Evaluation of the toxicity, pathology, and treatment of cyclohexylmethylphosphonofluoridate (CMPF) poisoning in rhesus monkeys

Synergistic action of organophosphates and COVID-19 on inflammation, oxidative stress, and renin-angiotensin system can amplify the risk of cardiovascular maladies

Organophosphates (OPs) are ubiquitous environmental contaminants, widely used as pesticides in agricultural fields. In addition, they serve as flame-retardants, plasticizers, antifoaming or antiwear agents in lacquers, hydraulic fluids, and floor polishing agents. Therefore, world-wide and massive application of these compounds have increased the risk of unintentional exposure to non-targets including the human beings. OPs are neurotoxic agents as they inhibit the activity of acetylcholinesterase at synaptic cleft. Moreover, they can fuel cardiovascular issues in the form of myocardities, cardiac oedema, arrhythmia, systolic malfunction, infarction, and altered electrophysiology. Such pathological outcomes might increase the severity of cardiovascular diseases which are the leading cause of mortality in the developing world. Coronavirus disease-19 (COVID-19) is the ongoing global health emergency caused by SARS-CoV-2 infection. Similar to OPs, SARS-CoV-2 disrupts cytokine homeostasis, redox-balance, and angiotensin-II/AT₁R axis to promote cardiovascular injuries. Therefore, during the current pandemic milieu, unintentional exposure to OPs through several environmental sources could escalate cardiac maladies in patients with COVID-19.

Substrate Analogues for the Enzyme-Catalyzed Detoxification of the Organophosphate Nerve Agents-Sarin, Soman, and Cyclosarin

The G-type nerve agents, sarin (GB), soman (GD), and cyclosarin (GF), are among the most toxic compounds known. Much progress has been made in evolving the enzyme phosphotriesterase (PTE) from Pseudomonas diminuta for the decontamination of the G-agents; however, the extreme toxicity of the G-agents makes the use of substrate analogues necessary. Typical analogues utilize a chromogenic leaving group to facilitate high-throughput screening, and substitution of an O-methyl for the P-methyl group found in the G-agents, in an effort to reduce toxicity. Till date, there has been no systematic evaluation of the effects of these substitutions on catalytic activity, and the presumed reduction in toxicity has not been tested. A series of 21 G-agent analogues, including all combinations of O-methyl, p-nitrophenyl, and thiophosphate substitutions, have been synthesized and evaluated for their ability to unveil the stereoselectivity and catalytic activity of PTE variants against the authentic G-type nerve agents. The potential toxicity of these analogues was evaluated by measuring the rate of inactivation of acetylcholinesterase (AChE). All of the substitutions reduced inactivation of AChE by more than 100-fold, with the most effective being the thiophosphate analogues, which reduced the rate of inactivation by about 4-5 orders of magnitude. The analogues were found to reliably predict changes in catalytic activity and stereoselectivity of the PTE variants and led to the identification of the BHR-30 variant, which has no apparent stereoselectivity against GD and a k_cat/K_m of 1.4 × 10⁶, making it the most efficient enzyme for GD decontamination reported till date.

The evolution of phosphotriesterase for decontamination and detoxification of organophosphorus chemical warfare agents

The organophosphorus chemical warfare agents were initially synthesized in the 1930's and are some of the most toxic compounds ever discovered. The standard means of decontamination are either harsh chemical hydrolysis or high temperature incineration. Given the continued use of chemical warfare agents there are ongoing efforts to develop gentle environmentally friendly means of decontamination and medical counter measures to chemical warfare agent intoxication. Enzymatic decontamination offers the benefits of extreme specificity and mild conditions, allowing their use for both environmental and medical applications. The most promising enzyme for decontamination of the organophosphorus chemical warfare agents is the enzyme phosphotriesterase from Pseudomonas diminuta. However, the catalytic activity of the wild-type enzyme with the chemical warfare agents falls far below that seen with its best substrates, and its stereochemical preference is for the less toxic enantiomer of the chiral phosphorus center found in most chemical warfare agents. Rational design efforts have succeeded in the dramatic improvement of the stereochemical preference of PTE for the more toxic enantiomers. Directed evolution experiments, including site-saturation mutagenesis, targeted error-prone PCR, computational design, and quantitative library analysis, have systematically improved the catalytic activity against the chemical warfare nerve agents. These efforts have resulted in greater than 4-orders of magnitude improvement in catalytic activity and have led to the identification of variants that are highly effective at detoxifying both G-type and V-type nerve agents. The best of these variants have the ability to prevent intoxication when delivered as a post-exposure treatment for VX and as a pre-exposure treatment for G-agent intoxication with observed protective factors up to 60-fold. Combining the best variant, H257Y/L303T, with a PCB polymer coating has enabled the development of a long lasting circulating prophylactic treatment that is highly effective against sarin.

Therapeutic Efficacy of Different Antidotal Mixtures Against Poisoning with GF-Agent in Mice

The toxicity of cyclohexyl methylphosphonofluoridate (GF-agent; cyclosarin) and therapeutic efficacy of four oximes (trimedoxime, methoxime, obidoxime and HI-6) in combination with atropine or benactyzine (BNZ) was studied in mice. The oxime therapy combined with anticholinergic drug was administered intramusculary (i.m.) 1 or 2 min after i.m. GF-agent challenge. All the drugs were applied in dose of 20% of LD50. Obidoxime and trimedoxime that were combined with atropine were less effective than methoxime and HI-6 in combination with BNZ when applied 2 minutes after GF-agent poisoning. When the drugs were administered 1 min after GF-agent challenge already, in case of methoxime, the faster application of therapy resulted in significantly higher protective ratio, while for obidoxime the therapeutic effectivity did not depend significantly on the seasonableness of therapeutic intervention. The present findings show that all four combinations of oxime with anticholinergic drug decrease the GF-agent toxicity more than twofold regardless of the time of treatment administration.

Superior efficacy of HI-6 dimethanesulfonate over pralidoxime methylsulfate against Russian VX poisoning in cynomolgus monkeys (Macaca fascicularis)

Organophosphorus nerve agents still represent a serious risk to human health. In the French armed forces, the current emergency treatment against OP intoxications is a fully licensed wet-dry dual-chambered autoinjector (Ineurope ®), that contains pralidoxime methylsulfate (2-PAM) to reactivate inhibited acetylcholinesterase (AChE), atropine sulfate (AS) and avizafone chlorhydrate (AVZ). While this treatment is effective against several of the known nerve agents, it shows little efficacy against the Russian VX (VR), one of the most toxic compounds. HI-6 dimethanesulfonate (HI-6 DMS) is an oxime able to reactivate in vitro and in vivo VR-inhibited AChE. To confirm the superiority of HI-6 DMS towards 2-PAM prior to licensing, we compared the two 3-drug-combinations (HI-6 vs 2-PAM, 33 and 18 mg/kg respectively, equimolar doses; AS/AVZ 0.25/0.175 mg/kg respectively) in VR-poisoned cynomolgus macaques, the model required by the French drug regulatory agency. In parallel we performed HI-6 pharmacokinetics analysis using a one compartment model. A better efficacy of the HI-6 DMS combination was clearly observed: up to 5 LD₅₀ of VR (i.m.), a single administration of the HI-6 DMS combination, shortly after the onset of clinical signs, prevented death of the four intoxicated animals. Conversely 2-PAM only prevented death in one out of three subjects exposed to the same amount of VR. As expected with V agents, reinhibition of blood AChE was observed but without any apparent impact on the clinical recovery of the animals. A single administration of the HI-6 DMS combination was still but partially effective at 15 LD₅₀ of VR, allowing a 50% survival rate.

The role of genetic background in susceptibility to chemical warfare nerve agents across rodent and non-human primate models

Genetics likely play a role in various responses to nerve agent exposure, as genetic background plays an important role in behavioral, neurological, and physiological responses to environmental stimuli. Mouse strains or selected lines can be used to identify susceptibility based on background genetic features to nerve agent exposure. Additional genetic techniques can then be used to identify mechanisms underlying resistance and sensitivity, with the ultimate goal of developing more effective and targeted therapies. Here, we discuss the available literature on strain and selected line differences in cholinesterase activity levels and response to nerve agent-induced toxicity and seizures. We also discuss the available cholinesterase and toxicity literature across different non-human primate species. The available data suggest that robust genetic differences exist in cholinesterase activity, nerve agent-induced toxicity, and chemical-induced seizures. Available cholinesterase data suggest that acetylcholinesterase activity differs across strains, but are limited by the paucity of carboxylesterase data in strains and selected lines. Toxicity and seizures, two outcomes of nerve agent exposure, have not been fully evaluated for genetic differences, and thus further studies are required to understand baseline strain and selected line differences.

Bispyridinium Oximes As Antidotal Treatment of Cyclosarin Poisoning—In Vitro and In Vivo Testing

The mechanism of intoxication with organophosphorus compounds, including highly toxic nerve agents and less toxic pesticides, is based on the formation of irreversibly inhibited acetylcholinesterase, which causes cumulation of neuromediator acetylcholine in synaptic clefts and subsequent overstimulation of cholinergic receptors, that is followed by a generalized cholinergic crisis. Nerve agent poisoning is conventionally treated using a combination of a cholinolytic (atropine mostly) to counteract the accumulation of acetylcholine and acetylcholinesterase reactivators (pralidoxime or obidoxime) to reactivate inhibited acetylcholinesterase. In this study of cyclosarin poisoning treatment, oximes of different chemical structures (obidoxime, HI-6, BI-6, and HS-6) were tested in vitro on rat brain acetylcholinesterase (enzyme source: rat brain homogenate), and afterwards, they were tested in vivo in equimolar doses, in mice and rats. The HI-6 oxime appeared to be the most effective oxime in vitro and in vivo.

Pretreatment and prophylaxis against nerve agent poisoning: Are undesirable behavioral side effects unavoidable?

The threat of chemical warfare agents like nerve agents requires life saving measures of medical pretreatment combined with treatment after exposure. Pretreatment (pyridostigmine) may cause some side effects in a small number of individuals. A comprehensive research on animals has been performed to clarify effects on behavior. The results from these studies are far from unambiguous, since pyridostigmine may produce adverse effects on behavior in animals in relatively high doses, but not in a consistent way. Other animal studies have examined the potential of drugs like physostigmine, galantamine, benactyzine, trihexyphenidyl, and procyclidine, but they all produce marked behavioral impairment at doses sufficient to contribute to protection against a convulsant dose of soman. Attempts have also been made to develop a combination of drugs capable of assuring full protection (prophylaxis) against nerve agents. However, common to all combinations is that they at anticonvulsant doses cause behavioral deficits. Therefore, the use of limited pretreatment doses may be performed without marked side effects followed by post-exposure therapy with a combination of drugs.

Efficacies of Atropine/2-PAM and Atropine/HI-6 in Treating Monkeys Intoxicated with Organophosphonate Nerve Agents

The efficacies of atropine (ATR)/ 2-PAM and ATRl HI-6 in treating male rhesus monkeys injected with the neurotoxic organophosphonate (OP) agents GA, GB, GD, GF, or VX were compared. Experiments were conducted using no more than 8 monkeys per OP and treatment regimen. Doses were selected using a modified up-and-down experimental design, challenging one monkey per day per OP and treatment. Results were used to approximate the median lethal dose (MLD) for groups of treated monkeys or monkeys given only a vehicle following challenge with an OP. Mortality and signs of intoxication with each treatment were statistically compared. Doses of 2-PAM (25.7 mgl kg) or HI-6 (50 mgl kg) and atropine (0.4 mg free base! kg) were given in a single intramuscular (IM) injection 1 min following challenge with an OP. Strong and well-defined relationships between agent dose and 10-h lethality were observed in untreated animals. The 10-h IM OP MLDs for untreated monkeys were estimated to be 80, 43, 8.0, 22, and 7.4 μg/kg for GA, GB, GD, GF, and VX, respectively. No statistical differences w ere found between AT Rl 2-PAM and AT Rl HI-6 treatment efficacies in preventing lethality for any of the OPs. Both oxime combinations appear to provide protection against a 2 × 10-h MLD of GA, GF, or VX; only

In vivo acetylcholinesterase reactivation in male guinea pigs and rhesus macaques following cyclosarin exposure and treatment with 1,1'-methylenebis{4-[(hydroxyimino)methyl] pyridinium} dimethanesulfonate

Acetylcholinesterase (AChE) reactivation studies were conducted in guinea pigs (GPs) and nonhuman primates (NHPs) to determine the 1,1'-methylenebis{4-[(hydroxyimino)methyl] pyridinium} dimethanesulfonate (MMB4 DMS) dose that reactivated at least 20% of blood AChE within 15 minutes following cyclosarin (GF) dosing (used as the criterion for efficacy). Male GPs and male rhesus macaques (NHPs) were pretreated with atropine 15 minutes prior to GF administration (1 × median lethal dose [LD50]) and MMB4 DMS 15 minutes following GF administration. The GP survival was 5 of 8, 8 of 8, 8 of 8, and 6 of 8 for the 0.75, 3.0, 6.0, or 12.0 mg/kg MMB4 DMS treatment groups, respectively. In NHPs, survival was 6 of 6 at 0.5, 1.2, 3.0, or 9.3 mg/kg MMB4 DMS, respectively, 24 hours post-challenge, with the majority of animals noted as clinically normal by 24 hours. Pharmacokinetic/pharmacodynamic modeling revealed that 1.8 mg/kg in GPs or 0.013 mg/kg in NHPs would result in an average 20% reactivation; human equivalent doses were calculated as 0.39 mg/kg (based on GP data) and 0.004 mg/kg (based on NHP data). The model suggested that MMB4 plasma concentrations of 1000 ng/mL and AChE reactivation of 80% would be most effective. Although a 0.5 mg/kg MMB4 DMS dose in NHPs resulted in 100% survival and an average of 78% AChE reactivation, adverse effects associated with GF administration were still observed 24 hours post-challenge (tremors, mydriasis, and weakness were observed in 3 of 6 animals). In comparison, 6 of 6 animals treated with 1.2 mg/kg MMB4 DMS were observed as clinically normal 24 hours post-challenge.

Efficacy and Pharmacokinetic/Pharmacodynamic Study of 1,1′-Methylenebis{4-[(hydroxyimino)methyl] pyridinium} Dimethanesulfonate in Guinea Pigs and Rhesus Macaques Exposed to Cyclosarin

Male Hartley guinea pigs and male rhesus macaques were used to determine an efficacious dose of 1,1′-methylenebis{4-[(hydroxyimino)methyl] pyridinium} dimethanesulfonate (MMB4 DMS) that would result in 80% survival, 24 hours following a single exposure to cyclosarin (GF). The pharmacokinetic/pharmacodynamic relationship between acetylcholinesterase activity and MMB4 plasma concentrations relative to survival was evaluated. Guinea pigs and non-human primates (NHPs) were concurrently administered MMB4 DMS (guinea pigs: 0, 10, 30, or 40 mg/kg, intramuscular [IM] and NHPs: 0.1, 1, 5, 10, or 20 mg/kg, IM), atropine, and diazepam following a 3 × median lethal dose (LD50) GF challenge. Clinical observations were evaluated using a quality-of-life (QOL) scoring system. All GF-exposed animals exhibited typical signs of nerve agent poisoning immediately following challenge. In guinea pigs, 24-hour survival was 0%, 50%, 90%, and 90% for 0, 10, 30, and 40 mg/kg MMB4 DMS groups, respectively. In addition, nearly all animals surviving to 24 hours were clinically normal, with many in the 30 and 40 mg/kg MMB4 DMS dose group observed as normal by 4 hours post-challenge. In NHPs, survival was 100% for all treatment groups, with all animals noted as clinically normal by 48 hours. Following treatment with atropine/MMB4 DMS/diazepam, NHPs exhibited dose- and temporal-related decreases in incidence and duration of the clinical signs of toxicity. The QOL scores improved with increasing MMB4 DMS dose in both species. The estimated ED80s were 25.5 mg/kg MMB4 DMS (human equivalent dose [HED] of 5.5 mg/kg) and ≤0.1 mg/kg (HED of 0.03 mg/kg) in guinea pigs and NHPs, respectively.

Acute toxicity of organophosphorus compounds in guinea pigs is sex- and age-dependent and cannot be solely accounted for by acetylcholinesterase inhibition

This study was designed to test the hypothesis that the acute toxicity of the nerve agents S-[2-(diisopropylamino)ethyl]-O-ethyl methylphosphonothioate (VX), O-pinacolyl methylphosphonofluoridate (soman), and O-isopropyl methylphosphonofluoridate (sarin) in guinea pigs is age- and sex-dependent and cannot be fully accounted for by the irreversible inhibition of acetylcholinesterase (AChE). The subcutaneous doses of nerve agents needed to decrease 24-h survival of guinea pigs by 50% (LD(50) values) were estimated by probit analysis. In all animal groups, the rank order of LD(50) values was sarin > soman > VX. The LD(50) value of soman was not influenced by sex or age of the animals. In contrast, the LD(50) values of VX and sarin were lower in adult male than in age-matched female or younger guinea pigs. A colorimetric assay was used to determine the concentrations of nerve agents that inhibit in vitro 50% of AChE activity (IC(50) values) in guinea pig brain extracts, plasma, red blood cells, and whole blood. A positive correlation between LD(50) values and IC(50) values for AChE inhibition would support the hypothesis that AChE inhibition is a major determinant of the acute toxicity of the nerve agents. However, such a positive correlation was found only between LD(50) values and IC(50) values for AChE inhibition in brain extracts from neonatal and prepubertal guinea pigs. These results demonstrate for the first time that the lethal potencies of some nerve agents in guinea pigs are age- and sex-dependent. They also support the contention that mechanisms other than AChE inhibition contribute to the lethality of nerve agents.

The comprehensive medical preparedness in chemical emergencies: 'the chain of chemical survival'

Medical management of victims of chemical incidents includes supportive therapy, decontamination and antidote administration. Chemical weapons of mass destruction are available to many countries and are a possible alternative to conventional weapons for terrorist groups. During the last 5 years, some Italian institutions have made big efforts to establish a national system of antidote stockpiling and distribution. Little or no efforts have been addressed to other aspects of the medical management of patients exposed to chemical agents, such as decontamination, personal protective equipment, and specific supportive therapy. Although antidotes are indispensable instruments for some poisonings, as nerve agent and botulin intoxication, antidote stockpiling cannot be considered the only objective of a comprehensive medical preparedness for chemical emergencies. This paper addresses the medical priority when approaching victims of chemical emergencies. The priority actually is to establish a chain of chemical survival in which antidote administration is one out of several links.

Effects of low-level inhalation exposure to cyclosarin on learned behaviors in Sprague-Dawley rats

Behavioral and biochemical effects of low-level whole-body inhalation exposure to the chemical warfare nerve agent cyclosarin (GF) were evaluated. Sprague-Dawley rats were first trained on a variable-interval, 56-s (VI56) schedule of food reinforcement. The VI56 schedule specifies that a single lever press, following an average interval of 56 s, produces food reinforcement (i.e., a single food pellet). Subjects were then exposed to GF vapor at concentrations of 1.6-5.2 mg/m3, or air control, for 60 min. Following exposures, performance on the VI56 and acquisition and maintenance of a radial-arm maze (RAM) spatial memory task were evaluated during 55 test sessions over approximately 11 wk. GF exposures produced miosis in all subjects and other mild clinical signs of toxicity at the highest concentration. Convulsions were not observed in any subjects. GF exposures produced concentration-dependent decreases in acetylcholinesterase and butyrylcholinesterase activity. Additionally, blood assays revealed concentration-dependent levels of regenerated GF, thus verifying systemic exposure. The largest concentration of GF disrupted performance on the VI56 task. The deficit, however, resolved by the third postexposure test session. All subjects acquired, and maintained, performance on the RAM task, and no significant differences were seen as a result of GF exposure. No delayed effects from exposures were observed. These results demonstrate that, in rats, inhalation exposure to GF at levels below those producing convulsions and other severe clinical signs of toxicity may produce performance deficits on learned behaviors, but the deficits appear to not be persistent.

Cerebral acetylcholine and choline contents and turnover following low-dose acetylcholinesterase inhibitors treatment in rats

Male Sprague-Dawley rats were treated for 3 weeks with (1) regular tap drinking water plus subcutaneous (s.c.) saline (0.5 ml/kg) injections three times/week, (2) pyridostigmine bromide (PB) in drinking water (80 mg/L) plus s.c. saline injections three times/week, (3) regular tap drinking water plus s.c. sarin (0.5 x LD(50)) injections three times/week, or (4) PB in drinking water plus s.c. sarin injections three times/week. Repeated doses of sarin, in the presence or absence of PB, were devoid of acute toxicity during the three-week treatment period. Two, 4, and 16 weeks post-treatment, animals were given an intravenous pulse injection of choline labeled with 4 deuterium atoms (D4Ch) followed, after 1 min, by microwave fixation of the brain in vivo. Tissue levels of endogenous acetylcholine (D0ACh), endogenous choline (D0Ch), D4Ch, and ACh synthesized from D4Ch (D4ACh) were measured by gas-chromatography mass-spectrometry in hippocampus, infundibulum, mesencephalon, neocortex, piriform cortex, and striatum. Ch uptake from blood and ACh turnover were estimated from D4Ch and D4ACh concentrations in brain tissue, respectively. Statistically significant differences among brain regions were found for D0Ch, D4Ch, D0ACh and D4ACh at 2, 4 and 16 weeks post-treatment. However, differences in the values of these parameters between control and drug treatments were found only for D0ACh and D0Ch at 2 and 4 weeks, but not at 16 weeks post-treatment. In conclusion, the results from these experiments do not support a delayed or persistent alteration in cholinergic function after exposure to low doses of PB and/or sarin.

Development of the Bisquaternary Oxime HI-6 Toward Clinical Use in the Treatment of Organophosphate Nerve Agent Poisoning

The traditional therapeutic treatment of organophosphate cholinesterase inhibitor (nerve agents) poisoning consists of co-treatment with an antimuscarinic (atropine) and a reactivator of inhibited acetylcholinesterase (AChE), which contains a nucleophilic oxime function. Two oximes are presently widely available for clinical use, pralidoxime and obidoxime (toxogonin), but both offer little protection against important nerve agent threats. This has highlighted the real need for the development and availability of more effective oximes for human use, a search that has been going on for up to 30 years. However, despite the demonstration of more effective and safe oximes in animal experiments, no additional oximes have been licensed for human use. HI-6, (1-[[[4(aminocarbonyl)-pyridinio]methoxy]methyl]-2(hydroxyimino)pyridinium dichloride; CAS 34433-31-3) has been studied intensively and has been proved effective in a variety of species including non-human primates and appears from clinical experience to be safe in humans. These studies have led to the fielding of HI-6 for use against nerve agents by the militaries of the Czech republic, Sweden, Canada and under certain circumstances the Organisation for the Prohibition of Chemical Weapons. Nevertheless HI-6 has not been granted a license for clinical use, must be used only under restricted guidelines and is not available for civilian use as far as is known. This article will highlight those factors relating to HI-6 that pertain to the licensing of new compounds of this type, including the mechanism of action, the clinical and pre-clinical demonstration of safety and its efficacy against a variety of nerve agents particularly in non-human primates, since no relevant human population exists. This article also contains important data on the use of HI-6 in baboons, which has not been available previously. The article also discusses the possibility of successful therapy with HI-6 against poisoning in humans relative to doses used in non-human primates and relative to its ability to reactivate inhibited human AChE.

The Role of Oximes in the Treatment of Nerve Agent Poisoning in Civilian Casualties

There are important differences between on-target military attacks against relatively well protected Armed Forces and nerve agent attacks initiated by terrorists against a civilian population. In contrast to military personnel, civilians are unlikely to be pre-treated with pyridostigmine and protected by personal protective equipment. Furthermore, the time after exposure when specific therapy can first be administered to civilians is likely to be delayed. Even conservative estimates suggest a delay between exposure and the first administration of atropine/oxime of at least 30 minutes. The organophosphorus nerve agents are related chemically to organophosphorus insecticides and have a similar mechanism of toxicity, but a much higher mammalian acute toxicity, particularly via the dermal route. Nerve agents phosphonylate a serine hydroxyl group in the active site of the enzyme, acetylcholinesterase (AChE), which results in accumulation of acetylcholine and, in turn, causes enhancement and prolongation of cholinergic effects and depolarisation blockade. The rate of spontaneous reactivation of AChE is variable, which partly accounts for differences in acute toxicity between the nerve agents. With soman in particular, an additional reaction occurs known as 'aging'. This consists of monodealkylation of the dialkylphosphonyl enzyme, which is then resistant to spontaneous hydrolysis and reactivation by oximes. Monodealkylation occurs to some extent with all dialkylphosphonylated AChE complexes; however, in general, is only of clinical importance in relation to the treatment of soman poisoning, where it is a very serious problem. With soman, aging occurs so fast that no clinically relevant spontaneous reactivation of AChE occurs before aging has taken place. Hence, recovery of function depends on resynthesis of AChE. As a result, it is important that an oxime is administered as soon after soman exposure as possible so that some reactivation of AChE occurs before all the enzyme becomes aged. Even though aging occurs more slowly and reactivation occurs relatively rapidly in the case of nerve agents other than soman, early oxime administration is still clinically important in patients poisoned with these agents. Experimental studies on the treatment of nerve agent poisoning have to be interpreted with caution. Some studies have used prophylactic protocols, whereas the drugs concerned (atropine, oxime, diazepam) would only be given to a civilian population after exposure. The experimental use of pyridostigmine before nerve agent exposure, although rational, is not of relevance in the civilian context. With the possible exception of the treatment of cyclosarin (GF) and soman poisoning, when HI-6 might be preferred, a review of available experimental evidence suggests that there are no clinically important differences between pralidoxime, obidoxime and HI-6 in the treatment of nerve agent poisoning, if studies employing pre-treatment with pyridostigmine are excluded.

Effectivity of new acetylcholinesterase reactivators in treatment of cyclosarin poisoning in mice and rats

The present study was performed to assess and compare a therapeutic efficacy of obidoxime, HI-6, BI-6 and HS-6 administered in equimolar doses and combined with atropine in cyclosarin-poisoned mice and rats. It was demonstrated that all the therapeutic regimens tested, were able to decrease the cyclosarin-induced toxicity significantly - at least 1.5 times. Higher therapeutic ratios, almost three times, were achieved in rats in comparison with mice. The highest therapeutic ratio was achieved for therapeutic regimen consisting of HI-6 and atropine in both mice and rats. Obidoxime was the least effective oxime in the treatment of cyclosarin intoxication. The BI-6 oxime was significantly more efficacious than obidoxime (in both mice and rats) and HS-6 (in rats) but its effectiveness did not reach the efficacy of HI-6.

Antidotal treatment of GF-agent intoxication in mice with bispyridinium oximes

It was shown that intoxications with GF-agent are rather resistant to convential oxime therapy; therefore, the development of new oximes in an effort to improve this unsatisfactory situation continues. Upon screening in vitro reactivation test for oximes, that were either newly synthesized at our department, or those that have never been tested for reactivation of GF-inhibited acetylcholinesterase (AChE), three oximes {(1,4-bis(4-hydroxyiminomethylpyridinium)butane dibromide) (K033); (1-(2-hydroxyiminomethylpyridinium)-3-(3-carbamoylpyridinium)-2-oxa-propane dichloride) (HS-6); and (1-(2-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium)-but-2-ene dibromide) (BI-6)} with the highest reactivation potency were chosen for in vivo testing in our study. 1,3-Bis(4-hydroxyiminomethylpyridinium)-2-oxa-propane dibromide) (obidoxime); (1-(2-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium)-2-oxa-propane dichloride) (HI-6); and (1,1-bis(4-hydroxyiminomethylpyridinium)-methane dibromide) (methoxime) were chosen for comparison as a standard antidotal treatment. All the oximes were applied at the same proportion of their LD50 value (5%), and because of the different acute toxicity of the oximes, the molar concentrations of their solutions for intramuscular (i.m.) administration were considerably different. The highest therapeutic ratio was achieved for therapeutic regimen consisting of HI-6 and atropine. The significantly (P < 0.05) lowest effectivity in treatment of supralethal GF-agent poisoning in comparison with all the other therapeutic regimens, was surprisingly observed for methoxime. HS-6, K033 and BI-6 as well as obidoxime were comparably effective antidotes against GF-agent intoxication and their therapeutic ratios were similar.

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.

Delayed neurologic and behavioral effects of subtoxic doses of cholinesterase inhibitors

We tested the hypothesis that pyridostigmine bromide (PB) intake and/or low-level sarin exposure, suggested by some as causes of the symptoms experienced by Persian Gulf War veterans, induce neurobehavioral dysfunction that outlasts their effects on cholinesterase. Adult male Sprague-Dawley rats were treated during 3 weeks with s.c. saline, PB in drinking water (80 mg/l), sarin (62.5 microg/kg; 0.5x LD(50), three times/week s.c.), or PB in drinking water + sarin. Animals were tested for passive avoidance, nociceptive threshold, acoustic startle, and open field activity 2, 4, or 16 weeks after treatment. Two weeks after sarin, acoustic startle was enhanced, whereas distance explored in the open field decreased. These effects were absent with PB + sarin or PB by itself. No effect on any variable was found at 4 weeks, whereas at 16 weeks sarin induced a decrease and PB + sarin induced an increase in habituation in the open field test. Nociceptive threshold was elevated in the PB + sarin group at 16 weeks. No effect of treatment on passive avoidance was noted in any group. Brain regional acetylcholinesterase and cholineacetyltransferase activities were not affected at any time after treatment, but muscarinic receptors were down-regulated in hippocampus, caudate putamen, and mesencephalon in the sarin group at 2 weeks. In conclusion, this study gives further support to the use of PB against nerve agent poisoning and does not support the hypothesis that delayed symptoms experienced by Persian Gulf War veterans could be due to PB, alone or in association with low-level sarin exposure.

Review of oximes in the antidotal treatment of poisoning by organophosphorus nerve agents

The cholinesterase-inhibiting organophosphorus compounds referred to as nerve agents (soman, sarin, tabun, GF agent, and VX) are particularly toxic and are considered to be among the most dangerous chemical warfare agents. Included in antidotal medical countermeasures are oximes to reactivate the inhibited cholinesterase. Much experimental work has been done to better understand the properties of the oxime antidotal candidates including the currently available pralidoxime and obidoxime, the H oximes HI-6 and Hlö-7, and methoxime. There is no single, broad-spectrum oxime suitablefor the antidotal treatment of poisoning with all organophosphorus agents. If more than one oxime is available, the choice depends primarily on the identity of the responsible organophosphorus compound. The H oximes appear to be very promising antidotes against nerve agents because they are able to protect experimental animals from toxic effects and improve survival of animals poisoned with supralethal doses. They appear more effective against nerve agent poisoning than the currently used oximes pralidoxime and obidoxime, especially in the case of soman poisoning. On the other hand, pralidoxime and especially obidoxime seem sufficiently effective to treat poisonings with organophosphorus insecticides that have relatively less toxicity than nerve agents.

A comparison of the efficacy of acetylcholinesterase reactivators against cyclohexyl methylphosphonofluoridate (GF agent) by in vitro and in vivo methods

The purpose of this study was to compare the therapeutic efficacy of a new acetylcholinesterase reactivator, designated BI-6 (1-(2-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium )-2-butene dibromide), with presently used oximes (pralidoxime, obidoxime, methoxime) and H-oximes (HI-6, HLö-7) by in vitro and in vivo methods. In vitro, methoxime seems to be the most efficacious reactivator of GF agent-inhibited acetylcholinesterase because the phosphorylation of acetylcholinesterase by GF agent markedly increases its affinity for the enzyme. The oxime BI-6 is more efficacious than other presently used oximes (pralidoxime, obidoxime) but its reactivating efficacy does not reach the efficacy of H-oximes tested. On the other hand, obidoxime and pralidoxime appear to be very poor reactivators of GF agent-inhibited acetylcholinesterase because the phosphonylation of acetylcholinesterase by GF agent markedly decreases their affinity to the enzyme. In vivo, H oximes (HI-6, HLö-7) are the most efficacious antidotes for the treatment of acute poisoning with GF agent in rats while the presently used oximes such as pralidoxime and obidoxime are practically ineffective. BI-6 and methoxime are more efficacious than pralidoxime and obidoxime, nevertheless their therapeutic efficacy does not reach the efficacy of H oximes. Our results show that the ability of oximes to reactivate GF agent-inhibited acetylcholinesterase in vitro usually corresponds to their therapeutic effects against GF agent in vivo.

Evaluation of combined toxic effects of GB/GF and efficacy of jielin injection against combined poisoning in mice

A computer program (Q-test) was used to evaluate the combined toxic effects of nerve agent GF and its combined form with sarin (GB/GF) in mice. Efficacy of Jielin Injection, the 2-PAM-containing antidote used successfully in China for the treatment of organophosphate pesticide poisoning, was also evaluated and compared with HI-6 against single and combined poisonings. The two agents were basically additive in toxicity when combined. However, toxic signs (convulsions) appeared later in combined poisoning than after exposure to each agent alone. The protective ratio of Jielin Injection against GF poisoning was low but significantly higher when against poisoning by GB or combined agent. When HI-6 was substituted for 2-PAM, the antidote was more effective against poisoning by both single and combined agents. Results of in vitro reactivation of GF-inhibited human erythrocyte acetylcholinesterase by these oximes agreed with the in vivo antidotal efficacy.

Comparison of the efficacy of HI-6 and obidoxime against cyclohexyl methylphosphonofluoridate (GF) in rats

1. The efficacy of HI-6 and obidoxime in combination with atropine on cyclohexyl methylphosphonofluoridate (GF)-induced cholinergic and stressogenic effects in rats was studied. 2. HI-6 sufficiently reactivated cholinesterase activity in blood as well as acetylcholinesterase activity in brain and diaphragm following GF intoxication, and practically eliminated stressogenic effects of GF (an increase in plasma corticosterone level and liver tyrosine aminotransferase activity). 3. Obidoxime had practically no effect on enzyme activity or stressogenic effects of GF agent. 4. These findings confirm that HI-6 has definite advantages over obidoxime in the treatment of intoxication with GF.

Acute toxicity of cyclohexylmethylphosphonofluoridate (CMPF) in rhesus monkeys: serum biochemical and hematologic changes

Changes in serum biochemical and hematological parameters were studied in 20 male rhesus monkeys following acute poisoning by the organophosphate nerve agent cyclohexylmethylphosphonofluoridate (CMPF or GF). Animals were challenged with 5 x LD50 GF (233 micrograms/kg, IM) following pretreatment with pyridostigmine (0.3-0.7 mg/kg per 24 h) and treated with atropine (0.4 mg/kg, IM) and either 2-PAM (25.7 mg/kg, IM) or H16 (37.8 mg/kg, IM) at the onset of clinical signs or at 1 min after exposure. Muscle fasciculations, tremors, or convulsions occurred in 19 of 20 animals. Serum biochemical and hematologic parameters were analyzed 2 days and 7 days after exposure and compared to pre-exposure baseline values. Significant increases in creatine kinase (CK), lactate dehydrogenase (LD), aspartate transaminase (AST), alanine transaminase (ALT) and potassium ion (K+), associated with damage to striated muscle and metabolic acidosis, occurred in both oxime-treated groups 2 days after exposure. Total protein, albumin, red blood cell (RBC) count, hemoglobin concentration (Hb) and hematocrit (Hct), were decreased in both oxime-treated groups at 7 days. The results demonstrate that animals exposed to a single high dose of GF and treated with standard therapy exhibit changes in serum biochemical and hematological indices directly and indirectly associated with their clinical presentations.

Review of oximes available for treatment of nerve agent poisoning

A review was conducted of papers describing the use of N-methyl-2-pyridinealdoxime (PAM), toxogonin or HI-6 as antidotes to the nerve agents tabun, sarin, soman and VX. The review included use of the oxime alone, oxime plus atropine and oxime plus atropine plus diazepam, given therapeutically, i.e. after nerve agent, in all cases. Experiments with any of these compounds given prophylactically were not considered. The review also included protocols of pyridostigmine prophylaxis and oxime-atropine therapy (with or without diazepam). It was difficult to draw conclusions as to the best oxime to use, because of lack of data in many cases. The identity of the oxime did not appear to be important when pyridostigmine prophylaxis was combined with atropine-oxime-diazepam therapy; in these cases, very good protection was observed in guinea pigs against all four nerve agents. The choice of oxime based on the data presently available may well depend on factors other than protection against lethality, such as cost and availability of the oxime and human toxicity of the oxime. This last factor was also reviewed, and the results showed that toxogonin is likely to cause more side-effects than PAM or HI-6. The efficacy of the oximes against the emerging threat agent GF was also reviewed.

Toxicity of the combined nerve agents GB/GF in mice: efficacy of atropine and various oximes as antidotes

The toxicity of a combination of isopropyl methylphosphonofluoridate (sarin; GB) and cyclohexyl methylphosphonofluoridate (GF) and the efficacy of various oxime reactivators in combination with atropine against the combined GB/GF challenge were evaluated in mice. The 24-h s.c. LD50 of the GB/GF combination was 1.15 mumol/kg (1.10-1.21; 95% confidence limits). Mice administered GB/GF displayed typical signs of nerve agent poisoning such as tremors and convulsions, with death most likely due to anoxia subsequent to respiratory arrest. The GB/GF LD50 value was comparable to the s.c. LD50 of 1.35 and 1.21 mumol/kg for GF and GB in mice, respectively. Combining the two nerve agents did not result in potentiation of the toxicity. In combination with atropine sulfate (17.4 mg/kg, i.p.), which alone did not reduce mortality, the oximes tested, 2-PAM, obidoxime and HI-6, were all effective when administered 5 min before 3 x LD50 dose of GB/GF with 24-h ED50 values of 102.5, 18.22 and 1.96 mumol/kg, respectively. Use of the GB/GF combination does not appear to confer any unique toxicity profile and appears to be easily treated with the standard therapy of a cholinolytic and oxime.