Kinetic analysis of interactions between human acetylcholinesterase, structurally different organophosphorus compounds and oximes

Reversible cholinesterase inhibitors as pretreatment for exposure to organophosphates. A review

Organophosphorus compounds (OPCs), inhibitors of acetylcholinesterase (AChE), are useful agents as pesticides, but also represent a serious health hazard. Standard therapy with atropine and established oxime-type enzyme reactivators (pralidoxime, obidoxime) is unsatisfactory. Better therapeutic results are obtained, when reversible AChE inhibitors are administered before OPC exposure. This review summarizes the history of such a pretreatment approach and sums up a set of experiments undertaken in search of compounds that are efficacious when given before a broad range of OPCs. The prophylactic efficacy of 10 known AChE inhibitors, either already used clinically for different indications (physostigmine, pyridostigmine, ranitidine, tiapride, tacrine, amiloride, metoclopramide, methylene blue) or developed for possible therapeutic use in the future (7-methoxytacrine, K-27) was compared, when administered before exposure to six chemically diverse OPCs in the same experimental setting: ethyl-paraoxon, methyl-paraoxon, diisopropylfluorophosphate, terbufos sulfone, azinphos-methyl and dicrotophos. The experimental oxime K-27 was the most efficacious compound, affording best protection, when administered before terbufos sulfone, azinphos-methyl and dicrotophos, second best before ethyl- and methyl-paraoxon exposure and third best before diisopropylfluorophosphate administration. This ranking was similar to that of physostigmine, which was superior to the Food and Drug Administration-approved pretreatment for soman with pyridostigmine. Tiapride, amiloride, metoclopramide, methylene blue and 7-methoxytacrine did not achieve protection. No correlation was observed between the IC₅₀ of the reversible AChE inhibitors and their protective efficacy. These studies indicate that K-27 can be considered a very promising broad-spectrum prophylactic agent in case of imminent organophosphate exposure, which may be related to its AChE reactivating activity rather than its AChE inhibition.

Water structure changes in oxime-mediated reactivation process of phosphorylated human acetylcholinesterase

The role of water in oxime-mediated reactivation of phosphylated cholinesterases (ChEs) has been asked with recurrence. To investigate oximate water structure changes in this reaction, reactivation of paraoxon-inhibited human acetylcholinesterase (AChE) was performed by the oxime asoxime (HI-6) at different pH in the presence and absence of lyotropic salts: a neutral salt (NaCl), a strong chaotropic salt (LiSCN) and strong kosmotropic salts (ammonium sulphate and phosphate HPO42-). At the same time, molecular dynamic (MD) simulations of enzyme reactivation under the same conditions were performed over 100 ns. Reactivation kinetics showed that the low concentration of chaotropic salt up to 75 mM increased the percentage of reactivation of diethylphosphorylated AChE whereas kosmotropic salts lead only to a small decrease in reactivation. This indicates that water-breaker salt induces destructuration of water molecules that are electrostricted around oximate ions. Desolvation of oximate favors nucleophilic attack on the phosphorus atom. Effects observed at high salt concentrations (>100 mM) result either from salting-out of the enzyme by kosmotropic salts (phosphate and ammonium sulphate) or denaturing action of chaotropic LiSCN. MDs simulations of diethylphosphorylated hAChE complex with HI-6 over 100 ns were performed in the presence of 100 mM (NH4)2SO4 and 50 mM LiSCN. In the presence of LiSCN, it was found that protein and water have a higher mobility, i.e. water is less organized, compared with the ammonium sulphate system. LiSCN favors protein solvation (hydrophobic hydration) and breakage of elelectrostricted water molecules around of oximate ion. As a result, more free water molecules participated to reaction steps accompanying oxime-mediated dephosphorylation.

Demonstration of In Vitro Resurrection of Aged Acetylcholinesterase after Exposure to Organophosphorus Chemical Nerve Agents

After the inhibition of acetylcholinesterase (AChE) by organophosphorus (OP) nerve agents, a dealkylation reaction of the phosphylated serine, referred to as aging, can occur. When aged, known reactivators of OP-inhibited AChE are no longer effective. Realkylation of aged AChE may provide a route to reversing aging. We designed and synthesized a library of quinone methide precursors (QMPs) as proposed realkylators of aged AChE. Our lead compound (C8) from an in vitro screen successfully resurrected 32.7 and 20.4% of the activity of methylphosphonate-aged and isopropyl phosphate-aged electric-eel AChE, respectively, after 4 days. C8 displays properties of both resurrection (recovery from the aged to the native state) and reactivation (recovery from the inhibited to the native state). Resurrection of methylphosphonate-aged AChE by C8 was significantly pH-dependent, recovering 21% of activity at 4 mM and pH 9 after only 1 day. C8 is also effective against isopropyl phosphate-aged human AChE.

Nerve Agents: What They Are, How They Work, How to Counter Them

Nerve agents are organophosphorus chemical warfare agents that exert their action through the irreversible inhibition of acetylcholinesterase, with a consequent overstimulation of cholinergic transmission followed by its shutdown. Beyond warfare, they have notoriously been employed in acts of terrorism as well as high profile assassinations. After a brief historical introduction on the development and deployment of nerve agents, this review provides a survey of their chemistry, the way they affect cholinergic transmission, the available treatment options, and the current directions for their improvement. As the review illustrates, despite their merits, the currently available treatment options present several shortcomings. Current research directions involve the search for improved antidotes, antagonists of the nicotinic receptors, small-molecule pretreatment options, as well as bioscavengers as macromolecular pretreatment options. These efforts are making good progress in many different directions and, hopefully, will lead to a lower target susceptibility, thus reducing the appeal of nerve agents as chemical weapons.

Mechanisms of acetylcholinesterase protection against sarin and soman by adenosine A1 receptor agonist N6-cyclopentyladenosine

Organophosphorus nerve agents (NAs) irreversibly inhibit acetylcholinesterase (AChE), the enzyme responsible for breaking down the neurotransmitter acetylcholine (ACh). The over accumulation of ACh after NA exposure leads to cholinergic toxicity, seizure, and death. Current medical countermeasures effectively mitigate peripheral symptoms, however; the brain is often unprotected. Alternative acute treatment with the adenosine A₁ receptor agonist N⁶-cyclopentyladensosine (CPA) has previously been demonstrated to prevent AChE inhibition as well as to suppress neuronal activity. The mechanism of AChE protection is unknown. To elucidate the feasibility of potential CPA-AChE interaction mechanisms, we applied a truncated molecular model approach and density functional theory. The candidate mechanisms studied are reversible enzyme inhibition, enzyme reactivation, and NA blocking prior to enzyme conjugation. Our thermodynamic data suggest that CPA can compete with the NAs sarin and soman for the active site of AChE, but may, in contrast to NAs, undergo back-reaction. We found a strong interaction between CPA and NA conjugated AChE, making enzyme reactivation unlikely but possibly allowing for CPA protection through the prevention of NA aging. The data also indicates that there is an affinity between CPA and unbound NAs. The results from this study support the hypothesis that CPA counters NA toxicity via multiple mechanisms and is a promising therapeutic strategy that warrants further development.

Kinetic analysis of oxime-assisted reactivation of human, Guinea pig, and rat acetylcholinesterase inhibited by the organophosphorus pesticide metabolite phorate oxon (PHO)

Phorate is a highly toxic agricultural pesticide currently in use throughout the world. Like many other organophosphorus (OP) pesticides, the primary mechanism of the acute toxicity of phorate is acetylcholinesterase (AChE) inhibition mediated by its bioactivated oxon metabolite. AChE reactivation is a critical aspect in the treatment of acute OP intoxication. Unfortunately, very little is currently known about the capacity of various oximes to rescue phorate oxon (PHO)-inhibited AChE. To help fill this knowledge gap, we evaluated the kinetics of inhibition, reactivation, and aging of PHO using recombinant AChE derived from three species (rat, guinea pig and human) commonly utilized to study the toxicity of OP compounds and five oximes that are currently fielded (or have been deemed extremely promising) as anti-OP therapies by various nations around the globe: 2-PAM Cl, HI-6 DMS, obidoxime Cl₂, MMB4-DMS, and HLö7 DMS. The inhibition rate constants (k_i) for PHO were calculated for AChE derived from each species and found to be low (i.e., 4.8×10³ to 1.4×10⁴M^-1min^-1) compared to many other OPs. Obidoxime Cl₂ was the most effective reactivator tested. The aging rate of PHO-inhibited AChE was very slow (limited aging was observed out to 48h) for all three species.

CONCLUSIONS

(1) Obidoxime Cl₂ was the most effective reactivator tested. (2) 2-PAM Cl, showed limited effectiveness in reactivating PHO-inhibited AChE, suggesting that it may have limited usefulness in the clinical management of acute PHO intoxication. (3) The therapeutic window for oxime administration following exposure to phorate (or PHO) is not limited by aging.

Assessment of brain oxygenation imbalance following soman exposure in rats

Nerve agents (NAs) are potent organophosphorus (OP) compounds with applications in chemical warfare. OP compounds act by inhibiting acetylcholinesterase (AChE). Soman (O-pinacolyl methylphosphonofluoridate) is one of the most potent NAs. It is well known that small doses of NAs can be lethal, and that even non-lethal exposure leads to long-term mental debilitation/neurological damage. However, the neuropathology following exposure to sub-lethal nerve agents is not well understood. In this study, we examined changes in tissue oxygenation (pO₂) in the cortex and hippocampus after a sub-lethal dose of soman [80-90 μg/kg; subcutaneous]. pO₂ changes can provide information regarding oxygen delivery and utilization and may be indicative of a disruption in cerebral blood flow and/or metabolism. Changes in oxygenation were measured with chronically implanted oxygen sensors in awake and freely moving rats. Measurements were taken before, during, and after soman-induced convulsive seizures. Soman exposure resulted in an immediate increase in pO₂ in the cortex, followed by an even greater increase that precedes the onset of soman-induced convulsive seizures. The rise in hippocampus pO₂ was delayed relative to the cortex, although the general pattern of brain oxygenation between these two regions was similar. After convulsive seizures began, pO₂ levels declined but usually remained hyperoxygenated. Following the decline in pO₂, low frequency cycles of large amplitude changes were observed in both the cortex and hippocampus. This pattern is consistent with recurring seizures. Measuring real-time changes in brain pO₂ provides new information on the physiological status of the brain following soman exposure. These results highlight that the measurement of brain oxygenation could provide a sensitive marker of nerve agent exposure and serve as a biomarker for treatment studies.

Counteracting desensitization of human α7-nicotinic acetylcholine receptors with bispyridinium compounds as an approach against organophosphorus poisoning

Irreversible inhibition of acetylcholinesterase (AChE) resulting in accumulation of acetylcholine and overstimulation of muscarinic and nicotinic receptors accounts for the acute toxicity of organophosphorus compounds (OP). Accordingly, the mainstay pharmacotherapy against poisoning by OP comprises the competitive muscarinic acetylcholine receptor antagonist atropine to treat muscarinic effects and, in addition, oximes to reactivate inhibited AChE. A therapeutic gap still remains in the treatment of desensitized nicotinic acetylcholine receptors following OP exposure. Hereby, nicotinic effects result in paralysis of the central and peripheral respiratory system if untreated. Thus, these receptors pose an essential target for therapeutic indication to address these life-threatening nicotinic symptoms of the cholinergic crisis. Identification of ligands regulating dynamic transitions between functional states by binding to modulatory sites appears to be a promising strategy for therapeutic intervention. In this patch clamp study, the ability of differently substituted bispyridinium non-oximes to "resensitize" i.e. to recover the activity of desensitized human homomeric α7-type nAChRs stably transfected in CHO cells was investigated and compared to the already described α7-specific positive allosteric modulator PNU-120596. The structures of these bispyridinium analogues were based on the lead structure of the tert-butyl-substituted bispyridinium propane MB327, which has been shown to have a positive therapeutic effect due to a non-competitive antagonistic action at muscle-type nAChRs in vivo and has been found to have a positive allosteric activity at neuronal receptors in vitro. Prior to test compounds, desensitization of hα7-nAChRs was verified by applying an excess of nicotine revealing activation at low, and desensitization at high concentrations. Thereby, desensitization could be reduced by modulation with PNU-120596. Desensitization was further verified by dose-response profiles of agonists, carbamoylcholine and epibatidine in the absence and presence of PNU-120596. Although less pronounced than PNU-120596 and the lead structure MB327, bispyridinium compounds, particularly those substituted at position 3 and 4, resensitized the nicotine desensitized hα7-nAChRs in a concentration-dependent manner and prolonged the mean channel open time. In summary, identification of more potent compounds able to restore nAChR function in OP intoxication is needed for development of a putative efficient antidote.

Application of the Ugi Multicomponent Reaction in the Synthesis of Reactivators of Nerve Agent Inhibited Acetylcholinesterase

Recently, a new class of reactivators of chemical warfare agent inhibited acetylcholinesterase (AChE) with promising in vitro potential was developed by the covalent linkage of an oxime nucleophile and a peripheral site ligand. However, the complexity of these molecular structures thwarts their accessibility. We report the compatibility of various oxime-based compounds with the use of the Ugi multicomponent reaction in which four readily accessible building blocks are mixed together to form a product that links a reactivating unit and a potential peripheral site ligand. A small library of imidazole and imidazolium reactivators was successfully synthesized using this method. Some of these compounds showed a promising ability to reactivate AChE inhibited by various types of CWA in vitro. Molecular modeling was used to understand differences in reactivation potential between these compounds. Four compounds were evaluated in vivo using sarin-exposed rats. One of the reactivators showed improved in vivo efficacy compared to the current antidote pralidoxime (2-PAM).

The oximes HI-6 and MMB-4 fail to reactivate soman-inhibited human and guinea pig AChE: A kinetic in vitro study

Acetylcholinesterase (AChE) inhibited by the organophosphorus nerve (OP) agent soman underlies a spontaneous and extremely rapid dealkylation ("aging") reaction which prevents reactivation by oximes. However, in vivo studies in various, soman poisoned animal species showed a therapeutic effect of oximes, with the exact mechanism of this effect remaining still unclear. In order to get more insight and a basis for the extrapolation of animal data to humans, we applied a dynamic in vitro model with continuous online determination of AChE activity. This model allows to simulate the in vivo toxico- and pharmacokinetics between human and guinea pig AChE with soman and the oximes HI-6 and MMB-4 in order to unravel the species dependent kinetic interactions. It turned out that only HI-6 was able to slow down the ongoing inhibition of human AChE by soman without preventing final complete inhibition of the enzyme. Continuous perfusion of AChE with soman and simultaneous or delayed (8, 15 or 40min) oxime perfusion did not result in a relevant reactivation of AChE (less than 2%). In conclusion, the results of the present study indicate a negligible reactivation of soman-inhibited AChE by oximes at conditions simulating the in vivo poisoning by soman. The observed therapeutic effect of oximes in soman poisoned animals in vivo must be attributed to alternative mechanisms which may not be relevant in humans.

UV/H2O2 advanced oxidation for abatement of organophosphorous pesticides and the effects on various toxicity screening assays

Advanced oxidation processes (AOPs) are utilized due to their ability to treat emerging contaminants with the fast reacting and non-selective hydroxyl radical (OH). Organophosphorous insecticides are common drinking water contaminants, with 12 different compounds of this class being found on the US EPA's most recent Candidate Contaminant List (CCL4). The use of the AOP UV/H₂O₂ for the treatment of organophosphorous insecticides was explored in this study, by coupling biological and analytical tools to follow the abatement of the target compounds. Four insecticides were explored for advanced oxidation treatment: acephate, dicrotophos, fenamiphos, and methamidophos. All four compounds were fast reacting with OH, all reacting with second order rate constants ≥5.5 × 10⁹ M^-1s^-1. Three major endpoints of toxicity were studied: estrogenicity, genotoxicity (mutagenicity) and neurotoxicity. None of the target compounds showed any estrogenic activity, while all compounds showed an active genotoxic (mutagenic) response (AMES II assay) and most compounds had some level of neurotoxic activity. AOP treatment did not induce any estrogenic activity, and reduced the compounds' neurotoxicity and genotoxicity in all but one case. Methamidophos degradation by UV/H₂O₂ resulted in an increase in genotoxicity, likely due to the formation of toxic transformation products. The increase in toxicity gradually decreased with time, possibly due to hydrolysis of the transformation products formed. This study provides insights into parent compound abatement and the changes in toxicity due to transformation products.

Evaluation of Potency of Known Oximes (Pralidoxime, Trimedoxime, HI-6, Methoxime, Obidoxime) to in vitro Reactivate Acetylcholinesterase Inhibited by Pesticides (Chlorpyrifos and Methylchlorpyrifos and Nerve Agent (Russian VX)

Nerve agents and pesticides belong to the group of organophosphates. They are able to inhibit irreversibly the enzyme acetylcholinesterase (AChE). Acetylcholinesterase reactivators were designed for the treatment of nerve agent intoxications. Their potency to reactivate pesticide-inhibited AChE was many times evaluated. In this study, five commonly used AChE reactivators (pralidoxime, methoxime, HI-6, obidoxime, trimedoxime) for the reactivation of AChE inhibited by two pesticides (chlorpyrifos and methylchlorpyrifos) were used. Russian VX (nerve agent) as a member of nerve agents’ family was taken for comparison. Obtained results show that oximes developed against nerve agent intoxication are less effective for intoxication with organophosphorus pesticides. Especially, methylchlorpyrifos-inhibited AChE was found to be poorly reactivated by the compounds used.

Study of para-Quinone Methide Precursors toward the Realkylation of Aged Acetylcholinesterase

Acetylcholinesterase (AChE) is an essential enzyme that can be targeted by organophosphorus (OP) compounds, including nerve agents. Following exposure to OPs, AChE becomes phosphylated (inhibited) and undergoes a subsequent aging process where the OP-AChE adduct is dealkylated. The aged AChE is unable to hydrolyze acetylcholine, resulting in accumulation of the neurotransmitter in the central nervous system (CNS) and elsewhere. Current therapeutics are only capable of reactivating inhibited AChE. There are no known therapeutic agents to reverse the aging process or treat aged AChE. Quinone methides (QMs) have been shown to alkylate phosphates under physiological conditions. In this study, a small library of novel quinone methide precursors (QMPs) has been synthesized and examined as potential alkylating agents against model nucleophiles, including a model phosphonate. Computational studies have been performed to evaluate the affinity of QMPs for the aged AChE active site, and preliminary testing with electric eel AChE has been performed.

Progress in acetylcholinesterase reactivators and in the treatment of organophosphorus intoxication: a patent review (2006-2016)

INTRODUCTION

organophosphorus compounds act as irreversible inhibitors of the vital enzyme acetylcholinesterase (AChE). this leads in the accumulation of acetylcholine (ACh) leading to cholinergic crisis and death. The main therapeutic approach is based on immediate administration of an ache reactivator as an antidote enabling recovery of the ache function. Areas covered: This review covers the development of AChE reactivators in order to introduce a new efficient drug that will overcome significant failures of common antidotes. Further options together with methods of detection are also discussed in order to assure a complete insight into the treatment of intoxication. Expert opinion: Since organophosphates belong to the most toxic chemical warfare agents, efficient antidotes are a matter of importance. The solution of how to limit the basic drawbacks of clinically used reactivators remained a spotlight for many researches worldwide. Recent strategies of the treatment of OP exposure bring us new possibilities which may overcome classic antidotes. The importance of detection of OP also has to be taken into consideration. Especially, with the fast spreading toxic effect when death can occur within minutes.

Structure-efficiency relationships of cyclodextrin scavengers in the hydrolytic degradation of organophosphorus compounds

New derivatives of cyclodextrins were prepared in order to determine the relative importance of the structural key elements involved in the degradation of organophosphorus nerve agents. To avoid a competitive inclusion between the organophosphorus substrate and the iodosobenzoate group, responsible for its degradation, the latter group had to be covalently bound to the cyclodextrin scaffold. Although the presence of the α nucleophile iodosobenzoate was a determinant in the hydrolysis process, an imidazole group was added to get a synergistic effect towards the degradation of the agents. The degradation efficiency was found to be dependent on the relative position of the heterocycle towards the reactive group as well as on the nature of the organophosphorus derivative.

Substrate Specificity and Leaving Group Effect in Ester Cleavage by Metal Complexes of an Oximate Nucleophile

Deprotonated zinc(II) and cadmium(II) complexes of a tridentate oxime nucleophile (1, OxH) show a very high reactivity, breaking by 2-3 orders of magnitude the previously established limiting reactivity of oximate nucleophiles in the cleavage of substituted phenyl acetates and phosphate triesters, but are unreactive with p-nitrophenyl phosphate di- and monoesters. With reactive substrates, these complexes operate as true catalysts through an acylation-deacylation mechanism. Detailed speciation and kinetic studies in a wide pH interval allowed us to establish as catalytically active forms [Cd(Ox)]⁺, [Zn(Ox)(OH)], and [Zn(Ox)(OH)₂]^- complexes. The formation of an unusual and most reactive zinc(II) oximatodihydroxo complex was confirmed by electrospray ionization mass spectrometry data and supported by density functional theory calculations, which also supported the previously noticed fact that the coordinated water in [Zn(OxH)(H₂O)₂]²⁺ deprotonates before the oxime. Analysis of the leaving group effect on the cleavage of phenyl acetates shows that the rate-determining step in the reaction with the free oximate anion is the nucleophilic attack, while with both zinc(II) and cadmium(II) oximate complexes, it changes to the expulsion of the leaving phenolate anion. The major new features of these complexes are (1) a very high esterolytic activity surpassing that of enzyme hydrolysis of aryl acetate esters and (2) an increased reactivity of coordinated oxime compared to free oxime in phosphate triester cleavage, contrary to the previously observed inhibitory effect of oxime coordination with these substrates.

The Reactivating and Therapeutic Efficacy of Oximes to Counteract Russian VX Poisonings

Russian VX ( O-isobutyl- S-(2-diethylaminoethyl)methylphosphonothioate) is the structural analogue of VX agent. It differs from VX agent ( O-ethyl- S-(2-diisopropylaminoethyl) methylphosphonothioate) by two alkyl groups. The potency of currently available oximes (pralidoxime, obidoxime, HI-6) to reactivate Russian VX–inhibited acetylcholinesterase and to eliminate Russian VX–induced acute toxic effects was evaluated using in vivo methods. In vivo determined percentage of reactivation of Russian VX–inhibited blood and brain acetylcholinesterase in poisoned rats shows that HI-6 seems to be the most efficacious reactivator of Russian VX–inhibited acetylcholinesterase among currently used oximes in the peripheral compartment, whereas no difference between reactivating efficacy of all tested oximes was observed in the central compartment. The oxime HI-6 was also found to be the most efficacious oxime in the elimination of acute lethal toxic effects in Russian VX–poisoned mice among all studied oximes. Thus, the oxime HI-6 seems to be the most suitable oxime for the antidotal treatment of acute poisonings with Russian VX as in the case of VX, sarin, cyclosarin, and soman poisonings.

Pyridoxal oxime derivative potency to reactivate cholinesterases inhibited by organophosphorus compounds

Organophosphorus (OP) nerve agents (sarin, tabun VX and soman) inhibit the enzyme acetylcholinesterase (AChE, EC 3.1.1.7) by binding to its active site while preventing neurotransmission in the cholinergic synapses. The protection and treatment of this kind of poisoning are still a challenge as we are yet to discover an antidote that would be effective in all cases of poisoning. To aid the search for more efficient antidotes, we evaluated the ability of nine pyridoxal oxime derivatives, prepared by a novel synthetic pathway, to reactivate recombinant human AChE and the related purified human plasma butyrylcholinesterase (BChE, EC 3.1.1.8) inhibited by VX, tabun and paraoxon. Oximes are derivatives of vitamin B6 bearing a phenacyl moiety attached to the quaternary nitrogen atom and having various substituents on the phenyl ring. As the results have shown, the tested oximes were in general more efficient in the reactivation of OP-inhibited BChE than AChE. The highest observed rate was in the case of VX-inhibited BChE reactivation, where k_obs was 0.0087min^-1 and the reactivation maximum of 90% was achieved within 5h. The cholinesterases displayed a binding affinity for these derivatives in a μmolar range no matter the substituent on their rings which was in accordance with the molecular modelling results showing a similar binding pattern for all oximes within the active site of both AChE and BChE. Such a positioning reveals also that hydroxy and a metoxy substituents at the vicinity of the oxime moiety present a possible steric hindrance explaining the reactivation results.

SAR study to find optimal cholinesterase reactivator against organophosphorous nerve agents and pesticides

Irreversible inhibition of acetylcholinesterase (AChE) by organophosphates leads to many failures in living organism and ultimately in death. Organophosphorus compounds developed as nerve agents such as tabun, sarin, soman, VX and others belong to the most toxic chemical warfare agents and are one of the biggest threats to the modern civilization. Moreover, misuse of nerve agents together with organophosphorus pesticides (e.g. malathion, paraoxon, chlorpyrifos, etc.) which are annually implicated in millions of intoxications and hundreds of thousand deaths reminds us of insufficient protection against these compounds. Basic treatments for these intoxications are based on immediate administration of atropine and acetylcholinesterase reactivators which are currently represented by mono- or bis-pyridinium aldoximes. However, these antidotes are not sufficient to ensure 100 % treatment efficacy even they are administered immediately after intoxication, and in general, they possess several drawbacks. Herein, we have reviewed new efforts leading to the development of novel reactivators and proposition of new promising strategies to design novel and effective antidotes. Structure-activity relationships and biological activities of recently proposed acetylcholinesterase reactivators are discussed and summarized. Among further modifications of known oximes, the main attention has been paid to dual binding site ligands of AChE as the current mainstream strategy. We have also discussed new chemical entities as potential replacement of oxime functional group.

Assessing the therapeutic efficacy of oxime therapies against percutaneous organophosphorus pesticide and nerve agent challenges in the Hartley guinea pig

Given the rapid onset of symptoms from intoxication by organophosphate (OP) compounds, a quick-acting, efficacious therapeutic regimen is needed. A primary component of anti-OP therapy is an oxime reactivator to rescue OP-inhibited acetylcholinesterases. Male guinea pigs, clipped of hair, received neat applications of either VR, VX, parathion, or phorate oxon (PHO) at the 85(th) percentile lethal dose, and, beginning with presentation of toxicosis, received the human equivalent dose therapy by intramuscular injection with two additional follow-on treatments at 3-hr intervals. Each therapy consisted of atropine free base at 0.4 mg/kg followed by one of eight candidate oximes. Lethality rates were obtained at 24 hr after VR, VX and PHO challenges, and at 48 hr after challenge with parathion. Lethality rates among symptomatic, oxime-treated groups were compared with that of positive control (OP-challenged and atropine-only treated) guinea pigs composited across the test days. Significant (p ≤ 0.05) protective therapy was afforded by 1,1-methylene bis(4(hydroxyimino- methyl)pyridinium) dimethanesulfonate (MMB4 DMS) against challenges of VR (p ≤ 0.001) and VX (p ≤ 0.05). Lethal effects of VX were also significantly (p ≤ 0.05) mitigated by treatments with oxo-[[1-[[4-(oxoazaniumylmethylidene)pyridin-1-yl]methoxymethyl]pyridin-4-ylidene]methyl]azanium dichloride (obidoxime Cl2) and 1-(((4-(aminocarbonyl) pyridinio)methoxy)methyl)-2,4-bis((hydroxyimino)methyl)pyridinium dimethanesulfonate (HLö-7 DMS). Against parathion, significant protective therapy was afforded by obidoxime dichloride (p ≤ 0.001) and 1,1'-propane-1,3-diylbis{4-[(E)-(hydroxyimino)methyl]pyridinium} dibromide (TMB-4, p ≤ 0.01). None of the oximes evaluated was therapeutically effective against PHO. Across the spectrum of OP chemicals tested, the oximes that offered the highest level of therapy were MMB4 DMS and obidoxime dichloride.

In vivo protection studies of bis-quaternary 2-(hydroxyimino)-N-(pyridin-3-yl) acetamide derivatives against sarin poisoning in mice

In vivo antidotal efficacy of new bis- quaternary 2-(hydroxyimino)- N-(pyridin-3yl) acetamide derivatives (HNK series), to counter multiples of lethal doses of nerve agent sarin (GB) and reactivation of acetylcholinesterase (AChE), was evaluated in Swiss albino mice. [Protection index PI; median lethal dose (LD50) of sarin with treatment/LD50 of sarin] was estimated, using 0.05, 0.10, and 0.20 LD50 as treatment doses of all the oximes with atropine against sarin poisoning. Dose-dependent time course study was conducted at 0.2, 0.4 and 0.8 LD50 dose of sarin for estimating maximum AChE inhibition. At optimized time (15 min), in vivo enzyme half inhibition concentration (IC50) was calculated. AChE reactivation efficacy of HNK series and pralidoxime (2-PAM) were determined by plotting shift of log IC50 doses. HNK-102 with atropine showed three fold higher PI compared to 2-PAM. In vivo IC50 of sarin for brain and serum AChE was found to be 0.87 LD50 (139.2 µg/kg) and 0.48 LD50 (77.23 µg/kg), respectively. Treatment with HNK-102 and HNK-111 (equal to their 0.20LD50) significantly reactivated sarin-intoxicated AChE ( p < 0.05) at 2× IC50 dose of sarin, compared to 2-PAM. The study revealed that HNK-102 oxime was three times more potent as antidote, for acute sarin poisoning compared to 2-PAM in vivo.

Pseudocatalytic scavenging of the nerve agent VX with human blood components and the oximes obidoxime and HI-6

Despite six decades of extensive research in medical countermeasures against nerve agent poisoning, a broad spectrum acetylcholinesterase (AChE) reactivator is not yet available. One current approach is directed toward synthesizing oximes with high affinity and reactivatability toward butyrylcholinesterase (BChE) in plasma to generate an effective pseudocatalytic scavenger. An interim solution could be the administration of external AChE or BChE from blood products to augment pseudocatalytic scavenging with slower but clinically approved oximes to decrease nerve agent concentrations in the body. We here semiquantitatively investigate the ability of obidoxime and HI-6 to decrease the inhibitory activity of VX with human AChE and BChE from whole blood, erythrocyte membranes, erythrocytes, plasma, clinically available fresh frozen plasma and packed red blood cells. The main findings are that whole blood showed a VX concentration-dependent decrease in inhibitory activity with HI-6 being more potent than obidoxime. Using erythrocytes and erythrocyte membranes again, HI-6 was more potent compared to obidoxime. With freshly prepared plasma, obidoxime and HI-6 showed comparable results for the decrease in VX. The use of the clinically available blood products revealed that packed red blood cells showed similar kinetics as fresh erythrocytes. Fresh frozen plasma resulted in a slower and incomplete decrease in inhibitory plasma compared to freshly prepared plasma. In conclusion, the administration of blood products in combination with available oximes augments pseudocatalytic scavenging and might be useful to decrease the body load of persistent, highly toxic nerve agents.

Efforts toward treatments against aging of organophosphorus-inhibited acetylcholinesterase

Aging is a dealkylation reaction of organophosphorus (OP)-inhibited acetylcholinesterase (AChE). Despite many studies to date, aged AChE cannot be reactivated directly by traditional pyridinium oximes. This review summarizes strategies that are potentially valuable in the treatment against aging in OP poisoning. Among them, retardation of aging seeks to lower the rate of aging through the use of AChE effectors. These drugs should be administered before AChE is completely aged. For postaging treatment, realkylation of aged AChE by appropriate alkylators may pave the way for oxime treatment by neutralizing the oxyanion at the active site of aged AChE. The other two strategies, upregulation of AChE expression and introduction of exogenous AChE, cannot resurrect aged AChE but may compensate for lowered active AChE levels by in situ production or external introduction of active AChE. Upregulation of AChE expression can be triggered by some peptides. Sources of exogenous AChE can be whole blood or purified AChE, either from human or nonhuman species.

Novel brain-penetrating oximes for reactivation of cholinesterase inhibited by sarin and VX surrogates

Current oxime reactivators for organophosphate-inhibited cholinesterase (ChE) do not effectively cross the blood-brain barrier and therefore cannot restore brain ChE activity in vivo. Our laboratories have studied highly relevant sarin and VX surrogates, which differ from their respective nerve agents only in the leaving group and thereby leave ChE phosphylated with the same chemical moiety as sarin and VX. Our laboratories have developed novel substituted phenoxyalkyl pyridinium oximes that lead to reduced ChE inhibition in the brains of rats challenged with a high sublethal dosage of the sarin surrogate, whereas 2-PAM did not, using a paradigm designed to demonstrate brain penetration. In addition, treatment of rats with these novel oximes is associated with attenuation of seizure-like behavior compared to rats treated with 2-PAM, providing additional evidence that the oximes penetrate the blood-brain barrier. Further, some of the oximes provided 24-h survival superior to 2-PAM, and shortened the duration of seizure-like behavior when rats were challenged with lethal dosages of the sarin and VX surrogates, providing additional support for the conclusion that these oximes penetrate the brain.

Reactivation of nerve agent-inhibited human acetylcholinesterase by obidoxime, HI-6 and obidoxime+HI-6: Kinetic in vitro study with simulated nerve agent toxicokinetics and oxime pharmacokinetics

Despite extensive research for decades no effective broad-spectrum oxime for the treatment of poisoning by a broad range of nerve agents is available. Previous in vitro and in vivo data indicate that the combination of in service oximes could be beneficial. To investigate the ability of obidoxime, HI-6 and the combination of both oximes to reactivate inhibited human AChE in the presence of sarin, cyclosarin or tabun we adopted a dynamic in vitro model with real-time and continuous determination of AChE activity to simulate inhalation nerve agent exposure and intramuscular oxime administration. The major findings of this kinetic study are that the extent and velocity of reactivation is dependent on the nerve agent and the oxime-specific reactivating potency. The oxime-induced reactivation of inhibited human AChE in the presence of nerve agents is markedly impaired and the combination of obidoxime and HI-6 had no additive effect but could broaden the spectrum. In conclusion, these data indicate that a combination of obidoxime and HI-6 would be beneficial for the treatment of poisoning by a broad spectrum of nerve agents and could present an interim solution until more effective and broad-spectrum reactivators are available.

Oximes in organophosphate poisoning: 60 years of hope and despair

The high number of annual fatalities following suicidal poisoning by organophosphorus (OP) pesticides and the recent homicidal use of the chemical warfare nerve agent sarin against civilian population in Syria underlines the continuous threat by these highly toxic agents. The need for an effective treatment of OP poisoning resulted in the implementation of a combination therapy with the muscarinic receptor antagonist atropine and an oxime for the reactivation of OP-inhibited acetylcholinesterase (AChE). Since the invention of the first clinically used oxime pralidoxime (2-PAM) in the 1950s ongoing research attempted to identify more effective oximes. In fact, several thousand oximes were synthesized in the past six decades. These include charged and non-charged compounds, mono- and bispyridinium oximes, asymmetric oximes, oximes with different substitutes and more recently non-oxime reactivators. Multiple in vitro and in vivo studies investigated the potential of oximes to reactivate OP-inhibited AChE and to reverse OP-induced cholinergic signs. Depending on the experimental model, the investigated species and the tested OP largely variable results were obtained by different laboratories. These findings and the inconsistent effectiveness of oximes in the treatment of OP-pesticide poisoned patients led to a continuous discussion on the value of oximes. In order to provide a forward-looking evaluation of the significance of oximes in OP poisoning multiple aspects, including intrinsic toxicity, in vitro reactivation potency, efficacy and pharmacokinetics, as well as the impact of the causative OP have to be considered. The different influencing factors in order to define the benefit and limitations of oximes in OP poisoning will be discussed.

Modeling and simulation of organophosphate-induced neurotoxicity: Prediction and validation by experimental studies

Exposure to organophosphorus (OP) compounds, either pesticides or chemical warfare agents, represents a major health problem. As potent irreversible inhibitors of cholinesterase, OP may induce seizures, as in status epilepticus, and occasionally brain lesions. Although these compounds are extremely toxic agents, the search for novel antidotes remains extremely limited. In silico modeling constitutes a useful tool to identify pharmacological targets and to develop efficient therapeutic strategies. In the present work, we developed a new in silico simulator in order to predict the neurotoxicity of irreversible inhibitors of acetyl- and/or butyrylcholinesterase (ChE) as well as the potential neuroprotection provided by antagonists of cholinergic muscarinic and glutamate N-methyl-d-aspartate (NMDA) receptors. The simulator reproduced firing of CA1 hippocampal neurons triggered by exposure to paraoxon (POX), as found in patch-clamp recordings in in vitro mouse hippocampal slices. In the case of POX intoxication, it predicted a preventing action of the muscarinic receptor antagonist atropine sulfate, as well as a synergistic action with the non-competitive NMDA receptor antagonist memantine. These in silico predictions relative to beneficial effects of atropine sulfate combined with memantine were recapitulated experimentally in an in vivo model of POX in adult male Swiss mice using electroencephalic (EEG) recordings. Thus, our simulator is a new powerful tool to identify protective therapeutic strategies against OP central effects, by screening various combinations of muscarinic and NMDA receptor antagonists.

Oxime-mediated in vitro reactivation kinetic analysis of organophosphates-inhibited human and electric eel acetylcholinesterase

Organophosphate (OP)-based pesticides and nerve agents are highly toxic compounds which interrupt the catalytic mechanism of acetylcholinesterase (AChE) by phosphorylating the hydroxyl moiety of serine residue. The inhibited enzyme can be reactivated by the nucleophilic action of oxime reactivators. To analyze the effect of different AChE sources on reactivation efficacy of reactivators, several in vivo studies have carried out using variety of AChE sources like pig, rat and monkey. Investigations on species differences provide a better insight for the development of new reactivators. Hence, present study was mainly targeted on comparative analysis of the reactivation of electric eel and human AChE inhibited by different OP. A series of butene-linked bis-pyridinium mono oximes which vary in functional groups present at the second pyridinium ring have been examined against sarin, VX, tabun and ethyl-paraoxon-poisoned AChE. In case of tabun-inhibited AChEs, tested oximes were better than reference oximes. For VX-poisoned human AChE, reactivator K251 (kr2;1.51 mM (-) (1 )min (-) (1)) showed good reactivation efficacy with standard oximes. Studies stipulated that butene-linked oximes consisting of different functional moieties are good reactivators and found to have better efficacy to reactivate nerve agent-inhibited human AChE in comparison to eel AChE.

Efficacy of Recommended Prehospital Human Equivalent Doses of Atropine and Pralidoxime Against the Toxic Effects of Carbamate Poisoning in the Hartley Guinea Pig

PURPOSE

Aldicarb and methomyl are carbamate pesticides commonly implicated in human poisonings. The primary toxic mechanism of action for carbamate poisoning is cholinesterase (ChE) inhibition. As such, it is logical to assume that the currently accepted therapies for organophosphate poisoning (muscarinic antagonist atropine and the oxime acetylcholinesterase reactivator pralidoxime chloride [2-PAM Cl]) could afford therapeutic protection. However, oximes have been shown to be contraindicated for poisoning by some carbamates.

METHODS

A protective ratio study was conducted in guinea pigs to evaluate the efficacy of atropine and 2-PAM Cl. The ChE activity was determined in both the blood and the cerebral cortex.

RESULTS

Coadministration of atropine free base (0.4 mg/kg) and 2-PAM Cl (25.7 mg/kg) demonstrated protective ratios of 2 and 3 against aldicarb and methomyl, respectively, relative to saline. The data reported here show that this protection was primarily mediated by the action of atropine. The reactivator 2-PAM Cl had neither positive nor negative effects on survival. Both blood acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activities were significantly reduced at 15 minutes postchallenge but gradually returned to normal within 24 hours. Analysis of cerebral cortex showed that BChE, but not AChE, activity was reduced in animals that succumbed prior to 24 hours after challenge.

CONCLUSION

The results suggest that coadministration of atropine and 2-PAM Cl at the currently recommended human equivalent doses for use in the prehospital setting to treat organophosphorus nerve agent and pesticide poisoning would likely also be effective against aldicarb or methomyl poisoning.

Probing the activity of a non-oxime reactivator for acetylcholinesterase inhibited by organophosphorus nerve agents

Currently fielded treatments for nerve agent intoxication include atropine, an acetylcholine receptor antagonist, and pralidoxime (2PAM), a small molecule reactivator of acetylcholinesterase (AChE). 2PAM reactivates nerve agent-inhibited AChE via direct nucleophilic attack by the oxime moiety on the phosphorus center of the bound nerve agent. Due to a permanently charged pyridinium motif, 2PAM is not thought to cross the blood brain barrier and therefore cannot act directly in the neuronal junctions of the brain. In this study, ADOC, a non-permanently charged, non-oxime molecule initially identified using pesticide-inhibited AChE, was characterized in vitro against nerve agent-inhibited recombinant human AChE. The inhibitory and reactivation potentials of ADOC were determined with native AChE and AChE inhibited with tabun, sarin, soman, cyclosarin, VX, or VR and then compared to those of 2PAM. Several structural analogs of ADOC were used to probe the reactivation mechanism of the molecule. Finally, guinea pigs were used to examine the protective efficacy of the compound after exposure to sarin. The results of both in vitro and in vivo testing will be useful in the design of future small molecule reactivators.

Activity-Based Protein Profiling Reveals Broad Reactivity of the Nerve Agent Sarin

Elucidation of noncholinesterase protein targets of organophosphates, and nerve agents in particular, may reveal additional mechanisms for their high toxicity as well as clues for novel therapeutic approaches toward intoxications with these agents. Within this framework, we here describe the synthesis of the activity-based probe 3, which contains a phosphonofluoridate moiety, a P-Me moiety, and a biotinylated O-alkyl group, and its use in activity-based protein profiling with two relevant biological samples, that is, rhesus monkey liver and cultured human A549 lung cells. In this way, we have unearthed eight serine hydrolases (fatty acid synthase, acylpeptide hydrolase, dipeptidyl peptidase 9, prolyl oligopeptidase, carboxylesterase, long-chain acyl coenzyme A thioesterase, PAF acetylhydrolase 1b, and esterase D/S-formyl glutathione hydrolase) as targets that are modified by the nerve agent sarin. It is also shown that the newly developed probe 3 might find its way into the development of alternative, less laborious purification protocols for human butyrylcholinesterase, a potent bioscavenger currently under clinical investigation as a prophylactic/therapeutic for nerve agent intoxications.

Comparison of inhibition kinetics of several organophosphates, including some nerve agent surrogates, using human erythrocyte and rat and mouse brain acetylcholinesterase

Because testing of nerve agents is limited to only authorized facilities, our laboratory developed several surrogates that resemble nerve agents because they phosphylate the acetylcholinesterase (AChE) with the same moiety as the actual nerve agents. The inhibition kinetic parameters were determined for AChE by surrogates of cyclosarin (NCMP), sarin (NIMP, PIMP and TIMP) and VX (NEMP and TEMP) and other organophosphorus compounds derived from insecticides. All compounds were tested with rat brain and a subset was tested with mouse brain and purified human erythrocyte AChE. Within the compounds tested on all AChE sources, chlorpyrifos-oxon had the highest molecular rate constant followed by NCMP and NEMP. This was followed by NIMP then paraoxon and DFP with rat and mouse brain AChE but DFP was a more potent inhibitor than NIMP and paraoxon with human AChE. With the additional compounds tested only in rat brain, TEMP was slightly less potent than NEMP but more potent than PIMP which was more potent than NIMP. Methyl paraoxon was slightly less potent than paraoxon but more potent than TIMP which was more potent than DFP. Overall, this study validates that the pattern of inhibitory potencies of our surrogates is comparable to the pattern of inhibitory potencies of actual nerve agents (i.e., cyclosarin>VX>sarin), and that these are more potent than insecticidal organophosphates.

Kinetic analysis of interactions of amodiaquine with human cholinesterases and organophosphorus compounds

Standard therapy of poisoning by organophosphorus compounds (OP) is a combined administration of an anti-muscarinic drug (e.g. atropine) and an oxime as reactivator of inhibited acetylcholinesterase (AChE). Limited efficacy of clinically used oximes against a variety of OPs was shown in numerous studies, calling for research on novel reactivators of OP-inhibited AChE. Recently, reactivation of OP-inhibited AChE by the antimalarial drug amodiaquine was reported. In the present study, amodiaquine and its interactions with human cholinesterases in presence or absence of OP nerve agents was investigated in vitro. Thereby, reversible inhibition of human cholinesterases by amodiaquine (AChE ≫ BChE) was observed. Additionally, a mixed competitive-non-competitive inhibition type of amodiaquine with human AChE was determined. Slow and partial reactivation of sarin-, cyclosarin- and VX-inhibited cholinesterases by amodiaquine was recorded, amodiaquine failed to reactivate tabun-inhibited human cholinesterases. Amodiaquine, being a potent, reversible AChE inhibitor, was tested for its potential benefit as a pretreatment to prevent complete irreversible AChE inhibition by the nerve agent soman. Hereby, amodiaquine failed to prevent phosphonylation and resulted only in a slight increase of AChE activity after removal of amodiaquine and soman. At present the molecular mechanism of amodiaquine-induced reactivation of OP-inhibited AChE is not known, nevertheless amodiaquine could be considered as a template for the design of more potent non-oxime reactivators.

Ultra-sensitive biosensor based on genetically engineered acetylcholinesterase immobilized in poly (vinyl alcohol)/Fe-Ni alloy nanocomposite for phosmet detection in olive oil

An ultra-sensitive screen-printed biosensor was successfully developed for phosmet detection in olive oil, based on a genetically-engineered acetylcholinesterase (AChE) immobilized in a azide-unit water-pendant polyvinyl alcohol (PVA-AWP)/Fe-Ni alloy nanocomposite. Fe-Ni not only allowed amplifying the response current but also lowering the applied potential from 80 mV to 30 mV vs Ag/AgCl. The biosensor showed a very good analytical performance for phosmet detection, with a detection limit of 0.1 nM. This detection limit is lower than the allowable concentrations set by international regulations. In addition to the good reproducibility, operational and storage stability, the developed biosensor was successfully used for the determination of phosmet in olive oil samples without any laborious pre-treatment. The phosmet recovery rate was about 96% after a simple liquid-liquid extraction.