyno macaques based on rabbit efficacy data.