In a new approach to treating anthrax exposure, a team of scientists has created an inhibitor designed to tackle the growing threat of antibiotic-resistant strains. Reporting in this week’s online early edition of the Proceedings of the National Academy of Sciences (PNAS), researchers from Rensselaer Polytechnic Institute and the University of Toronto describe the new anthrax toxin inhibitor, which performed successfully in both laboratory and animal tests.
Anthrax toxin, secreted by the anthrax bacterium, is made of proteins and toxic enzymes that bind together to inflict damage on a host organism. Rather than targeting the anthrax bacterium or toxin — the approach taken by the majority of current therapies — the new inhibitor blocks the receptors where anthrax toxin attaches in the body. And because the nanoscale assembly of molecules is designed to bind to multiple sites on the host receptor, it is naturally more potent.
The new approach led to a 50,000-fold increase in potency in cell culture, and the inhibitor protected rats from anthrax toxin in the study. The general concept also could be applied to designing inhibitors for other pathogens, including SARS, influenza, and AIDS, the researchers note.
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 the emergence of antibiotic-resistant strains calls for new approaches to designing therapeutics for bioterrorism agents, according to Ravi Kane, the Merck Associate Professor of Chemical and Biological Engineering at Rensselaer and corresponding author of the PNAS paper.
Pathogens such as anthrax can become resistant to antibiotics through natural processes, but resistance also can be engineered intentionally. The team’s new approach could help address this threat by making inhibitors that target the receptors where anthrax toxin attaches in the body, rather than at the anthrax bacterium or toxin directly.
Blocking “host receptors” is a better approach, Kane suggested. “Think about how a virus-like HIV becomes resistant to an inhibitor that binds to it,” he said. “A subtle change in the viral proteins can drastically reduce the affinity of the drug without compromising the ability of the virus to bind to its target cell. However, a host protein is not mutating like proteins on the pathogen. So it’s a stationary target versus a moving one.”
The inhibitor designed by the Rensselaer-Toronto team is “polyvalent,” which means that it displays multiple copies of receptor-binding peptides, allowing it to bind at multiple sites and become more potent than an inhibitor that binds to a single site. For the current experiment, the researchers made four different polyvalent inhibitors and then tested each in cell culture. They found that the most potent of the four inhibitors enabled more than a 50,000-fold enhancement in activity compared to an inhibitor that was not based on polyvalency.
This potent inhibitor was then characterized more fully and tested in rats. Five out of six rats injected only with anthrax toxin died; all six rats injected with toxin and a non-polyvalent inhibitor died. But the new polyvalent inhibitor protected all six rats in the experiment, with no signs of adverse side effects.
Once fully developed, administering the new inhibitor to patients could help reduce the high mortality rates associated with inhalational anthrax, according to the researchers. Antibiotics slow the progression of infection by targeting the anthrax bacterium, but they do not counter the advanced destructive effects of anthrax toxin in the body. Inhalation anthrax still has a fatality rate of 75 percent even after antibiotics are given, according to the Centers for Disease Control and Prevention. “Combining the inhibitor with antibiotic therapy may increase the likelihood of survival for an infected person,” Kane said.
The research team is led by Kane and Jeremy Mogridge, Canada Research Chair and assistant professor of laboratory medicine and pathobiology at the University of Toronto. Rensselaer graduate students and post-doctoral researchers who contributed to the work include Saleem Basha, Prakash Rai, Arundhati Saraph, and Kunal Gujraty. University of Toronto researchers included Vincent Poon, Mandy Go, Skanda Sadacharan, and Mia Frost. Funding for the research was provided by the National Institutes of Health.
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