Pharmaceutical Countermeasures to Chemical Warfare Agents

David H. Moore, D.V.M., Ph.D. (1)
Barbara B. Saunders-Price, Ph.D. (1)
(1) Battelle Memorial Institute

Introduction

With the increased threat of terrorism worldwide, the topic of the medical management of casualties from exposure to chemical warfare is timely. Of particular relevance are the pharmaceutical countermeasures available against chemical agents. To fully appreciate these countermeasures, a brief discussion of the agents and their mechanisms of actions is appropriate.

Chemical warfare agents have characteristics that make them uniquely suited to warfare. Toxic effects follow exposure to chemical agents (cont. p. 27 - Countermeasures) (Countermeasures - from p. 1)

dispersed as liquids, aerosols or vapor. Chemical warfare agents can be classified by their effects and also by the length of time they remain in the environment. Chemical warfare agents are either lethal in their effects or incapacitating, depending upon the class of agent, the concentration and the period of exposure. Persistent chemical agents can remain on environmental surfaces for an extended period of time; the vesicants such as sulfur mustard (HD) and Lewisite (L) and the nerve agents VX are persistent agents. Non-persistent agents are more volatile and do not remain in an open environment for more than a few hours. The lung damaging agents, such as, phosgene, cyanide, and the nerve agents, Tabun (GA), sarin (GB), soman (GD) and cyclosarin (GF) are considered non-persistent.

Table 1. Comparison of Potencies of Chemical Warfare Agents

CW Agent ECt50 LCt50

Nerve agents 3-5 10-200

Mustard 50-100 1500

Cyanide >1000 2500-5000

Phosgene >1000 3000

• Ct is Concentration of vapor (mg/m3) x time (minutes of exposure).

• ECt50 is the Ct producing clinical symptoms in 50% of the exposed population.

• LCt50 is the Ct that is lethal for 50% of the exposed population

The lethal agents: nerve, blood will be covered below. Excluded from this discussion are the pulmonary or choking agents, and other incapacitating vesicant agents and riot controls agents.

Nerve Agents and Pharmaceutical Countermeasures

Nerve agents exert their toxic effects by inhibition of the enzyme acetylcholinesterase (AChE), leading to accumulation of excess levels of the neurotransmitter acetylcholine (ACh) at cholinergic synapses. Enzyme inhibition is both rapid and irreversible, thus making organophosphorus (OP) nerve agents highly toxic and extremely dangerous chemicals (Table 1). The OP nerve agents bind with the AChE with a bond that can be reversed or can bind more tightly causing and aging of the OP-enzyme complex. Nerve agents gain entry by absorption through the lungs or skin and impair the activity of cholinergic synapses, including those of smooth and skeletal muscle, autonomic ganglia and the central nervous system (CNS). Acute toxic effects of nerve agents can be elicited at very low concentrations while lethal effects are observed at somewhat higher concentrations. Threshold symptoms for vapor exposure are commonly stated to be miosis, rhinorrhea and airway constriction. Lethal amounts of vapor or liquid cause a rapid cascade of events culminating within minutes in convulsion, loss of consciousness, apnea, paralysis and death. Seizure activity and a resulting CNS injury are common sequelae to a high level exposure to nerve agents.

Table 2. Nerve Agent Toxicity Comparison

LCt50 LD50

mg-min/m3 mg/70kg

Tabun (GA) 400 1,000

Sarin (GB) 100 1,700

Soman (GD) 70 50

GF 50 30

VX 10 10

Antidotes

Antidotal therapy includes the use of atropine to block the effects of excess ACh primarily at peripheral muscarinic receptor sites. Following atropine use, secretions are reduced and constriction of smooth muscle is reversed. Since it has little effect at nicotinic sites, skeletal muscle fasciculation will continue. Similarly, miosis will not be reversed.

Oximes are used to break the OP-enzyme bond and restore normal activity of the inhibited AChE. Pralidoxime chloride (2-PAM Cl) is the oxime of choice in the U.S. for reactivation of nerve agent inhibited AChE. Clinically, restoration of enzyme activity is noticeable in those organs with nicotinic receptors. Abnormal activity in skeletal muscle decreases and normal strength returns. Oximes of choice differ in various countries of the world. For example, P2S (pralidoxime mesylate) is used in the U.K. while obidoxime is preferred by most other European countries. In Israel, TMB4 is the choice enzyme reactivator. The effectiveness of an oxime can vary with the nerve agent in the OP-enzyme complex.

A number of other oximes are being used or being considered for use by various countries. The pyridinium oxime HI-6 is a good example. This drug, developed in Europe several decades ago, has been shown to be more effective as an antidote under various conditions. HI-6 is being considered for wide usage by Canada, the U.S. and a number of European countries. The compound’s solubility and stability have caused significant developmental challenges for a packaged antidote available for use in the field.

Nerve Agent Pre-treatment

A pre-treatment is a drug taken before exposure that when combined with post-exposure therapy decreases symptoms and increases survival.

Pyridostigmine Bromide (PB) pre-treatment is recommended for the specific indication of potential soman exposure. Following exposure to soman, the enzyme-agent complex undergoes a rapid conformational change that makes treatment a serious challenge. The rational for PB pre-treatment is based on the fact that traditional therapy (Mark I Kit) is relatively ineffective against soman. PB induces a transient carbamylation (reversible inhibition) of AChE and this inhibition protects the active site of the enzyme from nerve agent (irreversible) inhibition. Carbamylation of only a small amount (30%) of ChE is needed. However, PB pretreatment alone, without subsequent therapy provides no benefit.

Another nerve agent pre-treatment concept explored by scientists in Israel and the U.K. is the combination of a cholinolytic and a carbamate to be used in a transdermal application. While efficacy tests in humans have not been conducted, the combination of scopolamine and physostigmine (carbamate) in a transdermal patch has been demonstrated in humans and found to have minimal effects on behavior and performance

Although there are effective therapies for organophosphorus nerve agents, they often involve undesirable side effects associated with the cholinolytic drug. To address this issue and to add another concept for pre-treatment and post-exposure treatment, bioscavengers have been investigated. The theory of bioscavengers is that these are cholinesterases that will react with the nerve agent to remove it from reacting with the body’s AChE. Bioscavengers fall into two broad categories; stoichiometric and catalytic. New therapeutics and pre-treatments must offer advantages for their use and a new biological scavenger should provide protection against one or more nerve agents up to 5 times the median lethal dose without causing behavioral or physiological side effects. Studies with equine or human butyrylcholinesterase or fetal bovine serum acetylcholinesterase show that none of these scavengers elicit behavioral side effects when administered to rats or monkeys. These three scavengers as well as carboxylesterase are each capable of providing protection against 2 to 16 LD50s of GD, GB, or VX. Although numerous developmental challenges still remain before bioscavengers will be an acceptable therapeutic for nerve agent intoxication, the results to date offer impressive evidence for the value of this approach.

Nerve Agent Anticonvulsants

Anticonvulsant therapy is indicated with other therapy at the onset of severe effects from a nerve agent whether convulsions are present or not. Diazepam anticonvulsant therapy reduces brain damage caused by prolonged seizure activity seen in severe poisoning from nerve agents.

Anticonvulsants that are more effective against the characteristic nerve-agent induced seizure activity are under investigation. In the U.S. these include: midazolam and trihexyphenidyl and biperiden.

Blood Agents and Pharmaceutical Countermeasures

Chemical warfare blood agents are hydrogen cyanide (hydrocyanic acid, AC) and cyanogen chloride (CK). Cyanide is a rapidly acting lethal agent causing death in 6-8 minutes after inhalation of a high concentration (table 1). However, few toxic effects are seen below a lethal concentration. Once absorbed, the cyanide ion rapidly combines with the active site of the enzyme cytochrome oxidase interfering with aerobic metabolism, creating excess lactic acid and metabolic acidosis. Cell death is the final outcome. The organs most susceptible to cyanide are the CNS and the heart. The onset and progression of signs and symptoms are slower after ingestion of cyanide or after inhalation of a low concentration of vapor.

Antidotes

Detoxification of cyanide is preceded by its removal from the cytochrome oxidase complex by intravenous injection of sodium nitrite. This treatment forms methemoglobin to which cyanide preferentially binds. Intravenous injection of sodium thiosulfate, follows. This sulfate combines with cyanide to produce thiocyanate, which is excreted by the kidneys. The combination of sodium nitrite and sodium thiosulfate is the best therapy against cyanide and hydrocyanic acid poisoning. These are injected intravenouslyand sequentially, the nitrite followed by the thiosulfate. The combination is capable of detoxifying approximately 20 lethal doses of sodium cyanide in dogs and is effective even after respiration has stopped. Supportive care consists of providing oxygen and correcting any metabolic acidosis. Full recovery is relatively rapid following cyanide intoxication if the antidotes are given before cessation of cardiac activity.

Other pharmaceuticals that have shown efficacy in treating cyanide poisoning include other methemoglobin formers such as 4-Dimethylaminophenol (4-DMAP). The compound forms methemoglobin more rapidly than do nitrites, but often the methemoglobinemia reaches toxic levels. The drug is for intravenous use and is approved and used in Germany. Other methemoglobin formers that have been investigated include; aminoproplophenone (PAPP), para-aminooctanoylphenone (PAOP), hydroxylamine and primaquine analogs such as the 8-aminoquinolines.

Cobalt compounds also have efficacy as cyanide antidotes. Dicobalt edetate (Co2 EDTA, Kelocyanor) chelates cyanide, but has been associated with serious side effects such as angina pectoris, ventricular dysrhythmias, periorbital and laryngeal edema and convulsions. Dicobalt-EDTA can cause severe hypotension, particularly in patients without cyanide poisoning. The drug is used in the U.K., France, and the Netherlands.

A cyanide antidote compound approved in France is the vitamin B12 precursor hydroxocobalamin (vitamin B12a). This drug reacts stoichiometrically with cyanide to form cyanocobalamin (vitamin B12). Based on data from France, hydroxocobalamin appears to be safe and effective. Confirmatory studies on animal efficacy and healthy volunteer safety are underway in the US. Hydroxocobalamin does not affect hemoglobin or otherwise reduce the ability of the blood to carry oxygen. Hydroxocobalamin does not cause hemodynamic instability but instead appears to improve hemodynamic parameters in victims. Toxic effects are not typically observed with antidotal use.

Because the Taylor Cyanide Antidote Kit and several of the other antidotes is poorly suited for the time-critical intervention required in acute cyanide poisoning, hydroxocobalamin, is being investigated for possible introduction in the US. While hydroxocobalamin appears to be effective in the treatment of cyanide poisoning, it also appears to have a safety profile that makes it appropriate for use in the pre-hospital setting, particularly for smoke-inhalation victims.

Conclusion

Prompt recognition of the signs and the appropriate intervention for acute chemical agent poisoning are necessary for saving lives following intoxication. Because both nerve agent and cyanide poisoning can culminate in death so quickly, presumptive diagnosis and empiric treatment are required. Acute management primarily entails supportive care and administration of an antidote.

The nerve agent antidotes and therapies and some cyanide antidotes are appropriate for empiric pre-hospital treatment of poisoning, however in most areas of the world, adequate supplies of these life saving antidotes and therapies are not available in sufficient quantities in the locations where they will be required.

Work continues to educate emergency planners of the requirements for and availability of pharmaceutical countermeasures to chemical agents.

Pharmaceutical Countermeasures to Chemical Warfare Agents

Introduction

Conclusion

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