Although found in common household insecticides, chemicals called organophosphates can be just as harmful to people as to insects. That fact did not escape the notice of German scientists during World War II, who used organophosphates to make a collection of deadly liquids and gases that can cause lethal damage to the nervous system. These nerve agents still exist, and pose a potential threat to people around the world.
The good news is that the U.S. is working with other governments to destroy nerve agent stockpiles. Meanwhile, scientists are devising drugs to treat and prevent the toxic effects of organophosphates and related chemicals.
Biochemist Dan S. Tawfik, Ph.D., and his colleagues at the Weizmann Institute of Science in Rehovot, Israel are working to turn a natural enzyme into a drug capable of breaking down organophosphates. His research is supported by the NIH Countermeasures Against Chemical Threats (CounterACT) program, which aims to enhance medical responsiveness to chemical disasters.
'Drugs based on Dr. Tawfik's approach would be a valuable addition to our nation’s ability to mount an effective medical response in the event of a chemical emergency,' said David A. Jett, Ph.D., the lead contact for CounterACT and a program director at the National Institute of Neurological Disorders and Stroke (NINDS).
German, or 'G-type,' nerve agents were never deployed during World War II, but they have been used for malicious ends. In 1995, a terrorist group in Tokyo released the G-type agent sarin into the subway system, killing 12 people and causing thousands to seek medical attention.
Although not as lethal as nerve agents, organophosphate insecticides can be harmful in large or prolonged doses. Moreover, they could be released on an industrial scale, through an act of terror or accident.
Organophosphates attack the nervous system by inactivating an enzyme called acetylcholinesterase (AChE). AChE controls levels of the chemical acetylcholine (ACh), which regulates brain function, muscle contraction, heartbeat and breathing. When AChE is inactivated, ACh bombards nerve cells and muscles. At lower doses, exposure to organophosphates can therefore cause difficulty concentrating, muscle spasms and asthma-like symptoms. At higher doses – such as might occur in a terrorist attack or large-scale accident – they cause seizures, paralysis, respiratory failure and death.
There are limited drug treatments for nerve agent exposure. Diazepam is the only drug currently approved for nerve agent-induced seizures. Two other treatments – atropine and compounds called oximes – are meant to work as antidotes. Atropine prevents nerve and muscle cells from responding to ACh, and oximes reduce ACh levels by re-activating AChE. Unfortunately, these two drugs may not prevent lasting damage to the nervous system, Dr. Tawfik said.
'Atropine aims at the symptoms of nerve agent exposure, and oximes minimize the damage after it has already begun,' he said. 'Our goal is to develop a drug that can intercept organophosphates before they cause damage.'
In Nature Chemical Biology,* Dr. Tawfik and his team report progress toward a drug based on an enzyme called serum paraoxonase (PON1). The normal business of PON1 is to break down compounds derived from fats, but as biochemists like to say, it is promiscuous. It occasionally hooks up with and destroys organophosphates. The researchers theorized that they could amplify this behavior to turn PON1 into an organophosphate-busting machine.
Their efforts began nearly a decade ago. They purified PON1, solved its 3-D structure, and introduced dozens of mutations into it – some random and others directed at key sites within the enzyme. They tested many versions of this recombinant PON1 (rePON1) for the ability to neutralize organophosphates. They repeated the mutation process and the testing again and again.
The researchers have now succeeded in making rePON1 highly reactive against cyclosarin, a more toxic cousin of sarin. In test tube experiments, they show that rePON1 can break down cyclosarin with an efficiency 100,000 times higher than that of natural PON1.
The researchers also tested the ability of rePON1 to protect mice against exposure to a cyclosarin analogue. When injected with the enzyme one hour before exposure, 75 percent of the mice were alive at 24 hours post-exposure and more than 60 percent were alive at two weeks. In contrast, when the mice were given atropine/oxime five minutes before the exposure, just 22 percent were alive at 24 hours and none were alive at two weeks.
One drawback of rePON1 is that it has a short time window of effectiveness. If cyclosarin exposure was delayed until 24 hours after the mice received the enzyme, none of the mice lived. By that point, the enzyme had been almost completely cleared from their blood. To protect people against a nerve agent attack or accident, the enzyme needs to be hardier.
'We would like to see the enzyme sustained in the blood for several days and ideally two weeks,' he said. His team is collaborating with others to tinker further with the protein's chemistry and increase its longevity. There is also hope of using rePON1 to treat people after they are exposed to cyclosarin and related compounds, he said.
Finally, Dr. Tawfik and his team are working to develop a drug that can protect against V-type agents, a class of organophosphate-based nerve toxins developed during the cold war era. (According to the U.S. Army's Chemical Materials Agency, the 'V' stands for venomous.) VX is the most potent of all the organophosphate-based nerve agents. Less than one drop can be lethal.
- By Daniel Stimson, Ph.D.
*Gupta RD et al. “Directed evolution of hydrolases for prevention of G-type nerve agent intoxication.” Nature Chemical Biology, February 2011, Vol. 7, pp. 65-125.
Date Last Modified: Friday, July 01, 2011
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