Thursday, September 1, 2005

Vaccine for ricin toxin developed at Detrick lab


Courtesy photo

Ricin is a toxin made from the beans of the castor plant, left. Since 1989, toxin experts at the U.S. Army Medical Research Institute of Infectious Diseases have worked on finding a vaccine to combat ricin exposure.

Jack--of beanstalk fame--can attest to the fact that a few little beans can cause a whole bunch of problems.

Ricin, a toxin made from castor beans, makes Jack's problems look trivial and has no fairytale ending.

"Inhaling the toxin causes severe breathing problems as the lungs fill with fluids because the toxin attacks cells in the lung," said Dr. Leonard Smith of the Division of Integrated Toxicology at the U.S. Army Medical Research Institute of Infectious Diseases.

Ingesting ricin causes vomiting and diarrhea that may become bloody and result in dehydration, according to the Centers for Disease Control Web site. Hallucinations, seizures, blood in the urine--Jack never had to deal with those, either.

Since 1989, Smith and other toxin experts at the institute have worked on finding a vaccine to combat ricin exposure, whether it comes through the air or deliberate contamination of the food or water supply. No antidote exists for people who have been exposed.

"It's a heck of a lot easier to protect someone with a vaccine before a ricin exposure rather than to treat them with a drug afterward," Smith said. "Once ricin gets in the cells and has done the damage, it's going to be very, very difficult--if not impossible--to treat someone who has been exposed to a large dose. The damage has been done by the time one knows it. When somebody starts to have symptoms ... it may be impossible to save those people with any kind of therapy."

Ricin has had its fair share of the media spotlight in recent years. Press reports said the toxin turned up in an envelope in the mailroom that serves Senator Bill Frist's office and a postal handling facility in Greenville, S.C. It was also at the center of a plot in London where suspected al-Qaeda members were trying to make it.

Listed as a category B bioterrorism agent by the Centers for Disease Control, ricin is a threat to both servicemembers and the public.

"It can be obtained quite readily as a by-product of the castor beans," said Smith, who has worked for USAMRIID for 24 years. "After you extract what you need from the beans, like castor oil, there's quite a bit of ricin left behind. We have no medical solutions to defend against ricin intoxication, and so we are vulnerable."

According the CDC, ricin is also a stable substance that's not affected much by very hot or cold temperatures.

Because of ricin's sinister traits, researchers at USAMRIID have been heartened by recent results they've had with their latest attempt at a vaccine candidate. Work on a ricin vaccine began in 1989, and the quality attributes of two vaccine candidates the institute developed early on didn't meet Food and Drug Administration expectations. The third, a recombinant vaccine, capitalized on the lessons learned from the earlier attempts.

Ricin is composed of two protein subunits, the A chain and the B chain. When the B chain binds the toxin to a cell's surface, it permits the A chain to enter the cell. Once it's inside, the A chain stops new protein synthesis and causes cell death.

In earlier attempts to develop a ricin vaccine, researchers thought that isolating the entire ricin A chain could produce immunity. But they found that the chain wasn't stable, a key element for getting a vaccine approved for use.

By using molecular modeling and protein engineering, researchers--including Drs. Mark Olson, John Carra, Virginia Roxas-Duncan, Robert Wannemacher, Smith and Charles Millard--designed the new vaccine. The team started with a computer-aided analysis of the toxin structure, using a three-dimensional model provided by colleagues at the University of Texas, Austin.

"We compared ricin with other proteins of the same family," Olson said. "We tried to figure out where the protein molecules are diverging within the family--to see what changes were made by nature so we could make the changes we needed to make."

To improve the vaccine's stability, Olson and his team modeled changes in the structure of the ricin A chain molecule. Once they predicted which genetic sequences required alterations, they handed them off to Smith and others at USAMRIID for protein engineering.

"We went straight from the computer to molecular biology," Smith said. "We had to clone and purify the proteins, and test them in animals for toxicity and protection."

Four years later, the vaccine called RTA 1-33/44-198 is one the FDA should be pleased with, Smith said.

"Unlike earlier versions, this recombinant vaccine has no biological activity except for the immunity it elicits, which inactivates the toxin. It's produced and purified from E. coli and is highly stable and safe," he said.

Researchers in July tested the vaccine on eight monkeys that received three shots of the vaccine over an eight-week period, then challenged them with an aerosol version of ricin. Final results of the study will be published in the scientific literature later this year, but in the meantime, Smith said he's pleased with the results.

"The bottom line is the vaccine works," he said. .

Getting the vaccine into a clinical trial is the next hurdle, Smith said. Currently the USAMRIID vaccine is being considered for funding along with two other vaccines, said Andrea Atkinson, vaccine manager with the Joint Vaccine Acquisition Program. The organization manages biological defense vaccines through advanced development and FDA licensure.

"We are looking at schedules, who can be licensed fastest and which one meets our requirements," Atkinson said, adding that the Joint Vaccine Acquisition Program has not yet picked the finalist for funding.

Once a funding stream opens up for a vaccine like ricin, many pharmaceutical companies suddenly want to put their canoe in the water. That's good news, Atkinson said.

"That's fantastic for the Soldier because you know there's always going to be something available. There's always a next-generation candidate that is out there," she said. "It's also risk reduction from our perspective. If we were to experience a failure with a candidate, then there's something else coming down the pipeline to mitigate that risk."

Meanwhile the USAMRIID team is developing an animal model that can be used under the FDA's animal rule to show the vaccine protects its recipients.

"You can't challenge humans so it was necessary to develop a surrogate model to show the human is protected by the vaccination, especially from these products that aren't normally found in the environment," Smith said.

While funding decisions are being deliberated, Smith said he and his team remain busy in their Biosafety Level 2 lab looking at other funding opportunities for clinical trials and laying the foundation for them. The Defense Threat Reduction Agency has approved funding for the continued tech base development of the vaccine for fiscal year 2006.

"We'll keep going. There's no question about that. My job is to ... try to partner with whoever we can to get resources to have a lot of vaccine made ... and get that clinical trial going," he said.