Comparative evaluation of benzodiazepines for control of soman-induced seizures

Chemical warfare agents

Chemical warfare agents (CWA's) are defined as any chemical substance whose toxic properties are utilised to kill, injure or incapacitate an enemy in warfare and associated military operations. Chemical agents have been used in war since times immemorial, but their use reached a peak during World War I. During World War II only the Germans used them in the infamous gas chambers. Since then these have been intermittently used both in war and acts of terrorisms. Many countries have stockpiles of these agents. There has been a legislative effort worldwide to ban the use of CWA's under the chemical weapons convention which came into force in 1997. However the manufacture of these agents cannot be completely prohibited as some of them have potential industrial uses. Moreover despite the remedial measures taken so far and worldwide condemnation, the ease of manufacturing these agents and effectiveness during combat or small scale terrorist operations still make them a powerful weapon to reckon with. These agents are classified according to mechanism of toxicity in humans into blister agents, nerve agents, asphyxiants, choking agents and incapacitating/behavior altering agents. Some of these agents can be as devastating as a nuclear bomb. In addition to immediate injuries caused by chemical agents, some of them are associated with long term morbidities and psychological problems. In this review we will discuss briefly about the historical background, properties, manufacture techniques and industrial uses, mechanism of toxicity, clinical features of exposure and pharmacological management of casualties caused by chemical agents.

Disasters and mass casualties: II. explosive, biologic, chemical, and nuclear agents

Terrorists' use of explosive, biologic, chemical, and nuclear agents constitutes the potential for catastrophic events. Understanding the unique aspects of these agents can help in preparing for such disasters with the intent of mitigating injury and loss of life. Explosive agents continue to be the most common weapons of terrorists and the most prevalent cause of injuries and fatalities. Knowledge of blast pathomechanics and patterns of injury allows for improved diagnostic and treatment strategies. A practical understanding of potential biologic, chemical, and nuclear agents, their attendant clinical symptoms, and recommended management strategies is an important prerequisite for optimal preparation and response to these less frequently used agents of mass casualty. Orthopaedic surgeons should be aware of the principles of management of catastrophic events. Stress is less an issue when one is adequately prepared. Decontamination is essential both to manage victims and prevent further spread of toxic agents to first responders and medical personnel. It is important to assess the risk of potential threats, thereby allowing disaster planning and preparation to be proportional and aligned with the actual casualty event.

Chemical Terrorism Attacks: Update on Antidotes

There is well-founded concern that a chemical or radioactive agent will at some point be used as a weapon of terror. There are several antidotes that, if used correctly in a timely fashion, can help lessen the harm caused by these agents. This article is meant to introduce the clinician to several such agents, along with the antidotes useful in the management of exposure to these. It covers the indications, administration, and precautions for using these antidotes.

Efficacy of the bone injection gun in the treatment of organophosphate poisoning

Immediate administration of antidotal treatment is crucial in severe organophosphate (OP) poisoning and the use of an open intravenous (i.v.) line might also be required. The state of casualties might prevent getting access to their veins. The bone injection gun (BIG) was established as a simple method for introducing an intraosseous (i.o.) line and could be applied while wearing a protective suit. The present study followed the pharmacokinetics of the anticonvulsive drug midazolam after i.o. administration in pigs compared with i.v. and the common intramuscular (i.m.) administration. A new method for monitoring midazolam concentrations in plasma was developed. Plasma concentrations following both i.v. and i.o. administrations peaked at 2 min post injection and only at 10 min following the i.m. route. In an antidotal treatment study against paraoxone poisoning, the anticonvulsive effect of midazolam appeared immediately following i.o. administration, while it took 5-10 min to exhibit a similar effect following i.m. administration. This study indicates that the use of i.o. administration after OP poisoning might provide the necessary fast response for rapid termination of convulsions. The BIG might offer a convenient method for treating casualties in the chemical arena by teams wearing full protective gear.

Nerve Agents

BACKGROUND

Nerve agents, the deadliest of the classic chemical warfare agents, primarily function as acetylcholinesterase inhibitors and cause a rapidly progressive cholinergic crisis. Originally developed for battlefield use, they have been used in terrorist attacks and are considered threats to the civilian population.

REVIEW SUMMARY

The pathophysiology and clinical presentation of acute nerve agent poisoning are summarized and acute treatment protocols reviewed. Timely support and antidotal treatment are crucial and may be lifesaving. Pyridostigmine bromide, recently approved by the Food and Drug Administration as a pretreatment for soman poisoning, forms part of battlefield doctrine but is unlikely to be used in the civilian sector. Aside from that, civilian recommendations for acute therapy derive, with only minor modifications, from military doctrine.

CONCLUSION

Neurologists should familiarize themselves with the pathophysiology and treatment principles for the syndromes caused by nerve agents, not only to assist with the hospital care of these patients but also to serve as resources to their local medical communities in preparation for chemical terrorism. Because nerve agents injure the nervous system, nonneurologists have a right to expect neurologists to have mastered these principles.

Development of a rat pilocarpine model of seizure/status epilepticus that mimics chemical warfare nerve agent exposure

We developed a rat pilocarpine seizure/status epilepticus (SE) model, which closely resembles 1.6-2.0 x LD50 soman exposure, to analyse the molecular mechanism of neuronal damage and to screen effective neuroprotectants against cholinergic agonist and chemical warfare nerve agent (CWNA) exposure. Rats implanted with radiotelemetry probes capable of recording electroencephalogram (EEG), electrocardiogram (ECG), temperature, and physical activity were treated with lithium chloride (5 mEq/kg, im), followed 24 h later by (ip) doses of pilocarpine hydrochloride. Based on radiotelemetry analysis, a dose of 240 mg/kg (ip) pilocarpine generated seizure/SE analogous to 1.6-2.0 x LD50 of soman. The model was refined by reducing the peripheral convulsions without affecting the central nervous system (CNS) by administering methylscopolamine bromide (1 mg/kg, ip), an anti-cholinergic that does not cross the blood-brain barrier. However, when methylscopolamine bromide was administered, a higher dose of pilocarpine (320 mg/kg, ip) was required to generate the equivalent seizure/SE. Histopathology data indicated that pilocarpine induces significant damage to the hippocampal region of the brain, with similar neuropathology to that of 1.6-2.0 x LD50 soman exposure. There was a reduction in body temperature after the administration of pilocarpine, as observed in organophosphate (OP) nerve agents exposure. The heart-rate of pilocarpine-treated animals increased compared to the normal range. The pilocarpine seizure/SE model was also reproducible in the absence of lithium chloride. These results support that pilocarpine seizure/SE model is useful in studying the molecular mechanisms of neuropathology and screening neuroprotectants following cholinergic agonist and CWNA exposure.

Anticonvulsants for nerve agent-induced seizures: The influence of the therapeutic dose of atropine

Two guinea pig models were used to study the anticonvulsant potency of diazepam, midazolam, and scopolamine against seizures induced by the nerve agents tabun, sarin, soman, cyclosarin, O-ethyl S-(2-(diisopropylamino)ethyl)methylphosphonothioate (VX), and O-isobutyl S-(2-diethylamino)ethyl)-methyl phosphonothioate (VR). Animals instrumented for electroencephalogram recording were pretreated with pyridostigmine bromide (0.026 mg/kg i.m.) 30 min before challenge with 2 x LD50 (s.c.) of a nerve agent. In model A, atropine sulfate (2.0 mg/kg i.m.) and pyridine-2-aldoxime methylchloride (2-PAM; 25.0 mg/kg i.m.) were given 1 min after nerve agent challenge, and the tested anticonvulsant was given (i.m.) 5 min after seizure onset. In model B, a lower dose of atropine sulfate (0.1 mg/kg i.m.) was given along with 2-PAM 1 min after nerve agent challenge, and the anticonvulsant was given at seizure onset. With the lower dose of atropine, seizure occurrence increased to virtually 100% for all agents; the time to seizure onset decreased for sarin, cyclosarin, and VX; the signs of nerve agent intoxication were more severe; and coma resulted frequently with cyclosarin. The anticonvulsant ED50 doses for scopolamine or diazepam were, in general, not different between the two models, whereas the anticonvulsant ED50 values of midazolam increased 3- to 17-fold with the lower atropine dose. Seizure termination times were not systematically effected by the different doses of atropine. The order of anticonvulsant effectiveness within each model was scopolamine > or = midazolam > diazepam. The findings indicate that the dose of atropine given as antidotal therapy can significantly influence measures of nerve agent toxicity and responsiveness to anticonvulsant therapy.

Extrapolating from animal studies to the efficacy in humans of a pretreatment combination against organophosphate poisoning

The extrapolation from animal data to therapeutic effects in humans, a basic pharmacological issue, is especially critical in studies aimed to estimate the protective efficacy of drugs against nerve agent poisoning. Such efficacy can only be predicted by extrapolation of data from animal studies to humans. In pretreatment therapy against nerve agents, careful dose determination is even more crucial than in antidotal therapy, since excessive doses may lead to adverse effects or performance decrements. The common method of comparing dose per body weight, still used in some studies, may lead to erroneous extrapolation. A different approach is based on the comparison of plasma concentrations at steady state required to obtain a given pharmacodynamic endpoint. In the present study, this approach was applied to predict the prophylactic efficacy of the anticholinergic drug caramiphen in combination with pyridostigmine in man based on animal data. In two species of large animals, dogs and monkeys, similar plasma concentrations of caramiphen (in the range of 60-100 ng/ml) conferred adequate protection against exposure to a lethal-dose of sarin (1.6-1.8 LD(50)). Pharmacokinetic studies at steady state were required to achieve the correlation between caramiphen plasma concentrations and therapeutic effects. Evaluation of total plasma clearance values was instrumental in establishing desirable plasma concentrations and minimizing the number of animals used in the study. Previous data in the literature for plasma levels of caramiphen that do not lead to overt side effects in humans (70-100 ng/ml) enabled extrapolation to expected human protection. The method can be applied to other drugs and other clinical situations, in which human studies are impossible due to ethical considerations. When similar dose response curves are obtained in at least two animal models, the extrapolation to expected therapeutic effects in humans might be considered more reliable.

LC–UV and LC–MS evaluation of stress degradation behaviour of avizafone

It has been known for many years that benzodiazepine compounds effectively antagonize seizures induced by organophosphorous nerve agents. In the event of poisoning, a combination of three drugs is commonly used: an anticholinergic drug (e.g. atropine), an oxime used as cholinesterase reactivator (e.g. pralidoxime or HI-6) and an anticonvulsant (i.e. benzodiazepine). Most of anticholinergics and oximes are freely soluble in water, whereas many benzodiazepines are not. However, a water-soluble prodrug form of diazepam, avizafone, has been adopted by French armed forces for the immediate treatment of nerve agent seizure. The degradation behaviour of this new drug was investigated under different stress degradation conditions (hydrolytic, oxidative, photolytic and thermal) as recommended by International Conference on Harmonization. Successful separation of the active pharmaceutical ingredient from decomposition products formed under stress conditions was achieved using liquid chromatography. The method was validated with respect to specificity, linearity, precision and accuracy.

Anticonvulsant effects of GABA(A) modulators microinfused into area tempestas or substantia nigra in rats exposed to soman

Enhancement of GABAergic neurotransmission has anticonvulsant effects against nerve agent-induced seizures. However, systemic administration of drugs with GABA(A) agonist-like effects does not differentiate well between their anticonvulsant impact. In the present study, GABA(A) modulating drugs (1 microl) were microinfused bilaterally into the seizure controlling substrates, substantia nigra (SN) or area tempestas (AT), of rats subjected to seizures induced systemically by soman (100 microg/kg). The results showed that infusion of ethanol (0.47 micromol) and propofol (20 microg) in both SN and AT icrohad anticonvulsant effects (prevention of seizures or increased latency to seizures). Anticonvulsant effects were also obtained when muscimol (120 ng) was infused into AT or when diazepam (5 microg) was infused into SN. Pentobarbital (50 microg) did not attenuate soman-elicited seizures in any of the injection sites. Results from control experiments showed that the effects from the microinfusions were site-specific, and that the absence of effects of pentobarbital was not a result of too low dose of the drug. The microinfusion technique may allow a more detailed examination of anticonvulsant properties of drugs than by the use of systemic administration.

Seizure duration following sarin exposure affects neuro-inflammatory markers in the rat brain

The current study was aimed to characterize for the first time the alterations in the characteristic neuro-inflammatory markers triggered by sarin exposure in the rat's brain, and to investigate its dependency on seizure duration. Centrally mediated seizures are a common consequence of exposure to organophosphates (OP) despite conventional treatment with atropine and an oxime. In the present study midazolam, was used to control duration and intensity of seizures. The levels of the pro-inflammatory cytokine peptides IL-1beta, IL-6, TNF-alpha and prostaglandin E2 (PGE2) were monitored at various times after sarin exposure in the hippocampus and cortex of rats treated with midazolam following 5 or 30 min of seizure activity. Biochemical evaluation of brain tissues revealed a significant increase in the level of the pro-inflammatory peptides starting at 2 h and peaking at 2-24 h following sarin. Hippocampal values of IL1-beta increased from 1.2+/-0.1 pg/mg tissue (control), to 2.4+/-0.3 at 2 h (5 min seizure) and to 9.3+/-2.5 at 8h (30 min seizure). PGE2 level in the hippocampus increased up to 24 h following exposure (from 56+/-3 to 175+/-26 and 277+/-28 pg/mg tissue) following 5 and 30 min of seizure activity respectively. Thus, unlike limitation of seizures to 5 min by midazolam, delayed treatment (30 min) resulted in prolonged seizures and pronounced increase in cytokines and PGE2. In addition, a second increase in inflammatory markers was observed 30 days following sarin exposure only in rats treated following 30 min of seizure activity. Histological evaluation of the rat brain, conducted in this study, revealed lack of damage in the hippocampus and piriform cortex with minor lateral ventricles enlargement in few animals following 5 min of sarin-induced seizure activity. In contrast, marked histological damage to the brain was demonstrated following 30 min of seizure activity, consisting severe damage to the hippocampus, piriform cortex and some thalamic nuclei. In summary, a novel characterization of the prolonged central neuro-inflammatory process that accompanies sarin exposure is presented. The timing of the anticonvulsive treatment was shown to be crucial in modulation of the neuro-inflammatory response, and may implicate the consequent long-term brain damage.

Efficacy of the ketamine-atropine combination in the delayed treatment of soman-induced status epilepticus

Nerve agent poisoning is known to induce full-blown seizures, seizure-related brain damage (SRBD), and lethality. Effective and quick management of these seizures is critical. In conditions of delayed treatment, presently available measures are inadequate calling for optimization of therapeutic approaches. The effects of ketamine/atropine sulfate (KET/AS) combinations were thus assessed as potential valuable delayed therapy in soman-poisoned male guinea pigs. Animals received pyridostigmine (26 microg/kg, i.m.) 30 min before soman (62 microg/kg, i.m.) followed by therapy consisting of atropine methyl nitrate (4 mg/kg) 1 min later. KET was then administered i.m. at different times after the onset of seizures, starting at 30 min post-poisoning. KET was always injected with atropine sulfate, itself given at a dose that was unable to modify seizures (2 to 10 mg/kg). Different treatment schemes (dose and time of injection) were evaluated. Sub-anesthetic doses of KET (10 mg/kg) could prevent lethality and stop ongoing seizures only when administered 30 min after challenge. An extended delay before treatment (up to 2 h) called for an increase in KET dose (up to 60 mg/kg three times), thus reaching anesthetic levels but without the need of any ventilation support. KET proved effective in stopping seizures, highly reducing SRBD and allowing survival with a progressive loss of efficacy when treatment was delayed beyond 1 h post-challenge. Preliminary results suggest that association with the benzodiazepine midazolam (1 mg/kg) might be interesting when treatment is initiated 2 h after poisoning, i.e., when KET efficacy is dramatically reduced. All in all, these observations suggest that KET, in association with atropine sulfate and possibly other drugs, may be highly effective in the delayed treatment of severe soman intoxication.

Anticonvulsant treatment of sarin-induced seizures with nasal midazolam: An electrographic, behavioral, and histological study in freely moving rats

Centrally mediated seizures and convulsions are common consequences of exposure to organophosphates (OPs). These seizures rapidly progress to status epilepticus (SE) and contribute to profound brain injury. Effective management of these seizures is critical for minimization of brain damage. Nasal application of midazolam (1.5 mg/kg) after 5 min of sarin-induced electrographic seizure activity (EGSA) ameliorated EGSA and convulsive behavior (238 +/- 90 s). Identical treatment after 30 min was not sufficient to ameliorate ECoG paradoxical activity and convulsive behavior. Nasal midazolam (1.5 mg/kg), together with scopolamine (1 mg/kg, im) after 5 min of EGSA, exerted a powerful and rapid anticonvulsant effect (53 +/- 10 s). Delaying the same treatment to 30 min of EGSA leads to attenuation of paroxysmal ECoG activity in all cases but total cessation of paroxysmal activity was not observed in most animals tested. Cognitive tests utilizing the Morris Water Maze demonstrated that nasal midazolam alone or together with scopolamine (im), administered after 5 min of convulsions, abolished the effect of sarin on learning. Both these treatments, when given after 30 min of convulsions, only decreased the sarin-induced learning impairments. Whereas rats which were not subject to the anticonvulsant agents did not show any memory for the platform location, both treatments (at 5 min as well as at 30 min) completely abolished the memory deficits. Both treatments equally blocked the impairment of reversal learning when given at 5 min. However, when administered after 30 min, midazolam alone reversed the impairments in reversal learning, while midazolam with scopolamine did not. Rats exposed to sarin and treated with the therapeutic regimen with the exclusion of midazolam exhibited severe brain lesions that encountered the hippocampus, pyriform cortex, and thalamus. Nasal midazolam at 5 min prevented brain damage, while delaying the midazolam treatment to 30 min of EGSA resulted in brain damage. The addition of scopolamine to midazolam did not alter the above observation. In summary, nasal midazolam treatment briefly after initiation of OP-induced seizure leads to cessation of EGSA and prevented brain lesions and behavioral deficiencies in the rat model.

The role of diazepam in the treatment of nerve agent poisoning in a civilian population

The main site of action of diazepam, as with other benzodiazepines, is at the GABA(A) receptor, although it has been suggested that some of the potentially beneficial actions of diazepam in nerve agent poisoning are mediated through other means. It is likely that convulsions may have long-term sequelae in the central nervous system, because of damage by anoxia and/or excitotoxicity. Numerous pharmacodynamic studies of the action of diazepam in animals experimentally poisoned with nerve agents have been undertaken. In nearly all of these, diazepam has been studied in combination with other antidotes, such as atropine and/or pyridinium oximes, sometimes in combination with pyridostigmine pretreatment. These studies show that diazepam is an efficacious anticonvulsant in nerve agent poisoning. There is considerable experimental evidence to support the hypothesis that diazepam (and other anticonvulsants) may prevent structural damage to the central nervous system as evidenced by neuropathological changes such as neuronal necrosis at autopsy. In instances of nerve agent poisoning during terrorist use in Japan, diazepam seems to have been an effective anticonvulsant. Consequently, the use of diazepam is an important part of the treatment regimen of nerve agent poisoning, the aim being to prevent convulsions or reduce their duration. Diazepam should be given to patients poisoned with nerve agents whenever convulsions or muscle fasciculation are present. In severe poisoning, diazepam administration should be considered even before these complications occur. Diazepam is also useful as an anxiolytic in those exposed to nerve agents.

Nerve Agents

Nerve agents cause a rapidly fatal cholinergic crisis, but rapid, appropriate antidotal treatment saves lives. Survivors of nerve-agent poisoning generally are healthy, unlike survivors of some other chemical agent attacks. Neurologists can assist first responders and mass casualty planners materially by serving as resources for information on nerve agents and the syndromes they cause. They also can help their communities by reinforcing that treatment for nerve-agent poisoning is effective.

Nerve agent toxicity and treatment

The clinical syndrome of nerve agent toxicity varies widely, ranging from the classic cholinergic syndrome to flaccid paralysis and status epilepticus. All nerve agents are capable of producing marked neuropathology. Seizure control is strongly associated with protection against acute lethality and brain pathology. The mainstays of therapy of nerve agent poisoned patients are atropine, pralidoxime, and benzodiazepines. Fosphenytoin provides little therapeutic anticonvulsant effectiveness for nerve agent-induced status epilepticus. Tachycardia is not a contraindication to treatment with atropine in nerve agent toxicity. Atropine should be administered to alleviate respiratory distress, symptomatic bradycardia, and as an adjunct to benzodiazepines and pralidoxime to alleviate seizure activity. In significant nerve agent toxicity, a continuous pralidoxime infusion may be considered.

Pharmacokinetic studies of intramuscular midazolam in guinea pigs challenged with soman

Studies have demonstrated that benzodiazepine compounds are effective at antagonizing seizure activity produced by the organophosphate (OP) cholinesterase inhibitor soman. In this present study we have investigated the pharmacokinetics of midazolam and its associated effects on electroencephalographic (EEG) activity following intramuscular (i.m.) injection to soman-exposed guinea pigs (Crl:(HA)BR). Prior to experiments, the animals were surgically implanted with EEG leads to monitor seizure activity. For the study, animals were administered the following pretreatment/OP/treatment regimen. Pyridostigmine bromide (0.026 mg/kg, i.m.) was given 30 min prior to soman (56 micrograms/kg, 2 x LD50; subcutaneously, s.c.), followed in one minute by atropine sulfate (2 mg/kg, i.m.) and pralidoxime chloride (25 mg/kg, i.m.). All animals receiving this regimen developed seizure activity. Midazolam 0.8 mg/kg, i.m., was administered 5 min after onset of seizure activity. Based on EEG data, animals were categorized as either seizure-terminated or seizure not-terminated at 30 min following anticonvulsant administration. Serial blood samples were collected for the plasma midazolam analysis; the assay was accomplished with a gas chromatograph/mass spectrometer. The mean time to seizure termination was 8.8 +/- 1.6 min. The mean time-plasma concentration data were fit to standard pharmacokinetic models. The following parameter estimates were determined from the model-fit for seizure terminated and not-terminated animals respectively: apparent volumes of distribution (Vd) were 1.4 and 1.7 l/kg; area under the time-concentration curves (AUC), 15,990 and 15,120 ng.min/ml; times to maximal plasma concentration (Tmax), 1.66 and 2.91 min and maximal plasma concentrations (Cmax) 535.1 and 436.6 ng/ml. These data indicate that i.m. injection of midazolam is effective at terminating ongoing soman-induced seizure activity. Additionally, the relatively short Tmax and latency to seizure termination demonstrate the rapidity of drug absorption and action respectively.

Organophosphates/nerve agent poisoning: mechanism of action, diagnosis, prophylaxis, and treatment

OP/nerve agents are still considered as important chemicals acting on living organisms and are widely used. They are characterized according to their action as compounds influencing cholinergic nerve transmission via inhibition of AChE. Modeling of this action and extrapolation of experimental data from animals to humans is more possible for highly toxic agents than for the OP. The symptoms of intoxication comprise nicotinic, muscarinic, and central symptoms; for some OP/nerve agents, a delayed neurotoxicity is observed. Cholinesterases (AChE and BuChE) are characterized as the main enzymes involved in the toxic effect of these compounds, including molecular forms. The activity of both enzymes (and molecular forms) is influenced by inhibitors (reversible, irreversible, and allosteric) and other factors, such as pathological states. There are different methods for cholinesterase determination; however, the most frequent is the method based on the hydrolysis of thiocholine esters and subsequent detection of free SH-group of the released thiocholine. The diagnosis of OP/nerve agent poisoning is based on anamnesis, the clinical status of the intoxicated organism, and on cholinesterase determination in the blood. For nerve agent intoxication, AChE in the red blood cell is more diagnostically important than BuChE activity in the plasma. This enzyme is a good diagnostic marker for intoxication with OP pesticides. Some other biochemical examinations are recommended, especially arterial blood gas, blood pH, minerals, and some other specialized parameters usually not available in all clinical laboratories. These special examinations are important for prognosis of the intoxication, for effective treatment, and for retrospective analysis of the agent used for exposure. Some principles of prophylaxis against OP/nerve agent poisoning comprising the administration of reversible cholinesterase inhibitors such as pyridostigmine (alone or in combination with other drugs), scavengers such as preparations of cholinesterases, some therapeutic drugs, and possible combinations are given. Basic principles of the treatment of nerve agent OP poisoning are described. They are based on the administration of anticholinergics (mostly atropine but some other anticholinergics can be recommended) as a symptomatic treatment, cholinesterase reactivators as a causal treatment (different types but without a universal reactivator against all OP/nerve agents) as the first aid and medical treatment, and anticonvulsants, preferably diazepam though some other effective benzodiazepines are available. New drugs for the treatment are under experimental study based on new approaches to the mechanism of action. Future trends in the complex research of these compounds, which is important not only for the treatment of intoxication but also for the quantitative and qualitative increase of our knowledge of toxicology, neurochemistry, neuropharmacology, clinical biochemistry, and analytical chemistry in general, are characterized.

Future considerations for the medical management of nerve-agent intoxication

The use of chemical warfare agents against civilians and unprotected troops in international conflicts or by terrorists against civilians is considered to be a real threat, particularly following the terrorist attacks on 11 September 2001 against the World Trade Center in New York and against the Pentagon in Washington, DC. Over the past 10 years, terrorists have been planning to use or have used chemical warfare agents on several occasions around the world, and the attacks in 2001 illustrate their willingness to use any means of warfare to cause death and destruction among civilians. In spite of new international treaties with strong verification measures and with an aim to prohibit and prevent the use of weapons of mass destruction, nevertheless, some countries and terrorist groups have been able to develop, produce, and use such weapons, particularly nerve agents, in domestic terrorist attacks or during warfare in international conflicts. This article reviews current medical therapy for nerve-agent intoxication and discusses possible future improvement of medical therapies. Present medical counter-measures against nerve agents are not sufficiently effective particularly in protecting the brain. Therefore, new and more effective countermeasures must be developed to enable better medical treatment of civilians and military personnel following exposure to nerve agents. Therefore, it is important with an enhanced effort by all countries, to improve and increase research in medical countermeasures, in the development of protective equipment, and in carrying out regular training of medical and emergency personnel as well as of military nuclear, biological, or chemical (NBC) units. Only then will nations be able to reduce the risk from and prevent the use of such weapons of mass destruction (WMD).

Control of nerve agent-induced seizures is critical for neuroprotection and survival

This study evaluated the potency and rapidity of some anticholinergics (atropine, biperiden, and trihexyphenidyl) and benzodiazepines (diazepam and midazolam) as an anticonvulsant treatment against seizures induced by six nerve agents (tabun, sarin, soman, cyclosarin, VR, and VX) and summarized the relationship between anticonvulsant activity and nerve agent-induced lethality and neuropathology. Guinea pigs, previously implanted with cortical electrodes for EEG recording, were pretreated with pyridostigmine bromide (0.026 mg/kg im) 30 min prior to challenge with 2x LD50 dose (sc) of a given nerve agent; in a separate experiment, animals were challenged with 5x LD50 sc of soman. One minute after agent challenge the animals were treated im with 2 mg/kg atropine SO(4) admixed with 25 mg/kg 2-PAM Cl. Five minutes after the start of EEG seizures, animals were treated im with different doses of anticholinergics or benzodiazepines and observed for seizure termination. The time to seizure onset, the time to seizure termination, and 24-h lethality were recorded. The anticonvulsant ED50 of each drug for termination of seizures induced by each agent was calculated and compared. Brain tissue from animals that survived 24 h was examined for pathology. All drugs were capable of terminating seizure activity, with midazolam and trihexyphenidyl being significantly more potent than the other drugs, and midazolam being more rapid in controlling seizure than atropine, trihexyphenidyl, or diazepam against each agent. Seizures induced by sarin or VX required lower doses of all the test anticonvulsants. The dose of a given drug that was an effective anticonvulsant against a 2x LD50 challenge of soman was equally effective against seizures induced by a 5x LD50 challenge. All nerve agents were capable of producing neuropathology. Seizure control was strongly associated with protection against acute lethality and brain pathology.

Nasal midazolam as a novel anticonvulsive treatment against organophosphate-induced seizure activity in the guinea pig

Seizures and status epilepticus, which may contribute to brain injury, are common consequences of exposure to organophosphorus (OP) cholinesterase inhibitors. Effective management of these seizures is critical. To investigate the efficacy of nasal midazolam as an anticonvulsive treatment for OP exposure, as compared to intramuscular midazolam, guinea pigs were connected to a recording swivel for electrocorticograph (ECoG) monitoring and clinical observation. The experimental paradigm consisted of pyridostigmine pretreatment (0.1 mg/kg i.m.) 20 min prior to sarin exposure (1.2x LD(50,) 56 micro g/kg i.m.). One minute post-exposure, atropine (3 mg/kg i.m.) and TMB-4 (1 mg/kg im) were administered. Within 3-8 min after sarin exposure all animals developed electrographic seizure activity (EGSA), with convulsive behavior. Treatment with midazolam (1 mg/kg i.m.) 10 min after the onset of EGSA abolished EGSA within 389+/-181 s. The same dose was not effective, in most cases, when given 30 min after onset. However, a higher dose (2 mg/kg) was found efficacious after 30 min (949+/-466 s). In contrast, nasal application of midazolam (1 mg/kg) was found most effective, with significant advantages, in amelioration of EGSA and convulsive behavior, when given 10 min (216+/-185 s) or 30 min (308+/-122 s) following the onset of EGSA ( P<0.001). Thus, nasal midazolam could be used as a novel, rapid and convenient route of application against seizure activity induced by nerve agent poisoning.

Pharmacokinetics of intramuscularly administered biperiden in guinea pigs challenged with soman

Biperiden is an anticholinergic compound that has demonstrated effectiveness for treating organophosphate-induced seizure/convulsions. The plasma levels of biperiden associated with this efficacy have not yet been defined. In this study, the pharmacokinetics and tissue distribution of biperiden after intramuscular administration of 0.5 mg/kg were conducted while monitoring pharmacodynamic (electroencephalographic) data in soman-exposed guinea pigs. Overall, 59% of the animals had seizures terminated within 30 min of the biperiden administration. The mean time to seizure termination was 15.9 min. The pharmacokinetics of biperiden after i.m. administration to guinea pigs were best described by a one-compartment model with first-order absorption and elimination. The maximal plasma biperiden concentration (34.4 ng/mL) in seizure-terminated animals occurred at 26.3 min. Extensive partitioning into peripheral tissues was noted supporting the relatively large volume of distribution observed. Maximal biperiden concentrations in the cortex and brain stem were found at 30 min and were 2.3 and 1.7 times greater, respectively, than that in plasma. The time for maximal plasma concentration was found to corresponded well with the mean time to seizure termination following drug administration.

Pyridophens: binary pyridostigmine-aprophen prodrugs with differential inhibition of acetylcholinesterase, butyrylcholinesterase, and muscarinic receptors

A series of "binary prodrugs" called carbaphens,(1) carbamylated derivatives on one or both of the aromatic rings of the muscarinic receptor antagonist aprophen [(N,N-diethylamino)ethyl 2,2-diphenylpropionate], were synthesized to develop binary prophylactic agents against organophosphorus intoxication. As a group, the carbaphens retained the muscarinic receptor antagonist properties of aprophen but also preferentially inhibited butyrylcholinesterase (BChE) in contrast to acetylcholinesterase (AChE). Therefore, a new series of compounds named pyridophens were designed and synthesized to achieve binary prodrugs to preferentially inhibit AChE over BChE, while still retaining the muscarinic receptor antagonism of aprophen. The pyridophens consist of the basic pyridostigmine skeleton combined with the 2,2-diphenylpropionate portion of aprophen by replacement of the diethylamino group. Three compounds, 9 (a tertiary pyridine), 10 (a quaternary pyridine), and 12 (a tertiary tetrahydropyridine), were found to be effective inhibitors of both BChE and AChE. However, 10, N-methyl-3-[[(dimethylamino)carbonyl]oxy]-2-(2'2'-diphenylpropionoxy-methyl)pyridinium iodide, inhibited AChE selectively over BChE, with a bimolecular rate constant similar to pyridostigmine. In contrast to their potent cholinesterase inhibitory activity, all of the pyridophen analogues were less potent antagonists of the muscarinic receptor than aprophen.

The determination of biperiden in plasma using gas chromatography mass spectrometry: pharmacokinetics after intramuscular administration to guinea pigs

A gas chromatographic-mass spectrometric (GC-MS) method has been developed for the analysis of the biperiden from plasma. The method utilizes 290 microl of plasma and a simple hexane extraction/clean-up procedure. Standard curves were linear over the range of 1.9-250 ng/mL. The range of correlation coefficients for the individual standard curves was 0.9984-0.9999; the largest coefficient of variation expressed as a percentage (% CV) was 11.5%. Precision and accuracy were examined by assessing between-day and within-day variability. For between-day precision, the % CVs ranged from 2.86 to 5.17%. Accuracy as expressed by percentage error ranging from -2.16 to 5.83%. The study for within-day precision demonstrated % CVs from 0.95 to 5.55% with accuracy from -3.37 to 2.45%. Applicability of the method was demonstrated by examining the pharmacokinetics of intramuscular (i.m.) biperiden as an anticonvulsant treatment in a guinea pig model for organophosphate (OP)-induced seizure activity. Mean pharmacokinetic parameter estimates were similar to literature values; selected mean pharmacokinetic parameter estimates were: apparent volume of distribution, 13.9 L/kg; half-life of elimination, 93 min; time to maximal plasma concentration, 27.4 min; and maximal plasma concentration, 32.22 eta g/mL. The time to maximal plasma concentration was found to be similar to the onset time for terminating OP-induced seizure activity in guinea pigs receiving biperiden as an anticonvulsant treatment. The studies indicate that the method affords the required precision, accuracy and sensitivity to assay biperiden at the doses utilized for these pharmacokinetic studies after i.m. administration to guinea pigs.

A simple method for measuring EEG in freely moving guinea pigs

This paper describes a method of measuring EEG in guinea pigs by means of temporarily attached superficial electrodes with radiotelemetry equipment. This represents an alternative approach to more conventional methods of monitoring EEG in animals, which necessitate surgical implantation of electrodes. The novel approach of using superficial electrodes was compared with the cortical screw electrode technique, in a nerve agent-induced seizure model. In both cases, radiotelemetry techniques were used for data capture to facilitate monitoring of nerve agent-induced seizures in freely moving non-tethered guinea pigs. The advantages and disadvantages of the technique are discussed, for example the saving in resources and the benefits to animal welfare.

Chromatographic method for the determination of diazepam, pyridostigmine bromide, and their metabolites in rat plasma and urine

This study describes a chromatographic method for the determination of diazepam, an anxiolytic drug that is also used as an antidote against nerve agent seizures, its metabolites N-desmethyldiazepam, and temazepam, the anti-nerve agent drug pyridostigmine bromide (PB; 3-dimethylaminocarbonyloxy-N-methyl pyridinium bromide) and its metabolite N-methyl-3-hydroxypyridinium bromide in rat plasma and urine. The compounds were extracted using C18 Sep-Pak Vac 3cc (500 mg) cartridges and separated using isocratic mobile phase of methanol, acetonitrile and water (pH 3.2) (10:40:50) at a flow-rate of 0.5 ml/min in a period of 12 min, and UV detection ranging between 240 and 280 nm. The limits of detection for all analytes ranged between 20 and 50 ng/ml, while limits of quantitation were 100 ng/ml. Average percentage extraction recoveries of five spiked plasma samples were 79.1+/-7.7, 83.5+/-6.4, 83.9+/-5.9, 71.3+/-6.0 and 77.7+/-5.6, and from urine 79.4+/-7.9, 83.1+/-6.9, 73.6+/-7.7, 74.3+/-7.1 and 77.6+/-5.9 for diazepam, N-desmethyldiazepam, temazepam, pyridostigmine bromide, and N-methyl-3-hydroxypyridinium bromide, respectively. The relationship between peak areas and concentration was linear over the range between 100 and 1000 ng/ml. This method was applied to determine the above analytes following a single oral administration in rats as a tool to study the pharmacokinetic profile of each compound, alone and in combination.