updated 1/30/2006 6:40:09 PM ET 2006-01-30T23:40:09

Initial tests indicate an experimental vaccine for ricin works and is safe, raising the possibility it might one day offer protection from a poison that authorities fear could be a weapon for terrorists.

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Deadly and easy to produce, ricin is extracted from castor beans. It can be added to food or water, injected or sprayed as an aerosol.

It has a long history of use in espionage. The FBI is investigating a pair of incidents involving the poison in 2003. In one, a threatening letter containing ricin was found at a South Carolina postal facility, and a second letter with ricin, addressed to the White House, turned up at a Washington mail processing facility.

Researchers led by Dr. Ellen Vitetta of the University of Texas Southwestern Medical Center in Dallas found a way to modify the toxic sections of the ricin molecule to disrupt its poisonous effect.

Their findings were reported Monday in the online edition of Proceedings of the National Academy of Sciences.

The new molecule, called RiVax, was given to mice and rabbits, which then developed antibodies and those antibodies protected the animals when they were later given doses of ricin.

After getting permission from the Food and Drug Administration to conduct a safety test on humans, the researchers gave doses of RiVax to 15 volunteers.

Five volunteers given a high dose of the vaccine all developed antibodies to ricin, four of the five given a moderate dose developed antibodies, as did one of the five given a low dose.

Mice injected with a mix of ricin and the human antibodies were protected from the poison, and other studies showed that mice given ricin orally were also protected.

“Our major concern in this trial was safety,” Vitetta said. “We have taken a very deadly toxin and genetically engineered it to be safe and to induce protective immunity in humans.”

“We have shown that this vaccine is safe and immunogenic,” she said. “Now we need to tinker with the dose and formulation to give the longest-lasting and most robust immunity.”

She declined to speculate how long it might be before a working vaccine might be available for human use.

Vitetta said DOR BioPharma has been licensed to produce RiVax and is working to develop enough for large-scale testing. Her next step is fine-tuning the vaccine to determine how long it is effective, how it can be improved and if it works against inhaled aerosol ricin.

Greg Evans, director of the Center for the Study of Bioterrorism and Emerging Infections at Saint Louis University, commented that “anything we do to increase our arsenal in terms of a potential bioterrorist agents is probably beneficial.”

But he was cautious about the usefulness of a vaccine that needs to be given in advance of exposure.

“I doubt we’re going to vaccinate large numbers of people because (ricin) might be used as a weapon,” said Evans, who was not part of Vitetta’s research group.

© 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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