Enzymes interdict nerve agents in 'bioscavenger' program

By Karen Fleming-Michael

July 20, 2005

Plasma, goats and plants may one day hold the key to protecting warfighters and the public from nerve agents.

Boosting the amounts of an enzyme called butyrylcholinesterase, normally present in small quantities in blood plasma as detoxifiers, can interdict nerve agents when they enter the bloodstream so the nerve agents can't reach their targets.

Knowing this, researchers for 20 years have been finding ways of producing large amounts of the enzyme they call a "bioscavenger."

"The bioscavenger is being tested against all known nerve agents," said Col. Michelle Ross, deputy commander of the Army Medical Research Institute of Chemical Defense in Aberdeen Proving Ground, Md.

"The objective is to develop a pretreatment that is broad spectrum and will work against all known nerve agents," she said.

Aberdeen researchers, working jointly with the Walter Reed Army Institute of Research in Silver Spring, Md., have come up with three versions of the bioscavenger. The most mature version may transition for funding under Project BioShield, the president's 2003 initiative that encourages companies to develop bioterrorism countermeasures.

Ross said the bioscavenger approach is revolutionary because it works by preventing and destroying the nerve agent entering the body before it can reach its physiological target.

"If you have people who are going into harm's way - whether it's the warfighter or the hospital worker who has incoming casualties or the first responder going into a hot zone (like the Tokyo subway after the 1995 liquid sarin attack) if the bioscavenger is in circulation, they're protected against the toxicity of nerve agents. If there's no vulnerability, there's no threat," Ross said.

Ross stressed the enzyme also lets the warfighter keep fighting.

"(Current) nerve agent antidotes all enhance survival and, in the best cases, reverse the toxicity of exposure, but they cause a performance decrement, and the recipient becomes a casualty (who needs to be) evacuated to a military treatment facility," she said. "In an operational context, what the combatant commander wants is a warfighter to continue the mission, not be a casualty, not be a logistical burden to the health care system but keep on trucking - the bioscavenger addresses that concern."

Having a bioscavenger circulating in the bloodstream may also allow first responders go into an area contaminated with a nerve agent-perhaps without wearing protective suits. Though today's chemical protective gear is certainly effective, asking a Soldier to wear it while doing a medic’s job in the heat has its challenges, said Staff Sgt. Edward Allen of the U.S. Army Medical Research Institute of Chemical Defense.

"As with anything else in MOPP (mission oriented protective posture) gear, the more you practice with it on the better you get," he said. "I would describe it like doing needlepoint in a snowsuit, dishwashing gloves and wearing a motorcycle helmet."

Use of the bioscavenger is similar in concept to the use of gamma globulin shots that travelers have taken for more than 50 years to boost their immunity.

"It's a passive protection," said Dr. David Lenz of the institute at Aberdeen Proving Ground. "You will be protected as soon as you get the shot and achieve adequate plasma levels if you're subsequently exposed to... nerve agents."

Researchers hope to get Food and Drug Administration approval for the plasma-derived bioscavenger version first. Made from outdated human plasma, the enzyme, butyrylcholinestrase, is extracted and purified by a process perfected by researchers at the Walter Reed Army Institute of Research.

Baxter Healthcare Corporation received a contract April 6 from Dynport Vaccine Corporation LLC to produce batches of the plasma-derived bioscavenger to undergo a preliminary human clinical safety trial. If the trials are successful, the FDA may grant investigational new drug status to the bioscavenger, allowing the Department of Health and Human Services to move the product toward full FDA licensure for BioShield.

Although several thousand tons of outdated human plasma are available for preparing the enzyme, a liter of human plasma contains just a couple milligrams of the enzyme, so there's not enough plasma to meet demand.

Bioscavenger's second generation form, however, uses recombinant technology to create the enzyme. Nexia, a Canadian company recently purchased by PharmAthene Inc., created genetically altered nanny goats that produce the enzyme in their milk. Their offspring also inherit that ability. A liter of the goats' milk may contain as much as 1 to 3 grams of the enzyme.

"This potentially gives us an unlimited source of the enzyme," Ross said "The objective is to have enough enzyme available for not only the Department of Defense to support four million warfighters, but now ... there's a potential to have a requirement to have 38 million doses for the population of the United States, hence the need to go with a different developmental strategy."

Lenz said as with adaptation of any new technology, one always proceeds with cautious optimism.

"It is indeed a human protein bioscavenger that's produced in the milk, but there are subtle differences in the form it takes versus the purified form that comes from human plasma, he said.

Because it comes from a goat and not a human, the enzyme may be a little different in terms of its structure, said Ashima Saxena of the Walter Reed Army Institute of Research. "The question is whether the material works differently because of these slight differences in chemical structure."

"The plasma-based protein is made in people, so it's expected to be compatible with people," she said. "Goats are different. The milk based protein because it's made in goats may cause a potential reaction."

While researchers are determining if the goat-derived bioscavenger is as effective as the human-derived form, they're also exploring a third approach to harvesting bioscavenger. They're interested in a catalytic form of the bioscavenger whose molecules bind not just one on one with nerve agents as the current bioscavengers do, but one that would speed up the breakdown of the nerve agent in the bloodstream and is able to do this again and again.

"When you have the situation where you have one-to-one binding, a large amount of the enzyme is needed for a small amount of nerve agent," Lenz said. "If you can get something that can continuously destroy nerve agents for as long as it's in circulation, you can use less of it and improve its ability to protect."

Researchers have several proteins that they think hold promise, including a mutant form of the bioscavenger whose amino acid sequence is altered so it catalyzes the breakdown of the nerve agent. They're also looking at a naturally occurring human enzyme called PON, for paraoxonase, which catalyzes the nerve agents sarin, soman and VX.

"You're better off going with Mother Nature," said Dr. Bhupendra Doctor of the Walter Reed Army Institute of Research. "Enzymes that scavenge or hydrolyze organophosphates are all 'universal' antidotes, but when you go the mutation route, you have to add five to 10 years to the project because technologically it becomes more difficult. I think we will find a catalytic scavenger; we just haven't looked hard enough."

Ross is pleased with how far research on bioscavengers has come.

"If you look at average years in pharmaceutical and drug companies, we're right in the developmental window of time; 10 to 15 years is the average," she said. "With program costs totaling $20 to $30 million over 20 years, the cost is much less than big (pharmaceutical companies)."