Reactivation of cyclosarin-inhibited rat brain acetylcholinesterase by pyridinium--oximes

Pralidoxime-like reactivator with increased lipophilicity - Molecular modeling and in vitro study

Acetylcholinesterase (AChE, EC 3.1.1.7) reactivators (2-PAM, trimedoxime, obidoxime, asoxime) have become an integral part of antidotal treatment in cases of nerve agent and organophosphorus (OP) pesticide poisonings. They are often referred to as specific antidotes due to their ability to restore AChE function when it has been covalently inhibited by an OP compound. Currently available commercial reactivators exhibit limited ability to penetrate the blood-brain barrier, where reactivation of inhibited AChE is crucial. Consequently, there have been numerous efforts to discover more brain-penetrating AChE reactivators. In this study, we examined a derivative of 2-PAM designed to possess increased lipophilicity. This enhanced lipophilicity was achieved through the incorporation of a benzyl group into its molecular structure. Initially, a molecular modeling study was conducted, followed by a comparison of its reactivation efficacy with that of 2-PAM against 10 different AChE inhibitors in vitro. Unfortunately, this relatively significant structural modification of 2-PAM resulted in a decrease in its reactivation potency. Consequently, this derivative cannot be considered as a broad-spectrum AChE reactivator.

Oxime-mediated reactivation of organophosphate-inhibited acetylcholinesterase with emphasis on centrally-active oximes

This review provides an overview of the global research leading to the large number of compounds developed as reactivators of acetylcholinesterase inhibited by a variety of organophosphate compounds, most of which are nerve agents but also some insecticides. A number of these organophosphates are highly toxic and effective therapy by reactivators contributes to saving lives. Two major challenges for more effective therapy with reactivators are identification of a broad spectrum reactivator efficacious against a variety of organophosphate structures, and a reactivator that can cross the blood-brain barrier to protect the brain. The most effective of the reactivators developed are the nucleophilic pyridinium oximes, which bear a permanent positive charge from the quaternary nitrogen in the pyridinium ring. The permanent positive charge retards the oximes from crossing the blood-brain barrier and therefore restoration of normal cholinergic function in the brain is unlikely. A number of laboratories have developed nucleophiles, mostly oximes, that are theorized to cross the blood-brain barrier by several strategies. At the present time, no reactivator is optimally broad spectrum across the wide group of organophosphate chemistries. Some oximes, including the substituted phenoxyalkyl pyridinium oximes invented by our laboratories, have the potential to provide neuroprotection in the brain and show evidence of efficacy against both nerve agent and insecticidal chemistries, so these novel oximes have promise for future development. This article is part of the special issue entitled 'Acetylcholinesterase Inhibitors: From Bench to Bedside to Battlefield'.

Slight difference in the isomeric oximes K206 and K203 makes huge difference for the reactivation of organophosphorus-inhibited AChE: Theoretical and experimental aspects

Studies with oximes have been extensively developed to design new reactivators with better efficiency, and greater spectrum of action. In this study, we aimed to analyze the influence of the Carbamoyl group position change in two isomeric oximes, K203 and K206, on the reactivation percentage of Mus musculus Acetylcholinesterase (MmAChE), inhibited by different nerve agents. Theoretical calculations were performed to assess the difference for the oxime activity with inhibited AChE-complexes and the factors that govern this difference. Comparing theoretical and experimental data, it is possible to observe that this change between the oximes results in different reactivation percentage for the same nerve agent, due to the different interaction modes and activation energy for the studied systems.

A Comparison of the Neuroprotective Efficacy of Newly Developed Oximes (K156, K203) and Currently Available Oximes (Obidoxime, HI-6) in Cyclosarin-poisoned Rats

The neuroprotective effects of newly developed oximes (K156, K203) and currently available oximes (obidoxime, HI-6) in combination with atropine in rats poisoned with cyclosarin were studied. The cyclosarin-induced neurotoxicity was monitored using a functional observational battery 24 hours after cyclosarin challenge. The results indicate that a newly developed oxime K156 is able to counteract slightly cyclosarin-induced neurotoxicity while another newly developed oxime K203 is completely ineffective in reducing cyclosarin-induced neurotoxic signs and symptoms. The neuroprotective efficacy of K156 is comparable with commonly used obidoxime and oxime HI-6. Thus, none of the newly developed oximes achieves better neuroprotective efficacy than both commonly used oximes. They are therefore not suitable replacements for antidotal treatment of acute poisonings with cyclosarin.

Tetroxime: reactivation potency – in vitro and in silico study

Tetroxime – a unique bisquaternary compound with four oxime groups.

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.

Effect of seven newly synthesized and currently available oxime cholinesterase reactivators on cyclosarin-intoxicated rats

Seven new oxime-based acetylcholinesterase reactivators were compared with three currently available ones (obidoxime, trimedoxime, HI-6) for their ability to lessen cholinesterase inhibition in blood and brain of cyclosarin-treated rats. Oximes were given at doses of 5% their LD₅₀ along with 21 mg/kg atropine five min before the LD₅₀ of cyclosarin (120 ug/kg) was administered. Blood and brain samples were collected 30 minutes later. The greatest difference between acetylcholinesterase inhibition in blood of cyclosarin-treated rats was found after administration of HI-6 (40%), compared to 22% for trimedoxime and 6% for obidoxime. Only two of the seven newly synthesized oximes had any effect (K203 at 7%, K156 at 5%). Effective oximes against cyclosarin-inhibited plasma butyrylcholinesterase were HI-6 (42%), trimedoxime (11%), and K156 (4%). The oximes were less effective in brain than in blood, with reactivation values for HI-6 30% against acetylcholinesterase and 10% against butyrylcholinesterase. Values for newly synthesized oximes were less than 10% for K206, K269 and K203.

Inhibition of AChE by malathion and some structurally similar compounds

Inhibition of bovine erythrocyte acetylcholinesterase (free and immobilized on controlled pore glass) by separate and simultaneous exposure to malathion and malathion transformation products which are generally formed during storage or through natural or photochemical degradation was investigated. Increasing concentrations of malathion, its oxidation product malaoxon, and its isomerisation product isomalathion inhibited free and immobilized AChE in a concentration-dependent manner. KI, the dissociation constant for the initial reversible enzyme inhibitor-complex, and k3, the first order rate constant for the conversion of the reversible complex into the irreversibly inhibited enzyme, were determined from the progressive development of inhibition produced by reaction of native AChE with malathion, malaoxon and isomalathion. KI values of 1.3 x 10(-4) M(-1), 5.6 x 10(-6) M(-1) and 7.2 x 10(-6)M(-1) were obtained for malathion, malaoxon and isomalathion, respectively. The IC50 values for free/immobilized AChE, (3.7 +/- 0.2) x 10(-4) M/(1.6 +/-0.1) x 10(-4), (2.4 +/- 0.3) x 10(-6)/(3.4 +/- 0.1) x 10(-6)M and (3.2 +/- 0.3) x 10(-6) M/(2.7 +/- 0.2) x 10(-6) M, were obtained from the inhibition curves induced by malathion, malaoxon and isomalathion, respectively. However, the products formed due to photoinduced degradation, phosphorodithioic O,O,S-trimethyl ester and O,O-dimethyl thiophosphate, did not noticeably affect enzymatic activity, while diethyl maleate inhibited AChE activity at concentrations > 10mM. Inhibition of acetylcholinesterase increased with the time of exposure to malathion and its inhibiting by-products within the interval from 0 to 5 minutes. Through simultaneous exposure of the enzyme to malaoxon and isomalathion, an additive effect was achieved for lower concentrations of the inhibitors (in the presence of malaoxon/isomalathion at concentrations 2 x 10(-7) M/2 x 10(-7) M, 2 x 10(-7) M/3 x 10(-7)M and 2 x 10(-7) M/4.5 x 109-7) M), while an antagonistic effect was obtained for all higher concentrations of inhibitors. The presence of a non-inhibitory degradation product (phosphorodithioic O,O,S-trimethyl ester) did not affect the inhibition efficiencies of the malathion by-products, malaoxon and isomalathion.

Potency of Five Structurally Different Acetylcholinesterase Reactivators to Reactivate Human Brain Cholinesterases Inhibited by Cyclosarin

Acetylcholinesterase (AChE; EC 3.1.1.7) reactivators are used as a part of the antidotal therapy of organophosphorus pesticide and nerve agent intoxications. Cyclosarin is one member of the nerve agent family. In this article, we compared the reactivation potency of five structurally different AChE reactivators (pralidoxime, trimedoxime, methoxime, HS-6, and BI-6) to reactivate cyclosarin-inhibited cholinesterases of human brain. The results demonstrate that the bisquaternary monooxime reactivator BI-6 seems to be the most potent reactivator of cyclosarin-inhibited cholinesterases. Moreover, according to the results, we can describe basic structural requirements, which are necessary for the efficacious reactivation process.

Potency of new structurally different oximes to reactivate cyclosarin-inhibited human brain acetylcholinesterases

Antidotes currently used for organophosphorus pesticide and nerve agent intoxications consist of anticholinergics (atropine mainly) and acetylcholinesterase (AChE, EC 3.1.1.7) reactivators called oximes. Owing to the wide-spread of these toxic compounds worldwide, development of antidotes in the case of first aid is needed. To select the most promising AChE reactivators is a very time consuming process, which is necessary before approval of these compounds to be used as human antidotes. Because of ethical reasons, many developing experiments have been conducted on laboratory animals. However, these results often could not be transferred directly to human. Here, we have tested five newly developed AChE reactivators--K027, K033, K048, K074 and K075, which showed promising reactivation activity on rodents, as reactivators of inhibited human brain cholinesterases. For this purpose, cyclosarin was used as member of the nerve agent family. Oxime HI-6 and pralidoxime were used as AChE reactivator standards. Two AChE reactivators, K027 and K033, achieved comparable reactivation potency as HI-6. Moreover, oxime K033 reached its maximal reactivation potency at the lowest concentration which could be attained in humans.

A comparison of reactivating efficacy of newly developed oximes (K074, K075) and currently available oximes (obidoxime, HI-6) in cyclosarin-and tabun-poisoned rats

The potency of newly developed oximes (K074, K075) and commonly used oximes (obidoxime, HI-6) to reactivate nerve agent-inhibited acetylcholinesterase was evaluated in rats poisoned with tabun or cyclosarin at a lethal dose corresponding to the LD50 value. In vivo determined percentage of reactivation of tabun-inhibited blood and brain acetylcholinesterase showed that obidoxime is the most efficacious reactivator of tabun-inhibited acetylcholinesterase among studied oximes in the peripheral compartment (blood) although the differences between obidoxime and newly developed oximes were not significant. On the other hand, one of the newly developed oximes (K074) seems to be a significantly more efficacious reactivator of tabun-inhibited acetylcholinesterase in the central compartment (brain) than the other studied oximes. In addition, the oxime HI-6 is unable to sufficiently reactivate tabun-inhibited acetylcholinesterase in rats. In vivo determined percentage of reactivation of cyclosarin-inhibited blood and brain acetylcholinesterase in poisoned rats showed that HI-6 is the most efficacious reactivator of cyclosarin-inhibited acetylcholinesterase among the studied oximes in the peripheral (blood) as well as central (brain) compartment although the differences between the oxime HI-6 and other tested oximes in the brain were not significant. Due to their reactivating effects, both newly developed K-oximes can be considered to be promising oximes for the antidotal treatment of acute tabun poisoning while the oximes HI-6 is still the most promising oxime for the treatment of acute cyclosarin poisonings due to its high potency in reactivating cyclosarin-inhibited acetylcholinesterase in the peripheral as well as central compartment.

Synthesis of a novel series of bispyridinium compounds bearing a xylene linker and evaluation of their reactivation activity against chlorpyrifos-inhibited acetylcholinesterase

Nine potential AChE reactivators were synthesized using a modification of currently known synthetic pathways. Their potency to reactivate AChE inhibited by insecticide chlorpyrifos was tested in vitro. 2,2'-Bis(hydroxyiminomethyl)-1,1'-(1,4-phenylenedimethyl)-bispyridinium dibromide seems to be the most potent AChE reactivator. The reactivation potency of these compounds depends on structural factors such as length of the linking chain between both pyridinium rings and position of the oxime moiety on the pyridinium ring.

New group of xylene linker-containing acetylcholinesterase reactivators as antidotes against the nerve agent cyclosarin

Nerve agents such as sarin, cyclosarin and tabun are organophosphorus substances able to inhibit the enzyme acetylcholinesterase (AChE; EC 3.1.1.7). AChE reactivators and anticholinergics are generally used as antidotes in the case of intoxication with these agents. None of the known AChE reactivators is able to reactivate AChE inhibited by all nerve agents used. In this work, reactivation potency of nine newly developed AChE reactivators with an incorporated xylene ring in their structure was measured in vitro. Cyclosarin was chosen as an appropriate member of the nerve agent family. Reactivation potency of the tested AChE reactivators was compared with the gold standard of AChE reactivators--pralidoxime. Two oximes (K107 and K108) surpassed the reactivation potency of pralidoxime. Moreover, from the obtained results it could be deduced that AChE reactivators with a functional oxime group in position-2 are the most potent AChE reactivators in the case of cyclosarin intoxications.

Reactivation of sarin-inhibited pig brain acetylcholinesterase using oxime antidotes

INTRODUCTION

Organophosphorus nerve agents inhibit the enzyme, acetylcholinesterase (AChE; EC 3.1.1.7). AChE reactivators (also known as oximes) are generally used for the reactivation of an inhibited enzyme.

METHODS

Two new AChE reactivators--K033 and K027--were tested for their in vitro reactivation of sarin-inhibited pig-brain AChE. Their reactivation potencies were compared with the commercially available AChE reactivators, pralidoxime, obidoxime, and HI-6.

RESULTS

Of the oximes tested, the newly developed oxime K027 achieved the highest reactivation potency (100%; concentration of the oxime -10(-2) M). However, oxime HI-6 (33%) and obidoxime (23%) seem to be the best AChE reactivators for human relevant doses (10(-4) M and lower).

CONCLUSION

For human relevant doses, newly developed oximes (K027 and K033) do not surpass the reactivation potency of the most promising oxime, HI-6.

Synthesis of a novel series of non-symmetrical bispyridinium compounds bearing a xylene linker and evaluation of their reactivation activity against tabun and paraoxon-inhibited acetylcholinesterase

Nine potential non-symmetrical xylene-bridged AChE reactivators were synthesized using modifications of currently known synthetic pathways. Their potency to reactivate AChE inhibited by the nerve agent tabun and the insecticide paraoxon together with nine symmetrical xylene-bridged compounds, was tested in vitro. Seven compounds were promising against paraoxon-inhibited AChE. Two compounds were found to be more potent against tabun-inhibited AChE than obidoxime at a concentration applicable in vivo.

The evaluation of neuroprotective efficacy of newly developed oximes (K074, K075) and currently available oximes (obidoxime, HI-6) in cyclosarin-poisoned rats

The neuroprotective effects of newly developed oximes (K074, K075) and currently available oximes (obidoxime, HI-6) in combination with atropine in rats poisoned with cyclosarin were studied. The cyclosarin-induced neurotoxicity was monitored using a functional observational battery at 24 h and 7 days following cyclosarin challenge. The results indicate that the oxime HI-6 combined with atropine seems to be the most effective antidote for a decrease in cyclosarin-induced neurotoxicity. Both newly developed oximes (K074, K075) as well as obidoxime are also able to counteract cyclosarin-induced acute neurotoxicity, but their neuroprotective potency is significantly lower compared with the oxime HI-6. Therefore, the oxime HI-6 is still the most suitable oxime for the antidotal treatment of acute poisonings with cyclosarin due to its neuroprotective as well as reactivating efficacy.

The Acute Toxicity of Acetylcholinesterase Reactivators in Mice in Relation to Their Structure

Oximes in combination with atropine, are an integral part of the treatment of acute intoxications with organophosphorus insecticides or with the nerve agents such as tabun, sarin, soman, cyclosarin or VX. Organophosphorus compounds are extremely potent inhibitors of the enzyme acetylcholinesterase (AChE, 3.1.1.7). The pharmacological action of oximes is multiple: they are able to reactivate the inhibited AChE, but they affect acetylcholine release in peripheral and central cholinergic synapses, allosterically modulate the muscarinic receptors in peripheral and central synapses, and influence the nicotinic receptor-associated ion-channels. In our study, we have determined the acute toxicity of different structures of oximes after intramuscular application in mice. The acute toxicity of oximes is crucial for the assesment of a dose applied as a treatment for organophosphorus intoxications. We have tested 7 oximes of different structures (HS-6, K033, BI-6, MMB-4, K048, HI-6 and obidoxime ) and during our experiments we have observed the intoxication process including typical signs of intoxication, and times of death. K033 was the most toxic oxime with an LD50 of only 48 mg/kg, while the least toxic oxime - HI-6 - has an LD50 value of 671 mg/kg. All the oximes tested were of the bispyridinium type, with different length or shape of the connecting chain and positions of oxime groups at the pyridinium rings. All these structural features play an important role in biological activity of these compounds performed by their acute toxicity as well as by their reactivation potency.

New methods in synthesis of acetylcholinesterase reactivators and evaluation of their potency to reactivate cyclosarin-inhibited AChE

Nine potential AChE reactivators were synthesized using modification of currently known synthetic pathways. Their potency to reactivate AChE inhibited by cyclosarin nerve agent was tested in vitro. According to the previous results, 1,4-bis(2-hydroxyiminomethylpyridinium)butane dibromide seems to be the most potent AChE reactivator. The reactivation potency of these compounds depends on structural factors such as presence of quaternary nitrogens, length of the linking chain between both pyridinium rings, and position of the oxime moiety at the pyridinium ring.

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.

Reactivation potency of new group of acetylcholinesterase reactivators and their comparison with currently available oximes

In this work, in vitro potency of novel serie of monoquaternary pyridinium oximes to reactivate cyclosarin-inhibited acetylcholinesterase (AChE) was tested. Currently available oximes (pralidoxime, obidoxime, trimedoxime, HI-6 and BI-6) were used as oximes for comparison. As resulted, none of tested new reactivators was able to reactivate AChE inhibited by cyclosarin. Also pralidoxime, obidoxime and trimedoxime did not reach good reactivation results. Only oximes HI-6 and BI-6 achieved sufficient reactivation potency. From obtained results, it can be deduced, that only reactivators with oxime group in position two are able to reactivate cyclosarin-inhibited AChE.

Synthesis of the novel series of bispyridinium compounds bearing (E)-but-2-ene linker and evaluation of their reactivation activity against chlorpyrifos-inhibited acetylcholinesterase

Six potential AChE reactivators were synthesized using modification of currently known synthetic pathways. Their potency to reactivate AChE inhibited by insecticide chlorpyrifos was tested in vitro. According to the results, (E)-1-(2-hydroxyiminomethylpyridinium)-4-(4-hydroxyiminomethylpyridinium)-but-2-ene dibromide seems to be the most potent AChE reactivator. The reactivation potency of these compounds depends on structural factors such as constitution of the linking chain between both pyridinium rings, position of the oxime moiety at the pyridinium ring and presence of quaternary nitrogens.

New quaternary pyridine aldoximes as casual antidotes against nerve agents intoxications

In this work, the ability of four newly synthesized oximes--K005 (1,3-bis(2-hydroxyiminomethylpyridinium) propane dibromide), K027 (1-(4-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium) propane dibromide), K033 (1,4-bis(2-hydroxyiminomethylpyridinium) butane dibromide) and K048 (1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium) butane dibromide) to reactivate acetylcholinesterase (AChE, EC 3.1.1.7) inhibited by nerve agents is summarized. Reactivation potency of these compounds was tested using standard in vitro reactivation test. Tabun, sarin, cyclosarin and VX agent were used as appropriate testing nerve agents. Rat brain AChE was used as a source of the enzyme. Efficacies of new reactivators to reactivate tabun-, sarin-, cyclosarin- and VX-inhibited AChE were compared with the currently used AChE reactivators (pralidoxime, obidoxime and HI-6). Oxime K048 seems to be promising reactivator of tabun-inhibited AChE. Its reactivation potency is significantly higher than that of HI-6 and pralidoxime and comparable with the potency of obidoxime. The best reactivator of sarin-inhibited AChE seems to be oxime HI-6. None of the new AChE reactivators reached comparable reactivation potency. The same results were obtained for cyclosarin-inhibited AChE. However, oxime K033 is also potent reactivator of AChE inhibited by this nerve agent. In the case of VX inhibition, obidoxime and new oximes K027 and K048 seem to be the best AChE reactivators. None from the currently tested AChE reactivators is able to reactivate AChE inhibited by all nerve agents used and, therefore, the search for new potential broad spectrum AChE reactivators is needed.

Design and synthesis of new bis-pyridinium oxime reactivators for acetylcholinesterase inhibited by organophosphorous nerve agents

New bis-pyridinium oxime reactivators connected with a CH(2)CH(2)OCH(2)CH(2) linker between two pyridinium rings were designed and synthesized. In the test of their potency to reactivate AChE inhibited by cyclosarin, the bis-pyridinium oxime 6b achieved reactivation potency higher than 10% at the lower concentration 10(-4)M.

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.

In vitro reactivation potency of some acetylcholinesterase reactivators against sarin- and cyclosarin-induced inhibitions

In our study, we have tested six acetylcholinesterase (AChE) reactivators (pralidoxime, obidoxime, HI-6, trimedoxime, BI-6 and Hlö-7) for reactivation of sarin- and cyclosarin-inhibited AChE using an in vitro reactivation test. We have used rat brain homogenate as the suitable source of enzyme. All oximes are able to reactivate sarin-inhibited AChE. On the other hand, only HI-6 is able to reactivate satisfactorily cyclosarin-inhibited AChE.