Design and structure-activity relationships of antidotes to organophosphorus anticholinesterase agents

Screening of efficient salicylaldoxime reactivators for DFP and paraoxon-inhibited acetylcholinesterase

Previously we reported two salicylaldoxime conjugates (L7R3 and L7R5) showing equal or even higher reactivating efficiency for both organophosphorus nerve agent and pesticide inhibited acetylcholinesterase in comparison to obidoxime and HI-6. In this study, L7R3 and L7R5 were selected as lead compounds and refined by employing a fragment-based drug design strategy, and a total of 32 novel salicylaldoxime conjugates were constructed and screened for DFP and paraoxon inhibited acetylcholinesterase. The findings demonstrate that the conjugate L73R3, which contains a 4-nitrophenyl group, exhibited a higher reactivation efficacy against paraoxon-inhibited acetylcholinesterase compared to obidoxime and HI-6. It was confirmed that the combination of a 4-pyridinyl or 4-nitrophenyl peripheral site ligand, a piperazine linker and a methyl or chloro-substituted salicylaldoxime could construct efficient nonquaternary oxime reactivators. The results hold promise for developing a new generation of highly effective antidotes for organophosphate poisoning.

Organophosphorus compounds and oximes: a critical review

Organophosphorus (OP) pesticides and nerve agents still pose a threat to the population. Treatment of OP poisoning is an ongoing challenge and burden for medical services. Standard drug treatment consists of atropine and an oxime as reactivator of OP-inhibited acetylcholinesterase and is virtually unchanged since more than six decades. Established oximes, i.e. pralidoxime, obidoxime, TMB-4, HI-6 and MMB-4, are of insufficient effectiveness in some poisonings and often cover only a limited spectrum of the different nerve agents and pesticides. Moreover, the value of oximes in human OP pesticide poisoning is still disputed. Long-lasting research efforts resulted in the preparation of countless experimental oximes, and more recently non-oxime reactivators, intended to replace or supplement the established and licensed oximes. The progress of this development is slow and none of the novel compounds appears to be suitable for transfer into advanced development or into clinical use. This situation calls for a critical analysis of the value of oximes as mainstay of treatment as well as the potential and limitations of established and novel reactivators. Requirements for a straightforward identification of superior reactivators and their development to licensed drugs need to be addressed as well as options for interim solutions as a chance to improve the therapy of OP poisoning in a foreseeable time frame.

Efficient detoxification of nerve agents by oxime-assisted reactivation of acetylcholinesterase mutants

The recent advancements in crystallography and kinetics studies involving reactivation mechanism of acetylcholinesterase (AChE) inhibited by nerve agents have enabled a new paradigm in the search for potent medical countermeasures in case of nerve agents exposure. Poisonings by organophosphorus compounds (OP) that lead to life-threatening toxic manifestations require immediate treatment that combines administration of anticholinergic drugs and an aldoxime as a reactivator of AChE. An alternative approach to reduce the in vivo toxicity of OP centers on the use of bioscavengers against the parent organophosphate. Our recent research showed that site-directed mutagenesis of AChE can enable aldoximes to substantially accelerate the reactivation of OP-enzyme conjugates while dramatically slowing down rates of OP-conjugate dealkylation (aging). Therefore, this review focuses on oxime-assisted catalysis by AChE mutants that provides a potential means for degradation of organophosphates in the plasma before reaching the cellular target site. This article is part of the special issue entitled 'Acetylcholinesterase Inhibitors: From Bench to Bedside to Battlefield'.

Evaluation of high-affinity phenyltetrahydroisoquinoline aldoximes, linked through anti-triazoles, as reactivators of phosphylated cholinesterases

Acetylcholinesterase (AChE) is a pivotal enzyme in neurotransmission. Its inhibition leads to cholinergic crises and could ultimately result in death. A related enzyme, butyrylcholinesterase (BChE), may act in the CNS as a co-regulator in terminating nerve impulses and is a natural plasma scavenger upon exposure to organophosphate (OP) nerve agents that irreversibly inhibit both enzymes. With the aim of improving reactivation of cholinesterases phosphylated by nerve agents sarin, VX, cyclosarin, and tabun, ten phenyltetrahydroisoquinoline (PIQ) aldoximes were synthesized by Huisgen 1,3 dipolar cycloaddition between alkyne- and azide-building blocks. The PIQ moiety may serve as a peripheral site anchor positioning the aldoxime moiety at the AChE active site. In terms of evaluated dissociation inhibition constants, the aldoximes could be characterized as high-affinity ligands. Nevertheless, high binding affinity of these oximes to AChE or its phosphylated conjugates did not assure rapid and selective AChE reactivation. Rather, potential reactivators of phosphylated BChE, with its enlarged acyl pocket, were identified, especially in case of cyclosarin, where the reactivation rates of the lead reactivator was 100- and 6-times that of 2-PAM and HI-6, respectively. Nevertheless, the return of the enzyme activity was affected by the nerve agent conjugated to catalytic serine, which highlights the lack of the universality of reactivators with respect to both the target enzyme and OP structure.

Counteracting tabun inhibition by reactivation by pyridinium aldoximes that interact with active center gorge mutants of acetylcholinesterase

Tabun represents the phosphoramidate class of organophosphates that are covalent inhibitors of acetylcholinesterase (AChE), an essential enzyme in neurotransmission. Currently used therapy in counteracting excessive cholinergic stimulation consists of a muscarinic antagonist (atropine) and an oxime reactivator of inhibited AChE, but the classical oximes are particularly ineffective in counteracting tabun exposure. In a recent publication (Kovarik et al., 2019), we showed that several oximes prepared by the Huisgen 1,3 dipolar cycloaddition and related precursors efficiently reactivate the tabun-AChE conjugate. Herein, we pursue the antidotal question further and examine a series of lead precursor molecules, along with triazole compounds, as reactivators of two AChE mutant enzymes. Such studies should reveal structural subtleties that reside within the architecture of the active center gorge of AChE and uncover intimate mechanisms of reactivation of alkylphosphate conjugates of AChE. The designated mutations appear to minimize steric constraints of the reactivating oximes within the impacted active center gorge. Indeed, after initial screening of the triazole oxime library and its precursors for the reactivation efficacy on Y337A and Y337A/F338A human AChE mutants, we found potentially active oxime-mutant enzyme pairs capable of degrading tabun in cycles of inhibition and reactivation. Surprisingly, the most sensitive ex vivo reactivation of mutant AChEs occurred with the alkylpyridinium aldoximes. Hence, although the use of mutant enzyme bio-scavengers in humans may be limited in practicality, bioscavenging and efficient neutralization of tabun itself or phosphoramidate mixtures of organophosphates might be achieved efficiently in vitro or ex vivo with these mutant AChE combinations.

The estimation of oxime efficiency is affected by the experimental design of phosphylated acetylcholinesterase reactivation

Reactivation of acetylcholinesterase (AChE), an essential enzyme in neurotransmission, is a key point in the treatment of acute poisoning by nerve agents and pesticides, which structurally belong to organophosphorus compounds (OP). Due to the high diversity of substituents on the phosphorous atom, there is a variety of OP-AChE conjugates deriving from AChE inhibition, and therefore not only is there no universal reactivator efficient enough for the most toxic OPs, but for some nerve agents there is still a lack of any reactivator at all. The endeavor of many chemists to find more efficient reactivators resulted in thousands of newly-designed and synthesized oximes-potential reactivators of AChE. For an evaluation of the oximés reactivation efficiency, many research groups employ a simple spectrophotometric Ellman method. Since parameters that describe reactivator efficiency are often incomparable among laboratories, we tried to emphasize the critical steps in the determination of reactivation parameters as well as in the experimental design of a reactivation assay. We highlighted the important points in evaluation of reactivation kinetic parameters with an aim to achieve better agreement and comparability between the results obtained by different laboratories and overall, a more efficient evaluation of in vitro reactivation potency.

HI-6 assisted catalytic scavenging of VX by acetylcholinesterase choline binding site mutants

The high toxicity of organophosphorus compounds originates from covalent inhibition of acetylcholinesterase (AChE), an essential enzyme in cholinergic neurotransmission. Poisonings that lead to life-threatening toxic manifestations require immediate treatment that combines administration of anticholinergic drugs and an aldoxime as a reactivator of AChE. An alternative approach to reduce the in vivo toxicity of OPs focuses on the use of bioscavengers against the parent organophosphate. Our previous research showed that AChE mutagenesis can enable aldoximes to substantially accelerate the reactivation of OP-enzyme conjugates, while dramatically slowing down rates of OP-conjugate dealkylation (aging). Herein, we demonstrate an efficient HI-6-assisted VX detoxification, both ex vivo in human blood and in vivo in mice by hAChE mutants modified at the choline binding site (Y337A and Y337A/F338A). The catalytic scavenging of VX in mice improved therapeutic outcomes preventing lethality and resulted in a delayed onset of toxicity symptoms.

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.

Synthesis and in vitro reactivation study of isonicotinamide derivatives of 2-(hydroxyimino)-N-(pyridin-3-yl)acetamide as reactivators of Sarin and VX inhibited human acetylcholinesterase (hAChE)

Previously (Karade et al., 2014), we have reported the synthesis and in vitro evaluation of bis-pyridinium derivatives of pyridine-3-yl-(2-hydroxyimino acetamide), as reactivators of sarin and VX inhibited hAChE. Few of the molecules showed superior in vivo protection efficacy (mice model) (Kumar et al., 2014; Swami et al., 2016) in comparison to 2-PAM against DFP and sarin poisoning. Encouraged by these results, herein we report the synthesis and in vitro evaluation of isonicotinamide derivatives of pyridine-3-yl-(2-hydroxyimino acetamide) (4a-4d) against sarin and VX inhibited erythrocyte ghost hAChE. Reactivation kinetics of these compounds was studied and the determined kinetic parameters were compared with that of commercial reactivators viz. 2-PAM and obidoxime. In comparison to 2-PAM and obidoxime, oxime 4a and 4b exhibited enhanced reactivation efficacy toward sarin inhibited hAChE while oxime 4c showed far greater reactivation efficacy toward VX inhibited hAChE. The acid dissociation constant and IC50 values of these oximes were determined and correlated with the observed reactivation potential.

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.

Prophylaxis with human serum butyrylcholinesterase protects Göttingen minipigs exposed to a lethal high-dose of sarin vapor

Serum-derived human butyrylcholinesterase (Hu BChE) is a stoichiometric bioscavenger that is being developed as a potential prophylactic nerve agent countermeasure. Previously, we reported the prophylactic efficacy of Hu BChE in Göttingen minipigs against a whole-body exposure to 4.1mg/m(3) of sarin (GB) vapor, which produced lethality over 60min. Since the toxicity of nerve agent is concentration-dependent, in the present study, we investigated the toxic effects of an almost 3-fold higher rate of GB vapor exposure and the ability of Hu BChE to protect minipigs against this exposure. Male minipigs were subjected to: (1) air exposure; (2) GB vapor exposure; or (3) pretreatment with 7.5mg/kg of Hu BChE by i.m. injection, 24h prior to whole-body exposure to 11.4mg/m(3) of GB vapor for 10min. Electrocardiogram, electroencephalogram, and pupil size were monitored throughout exposure. Blood drawn before and throughout exposure was analyzed for blood gases, electrolytes, metabolites, acetylcholinesterase and BChE activities, and amount of GB bound to red blood cells and plasma. A novel finding was that saline-treated animals exposed to GB vapor did not develop any seizures, but manifested a variety of cardiac and whole blood toxic signs and rapidly died due to respiratory failure. Strikingly, pre-treatment with 7.5mg/kg of Hu BChE not only prevented lethality, but also avoided all cardiac toxic signs manifested in the non-treated cohort. Thus, Hu BChE alone can serve as an effective prophylactic countermeasure versus a lethal high-dose exposure to GB vapor.

Synthesis and in vitro kinetic evaluation of N-thiazolylacetamido monoquaternary pyridinium oximes as reactivators of sarin, O-ethylsarin and VX inhibited human acetylcholinesterase (hAChE)

Presently available medications for treatment of organiphosphorus poisoning are not sufficiently effective due to various pharmacological and toxicological reasons. In this regard, herein we report the synthesis of a series of N-thiazolylacetamide monoquaternary pyridinium oximes and its analogs (1a-1b to 6a-6b) with diversely substituted thiazole ring and evaluation of their in vitro reactivation efficacies against nerve agent (sarin, O-ethylsarin and VX) inhibited human erythrocyte acetylcholinesterase (hAChE). Reactivation kinetics was performed to determine dissociation constant (KD), reactivity rate constant (kr) and the second order rate constant (kr2) for all the compounds and compared their efficacies with commercial antidotes viz. 2-PAM and obidoxime. All the newly synthesized oximes were evaluated for their physicochemical parameters (pKa) and correlated with their respective reactivation efficacies to assess the capability of the oxime reactivator. Three of these novel compounds showed promising reactivation efficacies toward OP inhibited hAChE. Molecular docking studies were performed in order to correlate the reactivation efficacies with their interactions in the active site of the AChE.

Preclinical studies of noncharged oxime reactivators for organophosphate exposure

A countermeasure that protects the brain from organophosphate toxicity is an unmet need. Few small molecule reactivators that can cross the blood brain barrier and reactivate brain acetyl cholinesterases have been reported. Herein, we describe preclinical investigations of a new class of amidine-oxime reactivator of cholinesterases with improved potency and blood brain barrier permeability. (Z)-N-((E)-1-(Dimethylamino)-2-(hydroxyimino)ethylidene)butan-1-aminium chloride, 1, is zwitterionic at physiological pH but possesses increased oxime nucleophilicity because of the adjacent amidine functionality. The amidine-oximes reported herein were observed to be nontoxic (up to 200 mg/kg in vivo) and are chemically and metabolically stable. The results presented herein show that uncharged amidine-oxime reactivators such as 1 can penetrate the blood brain barrier in animals and protect from the toxicity of nerve agent model compounds.

Toxicogenomic studies of human neural cells following exposure to organophosphorus chemical warfare nerve agent VX

Organophosphorus (OP) compounds represent an important group of chemical warfare nerve agents that remains a significant and constant military and civilian threat. OP compounds are considered acting primarily via cholinergic pathways by binding irreversibly to acetylcholinesterase, an important regulator of the neurotransmitter acetylcholine. Many studies over the past years have suggested that other mechanisms of OP toxicity exist, which need to be unraveled by a comprehensive and systematic approach such as genome-wide gene expression analysis. Here we performed a microarray study in which cultured human neural cells were exposed to 0.1 or 10 μM of VX for 1 h. Global gene expression changes were analyzed 6, 24, and 72 h post exposure. Functional annotation and pathway analysis of the differentially expressed genes has revealed many genes, networks and canonical pathways that are related to nervous system development and function, or to neurodegenerative diseases such as Alzheimer's disease, Huntington's disease, and Parkinson's disease. In particular, the neuregulin pathway impacted by VX exposure has important implications in many nervous system diseases including schizophrenia. These results provide useful information valuable in developing suitable antidotes for more effective prevention and treatment of, as well as in developing biomarkers for, VX-induced chronic neurotoxicity.

Reactivation kinetics of a homologous series of bispyridinium bis-oximes with nerve agent-inhibited human acetylcholinesterase

The reactivation of organophosphorus compound (OP)-inhibited acetylcholinesterase (AChE) by oximes is inadequate in case of different OP nerve agents. This fact led to the synthesis of numerous novel oximes by different research groups in order to identify more effective reactivators. In the present study, we investigated the reactivation kinetics of a homologous series of bispyridinium bis-oximes bearing a (E)-but-2-ene linker with tabun-, sarin-, and cyclosarin-inhibited human AChE. In part, marked differences in affinity and reactivity of the investigated oximes toward OP-inhibited human AChE were recorded. These properties depended on the position of the oxime groups and the inhibitor. None of the tested oximes was equally effective against all used OPs. In addition, the data indicate that a (E)-but-2-ene linker decreased in most cases the reactivating potency in comparison to oximes bearing an oxybismethylene linker, e.g., obidoxime and HI-6. The results of this study give further insight into structural requirements for oxime reactivators, underline the necessity to investigate the kinetic interactions of oximes and AChE with structurally different OP inhibitors, and point to the difficulty to develop an oxime reactivator which is efficient against a broad spectrum of OPs.

Amidine-oximes: reactivators for organophosphate exposure

A new class of amidine-oxime reactivators of organophosphate (OP)-inhibited cholinesterases (ChE) were designed, synthesized, and tested. These compounds represent a novel group of oximes with enhanced capabilities of crossing the blood-brain barrier. Lack of brain penetration is a major limitation for currently used oximes as antidotes of OP poisoning. The concept described herein relies on a combination of an amidine residue and oxime functionality whereby the amidine increases the binding affinity to the ChE and the oxime is responsible for reactivation. Amidine-oximes were tested in vitro and reactivation rates for OP-BuChE were greater than pralidoxime (2-PAM) or monoisonitrosoacetone (MINA). Amidine-oxime reactivation rates for OP-AChE were lower compared to 2-PAM but greater compared with MINA. After pretreatment for 30 min with oximes 15c and 15d (145 μmol/kg, ip) mice were challenged with a soman model compound. In addition, 15d was tested in a post-treatment experiment (145 μmol/kg, ip, administration 5 min after sarin model compound exposure). In both cases, amidine-oximes afforded 100% 24 h survival in an animal model of OP exposure.

Adenovirus-mediated human paraoxonase1 gene transfer to provide protection against the toxicity of the organophosphorus pesticide toxicant diazoxon

Human paraoxonase1 (hPON1) is a potential therapeutic against the toxicity of organophosphorus (OP) pesticides and chemical warfare nerve agents. We tested whether PON1 gene transfer using adenovirus provides protection against the toxicity of the OP diazoxon. Using an adenovirus construct containing hPON1 gene, we showed elevated levels of recombinant hPON1 in vitro in 293A cells and in vivo in mice. The recombinant enzyme was secreted by 293A cells into culture medium and into the systemic circulation of mice. Western blotting revealed that the virally expressed hPON1 had the expected molecular weight of 45 kDa. Recombinant hPON1 in mice was in complex with mouse high-density lipoprotein (HDL) and migrated more slowly than endogenous hPON1 in the human HDL complex. Mice injected with adenovirus expressed PON1 at 600-3480 U ml(-1) on day 5 post-treatment, which is 8-50-fold above endogenous. Six mice expressing hPON1 survived 2LD(50) doses of diazoxon. Four of the six mice survived a second dose of diazoxon (for a total of 4LD(50)) administered 24 h later. In contrast, none of the three mice in the control group survived one 2LD(50) dose. These results show that hPON1 in mice functions as a prophylactic and offers significant protection against lethal doses of diazoxon.

In search of a catalytic bioscavenger for the prophylaxis of nerve agent toxicity

A novel approach for treating organophosphorus (OP) poisoning is the use of enzymes, both stoichiometric and catalytic, as bioscavengers to sequester these compounds in circulation before they reach their physiological targets. Human serum butyrylcholinesterase and a recombinant form of this enzyme produced in the milk of transgenic goats have completed Phase I clinical trials as stoichiometric bioscavengers for the protection of humans against OP nerve agents. However, a major limitation of the first generation bioscavenger is the 1:1 stoichiometry between the enzyme and the OP. Therefore, efforts are underway to develop the second generation catalytic bioscavenger, which will neutralize/hydrolyze multiple OP molecules. To avoid any complications related to adverse immune reactions, three enzymes from human (Hu) sources are being considered for development as catalytic bioscavengers: (1) prolidase; (2) paraoxonase 1 (PON1); (3) senescence marker protein-30 (SMP-30). Towards this effort, native or recombinant (r) forms of candidate catalytic bioscavengers were isolated and characterized for their ability to hydrolyze G-type nerve agents at concentrations of 10muM and 1mM. Results show that mammalian enzymes were significantly less efficient at hydrolyzing nerve agents as compared to bacterial organophosphorus hydrolase (OPH) and organophosphorus acid anhydrolase (OPAA). Recombinant Hu prolidase was the most efficient and the only mammalian enzyme that hydrolyzed all four G-type nerve agents. On the other hand, both rHu PON1 and Mo SMP-30 showed 10-fold lower activity towards sarin compared to rHu prolidase and did not hydrolyze tabun. Based on these results, Hu prolidase appears to be the most promising candidate for further development: (1) it can be easily expressed in E. coli; (2) of the three candidate enzymes, it is the only enzyme that hydrolyzes all four G-type agents. Efforts to improve the catalytic efficiency of this enzyme towards OP nerve agents are underway.

Human carboxylesterase 1 stereoselectively binds the nerve agent cyclosarin and spontaneously hydrolyzes the nerve agent sarin

Organophosphorus (OP) nerve agents are potent toxins that inhibit cholinesterases and produce a rapid and lethal cholinergic crisis. Development of protein-based therapeutics is being pursued with the goal of preventing nerve agent toxicity and protecting against the long-term side effects of these agents. The drug-metabolizing enzyme human carboxylesterase 1 (hCE1) is a candidate protein-based therapeutic because of its similarity in structure and function to the cholinesterase targets of nerve agent poisoning. However, the ability of wild-type hCE1 to process the G-type nerve agents sarin and cyclosarin has not been determined. We report the crystal structure of hCE1 in complex with the nerve agent cyclosarin. We further use stereoselective nerve agent analogs to establish that hCE1 exhibits a 1700- and 2900-fold preference for the P(R) enantiomers of analogs of soman and cyclosarin, respectively, and a 5-fold preference for the P(S) isomer of a sarin analog. Finally, we show that for enzyme inhibited by racemic mixtures of bona fide nerve agents, hCE1 spontaneously reactivates in the presence of sarin but not soman or cyclosarin. The addition of the neutral oxime 2,3-butanedione monoxime increases the rate of reactivation of hCE1 from sarin inhibition by more than 60-fold but has no effect on reactivation with the other agents examined. Taken together, these data demonstrate that hCE1 is only reactivated after inhibition with the more toxic P(S) isomer of sarin. These results provide important insights toward the long-term goal of designing novel forms of hCE1 to act as protein-based therapeutics for nerve agent detoxification.

Structure-Activity Approach in the Reactivation of Tabun-Phosphorylated Human Acetylcholinesterase with Bispyridinium para-Aldoximes

We investigated interactions of bispyridinium para-aldoximes N,N'-(propano)bis(4-hydroxyiminomethyl) pyridinium bromide (TMB-4), N,N'-(ethano)bis(4-hydroxyiminomethyl)pyridinium methanosulphonate (DMB-4), and N,N'-(methano)bis(4-hydroxyiminomethyl)pyridinium chloride (MMB-4) with human erythrocyte acetylcholinesterase phosphorylated by tabun. We analysed aldoxime conformations to determine the flexibility of aldoxime as an important feature for binding to the acetylcholinesterase active site. Tabun-inhibited human erythrocyte acetylcholinesterase was completely reactivated only by the most flexible bispyridinium aldoxime - TMB-4 with a propylene chain between two rings. Shorter linkers than propylene (methylene or ethylene) as in MMB-4 and DMB-4 did not allow appropriate orientation in the active site, and MMB-4 and DMB-4 were not efficient reactivators of tabun-phosphorylated acetylcholinesterase. Since aldoximes are also reversible inhibitors of native acetylcholinesterase, we determined dissociation constants and their protective index against acetylcholinesterase inactivation by tabun.

Bioscavenger for protection from toxicity of organophosphorus compounds

Current antidotal regimens for organophosphorus compound (OP) poisoning consist of a combination of pretreatment with a spontaneously reactivating AChE inhibitor such as pyridostigmine bromide, and postexposure therapy with anticholinergic drugs such as atropine sulfate and oximes such as 2-PAM chloride (Gray, 1984). Although these antidotal regimens are effective in preventing lethality of animals from OP poisoning, they do not prevent postexposure incapacitation, convulsions, performance deficits, or, in many cases, permanent brain damage (Dunn and Sidell, 1989). These problems stimulated the development of enzyme bioscavengers as a pretreatment to sequester highly toxic OPs before they reach their physiological targets. Several studies over the last two decades have demonstrated that exogenously administered human serum butyrylcholinesterase (Hu BChE) can be used successfully as a safe, efficacious, and single prophylactic treatment to counteract the toxicity of OPs. It also has potential use for first responders (civilians) reacting to terrorist nerve gas release, pesticide overexposure, or succinylcholine-induced apnea. A dose of 200 mg of Hu BChE in humans is envisioned as a prophylactic treatment that can protect from exposure of 2-5 x LD50 of nerve agents (Ashani, 2000).

Structure—activity relationships for in vitro oxime reactivation of chlorpyrifos-inhibited acetylcholinesterase

Organophosphorus pesticides parathion, chlorpyrifos, and malathion inhibit the enzyme acetylcholinesterase (AChE; EC 3.1.1.7) via phosphorylation of its active site. AChE reactivators and anticholinergics are compounds used as antidotes in the case of intoxication by these AChE inhibitors. In this work, chlorpyrifos, a representative member of this pesticide family, was used to inhibit the AChE activity of rat brain. The effect of twenty-one structurally different AChE reactivators was tested in vitro and subsequently, the relationship between their chemical structure and biological activity was outlined.

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.

para- andortho-Pyridinium aldoximes in reaction with acetylthiocholine

In the oximolysis reaction para-aldoximes K027 and TMB-4 react faster with ATCh than ortho-aldoximes HI-6 and K033. The reaction rate constants at 25 degrees C were 22 M(-1) min(-1) for HI-6 and K033, 230 M(-1) min(-1) for TMB-4 and 306 M(-1) min(-1) for K027. Semi-empirical calculations showed that differences in rates do not origin from different electron density on the oxygen of the oxime group, but can be explained by the steric hindrance of the oxime group within the molecule. Thermodynamic parameters, DeltaG#, DeltaH# and DeltaS#, were also determined for oximolysis reaction.

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.

Bioscavengers for the protection of humans against organophosphate toxicity

Current antidotes for organophosphorus compounds (OP) poisoning consist of a combination of pretreatment with carbamates (pyridostigmine bromide), to protect acetylcholinesterase (AChE) from irreversible inhibition by OP compounds, and post-exposure therapy with anti-cholinergic drugs (atropine sulfate) to counteract the effects of excess acetylcholine and oximes (e.g., 2-PAM chloride) to reactivate OP-inhibited AChE. These antidotes are effective in preventing lethality from OP poisoning, but they do not prevent post-exposure incapacitation, convulsions, seizures, performance decrements, or in many cases permanent brain damage. These symptoms are commonly observed in experimental animals and are likely to occur in humans. The problems intrinsic to these antidotes stimulated attempts to develop a single protective drug, itself devoid of pharmacological effects, which would provide protection against the lethality of OP compounds and prevent post-exposure incapacitation. One approach is the use of enzymes such as cholinesterases (ChEs), beta-esterases in general, as single pretreatment drugs to sequester highly toxic OP anti-ChEs before they reach their physiological targets. This approach turns the irreversible nature of the OP: ChE interaction from disadvantage to an advantage; instead of focusing on OP as an anti-ChE, one can use ChE as an anti-OP. Using this approach, it was shown that administration of fetal bovine serum AChE (FBSAChE) or equine serum butyrylcholinesterase (EqBChE) or human serum BChE (HuBChE) protected the animals from multiple LD50s of a variety of highly toxic OPs without any toxic effects or performance decrements. The bioscavengers that have been explored to date for the detoxification of OPs fall into three categories: (A) those that can catalytically hydrolyze OPs and thus render them non-toxic, such as OP hydrolase and OP anhydrase; (B) those that stoichiometrically bind to OPs, that is, 1 mol of enzyme neutralizes one or 2 mol of OP inactivating both, such as ChEs and related enzymes; and (C) and those generally termed as "pseudo catalytic", e.g., a combination of ChE and an oxime pre-treatment such that the catalytic activity of OP-inhibited ChE can rapidly and continuously be restored in the presence of an oxime. Since the biochemical mechanism underlying prophylaxis by exogenous esterases such as ChEs is established and tested in several animal species, including non-human primates, this concept should allow a reliable extrapolation of results from animal experiments to human application. Having being extensively investigated by several groups, plasma derived HuBChE is judged to be the most suitable bioscavenger for its advancement for human use. The program is being developed at the present time for conducting a safety clinical trial in human volunteers. Several other candidate bioscavengers will follow; e.g., recombinant HuBChE expressed in the milk of transgenic goats, pseudo catalytic scavenger(s), e.g., a combination of ChE and oxime, and possibly PON 1 as a catalytic scavenger in the future.

Comparison ofin vitro potency of oximes (pralidoxime, obidoxime, HI-6) to reactivate sarin-inhibited acetylcholinesterase in various parts of pig brain

The potency of currently used oximes to reactivate sarin-inhibited acetylcholinesterase (AChE) in various parts of pig brain and whole pig brain was evaluated using in vitro methods. Significant differences in reactivation potency among all tested oximes were observed. At concentrations (10(-4) M) corresponding to recommended doses in vivo, the oxime HI-6 seems to be a more efficacious reactivator of sarin-inhibited AChE in whole pig brain as well as in cerebral hemispheres and cerebellum compared with the other oximes studied. Nevertheless, there are not any differences in the potency of oximes tested to reactivate sarin-inhibited AChE in medulla oblongata. Thus, the oxime HI-6 appears to be the most promising oxime among currently available oximes for the antidotal treatment of acute sarin poisoning, although it is not more efficacious than other currently used oximes in medulla oblongata, whose function is necessary for the vital functions of respiration and circulation.

Organophosphates/nerve agent poisoning: mechanism of action, diagnosis, prophylaxis, and treatment

OP/nerve agents are still considered as important chemicals acting on living organisms and are widely used. They are characterized according to their action as compounds influencing cholinergic nerve transmission via inhibition of AChE. Modeling of this action and extrapolation of experimental data from animals to humans is more possible for highly toxic agents than for the OP. The symptoms of intoxication comprise nicotinic, muscarinic, and central symptoms; for some OP/nerve agents, a delayed neurotoxicity is observed. Cholinesterases (AChE and BuChE) are characterized as the main enzymes involved in the toxic effect of these compounds, including molecular forms. The activity of both enzymes (and molecular forms) is influenced by inhibitors (reversible, irreversible, and allosteric) and other factors, such as pathological states. There are different methods for cholinesterase determination; however, the most frequent is the method based on the hydrolysis of thiocholine esters and subsequent detection of free SH-group of the released thiocholine. The diagnosis of OP/nerve agent poisoning is based on anamnesis, the clinical status of the intoxicated organism, and on cholinesterase determination in the blood. For nerve agent intoxication, AChE in the red blood cell is more diagnostically important than BuChE activity in the plasma. This enzyme is a good diagnostic marker for intoxication with OP pesticides. Some other biochemical examinations are recommended, especially arterial blood gas, blood pH, minerals, and some other specialized parameters usually not available in all clinical laboratories. These special examinations are important for prognosis of the intoxication, for effective treatment, and for retrospective analysis of the agent used for exposure. Some principles of prophylaxis against OP/nerve agent poisoning comprising the administration of reversible cholinesterase inhibitors such as pyridostigmine (alone or in combination with other drugs), scavengers such as preparations of cholinesterases, some therapeutic drugs, and possible combinations are given. Basic principles of the treatment of nerve agent OP poisoning are described. They are based on the administration of anticholinergics (mostly atropine but some other anticholinergics can be recommended) as a symptomatic treatment, cholinesterase reactivators as a causal treatment (different types but without a universal reactivator against all OP/nerve agents) as the first aid and medical treatment, and anticonvulsants, preferably diazepam though some other effective benzodiazepines are available. New drugs for the treatment are under experimental study based on new approaches to the mechanism of action. Future trends in the complex research of these compounds, which is important not only for the treatment of intoxication but also for the quantitative and qualitative increase of our knowledge of toxicology, neurochemistry, neuropharmacology, clinical biochemistry, and analytical chemistry in general, are characterized.

Spectrophotometric and electrochemical study of protolytic equilibria of some oximes-acetylcholinesterase reactivators

Newly synthesized oximes, mono and bis imidazole derivatives, which promise to be more effective acetylcholinesterase reactivators than standard antidotes used, were investigated by spectrophotometric and electrochemical methods. The electrochemical investigations confirmed the existence of overlapping equilibria, obtained by spectrophotometric methods. Dissociation constants of those oximes were also obtained by numerical treatment of overlapping equilibria, using the Lavendberg Marquardt least square method, and when compared with the same for some similar compounds, were found to be very effective acetylcholinesterase reactivators. The distribution of ionic forms of the investigated oximes, as a dependence of pH values, was calculated from the obtained values of dissociation constants. The results indicated that many oxime anions will be available at physiological pH 7.4 and a relative increased ability to reactivate inhibited acetylcholinesterase could be expected.

Experimental and clinical toxicology of anticholinesterase agents

Several organophosphorus compounds (OP) and carbamates (CA) are used as insecticides or warfare agents (OPs only). Their acute toxic effect in the central and peripheral nervous system is due to inhibition of acetylcholinesterase (AChE) at nerve endings which causes accumulation of acetylcholine and consequently overstimulation of the nicotinic and muscarinic receptors. The cholinergic syndrome appears at approximately 50% AChe inhibition whereas death is believed to occur at > 90%. Inhibition of AChE (phosphorylation) by most OPs is irreversible whereas CAs reversibly inhibit AChE (spontaneous reactivation with a t(1/2) of minutes); dimethylphosphorylated AChE partially and slowly (t(1/2) = 1-2 h) reactivates. Although long-term, mild neurobehavioural changes of questionable significance have been reported in some instances, recovery from the cholinergic syndrome appears to be complete, unless lesions develop in the central nervous system as a consequence of either convulsions or anoxia. Certain OPs and CAs have been reported to interact with cholinergic receptors in vitro. The toxicological relevance of these interactions is still not clear. Certain OPs cause OP-induced delayed polyneuropathy (OPIDP) which develops 2-5 weeks after an acute poisoning. The molecular target is believed to be neuropathy target esterase (NTE). OP insecticides are more potent AChE inhibitors rather than NTE inhibitors and therefore, the dose required to cause OPIDP is much higher than that causing the cholinergic syndrome. In the experimental animal, OPIDP is associated with > 70% NTE inhibition after single or repeated exposures. The threshold in man is not known, although there are indications that it is similar. Some non-neuropathic esterase inhibitors (OPs, CAs, sulfonyl fluorides) exacerbate the clinical outcome of OPIDP and other chemical axonopathies, and of nerve crush. The phenomenon has been called promotion and has so far been observed in experimental animals only.

Acceleration of oxime-induced reactivation of organophosphate-inhibited fetal bovine serum acetylcholinesterase by monoquaternary and bisquaternary ligands

Reactivation of organophosphate (OP)-inhibited acetylcholinesterase (AChE) by oximes is the primary reason for their effectiveness in the treatment of OP poisoning. Reactivation is reported to accelerate by quaternary ligands such as decamethonium, which is devoid of nucleophilicity. The mechanism of this enhancement is not known. To better understand the acceleration phenomenon, we examined ligand modulations of oxime-induced reactivation of methylphosphonylated AChE using 7-(methylethoxyphosphinyloxy)-1-methylquinolinium iodide and fetal bovine serum AChE. Edrophonium, decamethonium, and propidium, three quaternary AChE ligands of different types, were tested as potential accelerators. Experiments were carried out with both soluble enzyme preparation and AChE conjugated to polyurethane. Kinetic measurements with oximes 2-[hydroxyiminomethyl]-1-methylpyridinium chloride, 1,1'-trimethylene bis-(4-hydroxyimino methyl)-pyridinium dibromide, and 1, 1'-[oxybis-methylene)bis[4-(hydroxyimino)methyl]pyridiniu um dichloride showed that in the presence of 50 microM edrophonium, the reactivation rate constants increased 3.3-12.0-fold; 200 microM decamethonium produced a 1.6-3.0-fold enhancement of reactivation rate constants by the same oximes. Reactivation of the inhibited enzyme by 1-(2-hydroxyiminomethyl-1-pyridinium)-1-(4-carboxy-aminopyridinium )-d imethyl ether hydrochloride, 1-(2-hydroxyiminomethyl-1-pyridinium)-1-(3-carboxy-aminopyridinium )-d imethyl ether hydrochloride, and 1-[[[4-(aminocarbonyl)pyridino]methoxy]methyl]-2, 4, -bis(hydroxyimino)methyl pyridinium dichloride was not affected by either ligand. Propidium slowed the reactivation of 7-(methylethoxyphosphinyloxy)-1- methylquinolinium iodide-inhibited AChE by all oximes. Results suggest that the accelerator site may reside inside the catalytic gorge rather than at its entrance and acceleration may be due to the prevention of reinhibition of the regenerated enzyme by the putative product, the phosphonylated oxime. In addition to the nucleophilic property of the oximate anion, some of the reactivators may carry an accelerating determinant, as characterized with respect to edrophonium and decamethonium. Results offer possible explanations for the superiority of 1-(2-hydroxyiminomethyl-1-pyridinium)-1-(4-carboxy-aminopyridinium )-d imethyl ether hydrochloride over other oximes in the reactivation of specific AChE-OP conjugates.