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

This article is referred to by:

Development of human monoclonal antibodies to diphtheria toxin: A solution for the increasing lack of equine DAT for therapeutic use?

Abbreviations

(CHO)	=	Chinese hamster ovary
(DAT)	=	diphtheria anti-toxin
(FDA)	=	Food and Drug Administration
(GEE)	=	generalized estimating equation
(IU)	=	international unit
(mAb)	=	monoclonal antibody
(NIH)	=	National Institutes of Health

Disclosure of potential conflicts of interest

HLS, PC, ML, and DCM are employees of MassBiologics of the University of Massachusetts Medical School where the S315 monoclonal antibody was discovered.

Acknowledgments

The authors wish to thank Erica Messick and Jennifer Brennan for their technical expertise, Larry Allen for assistance with data preparation, and Su-Chun Cheng for additional statistical analyses.

Funding

These studies were funded by MassBiologics of the University of Massachusetts Medical School.