The FDA has approved a new indication for BioThrax (Anthrax Vaccine Adsorbed) to prevent disease following suspected or confirmed exposure to Bacillus anthracis, the bacterium that causes anthrax disease. The vaccine’s new use is approved for people 18 through 65 years of age in conjunction with recommended antibiotic treatment. BioThrax was initially approved by the FDA in 1970 for the prevention of anthrax disease in persons at high risk of exposure.

Anthrax disease, especially the inhalation form, often is fatal if not promptly treated. Anthrax is considered one of the more likely agents to be used in a biological attack, primarily because its spores are very stable and easy to disperse. Although it is rare, people may contract anthrax disease through natural exposures, such as contact with infected animals or contaminated animal products.

“With today’s approval of BioThrax, we now have a vaccine that can be used, together with antibiotic treatment, to prevent disease after exposure to anthrax spores,” said Karen Midthun, M.D., director of the FDA’s Center for Biologics Evaluation and Research.

BioThrax is the first vaccine to receive approval based on the Animal Rule, which allows animal efficacy data to be used as a basis for approval when human efficacy studies are not ethical or feasible.

Protective antibody levels, which were determined in rabbit and monkey studies, were used to predict efficacy in humans based on an assessment of the extent of antibody response achieved in human study participants. A 70% probability of survival in animal models from inhalational anthrax disease was deemed a reasonable level of protection and likely to provide reasonable benefit in humans.

The ability of BioThrax to increase the probability of survival after stopping post-exposure antibiotic treatment was assessed in rabbits. Rabbits treated with both antibiotics and BioThrax had a survival rate of 70% to 100%, depending on the vaccine dose administered. In contrast, in two studies of rabbits that received only antibiotic treatment, survival rates were 44% and 23%, respectively.

The safety and antibody responses to BioThrax in humans were evaluated in a multicenter study conducted in the U.S. Subcutaneous (under the skin) injections were given to 200 healthy adults in three doses at zero, two and four weeks. The majority of study participants generated antibody responses that correlated to a 70% probability of survival that was observed in animal models.

The observed adverse reactions were comparable with those observed when BioThrax is used for pre-exposure disease prevention. The safety profile for BioThrax is well-established, with the majority of localized adverse events reported as tenderness, pain, swelling, and redness at the injection site, as well as limited movement of the injected arm. The most common systemic adverse reactions were muscle aches, headache and fatigue.

BioThrax is manufactured by Emergent BioDefense Operations Lansing, based in Lansing, Mich.