July 16, 2004  


Scientists Step Closer to Blocking Anthrax Toxin

By Heather Rock Woods and Irimpan Mathews

Scientists working on SSRL’s macromolecular crystallography beam lines have taken a big step forward in developing a drug to stop the most deadly of the toxins secreted by Anthrax.

An x-ray crystal structure of an inhibitor (NSC 12155)
bound in the active site of the Lethal Factor protein.
(Image courtesy of Robert Liddington)

Anthrax makes a lethal cocktail of three toxin proteins that flood the bloodstream, leading to rapid death if the infection is not diagnosed and treated in its early stages.  Even antibiotic treatments can fail when the Anthrax bacterium, Bacillus anthracis, has already produced lethal levels of toxins.

“The bottom line is we need anti-toxin approaches to treating Anthrax in the late stages,” said faculty researcher Robert Liddington of The Burnham Institute in La Jolla, California. “Antibiotics kill the bacteria but are only effective if given early because they don’t take out the toxins.” Liddington’s group published two papers in the June 2 issue of Nature Structural & Molecular Biology, together with colleagues at the Harvard Medical School and the United States Army Medical Research Institute of Infectious Diseases (USAMRIID).

The poisonous protein called Lethal Factor (LF) is the greatest source of damage in highly fatal cases of inhalation anthrax.  LF swiftly blocks signals that recruit immune cells to fight the infection.  Another enzyme, Edema Factor (EF), causes the release of fluid into the lungs and is deadly on its own.  Protective Antigen (PA) protein acts as a transporter system, enabling LF and EF to enter target cells.

An anti-toxin that stops LF would be a vital addition to combined therapy with existing treatments (antibiotics, anti-PA antibodies, critical care).

“The other alternative is vaccination, which is used by the armed forces.  However, the side effects are significant enough that it’s unlikely you would want to vaccinate the whole population,” Liddington said.

The Burnham group screened small molecules from the National Cancer Institute Diversity Set to identify chemical compounds that can block LF.

“This was step one, to make inhibitors that work in a cell-based assay,” Liddington said.  “In other words, the cells don’t die when exposed to LF bound with an inhibitor in a lab setting.”

Then the researchers began working on chemically generating even better inhibitors.  Part of the process involves shining SSRL’s x-rays on LF-inhibitor co-crystals to find their atomic-resolution structure.  They also collected data at the National Synchrotron Light Source in New York.

“The structure is key because it tells how and where the drug binds with atomic precision, which allows us to determine ways to alter the drug molecules to bind more strongly and more specifically to LF,” he said.

The final phase would be to take the effective anti-toxin, “stockpile it and hope it never needs to be used,” Liddington said.




The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Friday July 16, 2004 by Emily Ball