Ambrogio Sara, Bertozzi Giulia
Nerve agents are a class of phosphorus-containing organic chemicals ( organophosphates ) that disrupt the mechanism by which nerves transfer messages to organs. The disruption is caused by blocking acetylcholinesterase , an enzyme that normally relaxes the activity of acetylcholine , a neurotransmitter.
OPs comprise a large and diverse family of compounds, many of which can be hydrolyzed in the presence of water or by specific enzymes, resulting in detoxification.
Poisoning by a nerve agent leads to contraction of pupils, profuse salivation, convulsions, involuntary urination and defecation, and eventual death by asphyxiation as control is lost over respiratory muscles. Some nerve agents are readily vaporized or aerosolized and the primary portal of entry into the body is the respiratory system. Nerve agents can also be absorbed through the skin, requiring that those likely to be subjected to such agents wear a full body suit in addition to a respirator.
Organophosphates (OPs) are chemical substances originally produced by the reaction of alcohols and phosphoric acid. In the 1930s, organophosphates were used as insecticides , but the German military developed these substances as neurotoxins in World War II. They function as cholinesterase inhibitors, thereby affecting neuromuscular transmission.
Organophosphate insecticides, such as diazinon, chlorpyrifos, disulfoton, azinphos-methyl, and fonofos, have been used widely in agriculture and in household applications as pesticides. Over 25,000 brands of pesticides are available in the United States, and their use is monitored by the Environmental Protection Agency (EPA).
Toxic nerve agents used by the military are often of the organophosphate group; an example is sarin , the nerve gas used in a terrorist action in Tokyo in 1995. During the Gulf War, the US military was given OPs agents which some believe caused some of the symptoms of Gulf War syndrome.
Controversy exists regarding the long-term effects of exposure to low levels of potentially neurotoxic substances.
Major nerve agents
As their name suggests, nerve agents attack the nervous system of the human body. All such agents function the same way: by inhibiting the enzyme acetylcholinesterase (AChE) which is responsible for the breakdown of acetylcholine (ACh) in the synapse. ACh gives the signal for muscles to contract, preventing them from relaxing.
There are two types of cholinesterase:
- (AChE), also known as RBC cholinesterase, erythrocyte cholinesterase, or (most formally) acetylcholine acetylhydrolase, found primarily in the blood and neural synapses.
- (BChE or BuChE), also known as plasma cholinesterase, butyrylcholinesterase, or (most formally) acylcholine acylhydrolase, found primarily in the liver.
The difference between the two types of cholinesterase has to do with their respective preferences for substrates: the former hydrolyses acetylcholine more quickly; the latter hydrolyses butyrylcholine more quickly.
Pseudocholinesterase levels may be reduced in patients with advanced liver disease.
Initial symptoms following exposure to nerve agents (like sarin) are a runny nose, tightness in the chest, and constriction of the pupils. Soon after, the victim will then have difficulty breathing and will experience nausea and drooling. As the victim continues to lose control of his or her bodily functions, he or she will involuntarily salivate, lacrimate, urinate, defecate, and experience gastrointestinal pain and vomiting. Blisters and burning of the eyes and/or lungs may also occur. This phase is followed by initially myoclonic jerks followed by status epilepticus. Death then comes via complete respiratory depression, most likely via the excessive peripheral activity at the neuromuscular junction of the diaphragm.
The effects of nerve agents are very long lasting and increase with successive exposures. Survivors of nerve agent poisoning almost invariably suffer chronic neurological damage. This neurological damage can also lead to continuing psychiatric effects.
Mechanism of action
When a normally functioning motor nerve is stimulated it releases the neurotransmitter acetylcholine, which transmits the impulse to a muscle or organ. Once the impulse is sent, the enzyme acetylcholinesterase immediately breaks down the acetylcholine in order to allow the muscle or organ to relax.
Nerve agents disrupt the nervous system by inhibiting the function of acetylcholinesterase by forming a covalent bond with the site of the enzyme where acetylcholine normally undergoes hydrolysis (breaks down). The result is that acetylcholine builds up and continues to act so that any nerve impulses are continually transmitted and muscle contractions do not stop.
This same action also occurs at the gland and organ levels , resulting in uncontrolled drooling, tearing of the eyes (lacrimation) and excess production of mucus from the nose (rhinorrhea).
Inhibition of serine hydrolases by OPs is mainly due to the formation of a covalent bond between the phosphor-alkoxy moiety of the OP compound and the hydroxyl residue of the catalytic serine of the active site of these enzymes, with the resulting release of the OP “leaving group”. The OP adduct can suffer further dealkylation through a process called “aging”, probably assisted by residues near the active site of the enzyme. Although the initial covalent bond is usually reversible, the aging enhances the stability of the OP adduct bound to the enzyme, preventing the dissociation of the enzyme-inhibitor complex. The OP-adducted protein will remain in circulation for a length of time dependent on the half-life of the enzyme, generally much longer than the resident time of the OP or its metabolites.
In vertebrates, butyrylcholinesterase (BChE) can also hydrolyze acetylcholine. BChE is also a tetrameric serine esterase present almost ubiquitously and in plasma. BChE is also inhibited by OP compounds, although this inhibition has no cholinergic symptoms and no known biological effect. The physiological function of BChE is still not clear, although some studies suggest that it could act as an AChE backup activity. This certainly appears to be the case in the AChE knockout mouse generated. Furthermore, BChE, as a stoichiometric scavenger, may protect from synthetic and naturally occurring poisons by preventing AChE inhibition by these poisons.
Biomonitoring OP exposures
Biomonitoring refers to the assessment of human exposure to chemicals and health risk by measuring the chemicals or their metabolites in body fluids, such as blood or urine. Early detection of exposures can enable interventions before severe symptoms occur and follow-up of the recovery of the intoxicated subject.
The term biomarker refers to biological substances that are indicators of exposure or disease, and that can be measured by laboratory techniques. There is an increasing interest in identifying and characterizing new biomarkers for a better health assessment.
Blood cholinesterases have long served as biomarkers of OP exposure, since they were the first OP targets discovered. It has been shown that inhibition of RBC AChE can be used as a valuable surrogate for inhibition of neuronal AChE, although plasma BChE is more sensitive to some OPs than RBC AChE. Furthermore, both cholinesterases are found in blood, an ideal matrix for biomarker analysis.
PubMed: Proteomic analysis of adducted butyrylcholinesterase for biomonitoring organophosphorus exposures, 2012.
Countermeasures in Development
Butyrylcholinesterase is a prophylactic countermeasure against organo-phosphorus nerve agents. It acts as a scavenger by binding nerve agent in the blood stream before it can exert effects in the nervous system. Because it is a biological scavenger (and universal target) it is currently the only therapeutic agent effective in providing complete stoichiometric protection against the entire spectrum of organo-phosphorus nerve agents.
Therapeutic uses of organophosphates
Several organophosphate agents are being tried therapeutically. Cholinesterase inhibition, which in large doses makes these agents effective pesticides, also may be useful in other doses for treating dementia. Metrifonate has been used to treat schistosomiasis and is undergoing trials for the treatment of primary degenerative dementia.
The organophosphates pyridostigmine and physostigmine are carbamate anticholinesterases that have been used for many years for the treatment of myasthenia gravis . Although the short-duration anticholinesterases are generally safe, reports of their abuse are associated with a picture similar to pesticide intoxication.
In 1986, testing began for tacrine, the first cholinesterase inhibitor to be tried for Alzheimer disease ; it was released for clinical use in 1993. It is no longer in use. The blood-brain barrier has been the limiting factor in developing a cholinesterase inhibitor for use in dementia. Drugs such as rivastigmine are now widely used.
Infact, the severity of complications depends on dose and mechanism of action for individual Alzheimer's desease drugs.
- effects: Patients with elevated ALT (alanine transaminase) levels were generally asymptomatic, although sometimes they experienced eosinophilia, rash, and fever. Few patients developed signs of severe hepatocellular injury (e.g., jaundice); no death attributable to liver toxicity was reported
- adverse events and loss of body weight: Nausea, vomiting and diarrhea are thought to reflect excessive activation of intestinal muscarinic cholinergic receptors and tend to be dose related. Anorexia and loss of body weight are associated gastrointestinal adverse events
- adverse event: Bradycardia and subsequent dizziness or syncope originates from central and peripheral muscarinic cholinergic stimulation. Cardiovascular adverse events can lead to falls and other types of injury-causing accidents
PubMed: Drug Class Review on Alzheimer's Drugs, 2006.
PubMed: Synthesis of organophosphates with fluorine-containing leaving groups as serine esterase inhibitors with potential for Alzheimer disease therapeutics, 2009.