Efficacy Projection of Obiltoxaximab for Treatment of Inhalational Anthrax across a Range of Disease Severity

Abstract

Inhalational anthrax has high mortality even with antibiotic treatment, and antitoxins are now recommended as an adjunct to standard antimicrobial regimens. The efficacy of obiltoxaximab, a monoclonal antibody against anthrax protective antigen (PA), was examined in multiple studies conducted in two animal models of inhalational anthrax. A single intravenous bolus of 1 to 32 mg/kg of body weight obiltoxaximab or placebo was administered to New Zealand White rabbits (two studies) and cynomolgus macaques (4 studies) at disease onset (significant body temperature increase or detection of serum PA) following lethal challenge with aerosolized Bacillus anthracis spores. The primary endpoint was survival. The relationship between efficacy and disease severity, defined by pretreatment bacteremia and toxemia levels, was explored. In rabbits, single doses of 1 to 16 mg/kg obiltoxaximab led to 17 to 93% survival. In two studies, survival following 16 mg/kg obiltoxaximab was 93% and 62% compared to 0% and 0% for placebo (P = 0.0010 and P = 0.0013, respectively). Across four macaque studies, survival was 6.3% to 78.6% following 4 to 32 mg/kg obiltoxaximab. In two macaque studies, 16 mg/kg obiltoxaximab reduced toxemia and led to survival rates of 31%, 35%, and 47% versus 0%, 0%, and 6.3% with placebo (P = 0.0085, P = 0.0053, P = 0.0068). Pretreatment bacteremia and toxemia levels inversely correlated with survival. Overall, obiltoxaximab monotherapy neutralized PA and increased survival across the range of disease severity, indicating clinical benefit of toxin neutralization with obiltoxaximab in both early and late stages of inhalational anthrax.

FIG 1

Toxin neutralization with obiltoxaximab confers survival benefit in the treatment of inhalational anthrax. NZW rabbits (studies R1 and R2) and cynomolgus macaques (studies M1 to M4) were aerosol challenged with targeted 200 LD₅₀ of B. anthracis spores. Treatment was administered at the indicated doses following the first detection of circulating PA by PA-ECL (all studies) or significant increase in body temperature (rabbits only). Forest plot represents differences between survival proportions in placebo (control) and obiltoxaximab treatment groups as vertical lines with corresponding confidence intervals (horizontal lines). Only animals bacteremic at the time of treatment were included in the analyses.

FIG 2

Neutralization of circulating PA following obiltoxaximab administration. Cynomolgus macaques in studies M2 to M4 were aerosol challenged with targeted 200 LD₅₀ of B. anthracis spores, and placebo or obiltoxaximab was administered following the first detection of circulating PA by PA-ECL. Peripheral blood samples were collected immediately prior to treatment (PTT) or at the indicated times posttreatment or postchallenge for the assessment of circulating free PA. (A) Shown are individual animal values for free PA values obtained immediately prior to treatment in each dose group. Horizontal lines indicate geometric means and 95% CI. Free PA was not measured in study M1. (B) Shown are means and 95% CI for free PA levels in samples collected at each indicated time point in study M2. Numbers of animals surviving to each sample collection are indicated on the bottom. The single control survivor and 50% of obiltoxaximab-treated survivors were sacrificed on study day 28. Terminal samples are not included on the graph. Upper limit of quantitation (ULOQ) and lower limit of quantitation (LLOQ) for free PA assay were 40,0000 and 9.68 ng/ml, respectively; LLOQ is indicated by the dotted line. For statistical computations, PA levels below the limit of detection were replaced with 4.84 ng/ml (1/2 LLOQ).

FIG 3

PTT bacteremia in survivors and nonsurvivors of cynomolgus macaque treatment studies. Cynomolgus macaques in studies M1 to M4 were aerosol challenged with targeted 200 LD₅₀ of B. anthracis spores, and placebo or obiltoxaximab was administered following the first detection of circulating PA by PA-ECL. Peripheral blood samples were collected immediately PTT for quantitative assessment of bacteremia. (A) Shown are geometric means and individual animal values in each study. Both placebo- and obiltoxaximab-treated animals were included in the analyses. (B) Shown are means and SEM for bacteremia levels in survivors (shaded bars) and nonsurvivors (white bars) obtained prior to treatment for each group that received obiltoxaximab. The limit of detection (LOD) was 33 CFU/ml in M1 and 3 CFU/ml in M2 to M4; the limit of quantification (LOQ) was 1,000 CFU/ml in M1 and 100 CFU/ml in M2 to M4. *, P < 0.05; **, P < 0.01; ***, P < 0.001 compared to the control (Mann-Whitney test). Bacteremia levels below the limit of detection were replaced with 1/2 LOD, and levels above limits of detection but below limits of quantitation were replaced with 1/2 LOQ.

FIG 4

Levels of bacteremia and toxemia at the time of toxin neutralization are prognostic of survival following antitoxin monotherapy. Shown are logistic regression analyses describing the relationship between survival and bacteremia (A and B) or survival and toxemia (C) for animals in studies M1 to M4 treated with obiltoxaximab at 16 mg/kg (A and C) or all doses (4 to 32 mg/kg) (B). Filled circles indicate the proportion of animals that survived within each half-log bin. Control animals were not included in the analyses shown in panels A to C. Panels A and B include data from studies M1 to M4, and panel C includes data from studies M2 to M4. (D) Shown are dose-response relationships after stratifying by level of pretreatment bacteremia. The four quadrants represent quartiles of the PTT bacteremia distribution across 5 studies (rabbit study R1 and cynomolgus macaque studies M1 to M4). Bacteremia levels in each quartile are shown on top of each quadrant. Numbers on the plot indicate the total number of animals comprising each observed mean data point, and the solid black line represents the prediction based on modeling of survival data. Gray vertical lines represent 95% confidence intervals. CFU, CFU/ml; BLQ, below the limit of quantitation.