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A Monoclonal Antibody for Preventing and Treating Inhalational Anthrax

A human monoclonal antibody, raxibacumab, was effective in the treatment and prevention of inhalational anthrax in two animal models.

The anthrax attacks of 2001 served as a reminder of the outbreak potential of Bacillus anthracis infection. Inhalational anthrax is often fatal, mainly because of toxin-mediated injury. Might we be able to protect against this disease?

Investigators recently conducted randomized, placebo-controlled studies of a human monoclonal antibody, raxibacumab, in two animal models of inhalational anthrax, each involving rabbits and monkeys. In one, raxibacumab was administered prophylactically before inhalational challenge with B. anthracis spores; in the other, it was administered after the onset of systemic disease. The researchers also conducted a randomized, single-blind, placebo-controlled trial to evaluate the antibody’s safety in humans. (All the researchers are employed by 1 of 2 biomedical companies; some also own equity in their company.)

In the prophylaxis experiments, the survival rates were higher and the median duration of survival was longer with raxibacumab than with placebo. In addition, monkeys that received raxibacumab and survived the initial inhalational challenge with B. anthracis were rechallenged approximately 1 year later, and all survived. Results of the treatment experiments were consistent with those of the prophylaxis studies: Raxibacumab conferred a survival advantage both in rabbits and in monkeys.

The safety studies included 333 healthy human volunteers who received intravenous raxibacumab at a dose of 40 mg/kg, the dose recommended for licensure. Most adverse events were mild to moderate in severity and short-lived; incidence was similar between the raxibacumab and placebo groups.

Comment: This investigation is important for at least two reasons. First, it shows that raxibacumab is effective adjunctive therapy in the treatment and prevention of inhalational anthrax in animals. Second, it fulfills the criteria required by the "animal rule," a federal regulatory tool described in an accompanying editorial. This rule permits results from animal models to serve as surrogate efficacy data when human trials are not feasible.

— Larry M. Baddour, MD

Published in Journal Watch Infectious Diseases July 8, 2009

Citation(s):

Migone T-S et al. Raxibacumab for the treatment of inhalational anthrax. N Engl J Med 2009 Jul 9; 361:135.

• Original article (Subscription may be required)
• Medline abstract (Free)

Nabel GJ. Protecting against future shock — Inhalational anthrax. N Engl J Med 2009 Jul 9; 361:191.

• Original article (Subscription may be required)
• Medline abstract (Free)