Current understanding of the application of pyridinium oximes as cholinesterase reactivators in treatment of organophosphate poisoning

This paper reviews the mechanisms of interaction of organophosphorus compounds with cholinesterases and clinical signs of acute poisoning. Further, we describe the current understanding of the mechanisms of action of pyridinium oximes pralidoxime (PAM-2), trimedoxime (TMB-4), obidoxime (LüH-6, Toxogonin), HI-6 and HLö-7 which are used as cholinesterase reactivators in the treatment of poisoning with organophosphorus compounds. We also review the most important literature data related to the efficacy of these oximes in the treatment of poisoning with warfare nerve agents soman, sarin, tabun, VX and cyclosarin and organophosphorus insecticides. Finally, we discuss the criteria for selection of oximes intended for further development as antidotes in poisoning with organophosphorus compounds and auto-injectors for their application in urgent situations.

In vitro biological efficiency of tenocyclidine – TCP and its adamantane derivative TAMORF

Tenocyclidine-TCP showing a broad spectrum of pharmacological activity including antidotal effect in organophosphorus compounds poisoning, radioprotective and anticancer effects. We investigated in vitro interactions of TCP and its adamantane derivative--TAMORF with human erythrocyte acetylcholinesterase (AChE). Moreover, their genotoxicity and radioprotective activity on human white blood cells were studied using the alkaline comet assay, viability testing and the analysis of the structural chromosome aberrations. The tested compounds were found to be weak inhibitors of AChE, for TCP IC(50)=1 x 10(-5)M and for TAMORF IC(50)>1 x 10(-3)M, without reactivating and protective effects on AChE inhibited by soman. Results suggest that TCP modified by the replacement of the cyclohexyl ring with an adamantly ring and piperidine with morpholine group (TAMORF) have lower toxicity. Both compounds possess low cytotoxicity and radioprotective activity, but TAMORF also shows cell growth inhibitory effects. To clarify differences in their biological efficiency observed in vitro and in vivo, additional analyses are necessary. Since TAMORF was found to significantly inhibit cell growth and proliferation in vitro, it is reasonably to consider it as a source molecule promising for further modifications and development of more potent substances with antitumor properties rather then radioprotector or antidote in organophosphorus poisoning.

Substituted monoquaternary oximes as reactivators of cyclosarin--and chlorpyrifos--inhibited acetylcholinesterase

This paper describes an in vitro study of three potential acetylcholinesterase (AChE; EC 3.1.1.7) reactivators derived from a monoquaternary reactivator pralidoxime. Compounds used were pyridinium-2-aldoxime-4-carbamoyl-N-methyl iodide (TO231), pyridinium-2-aldoxime-4-ethoxycarbonyl-N-methyl iodide (TO237), and pyridinium-2-aldoxime-5-ethoxycarbonyl-N-methyl iodide (TO238). Pralidoxime and obidoxime were used for comparison. Nerve agent cyclosarin and pesticide chlorpyrifos were used as organophosphorus cholinesterase inhibitors. The source of AChE was rat brain homogenate. None of the tested oximes was able to reactivate cyclosarin-inhibited AChE (at 1.0 mmol L(-1) oxime concentration). In case of chlorpyrifos, TO231 was the most potent AChE reactivator with an 82 % reactivation at 1.0 mmol L(-1) oxime concentration. This reactivating potency equals that of pralidoxime and obidoxime. TO238 was less effective, and TO237 did not reactivate chlorpyrifos-inhibited AChE at all. None of the tested AChE reactivators, reference compounds included, could be considered universal for both chlorpyrifos- and cyclosarin-inhibited AChE.

Acetylcholinesterase: converting a vulnerable target to a template for antidotes and detection of inhibitor exposure

Applications of recombinant DNA technology, chemical synthesis on biological templates and fluorescence detection of organophosphorylation provide unexplored avenues for development of antidotes and approaches for remote detection of organophosphate nerve agents and pesticides. We discuss here how acetylcholinesterase (AChE), through appropriate mutations, becomes more susceptible to oxime reactivation. Since the reaction between organophosphate and the mutated enzyme remains rapid, regeneration of active enzyme by oxime becomes the rate-limiting step in the process to complete a catalytic cycle for generation of active enzyme. Accordingly, "Oxime-assisted Catalysis" by AChE provides a potential means for catalyzing the hydrolysis of organophosphates in plasma prior to their reaching the cellular target site. In turn, AChE, when conjugated with organophosphate, is employed as a template for 'click-chemistry, freeze-frame' synthesis of new nucleophilic reactivating agents that could potentially prove useful in AChE reactivation at the target site as well as in catalytic scavenging of organophosphates in plasma. Finally, substituted AChE molecules can be conjugated to fluorophores giving rise to shifts in emission spectra for detection of dispersed organophosphates. Since external reagents do not have to be added to detect the fluorescence change, the modified enzyme would serve as a remote sensor.

New Safe Method for Preparation of Sarin-Exposed Human Erythrocytes Acetylcholinesterase Using Non-Toxic and Stable Sarin Analogue Isopropyl p-Nitrophenyl Methylphosphonate and its Application to Evaluation of Nerve Agent Antidotes

INTRODUCTION

A non-toxic and stable sarin analogue, isopropyl p-nitrophenyl methylphosphonate (INMP), was synthesized for safe preparation of sarin-exposed acetylcholinesterase (AChE).

RESULTS AND DISCUSSION

This agent was stable for years, able to be handled in an ordinary laboratory without special care, and its 50% inhibitory concentration (IC50) on 0.04 U/ml human erythrocytes AChE was 15 nM. This reagent was thought to be especially useful since it enables experiments that require sarin-inhibited AChE, such as the development of antidotes for sarin, in a usual laboratory. To demonstrate the usefulness of this method, 40 known and novel pyridinealdoxime methiodide (PAM)-type oxime antidotes were synthesized, and their reactivation activities to INMP-exposed AChE and structure-activities correlation were studied.

CONCLUSION

Among the antidotes tested in this experiment except for 2-PAM, the compound found to have the highest reactivation activity, was the novel hydrophobic 2-PAM-type compound, 2-[(hydroxyimino)methyl]-1-[4-(tert-butyl)benzyl] pyridinium bromide.

Hydrolysis of an acetylthiocholine by pralidoxime iodide (2-PAM)

Pralidoxime iodide (2-PAM), an antidote approved for the reactivation of inhibited acetylcholinesterase (AChE) in organophosphate poisoning, dose-dependently hydrolyzed an acetylthiocholine iodide (ASCh). The AChE (0.3 U) activity inhibited by VX analog (ENMP, 0.1 microM) increased to approximately 200% of normal levels after a dosage of 5 mM 2-PAM (control 0.132+/-0.012 U/ml, 5 mM 0.253+/-0.026 U/ml). This result indicates that 2-PAM produced a thiocholine from the ASCh by hydrolysis. High-performance liquid chromatography (HPLC) analysis was then performed to further clarify the hydrolysis of ASCh with 2-PAM. It was clear that 2-PAM was converted to acetylated 2-PAM with acetic acid produced from ASCh by hydrolysis. Next, we tried to compare this esterase-like activity of 2-PAM with that of obidoxime, which is known as a strong reactivator of inhibited AChE, and with diacetylmonoxime, known as a weak reactivator. All of these oximes showed esterase-like activity, and their strengths were consistent with those of known reactivators of inhibited AChE. These results indicate that a great deal of the data obtained previously with ASCh relating to the effects of oximes must be rechecked. It is clear that oximes easily hydrolyze ASCh. We therefore strongly caution that the method of determining AChE activity with ASCh is not suitable for examining the effects of oximes.

In vitro potency of H oximes (HI-6, HLö-7), the oxime BI-6, and currently used oximes (pralidoxime, obidoxime, trimedoxime) to reactivate nerve agent-inhibited rat brain acetylcholinesterase

The efficacy of H oximes (HI-6, HLö-7), the oxime BI-6, and currently used oximes (pralidoxime, obidoxime, trimedoxime) to reactivate acetylcholinesterase inhibited by two nerve agents (tabun, VX agent) was tested in vitro. Both H oximes (HI-6, HLö-7) and the oxime BI-6 were found to be more efficacious reactivators of VX-inhibited acetylcholinesterase than pralidoxime and obidoxime. On the other hand, their potency to reactivate tabun-inhibited acetylcholinesterase was low and did not reach the reactivating efficacy of trimedoxime and obidoxime. Thus, none of these compounds can be considered to be a broad-spectrum reactivator of nerve agent-inhibited acetylcholinesterase in spite of high potency to reactivate acetylcholinesterase inhibited by some nerve agents. More than one oxime may be necessary for the antidotal treatment of nerve agent-exposed individuals.

Antidotal efficacy of pyridinium chloride derivatives against soman poisoning

Acetylcholinesterase (AChE; EC 3.1.1.7.) is an extremely active enzyme necessary for terminating the action of acetylcholine in cholinergic synapses. The aim of this study was to evaluate the efficacy of four mono-pyridinium compounds 1-phenacylpyridinium chloride (I), 1-phenacyl-2-methylpyiridinium chloride (II), 1-benzoylethylpyridinium chloride (III), and 1-benzoylethylpyridinium-4-aldoxime chloride (IV) in the therapy of soman poisoning. Their effect was compared with HI-6 and TMB-4 oximes. The inhibitory potency (IC50) of compounds as well as reactivating (%R) and protective potency (P50) with respect to soman-inhibited AChE were determined for each of the compounds. Their acute intraperitoneal toxicity (LD50 with 95% confidence limits) was tested in mice and observed for 24 hr. The therapeutic effect was expressed as the protective index and as the therapeutic dose. The tested compounds were found to be reversible inhibitors of AChE. In vivo results show that the tested compounds are relatively toxic (their LD50 was from 74.9 to 210.0 mg/kg body weight). The best antidotal efficacy was obtained with compound II, which had the highest affinity for AChE (IC50 was 1.9 x 10(-5) mol l(-1)) and seems to be an adequate antidote in soman poisoning (its protective index and therapeutic dose were 2.8 and 2, respectively). Our results indicate that its antidotal effect is related to the reactivation or protection of AChE. The type of the substituent in the pyridinium ring generally has a significant influence on toxicity in vitro and in vivo, and on the antidotal efficacy of all new tested compounds.

Kinetic analysis of the protection afforded by reversible inhibitors against irreversible inhibition of acetylcholinesterase by highly toxic organophosphorus compounds

In organophosphate poisoning, the underlying mechanism of the therapeutic efficacy of carbamate prophylaxis, which was successfully tested in animal experiments, still awaits complete understanding. In particular, it is unclear whether survival is improved by increased acetylcholinesterase activity during the acute phase, when both carbamate and organophosphate are present. This question should be solved experimentally by means of a dynamically working in vitro model. Immobilized human erythrocytes were continuously perfused while acetylcholinesterase activity was monitored in real-time by a modified Ellman method. The concentrations of reversible inhibitors and of paraoxon were varied to assess the influence of both components on the enzyme activity under steady-state conditions. Physostigmine, pyridostigmine and huperzine A were tested for their prophylactic potential. Upon pretreatment with these reversible inhibitors the enzyme was inhibited by 20-90%. Additional perfusion with 1 microM paraoxon for 30 min resulted in a residual activity of 1-4%, at low and high pre-inhibition, respectively. The residual activity was significantly higher than in the absence of reversibly blocking agents (0.3%). After discontinuing paraoxon, the activity increased even in the presence of the reversible blockers. Stopping the reversibly blocking agents resulted in 10-35% recovery of the enzyme activity, depending on the degree of pre-inhibition. The experimental results agreed with computer simulations upon feeding with the essential reaction rate constants, showing that physostigmine was somewhat superior to pyridostigmine in enhancing residual activity in the presence of 1 microM paraoxon for 30 min. The model predicts that inhibitors with a faster dissociation rate, e.g. huperzine A, may be superior in case of a 'hit-and-run' poison such as soman.

Development of a dynamic model for real-time determination of membrane-bound acetylcholinesterase activity upon perfusion with inhibitors and reactivators

Quantitative predictions of the course of acetylcholinesterase (AChE) activity, following interference of inhibitors and reactivators, are usually obscured by the time-dependent changes of all reaction partners. To mimic these dynamics we developed an in vitro model. Immobilized human erythrocyte ghosts in a bioreactor were continuously perfused while AChE activity was monitored by a modified Ellman method. The perfusion system consisted of two HPLC pumps with integrated quaternary low-pressure gradient formers that were programmed by a computer using commercial HPLC software. The combined eluates passed a particle filter (Millex-GS, 0.22 microm) containing a thin layer of erythrocytes that was immersed in a temperature-controlled water bath. The effluent passed a flow cell in a UV-vis detector, the signal of which was digitized, written to disc and calculated with curve fitting programs. AChE activity decreased by 3.4% within 2.5 h. The day-to-day variation of the freshly prepared bioreactor using the same enzyme source was +/-3.3%. Residual activity of 0.2% marked the limit of quantification. Following perfusion with paraoxon, pseudo first-order rate constants of inhibition were established that did not differ from results obtained in conventional assays. The same holds true for reactivation with obidoxime. The set-up presented allows freely programmable time-dependent changes of up to eight solvents to mimic pharmacokinetic profiles without accumulation of products. Due to some hysteresis in the system, reaction half-lives should be >3 min and concentration changes in critical compounds should exceed half-lives of 5 min. Otherwise, the system offers much flexibility and operates with high precision.

Acetylcholinesterase inhibition: does it explain the toxicity of organophosphorus compounds?

The hypothesis that acetylcholinesterase (AChE) inhibition is the mechanism of toxicity of organophosphorus (OP) compounds was examined by mathematically modeling the in vivo lethal effects of OP compounds and determining the amount of variation in OP toxicity that is explained by AChE inhibition. Mortality dose-response curves for several OP compounds (i.e., VX, soman, cyclosarin, sarin, tabun, diisopropylfluorophosphate and paraoxon) exhibited steep probit slopes (> 9.6) in guinea pigs. Steep probit slopes were also observed when the mortality dose-response curves for soman were examined in mice, rats, rabbits and non-human primates. The consistently steep probit slopes of the dose-response curves for highly toxic OP compounds suggested that these compounds have a single specific mechanism of toxicity regardless of the OP compound or the species in which it was tested. Regression analysis indicated that 93% of the 3,280-fold variation in the median lethal doses (i.e., LD(50)) of OP compounds in rats was explained by the variation in their in vitro rate constants for inhibition of AChE. Conversely, 91% of the 23-fold variation in the ability of the oximes pralidoxime and obidoxime to protect against the toxicity of OP compounds in guinea pigs was explained by the variation in the in vitro ability of oximes to reactivate OP-inhibited AChE. The best explanation for this variety of observations was that the primary mechanism of in vivo toxicity for highly toxic OP compounds is the inhibition of AChE, and the residual unexplained variation in OP toxicity that might be explained by other mechanisms represents < 10% of the total variation in OP toxicity.

[Synthesis of reactivators of phosphorylated acetylcholinesterase of bis-pyridiniumdialdoxime type with a 3-oxapentane connecting chain and their testing in vitro on a model of the enzyme inhibited by chlorpyrifos and methylchlorpyrifos]

Insecticides (e.g., parathion, chlorpyrifos, methylchlorpyrifos) and nerve agents (e.g.. soman, sarin, tabun, VX) belong to the group of organophosphates. They are able to irreversibly inhibit the enzyme acetylcholinesterase (AChE). Three new reactivators with a 3-oxapentane connecting chain were prepared. The ability of the new compounds to reactivate AChE inhibited by pesticides was tested in vitro and compared to known oxime 10(-3) M which is unfortunately not applicable to in vivo experiments. All tested compounds are practically ineffective for methylchlorpyrifos-inhibited AChE at the physiological concentration (10(-5) M). On the other hand, the known reactivators surpass new substances in the case of chlorpyrifos-inhibited AChE at both concentrations.

Russian VX: Inhibition and Reactivation of Acetylcholinesterase Compared with VX Agent

Organophosphorus compounds such as nerve agents inhibit, practically irreversibly, cholinesterases by their phosphorylation in the active site of these enzymes. Current antidotal treatment used in the case of acute nerve agent intoxications consists of combined administration of anticholinergic drug (usually atropine) and acetylcholinesterase (AChE, EC 3.1.1.7) reactivator (HI-6, obidoxime, pralidoxime), which from a chemical view is a derivative from the group of pyridinium or bispyridinium aldoximes (commonly called "oxime"). Oximes counteract acetylcholine increase, resulting from AChE inhibition. In the human body environment these compounds are powerful nucleophiles and are able to break down the bond between AChE and nerve agent molecule. This process leads to renewal of enzyme functionality -- to its reactivation. The usefulness of oxime in the reactivation process depends on its chemical structure and on the nerve agent whereby AChE is inhibited. Due to this fact, selection of suitable reactivator in the treatment of intoxications is very important. In our work, we have compared differences in the in vitro inhibition potency of VX and Russian VX on rat, pig and human brain, and subsequently we have tested reactivation of rat brain cholinesterase inhibited by these agents using oxime HI-6, obidoxime, pralidoxime, trimedoxime and methoxime. The results showed that no major differences in the reactivation process of both VX and Russian VX-inhibited cholinesterase. The similarity in reactivation was caused by analogous chemical structure of either nerve agent; and that oxime HI-6 seems to be the most effective reactivator tested, which confirms that HI-6 is currently the most potent reactivator of AChE inhibited by nerve agents. The results obtained in our study should be considered in the future development of new AChE reactivators.

Quaternary salts of 4,3′ and 4,4′ bis-pyridinium monooximes. Part 2: Synthesis and biological activity

In continuation of our investigations of unsymmetrical bisquaternary monooximes, we synthesized four new series of compounds bridged by hexyl, heptyl, octyl and nonyl groups. All eight monooximes viz., dibromides of 1-(4-hydroxyiminomethylpyridinium)6-(3/4-carbamoylpyridinium)hexane, 1-(4-hydroxyiminomethylpyridinium)-7-(3/4-carbamoylpyridinium)heptane, 1-(4-hydroxyiminomethylpyridinium)-8-(3/4-carbamoylpyridinium)octane, 1-(4-hydroxyiminomethylpyridinium)-9-(3/4-carbamoylpyridinium)nonane as well as the corresponding bis-oximes were synthesized and characterized by spectral data. Their ability to reactivate tetraethylpyrophosphate (TEPP) inhibited mouse total brain cholinesterase was investigated and compared with the conventional oxime 2-pyridinealdoxime chloride (2-PAM). Mouse brain homogenate was used as the source of acetylcholinesterase. Among all the compounds, tested the compound with the hexylene bridge (6b) and a 3-carbamoyl group on the second pyridine ring was found to be the most active acetylcholinesterase reactivator (72%) which is greater than that of 2-PAM (56%). However, the activity was reversed; as the chain length increased from a heptylene to a nonylene bridge, they potentiated the inhibitory effect of TEPP rather than reactivation. It is interesting to note that compound 6b with a carbamoyl group at the 3rd position of the pyridine ring showed dose dependent reactivation whereas the corresponding compound 6a with the carbamoyl group present at the 4th position of the pyridine ring showed reactivation at lower concentration (30 microM) and potentiation of TEPP inhibition at higher concentrations (100 and 300 microM).

Effect of Sodium Bicarbonate in Rats Acutely Poisoned with Dichlorvos

The development of effective antidotes against organophosphates such as dichlorvos has been a persistent challenge over the past decades. Therapy of organophosphate poisoning is based on the administration of atropine and oxime as standard antidotes. The present study was undertaken to evaluate the ability of sodium bicarbonate to improve protective effects of standard antidotes in rats poisoned with dichlorvos. The aim of this experiment was to establish the correlation between protective effects and biochemical parameters relevant for acid-base status. In order to examine the protective effect of both standard antidotes and their combinations, groups of experimental animals were poisoned subcutaneously with increasing doses of dichlorvos. Immediately thereafter, rats were treated with atropine 10 mg/kg intramuscularly, oximes 10 mg/kg intramuscularly and sodium bicarbonate 3 mmol/kg intraperitoneally. These antidotes were administered either as single doses or in combinations. In the biochemical part of the experiments, rats were poisoned with dichlorvos 1.3 LD(50) (10.64 mg/kg) subcutaneously and immediately thereafter treated with atropine 10 mg/kg intramuscularly, oximes (trimedoxime or obidoxime) 10 mg/kg intramuscularly and sodium bicarbonate 3 mmol/kg intraperitoneally either as single doses or in combinations. Parameters relevant for acid-base status were measured 10 minutes after the administration of antidotes. The results of our study indicate that addition of sodium bicarbonate to standard antidotes significantly improves protective effects of atropine, obidoxime and trimedoxime. Correlation between protection and biochemical outcome is clearly evident when sodium bicarbonate is being added to atropine.

New evidences for old biomarkers: effects of several xenobiotics on EROD and AChE activities in Zebra mussel (Dreissena polymorpha)

The biomarker approach is widely used both in vertebrates and invertebrates for environmental biomonitoring, because it can supply an integrated response for multi-xenobiotics contamination. However, the use of biomarkers requires the identification of every possible variation that can influence the biochemical response, because ecosystems are generally subject to a mixture of pollutants, which can create additive, opposite or competitive effects. In recent years, there has been considerable interest in the use of biomarkers within marine bivalves, while very few data are available for freshwater molluscs. The aim of this research was to investigate changes on EROD and AChE activities in the freshwater bivalve Zebra mussel (Dreissena polymorpha) exposed to different pollutants (Arochlor 1260, CB 153 and 126, pp'DDT, chlorpyrifos, carbaryl) at laboratory conditions, in order to standardize the analytical procedures and to highlight eventual interferences on enzyme activities. Chemical concentrations in the mussel soft tissues were analyzed by GC/MS-MS. Main results showed a significant induction of EROD activity when mussels were exposed to 100 ng/l of PCB mixture of Arochlor 1260 and dioxin-like CB 126, but this congener showed also a clear competitive inhibition after 48 h of exposure. Surprisingly, pp'DDT determined a significant decrease of basal EROD activity after only 24 h of exposure, even if it was not possible to discriminate between the effect of the parent compound and that of its metabolites (DDD, DDE). We also found an interaction between the organophosphate insecticide chlorpyrifos, which does not directly decrease the AChE activity, and terbutilazine. This herbicide increased the biotransformation of the organophosphate compound to its oxidized metabolite (oxon), a much stronger AChE inhibitor. The possible use of the oxime Pyridine-2-Aldoxime Methochloride (2-PAM) to bring back the catalytic activity to basal levels was also demonstrated.

A comparison of the potency of newly developed oximes (K027, K048) and commonly used oximes (obidoxime, HI-6) to counteract tabun-induced neurotoxicity in rats

The neuroprotective effects of newly developed oximes (K027, K048) and currently available oximes (obidoxime, HI-6) in combination with atropine in rats poisoned with tabun at a sublethal dose (170 microg kg(-1) i.m.; 80% of LD(50) value) were studied. The tabun-induced neurotoxicity was monitored using a functional observational battery and an automatic measurement of motor activity. The neurotoxicity of tabun was monitored at 24 h and 7 days following tabun challenge. The results indicate that the oxime HI-6 in combination with atropine was not able to protect the rats from the lethal effects of tabun. Two non-treated tabun-poisoned rats and one tabun-poisoned rat treated with atropine combined with HI-6 died within 2 h. On the other hand, all other tested oximes combined with atropine allowed all the tabun-poisoned rats to survive 7 days following tabun challenge. Both newly developed oximes combined with atropine seem to be sufficiently effective antidotes for a decrease in tabun-induced neurotoxicity in the case of sublethal poisoning although they are not able to eliminate tabun-induced neurotoxicity completely. The neuroprotective efficacy of obidoxime in combination with atropine approached the potency of newly developed oximes but the ability of the oxime HI-6 to counteract tabun-induced acute neurotoxicity was significantly lower, especially at 24 h after tabun poisoning. Due to their neuroprotective effects, both newly developed oximes appear to be suitable oximes for the antidotal treatment of acute tabun poisoning.

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.

In vitro and in vivo evaluation of pyridinium oximes: mode of interaction with acetylcholinesterase, effect on tabun- and soman-poisoned mice and their cytotoxicity

The increased concern about terrorist use of nerve agents prompted us to search for new more effective oximes against tabun and soman poisoning. We investigated the interactions of five bispyridinium oximes: K027 [1-(4-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium) propane dibromide], K048 [1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium) butane dibromide], K033 [1,4-bis(2-hydroxyiminomethylpyridinium) butane dibromide], TMB-4 [1,3-bis(4-hydroxyiminomethylpyridinium) propane dibromide] and HI-6 [(1-(2-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium)-2-oxapropane dichloride)] with human erythrocyte acetylcholinesterase (AChE; E.C. 3.1.1.7) and their effects on tabun- and soman-poisoned mice. All the oximes reversibly inhibited AChE, and the enzyme-oxime dissociation constants were between 17 and 180 microM. Tabun-inhibited AChE was completely reactivated by TMB-4, K027 and K048, with the overall reactivation rate constants of 306, 376 and 673 min(-1)M(-1), respectively. The reactivation of tabun-inhibited AChE by K033 reached 50% after 24h, while HI-6 failed to reactivate any AChE at all. Soman-inhibited AChE was resistant to reactivation by 1mM oximes. All studied oximes protected AChE from phosphorylation with both soman and tabun. In vivo experiments showed that the studied oximes were relatively toxic to mice; K033 was the most toxic (LD50=33.4 mg/kg), while K027 was the least toxic (LD50=672.8 mg/kg). The best antidotal efficacy was obtained with K048, K027 and TMB-4 for tabun poisoning, and HI-6 for soman poisoning. Moreover, all tested oximes showed no cytotoxic effect on several cell lines in concentrations up to 0.8mM. The potency of the oximes K048 and K027 to protect mice from five-fold LD50 of tabun and their low toxicity make these compounds leading in the therapy of tabun poisoning. The combination of HI-6 and atropine is the therapy of choice for soman poisoning.

In vitro reactivation of sarin-inhibited brain acetylcholinesterase from different species by various oximes

In vitro as well as in vivo evaluation of the reactivating efficacy of various oximes against nerve agent-inhibited acetylcholinesterase has been usually done with the help of animal experiments. Nevertheless, previously published data indicate that the reactivation potency of oximes may be different in human and animal species, which may hamper the extrapolation of animal data to human data. Therefore, to better evaluate the efficacy of various oximes (pralidoxime, obidoxime, HI-6, K033) to reactivate brain acetylcholinesterase inhibited by sarin by in vitro methods, human, rat and pig brain acetylcholinesterase were used to calculate kinetic parameters for the reactivation. Our results show differences among the species, depending on the type of oxime, and indicate that data from animal experiments needs to be carefully evaluated before extrapolation to humans.

A new group of monoquaternary reactivators of acetylcholinesterase inhibited by nerve agents

Reactivators of acetylcholinesterase (AChE; EC 3.1.1.7) are able to treat intoxication by organophosphorus compounds, especially with pesticides or nerve agents. Owing to the fact that there exists no universal "broad-spectrum" reactivator of organophosphates-inhibited AChE, many laboratories have synthesized new AChE reactivators. Here, we synthesized five new and three previously known quaternary monopyridinium oximes as potential reactivators of AChE inhibited by nerve agents. Potencies to cleave p-nitrophenyl acetate (PNPA), which is commonly used as a model substrate of nerve agents, were measured. Their cleaving potencies were compared with 4-PAM (4-hydroxyiminomethyl-1-methylpyridinium iodide), which is derived from the structure of the currently used AChE-reactivator 2-PAM (2-hydroxyiminomethyl-1-methylpyridinium iodide). Three newly synthesized oximes achieved similar nucleophilicity at the similar pKa according to 4-PAM, which is very promising for using these derivatives as AChE reactivators.

A COMPARISON OF THE POTENCY OF TRIMEDOXIME AND OTHER CURRENTLY AVAILABLE OXIMES TO REACTIVATE TABUN-INHIBITED ACETYLCHOLINESTERASE AND ELIMINATE ACUTE TOXIC EFFECTS OF TABUN

Tabun (O-ethyl-N,N-dimethyl phosphoramidocyanidate) belongs to highly toxic organophosphorus compounds misused as chemical warfare agents for military as well as terroristic purposes. It differs from other highly toxic organophosphates by its chemical structure and by the fact that tabun-inhibited acetylcholinesterase is extraordinarily difficult to reactivate. The potency of trimedoxime and other commonly used oximes (pralidoxime, obidoxime, the oxime HI-6) to reactivate tabun-inhibited acetylcholinesterase and to eliminate tabun-induced acute effects was evaluated using in vitro and in vivo methods. In vitro calculated kinetic parameters of reactivation of tabun-inhibited acetylcholinesterase from rat brain homogenate and in vivo determined percentage of reactivation of tabun-inhibited blood and tissue acetylcholinesterase in poisoned rats show that trimedoxime seems to be the most efficacious reactivator in the case of tabun poisonings. Trimedoxime was also found to be the most efficacious oxime in the elimination of acute lethal toxic effects in tabun-poisoned rats and mice. The oxime HI-6, so efficacious against soman, does not seem to be sufficiently effective oxime to reactivate tabun-inhibited acetylcholinesterase and to counteract acute lethal effects of tabun.

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.

Novel neuroprotective effects with O-benzyl derivative of pralidoxime in soman-intoxicated rodents

Pharmacological properties of oxime reactivators, not related to its ability to regenerate or reactivate nerve agent-inhibited acetylcholinesterase located at nerve synapses, have been reported to be important in protecting against poisoning by the nerve agent soman. Such non-reactivation effects have thus far been associated only with bispyridinium oximes. This study investigated the possibility of creating similar non-reactivation therapeutic effects in the mono-pyridinium ring oxime, pralidoxime (2-PAM) through attachment of alkyl groups of increasing chain length to the oxime functional group. Of the 4 derivatives investigated, only the O-benzyl derivative displayed strong sedative effects in mice and mitigated the development of motor convulsions following soman challenge (1.8 x LD50, subcutaneous). Anticonvulsant effects of this compound were enhanced by co-administration of a non-anticonvulsant dose of atropine sulfate. Administration of equivalent amount of other O-derivatives of pralidoxime failed to elicit similar anticonvulsant actions. Electroencephalographic (EEG) and histopathological studies using the rat model, intoxicated with a lethal dose (1.6 x LD50, s.c.) of soman, confirmed O-benzyl derivative neuroprotective capabilities when used as a pretreatment drug. Microdialysis studies revealed that its neuroprotective effect is related to its ability to attenuate soman-induced increase in acetylcholine.