Anthrax Inhibitor Counteracts Toxin, May Lead To New Therapeutics

Anthrax toxin, secreted by the anthrax bacterium, is made of proteins and toxic enzymes that bind together to inflict damage on a host organism. The inhibitor, which is described by the Rensselaer-Toronto team in the April 23 online edition of the journal Nature Biotechnology, works by preventing the assembly of toxic enzyme components, thereby blocking the formation of fully assembled anthrax toxin and neutralizing its activity.

The inhibitor protected rats from anthrax toxin in the study.

"Our eventual goal is to use the inhibitor as a human therapeutic for anthrax exposure, one that can stop the toxin from functioning inside the body," says Ravi Kane, the Merck Associate Professor of Chemical and Biological Engineering at Rensselaer and a principal investigator of the project. "Combining the inhibitor with antibiotic therapy may increase the likelihood of survival for an infected person."

The 2001 intentional release of anthrax spores via postal mail in the United States led to increased research on possible therapeutics and vaccines to treat toxins that could be used as biological weapons. The current treatment for anthrax exposure is antibiotics, but inhalation anthrax still has a fatality rate of 75 percent even after antibiotics are given, according to the Centers for Disease Control and Prevention. Antibiotics slow the progression of infection by targeting the bacteria, but do not counter the advanced destructive effects of anthrax toxin in the body.

Anthrax toxin is a polyvalent protein complex in that it displays multiple copies of identical binding surfaces on the same structure. The inhibitor designed by the Rensselaer-Toronto team is also polyvalent and recognizes these surface patterns on the anthrax toxin molecular structure, allowing it to bind at multiple sites and become four orders of magnitude more potent than an inhibitor that binds to a single site.

"Think about how two Lego blocks snap together. A brick with four studs can interlock with a brick with four holes. These bricks will grip together better than if they had only one stud and one hole," says Jeremy Mogridge, Canada Research Chair and assistant professor of Laboratory Medicine and Pathobiology at the University of Toronto. "Furthermore, Lego works because the pattern of studs on one brick matches the pattern of holes on another."

Earlier work by other groups has shown that an inhibitor with a fixed pattern of chemical groups can recognize a protein with a similar fixed pattern of complementary groups. In this study, the team demonstrated that a therapeutic inhibitor displaying random patterns can recognize a target if its statistical characteristics match those of the toxin target. According to the researchers, endowing inhibitors with statistical pattern-matching capabilities is less difficult than designing inhibitors with fixed structures.

"The pattern matching-based approach used by our research team to neutralize anthrax toxin should be broadly applicable in designing potent therapeutics for a variety of pathogens and toxins, including influenza and HIV," says Kane.

The researchers tested their pattern-matching strategy by designing a polyvalent inhibitor for cholera toxin, demonstrating that this approach also could be used successfully to enhance the potency of polyvalent inhibitors directed to this target and, they suggest, others. They note the work also could be useful for creating specific target recognition in biological sensors.

The team says their work demonstrates for the first time that liposome-based polyvalent inhibitors are effective in animals and they are continuing development on the anthrax inhibitor through additional animal testing.

The research team is led by Kane and Mogridge. Rensselaer graduate students and post-doctoral researchers who contributed to the work include Prakash Rai, Chakradhar Padala, Arundhati Saraph, Saleem Basha, and Sandesh Kate. University of Toronto researchers included Vincent Poon and Kevin Tao. Funding for this research was provided by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health.