The role of oximes in the management of organophosphorus pesticide poisoning

Use of individual auto-injector kits 'IZAS-05' on the contemporary battlefield

In crisis situations, such as transportation catastrophes, terrorist attacks or contamination with chemical warfare agents, it is crucial to properly organize and sensibly conduct rescue operations. Among chemical warfare agents, the most toxic ones are the derivatives of organophosphorus compounds. An individual auto-injector kit 'IZAS-05' contains auto-injectors, which are devices designed for intramuscular administration of drugs in self-aid or buddy-aid on the battlefield. This paper describes in detail the components of the 'IZAS-05' kit, as well as its mode of use and possible contraindications.

Potential of two new oximes in reactivate human acetylcholinesterase and butyrylcholinesterase inhibited by organophosphate compounds: an in vitro study

Organophosphate (OP) compounds exert inhibition on cholinesterase (ChE) activity by irreversibly binding to the catalytic site of the enzyme. Oximes are compounds generally used to reverse the ChE inhibition caused by OP agents. In this study, we compared the in vitro reactivation potency of two new oximes (oxime 1: butane-2,3-dionethiosemicarbazone; oxime 2: 3-(phenylhydrazono) butan-2-one) against the inhibition on acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activities induced by chlorpyrifos, diazinon and malathion. Oximes used clinically (obidoxime and pralidoxime) were used as positive control. For this study, human blood (erythrocytes for AChE determination and plasma for BChE determination) was used and different concentrations of oximes (1-100 μM) were tested. The concentrations of OP used were based on the IC50 for AChE and BChE. Results demonstrated that obidoxime was more effective in reactivate the AChE inhibition induced by OP compounds. However, both newly developed oximes achieved similar reactivations rates that pralidoxime for chlorpyrifos and diazinon-inhibited AChE. For BChE reactivation, none of evaluated oximes achieved positives rates of reactivation, been obidoxime able to reactivate malathion-inhibited BChE only in 24% at the highest concentration. We conclude that both newly developed oximes seem to be promising reactivators of OP-inhibited AChE.

A study on quantum chemical calculations of 3-, 4-nitrobenzaldehyde oximes

The molecular geometry, vibrational frequencies, 1H and 13C NMR chemical shifts, UV-vis spectra, HOMO-LUMO analyses, molecular electrostatic potentials (MEPs), , thermodynamic properties and atomic charges of 3- and 4-Nitrobenzaldehyde oxime (C7H6N2O3) molecules have been investigated by using Hartree-Fock (HF) and density functional theory (DFT/B3LYP) methods with the 6-311++G(d, p) basis set. The calculated optimized geometric parameters (bond lengths and bond angles), the vibrational frequencies calculated and 13C and 1H NMR chemical shifts values for the mentioned compounds are in a very good agreement with the experimental data. Furthermore, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) have been simulated and the transition states, energy band gaps and molecular electrostatic potential (MEP) maps for each oxime compound have been determined. Additionally, we also report the infrared intensities and Raman activities for the compounds under study.

Regeneration of Red Cell Cholinesterase Activity Following Pralidoxime (2-PAM) Infusion in First 24 h in Organophosphate Poisoned Patients

Oximes such as pralidoxime chloride reactivate acetylcholinesterase. However their role in management of organophosphate poisoning is controversial. The study was carried out to find effectiveness of pralidoxime chloride (2-PAM) in regenerating red cell acetyl cholinesterase in first 24 h following administration of it in dose recommended by WHO. Eight patients with OPP [chlorpyriphos (3), phorate (3), dichlorvos (1) and monocrotophos (1) who fulfilled the criteria for inclusion were investigated. In addition to decontamination and atropine, all these patients were administered 30 mg/kg body wt of 2-PAM as bolus dose followed by 7.5 mg/kg body wt/h with maximum dose being 500 mg/h as continuous infusion till first 24 h. Red cell AChE activity was estimated every 15 min for first 4 h, one hourly for next 4 h and then 2 hourly till 24 h and subsequently without 2-PAM every 12 h till 7 days or discharge or death which ever earlier. In all the patients maximum increase in activity was observed in first 4 h following which rise was very slow despite continued 2-PAM infusion and reaching a steady state in 20 h in all the cases. The increase in red cell AChE activity observed in diethyl group at 24 h of 2-PAM infusion was 154% vs. 81% in dimethyl group. At 7 days the increase in activity was 215% vs. 118% respectively. However on multiple repeated ANOVA, no statistically significant difference was observed between diethyl and dimethyl groups at admission and discharge (P > 0.05). Similarly no significant difference was observed in three groups when patients were categorized according to WHO classification of organophosphates (P > 0.05). The maximum increase in red cell AChE activity occurs in first 4 h of 2-PAM administration followed by a slow increase despite 2-PAM infusion till 24 h.

Does modulation of organic cation transporters improve pralidoxime activity in an animal model of organophosphate poisoning?

OBJECTIVES

Pralidoxime is an organic cation used as an antidote in addition to atropine to treat organophosphate poisoning. Pralidoxime is rapidly eliminated by the renal route and thus has limited action. The objectives of this work were as follows. 1) Study the role of organic cation transporters in the renal secretion of pralidoxime using organic cation transporter substrates (tetraethylammonium) and knockout mice (Oct1/2⁻/⁻; Oct3⁻/⁻). 2) Assess whether sustained high plasma concentrations increase pralidoxime antidotal activity toward paraoxon-induced respiratory toxicity.

SETTING

INSERM U705, Faculté de Pharmacie, Université Paris Descartes, 4 Avenue de l'Observatoire, 75006 Paris, France.

SUBJECTS

Rodents: Knockout mice (Oct1/2⁻/⁻; Oct3⁻/⁻) and Sprague-Dawley rats.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

In rats, the renal clearance of pralidoxime was 3.6-fold higher than the creatinine clearance. Pretreatment with tetraethylammonium (75 mg/kg) in rats or deficiencies in organic cation transporters 1 and 2 in mice (Oct1/2⁻/⁻) resulted in a significant increase in plasma pralidoxime concentrations. Lack of Oct3 did not alter plasma pralidoxime concentrations. The antidotal activity of pralidoxime (50 mg/kg intramuscularly) was longer and with greater effect, resulting in a return to normal values when administered to rats pretreated with tetraethylammonium.

CONCLUSIONS

Pralidoxime is secreted in rats and mice by renal Oct1 and/or Oct2 but not by Oct3. Modulation of organic cation transporter activity increased the plasma pralidoxime concentrations and the antidotal effect of pralidoxime with sustained return within the normal range of respiratory variables in paraoxon-poisoned rats. These results suggest a promising approach in an animal model toward the increase in efficiency of pralidoxime. However, further studies are needed before these results are extended to human poisoning.

Effect of acute soman exposure on GABA(A) receptors in rat hippocampal slices and cultured hippocampal neurons

Exposure of the central nervous system to organophosphorus (OP) nerve agents causes seizures and neuronal cell death. Benzodiazepines are commonly used to treat seizures induced by OPs. However, it is known that soman-induced seizures are particularly resistant to benzodiazepine treatment, as compared with other OPs. This study investigated the effect of soman on γ-aminobutyric acid (GABA) neurotransmission in acute rat hippocampal slices and the surface expression of GABA(A) receptors in cultured rat hippocampal neurons. Results showed that GABA-mediated inhibitory post synaptic currents (IPSCs) are significantly reduced by soman in a concentration-dependent manner in acute rat hippocampal slices. Furthermore, confocal microscopic and cell-based ELISA assays revealed that soman caused rapid internalization of GABA(A) receptors in cultured rat hippocampal neurons. The effect of soman on GABA(A)R endocytosis was not due to inhibition of acetylcholinesterase (AChE) because (1) the acetylcholine muscarinic receptor antagonist atropine did not block soman-induced GABA(A)R endocytosis; and (2) physostigmine, at concentrations that completely inhibit AChE activity, did not cause GABA(A)R endocytosis. Moreover, blocking of the N-methyl-D-aspartate (NMDA) receptors by 2-amino-5-phosphonovalerate (APV) had no effect on soman-induced GABA(A)R endocytosis, suggesting that the soman effect was not secondary to glutamate receptor over activation. Regardless of the exact mechanism, the observation that soman induces rapid GABA(A)R endocytosis may have significant implications in the development of effective countermeasures against soman-induced seizures.

Cholinesterases: Structure, Role, and Inhibition

Acetilkolinesteraza (AChE; E.C. 3.1.1.7) i butirilkolinesteraza (BChE; E.C. 3.1.1.8) enzimi su koji se zbog svoje uloge u organizmu intenzivno istražuju unutar područja biomedicine i toksikologije. Iako strukturno homologni, ovi enzimi razlikuju se prema katalitičkoj aktivnosti, odnosno specifičnosti prema supstratima koje mogu hidrolizirati te selektivnosti za vezanje mnogih liganada. U ovom radu dan je pregled dosadašnjih istraživanja kolinesteraza i njihovih interakcija s ligandima i inhibitorima te su izdvojene aminokiseline aktivnog mjesta koje sudjeluju u tim interakcijama.

Long-term evaluation of organophosphate toxicity and antidotal therapy in co-cultures of spinal cord and muscle tissue

Victims of nerve agents basically require antidotal treatment. There is need for novel antidotes and for therapeutic procedures that are specifically adapted to these patients. To cope with this challenge, in vitro test systems which are easy to handle and allow for conducting long-term studies would be of great benefit. The present work introduces co-cultures of spinal cord and muscle tissue as ex vivo testing systems meeting these criteria. Cell cultures in which functional neuromuscular synapses formed ex vivo were prepared from embryonic mice. Spontaneous muscle activity was recorded by video microscopy. Muscle contractions involved intact neuromuscular transmission as indicated by the effect of succinylcholine, a muscle relaxant that completely abolished muscle activity. At a concentration of 0.75 μM the nerve agent VX reduced the frequency of spontaneous muscle contractions by about 75%. Subsequent application of obidoxime re-established muscle movements. After 24 h of antidotal treatment, muscle activity approached the level of sham-treated cultures and remained stable over the following week. In summary, co-cultures of spinal cord and muscle tissue are promising tools for evaluating the success of antidotal treatment following organophosphate intoxication over a period of at least seven days.

Evolution of and perspectives on therapeutic approaches to nerve agent poisoning

After more than 70 years of considerable efforts, research on medical defense against nerve agents has come to a standstill. Major progress in medical countermeasures was achieved between the 50s and 70s with the development of anticholinergic drugs and carbamate-based pretreatment, the introduction of pyridinium oximes as antidotes, and benzodiazepines in emergency treatments. These drugs ensure good protection of the peripheral nervous system and mitigate the acute effects of exposure to lethal doses of nerve agents. However, pyridostigmine and cholinesterase reactivators currently used in the armed forces do not protect/reactivate central acetylcholinesterases. Moreover, other drugs used are not sufficiently effective in protecting the central nervous system against seizures, irreversible brain damages and long-term sequelae of nerve agent poisoning.New developments of medical counter-measures focus on: (a) detoxification of organophosphorus molecules before they react with acetylcholinesterase and other physiological targets by administration of stoichiometric or catalytic scavengers; (b) protection and reactivation of central acetylcholinesterases, and (c) improvement of neuroprotection following delayed therapy.Future developments will aim at treatment of acute and long-term effects of low level exposure to nerve agents, research on alternative routes for optimizing drug delivery, and therapies. Though gene therapy for in situ generation of bioscavengers, and cell therapy based on neural progenitor engraftment for neuronal regeneration have been successfully explored, more studies are needed before practical medical applications can be made of these new approaches.

Optimal choice of acetylcholinesterase reactivators for antidotal treatment of nerve agent intoxication

The studies dealing with mechanism of organophosphates (OP)/nerve agent action, prophylaxis and treatment of intoxications is a very hot topic at present. Though the research is very intensive, unfortunately, up to now, there is not universal or significantly better reactivator sufficiently effective against all nerve agents/OP when compared with presently available oximes (pralidoxime, methoxime, obidoxime, trimedoxime, HI-6). The use of the most effective reactivator (HI-6) using simple type of autoinjector (e.g. ComboPen) is strictly limited because of decomposition of HI-6 in solution. Thanks to better solubility it is clear that another salt of HI-6 (dimethanesulfonate, HI-6 DMS) is more convenient for the use as antidote against nerve agents in the autoinjector than HI-6 chloride (Cl). It was clearly demonstrated that reactivation potency of HI-6 DMS in comparison with HI-6 Cl in vivo was the same and bioavailability of HI-6 DMS is better than that of HI-6 Cl. Three chambered autoinjector allows administration of all three antidotes (atropine, reactivator, diazepam) simultaneously. Moreover, the content of chambers can be changed according to proposed requirements. Possible way to solve the problem of universal reactivator could be the use of two reactivators. Three chambered autoinjector is an ideal device for this purpose.

A comparison of tabun-inhibited rat brain acetylcholinesterase reactivation by three oximes (HI-6, obidoxime, and K048) in vivo detected by biochemical and histochemical techniques

Tabun belongs to the most toxic nerve agents. Its mechanism of action is based on acetylcholinesterase (AChE) inhibition at the peripheral and central nervous systems. Therapeutic countermeasures comprise administration of atropine with cholinesterase reactivators able to reactivate the inhibited enzyme. Reactivation of AChE is determined mostly biochemically without specification of different brain structures. Histochemical determination allows a fine search for different structures but is performed mostly without quantitative evaluation. In rats intoxicated with tabun and treated with a combination of atropine and HI-6, obidoxime, or new oxime K048, AChE activities in different brain structures were determined using biochemical and quantitative histochemical methods. Inhibition of AChE following untreated tabun intoxication was different in the various brain structures, having the highest degree in the frontal cortex and reticular formation and lowest in the basal ganglia and substantia nigra. Treatment resulted in an increase of AChE activity detected by both methods. The highest increase was observed in the frontal cortex. This reactivation was increased in the order HI-6 < K048 < obidoxime; however, this order was not uniform for all brain parts studied. A correlation between AChE activity detected by histochemical and biochemical methods was demonstrated. The results suggest that for the mechanism of action of the nerve agent tabun, reactivation in various parts of the brain is not of the same physiological importance. AChE activity in the pontomedullar area and frontal cortex seems to be the most important for the therapeutic effect of the reactivators. HI-6 was not a good reactivator for the treatment of tabun intoxication.

In vitro ability of currently available oximes to reactivate organophosphate pesticide-inhibited human acetylcholinesterase and butyrylcholinesterase

We have in vitro tested the ability of common, commercially available, cholinesterase reactivators (pralidoxime, obidoxime, methoxime, trimedoxime and HI-6) to reactivate human acetylcholinesterase (AChE), inhibited by five structurally different organophosphate pesticides and inhibitors (paraoxon, dichlorvos, DFP, leptophos-oxon and methamidophos). We also tested reactivation of human butyrylcholinesterase (BChE) with the aim of finding a potent oxime, suitable to serve as a “pseudocatalytic” bioscavenger in combination with this enzyme. Such a combination could allow an increase of prophylactic and therapeutic efficacy of the administered enzyme. According to our results, the best broad-spectrum AChE reactivators were trimedoxime and obidoxime in the case of paraoxon, leptophos-oxon, and methamidophos-inhibited AChE. Methamidophos and leptophos-oxon were quite easily reactivatable by all tested reactivators. In the case of methamidophos-inhibited AChE, the lower oxime concentration (10⁻⁵ M) had higher reactivation ability than the 10⁻⁴ M concentration. Therefore, we evaluated the reactivation ability of obidoxime in a concentration range of 10⁻³–10⁻⁷ M. The reactivation of methamidophos-inhibited AChE with different obidoxime concentrations resulted in a bell shaped curve with maximum reactivation at 10⁻⁵ M. In the case of BChE, no reactivator exceeded 15% reactivation ability and therefore none of the oximes can be recommended as a candidate for “pseudocatalytic” bioscavengers with BChE.

Oximes for acute organophosphate pesticide poisoning

BACKGROUND

Acute organophosphorus pesticide poisoning causes tens of thousands of deaths each year across the developing world. Standard treatment involves administration of intravenous atropine and oxime to reactivate inhibited acetylcholinesterase. The clinical usefulness of oximes, such as pralidoxime and obidoxime, has been challenged over the past 20 years by physicians in many parts of the world.

OBJECTIVES

To quantify the effectiveness and safety of the administration of oximes in acute organophosphorus pesticide-poisoned patients.

SEARCH STRATEGY

We searched both English and Chinese databases: Cochrane Injuries Group Specialised Register, Cochrane Central Register of Controlled Trials (The Cochrane Library), MEDLINE (Ovid SP), EMBASE (Ovid SP), ISI Web of Science: Science Citation Index Expanded (SCI-EXPANDED), ISI Web of Science: Conference Proceedings Citation Index-Science (CPCI-S) and the Chinese language databases CNKI and WANGFANG. All searches were run in September 2009.

SELECTION CRITERIA

Articles that could possibly be RCTs were retrieved to determine if they were randomised.

DATA COLLECTION AND ANALYSIS

The published methodology of three RCTs was not clear. We contacted the principal authors of these, but did not obtain further information.

MAIN RESULTS

Seven pralidoxime RCTs were found. Three RCTs including 366 patients studied pralidoxime vs placebo and four RCTs including 479 patients compared two or more different doses. These trials found quite disparate results with treatment effects ranging from benefit to harm. However, many studies did not take into account several issues important for outcomes. In particular, baseline characteristics were not balanced, oxime doses varied widely, there were substantial delays to treatment, and the type of organophosphate was not taken into account. Only one RCT compared the World Health Organization (WHO) recommended doses with placebo. This trial showed no clinical benefits and a trend towards harm in all sub-groups, despite clear evidence that these doses reactivated acetylcholinesterase in the blood.

AUTHORS' CONCLUSIONS

Current evidence is insufficient to indicate whether oximes are harmful or beneficial. The WHO recommended regimen (30 mg/kg pralidoxime chloride bolus followed by 8 mg/kg/hr infusion) is not supported. Further RCTs are required to examine other strategies and regimens. There are many theoretical and practical reasons why oximes may not be useful, particularly for late presentations of dimethyl OP and those with a large excess of OP that simply re-inhibits reactivated enzymes. Future studies should screen for patient sub-groups that may benefit and may need flexible dosing strategies as clinical effectiveness and doses may depend on the type of OP.

Chromatographic analysis of toxic phosphylated oximes (POX): a brief overview

Poisoning with organophosphorus compounds (OP), e.g. pesticides and nerve agents, causes inhibition of acetylcholinesterase (AChE) by phosphylation of the active site serine residue. Consequently, accumulation of stimulating acetylcholine in the synaptic cleft induces cholinergic crisis which ultimately may lead to death. For standard causal therapy, enzyme reactivators are administered representing oxime derivatives of quarternary pyridinium compounds, e.g. pralidoxime (2-PAM), obidoxime and HI 6. The mechanism of action includes removal of the phosphyl moiety by a nucleophilic attack of the oximate molecule substituting the enzyme and forming a phosphylated oxime (POX). POX is produced in stoichiometric amounts of reactivated enzyme and exhibits a significantly enhanced toxicity (inhibition rate constant) when compared to the parent OP. However, stability of POX under physiological conditions appears to be highly limited. Nevertheless, the presence of POX reveals a potential critical issue for both therapeutic efficacy in vivo and pharmacokinetic and pharmacodynamic (PK-PD) modelling based on cholinesterase activity data. Detailed characterization represents an important need for elaboration of the entire oxime pharmacology.Nevertheless, reports on POX toxicity and analysis are quite rare and may therefore be indicative of the challenge of POX analysis. This review provides a concise overview of chromatographic approaches applied to POX separation. Chromatography represents the key technology for POX purification and quantification in kinetic in vitro studies using buffers and biological fluids. Applications based on reversed-phase chromatography (RPC), ion pair chromatography (IPC) and an affinity approach as well as thin layer chromatography (TLC) are discussed and novel applications and data are presented.

Acute aldicarb toxicity in dogs: 15 cases (2001-2009)

OBJECTIVE

To describe the common clinical signs, laboratory abnormalities, treatment, and prognosis associated with acute aldicarb toxicosis in dogs.

DESIGN

Retrospective observational study from 2001 to 2009.

SETTING

Urban referral hospital.

ANIMALS

Fifteen client-owned dogs.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The most common clinical signs associated with acute aldicarb toxicosis were vomiting, ptyalism, diarrhea, and tremors. Of the 15 dogs, 11 were admitted to the hospital for treatment, 2 were euthanized at presentation and 2 were discharged against medical advice following minimal treatment and lost to follow-up. Laboratory abnormalities included lactic acidosis and hyperglycemia in 12 and 9 patients, respectively. Treatment of hospitalized dogs included induction of emesis with apomorphine (4 dogs), activated charcoal (5), IV fluids (11), atropine (7), methocarbamol (3), diazepam (1), pralidoxime (1) and diphenhydramine (1). Ten of 11 hospitalized dogs survived to discharge; 1 was euthanized following a respiratory arrest after 36 hours of hospitalization. One patient received mechanical ventilation and treatment for pneumonia before discharge from the hospital. The median duration of hospitalization was 22 hours (range 12-168 h).

CONCLUSIONS

Acute aldicarb toxicosis carries a good prognosis for survival and hospital discharge with treatment. Supportive care should be considered for at least 18-24 hours to monitor for response to therapy and development of respiratory failure.

Kinetic analysis of interactions of paraoxon and oximes with human, Rhesus monkey, swine, rabbit, rat and guinea pig acetylcholinesterase

Previous in vitro studies showed marked species differences in the reactivating efficiency of oximes between human and animal acetylcholinesterase (AChE) inhibited by organophosphorus (OP) nerve agents. These findings provoked the present in vitro study which was designed to determine the inhibition, aging, spontaneous and oxime-induced reactivation kinetics of the pesticide paraoxon, serving as a model compound for diethyl-OP, and the oximes obidoxime, pralidoxime, HI 6 and MMB-4 with human, Rhesus monkey, swine, rabbit, rat and guinea pig erythrocyte AChE. Comparable results were obtained with human and monkey AChE. Differences between human, swine, rabbit, rat and guinea pig AChE were determined for the inhibition and reactivation kinetics. A six-fold difference of the inhibitory potency of paraoxon with human and guinea pig AChE was recorded while only moderate differences of the reactivation constants between human and animal AChE were determined. Obidoxime was by far the most effective reactivator with all tested species. Only minor species differences were found for the aging and spontaneous reactivation kinetics. The results of the present study underline the necessity to determine the inhibition, aging and reactivation kinetics in vitro as a basis for the development of meaningful therapeutic animal models, for the proper assessment of in vivo animal data and for the extrapolation of animal data to humans.

Effect of different buffers on kinetic properties of human acetylcholinesterase and the interaction with organophosphates and oximes

Acetylcholinesterase (AChE) is the primary target of organophosphorus compounds (OP). The investigation into interactions between AChE, OP and oximes in vitro may be affected by the experimental conditions, e.g. by the buffer system. Hence, it was tempting to investigate the Michaelis-Menten kinetics and the inhibition and reactivation kinetics of paraoxon-ethyl, sarin, soman and VX in the presence of phosphate, MOPS, Tyrode and TRIS buffer with human AChE. Compared to phosphate buffer, the inhibition and reactivation kinetics of human erythrocyte AChE were markedly changed by TRIS and in part by MOPS, whereas Tyrode showed similar results to phosphate buffer. These results indicate an effect of the tested buffers on the properties of AChE, and an interaction between OP and oximes has to be considered for the design of in vitro studies and may impair the comparison of data from different laboratories. In view of the comparability of human in vitro kinetic data determined with phosphate buffer with data from human OP poisoning, it seems to be a suitable buffer for the investigation into interactions between AChE, OP and oximes.

Development of a prolonged calcium plateau in hippocampal neurons in rats surviving status epilepticus induced by the organophosphate diisopropylfluorophosphate

Organophosphate (OP) compounds are among the most lethal chemical weapons ever developed and are irreversible inhibitors of acetylcholinesterase. Exposure to majority of OP produces status epilepticus (SE) and severe cholinergic symptoms that if left untreated are fatal. Survivors of OP intoxication often suffer from irreversible brain damage and chronic neurological disorders. Although pilocarpine has been used to model SE following OP exposure, there is a need to establish a SE model that uses an OP compound in order to realistically mimic both acute and long-term effects of nerve agent intoxication. Here we describe the development of a rat model of OP-induced SE using diisopropylfluorophosphate (DFP). The mortality, behavioral manifestations, and electroencephalogram (EEG) profile for DFP-induced SE (4 mg/kg, sc) were identical to those reported for nerve agents. However, significantly higher survival rates were achieved with an improved dose regimen of DFP and treatment with pralidoxime chloride (25 mg/kg, im), atropine (2 mg/kg, ip), and diazepam (5 mg/kg, ip) making this model ideal to study chronic effects of OP exposure. Further, DFP treatment produced N-methyl-D-aspartate (NMDA) receptor-mediated significant elevation in hippocampal neuronal [Ca(2+)](i) that lasted for weeks after the initial SE. These results provided direct evidence that DFP-induced SE altered Ca(2+) dynamics that could underlie some of the long-term plasticity changes associated with OP toxicity. This model is ideally suited to test effective countermeasures for OP exposure and study molecular mechanisms underlying neurological disorders following OP intoxication.

Tabun-inhibited rat tissue and blood cholinesterases and their reactivation with the combination of trimedoxime and HI-6 in vivo

Up to now, intensive attempts to synthesize a universal reactivator able to reactivate cholinesterases inhibited by all types of nerve agents/organophosphates were not successful. Therefore, another approach using a combination of two reactivators differently reactivating enzyme was used: in rats poisoned with tabun and treated with combination of atropine (fixed dose) and different doses of trimedoxime and HI-6, changes of acetylcholinesterase activities (blood, diaphragm and different parts of the brain) were studied. An increase of AChE activity was observed following trimedoxime treatment depending on its dose; HI-6 had very low effect. Combination of both oximes showed potentiation of their reactivation efficacy; this potentiation was expressed for peripheral AChE (blood, diaphragm) and some parts of the brain (pontomedullar area, frontal cortex); AChE in the basal ganglia was relatively resistant. These observations suggest that the action of combination of oximes in vivo is different from that observed in vitro.

In vivo oxime administration does not influence Ellman acetylcholinesterase assay results

Organophosphorus compounds (OPs) are potent inhibitors of acetylcholinesterase (AChE). Treatment for OP poisoning is by administration of atropine sulfate, an oxime, and diazepam. Oximes such as 2-PAM are used to reactivate OP-inhibited AChE so as to restore normal enzymatic function and serve as a true antidote. There are reports of non-enzymatic hydrolysis by oximes of acetylthiocholine in in vitro preparations in the widely used Ellman assay for AChE activity, which may confound the interpretation of AChE activity by producing elevated results. The purpose of this experiment was to determine if there is appreciable interference by therapeutic levels of oximes on the results of the Ellman assay in assessing AChE reactivation by oxime compounds in vivo. When therapeutic doses of oximes (2-PAM, HI-6, MMB-4, or MINA) were administered intramuscularly to guinea pigs and samples collected 60 min later, there was no statistical difference between oxime and saline control groups in measured AChE activity in various tissue samples, including blood. With appropriate dilution of samples prior to spectrophotometric assay, the Ellman assay is an acceptable method to measure in vivo oxime reactivation of inhibited AChE. Inclusion of an oxime control group to insure that this particular type of interference is not causing false readings in the assay is a prudent step.

Evaluation of nine oximes on in vivo reactivation of blood, brain, and tissue cholinesterase activity inhibited by organophosphorus nerve agents at lethal dose

The capability of several oximes (HI-6, HLö7, MMB-4, TMB-4, carboxime, ICD 585, ICD 692, ICD 3805, and 2-PAM) to reactivate in vivo AChE inhibited by the nerve agents sarin, cyclosarin, VX, or VR in blood, brain regions, and peripheral tissues in guinea pigs was examined and compared. Animals were injected subcutaneously with 1.0 LD(50) of sarin, cyclosarin, VR, or VX, and treated intramuscularly 5 min later with one of these compounds. Toxic signs and lethality were monitored, and tissue AChE activities were determined at 60 min after nerve agent. The animals exposed to sarin or cyclosarin, alone or with non-oxime treatment, some died within 60 min; however, when treated with an oxime, no animal died. For VR or VX, all animals survived for 60 min after exposure, with or without non-oxime or oxime therapy. These nerve agents caused differential degrees of inhibition: in whole blood sarin = cyclosarin > VR = VX; in brain regions sarin > cyclosarin > VX > VR; and in peripheral tissues sarin > VX > cyclosarin > VR. These oximes exhibited differential potency in reactivating nerve agent-inhibited AChE in various peripheral tissues, but not AChE activity in the brain regions. There was no difference in the AChE reactivating potency between the dichloride and dimethanesulfonate salts of HI-6. AChE inhibited by sarin was the most and cyclosarin the least susceptible to oxime reactivation. Overall, MMB-4 appeared to be, among all oximes tested, the most effective in vivo AChE reactivator against the broadest spectrum of nerve agents.

Minireview: does in-vitro testing of oximes help predict their in-vivo action after paraoxon exposure?

K-oximes have recently been developed in the search for efficacious broad-band reactivators of acetylcholinesterase (AChE) inhibited by organophosphorus compounds (OPC). Before clinical use, their toxicity and efficacy need to be assessed, and there is clear demand for simple in vitro tests that can predict in vivo performance. This article summarizes our in vitro data obtained for conventional and experimental oximes in human and rat blood exposed to the OPC paraoxon and correlates them with our in vivo results. The intrinsic AChE inhibitory activity of oximes, as reflected by their in vitro IC(50), is strongly correlated with their LD(50) (rat): oximes with a high IC(50) (K-27, K-48, pralidoxime and obidoxime) also show a high LD(50) and are thus relatively non-toxic, whereas oximes K-105, K-108 and K-113 have a low IC(50), a low LD(50) and are far more toxic. The IC(50) is also correlated with the in vivo capacity to protect from paraoxon-induced mortality: oximes with a higher IC(50) reduce the relative risk of death more. In contrast, the protective ability as assessed in vitro by the slope of the IC(50) shift (tanalpha), is not correlated with in vivo protection from paraoxon-induced mortality: the best in vivo protectors (K-27 and K-48) show a much lower tanalpha value (around 2) than K-110 and K-113 (tanalpha around 10), which hardly reduce the relative risk of death after paraoxon exposure. The partition coefficient logP of the individual oximes is inversely correlated with their IC(50) and with their LD(50) and is therefore an indicator of toxicity: strongly hydrophilic oximes tend to be less toxic than less hydrophilic ones. These data highlight the good predictive value of in vitro IC(50) testing for in vivo toxicity and the limited practical significance of in vitro assessment of protective potency.