Potency of a human monoclonal antibody to diphtheria toxin relative to equine diphtheria anti-toxin in a guinea pig intoxication model

Abstract

Prompt administration of anti-toxin reduces mortality following Corynebacterium diphtheriae infection. Current treatment relies upon equine diphtheria anti-toxin (DAT), with a 10% risk of serum sickness and rarely anaphylaxis. The global DAT supply is extremely limited; most manufacturers have ceased production. S315 is a neutralizing human IgG1 monoclonal antibody to diphtheria toxin that may provide a safe and effective alternative to equine DAT and address critical supply issues. To guide dose selection for IND-enabling pharmacology and toxicology studies, we dose-ranged S315 and DAT in a guinea pig model of diphtheria intoxication based on the NIH Minimum Requirements potency assay. Animals received a single injection of antibody premixed with toxin, were monitored for 30 days, and assigned a numeric score for clinical signs of disease. Animals receiving ≥ 27.5 µg of S315 or ≥ 1.75 IU of DAT survived whereas animals receiving ≤ 22.5 µg of S315 or ≤ 1.25 IU of DAT died, yielding a potency estimate of 17 µg S315/IU DAT (95% CI 16-21) for an endpoint of survival. Because some surviving animals exhibited transient limb weakness, likely a systemic sign of toxicity, DAT and S315 doses required to prevent hind limb paralysis were also determined, yielding a relative potency of 48 µg/IU (95% CI 38-59) for this alternate endpoint. To support advancement of S315 into clinical trials, potency estimates will be used to evaluate the efficacy of S315 versus DAT in an animal model with antibody administration after toxin exposure, more closely modeling anti-toxin therapy in humans.

Keywords: anti-toxin; diphtheria; monoclonal antibody; potency.

Figures

Figure 1.

Percent survival following receipt of fixed diphtheria toxin dose pre-mixed with varying amount of antibody by outcome for DAT and mAb treated cohorts. The proportion of animals surviving at each antibody dose is shown in the solid bars and the proportion surviving without symptoms of hind limb paralysis is shown in the hatched bars. The number of animals (n) evaluated in each cohort is noted on the graph. The results for cohorts receiving equine diphtheria anti-toxin (DAT) are shown in panel A and those for cohorts given S315 human monoclonal antibody to diphtheria toxin are shown in panel B.

Figure 2.

Survival over time by antibody cohort following diphtheria toxin challenge. The proportion of animals surviving over the 30-day study period by study day following receipt of toxin-antibody mixture is shown for equine diphtheria anti-toxin (DAT) dosing cohorts in panel A and for S315 human monoclonal antibody dosing cohorts in panel B.

Figure 3.

Survival over time without hind limb paralysis by antibody cohort following diphtheria toxin challenge. The proportion of animals surviving without hind limb paralysis over the 30-day study period by study day following receipt of toxin-antibody mixture is shown for equine diphtheria anti-toxin (DAT) dosing cohorts in panel A and S315 human monoclonal antibody dosing cohorts in panel B.