Potency of a Human Monoclonal Antibody to Diphtheria Toxin Relative to Equine Diphtheria Anti-toxin in an Animal Model

Abstract

Background. Morbidity and mortality from toxigenic Corynebacterium diphtheriae is greatly reduced by prompt administration of anti-toxin. Current treatment relies upon equine-derived antibodies to diphtheria toxin (Diphtheria Anti-Toxin [DAT]) that carry the risk of a severe allergic reaction. The global supply of DAT is extremely limited as most manufacturers have ceased production. MassBiologics of the University of Massachusetts Medical School has developed a human monoclonal antibody (S315) that neutralizes diphtheria toxin in vitro and protects toxin-exposed animals. A human monoclonal has the potential to provide a safer alternative to equine DAT and address critical supply issues for global use. Methods. To guide dose selection for investigational new drug-enabling pharmacology and toxicology studies, we dose-ranged S315 and DAT in a guinea pig model of diphtheria intoxication based on the National Institutes of Health (NIH) Minimum Requirements (1946) potency assay for DAT preparations. Guinea pigs (5 per cohort) were dosed with a single subcutaneous injection of DAT (1.25–2.5 IU) or S315 (8–100 µg) premixed with a fixed dose of toxin. Animals were monitored daily for 30 days and assigned a score for presence of symptoms (lethargy, dehydration, weak limbs) or death. A logistic analysis was performed using Fieller's theorem to calculate the 95% confidence intervals (CIs) around the ED50 for each group. Results. All animals receiving ≥27.5 µg of S315 or ≥1.75 IU of DAT survived toxin exposure whereas all 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 of the surviving animals exhibited transient limb weakness, possibly a manifestation of peripheral neuropathy due to diphtheria toxin, the ED50 required to prevent any signs of diphtheria intoxication were also calculated for DAT and S315, yielding a relative potency of 48 µg/IU (95% CI, 38–59). Conclusion. To support the advancement of S315 into clinical trials, these potency estimates will be used to evaluate the efficacy of S315 versus DAT in an animal model in which antibody will be administered after exposure to diphtheria toxin, more closely modeling the timing of anti-toxin administration in humans. Disclosures. H. Smith, MassBiologics of the University of Massachusetts Medical School: Employee, Salary; P. Cheslock, MassBiologics of the University of Massachusetts Medical School: Employee, Salary; M. Leney, MassBiologics of the University of Massachusetts Medical School: Employee, Salary; B. Barton, MassBiologics of the University of Massachusetts Medical School: Collaborator, Salary; D. Molrine, MassBiologics of the University of Massachusetts Medical School: Employee, Salary