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

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

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

Lessons to be learnt from organophosphorus pesticide poisoning for the treatment of nerve agent poisoning

The increasing threat of nerve agent use for terrorist purposes against civilian and military population calls for effective therapeutic preparedness. At present, administration of atropine and an oxime are recommended, although effectiveness of this treatment is not proved in clinical trials. Here, monitoring of intoxications with organophosphorus (OP) pesticides may be of help, as their actions are closely related to those of nerve agents and intoxication and therapy follow the same principles. To this end, the clinical course of poisoning and the effectiveness of antidotal therapy were investigated in patients requiring artificial ventilation being treated with atropine and obidoxime. However, poisoning with OP pesticides shows extremely heterogeneous pictures of cholinergic crisis frequently associated with clinical complications. To achieve valuable information for the therapy of nerve agent poisoning, cases resembling situations in nerve agent poisoning had to be extracted: (a) intoxication with OPs forming reactivatable OP-AChE-complexes with short persistence of the OP in the body resembling inhalational sarin intoxication; (b) intoxication with OPs resulting rapidly in an aged OP-AChE-complex resembling inhalational soman intoxication; (c) intoxications with OPs forming a reactivatable AChE-OP complex with prolonged persistence of the OP in the body resembling percutaneous VX intoxication. From these cases it was concluded that sufficient reactivation of nerve agent inhibited non-aged AChE should be possible, if the poison load was not too high and the effective oximes were administered early and with an appropriate duration. When RBC-AChE activity was higher than some 30%, neuromuscular transmission was relatively normal. Relatively low atropine doses (several milligrams) should be sufficient to cope with muscarinic symptoms during oxime therapy.

Synthesis of asymmetrical bispyridinium compounds bearing cyano-moiety and evaluation of their reactivation activity against tabun and paraoxon-inhibited acetylcholinesterase

Three asymmetrical AChE reactivators with cyano-moiety and propane linker were synthesized using modification of currently known synthetic pathways. Their potency to reactivate AChE inhibited by nerve agent tabun and insecticide paraoxon was tested in vitro and compared to pralidoxime, HI-6, obidoxime, K027, and K048. According to the results, three compounds seem to be promising against paraoxon-inhibited AChE. Better results were obtained for bisquaternary substances at least with one oxime group in position four. None of tested substances was able to satisfactorily reactivate tabun-inhibited AChE at concentration applicable for in vivo experiments.

Testing of antidotes for organophosphorus compounds: experimental procedures and clinical reality

According to current knowledge, inhibition of acetylcholinesterase (AChE) is a very important toxic action of organphosphorus compounds (OP). Hence, it is obvious to follow the AChE activity in order to quantify the degree of inhibition and to assess possible reactivation. Red blood cell (RBC)-AChE provides an easily accessible source to follow the AChE status also in humans. There are many reports underlining the appropriateness of RBC-AChE as a surrogate parameter that mirrors the synaptic enzyme. With this tool at hand, we can study the kinetic parameters of inhibition, spontaneous and oxime-induced reactivation, as well as aging with human RBCs under physiological conditions in vitro. Moreover, we can simulate the influence of inhibitor and reactivator on enzyme activity and can calculate what happens when both components change with time. Finally, we can correlate under controlled conditions the AChE-status in intoxicated patients with the clinical signs and symptoms and determine the time-dependent changes of the oxime and OP concentration. Data of a clinical trial performed in Munich to analyze the value of obidoxime has elucidated that obidoxime worked as expected from in vitro studies. Following a 250mg bolus, obidoxime was administered by continuous infusion at 750mg/24h aimed at maintaining a plasma concentration of 10-20microM obidoxime. This oxime concentration reactivated RBC-AChE>20% of normal in most cases of OP poisoning by diethylphosphoryl compounds within a few hours. The degree of reactivation fitted theoretical calculations very well when the obidoxime and paraoxon concentrations were fed into the model. Only in a few cases reactivation was much lower than expected. The reason for this effect is probably based on the polymorphism of paraoxonase (PON1) in that the (192)arginine phenotype does hardly hydrolyze the arising diethylphosphoryl obidoxime. While this variable may complicate a proper assessment even more, we are confident that the in vitro evaluation of all relevant kinetic data enables the prediction of probable effectiveness in humans. These studies also help to understand therapeutic failures and to define scenarios where oximes are virtually ineffective. These include poisonings with rapidly aging phosphylated AChE, late start with an effective oxime and too early discontinuation of oximes in poisonings with a persistent OP. The experience gathered with the experimental and therapeutic approaches to human poisoning by OP pesticides may be helpful when oximes have to be selected against nerve agents.

Pretreatment with pyridinium oximes improves antidotal therapy against tabun poisoning

Oximes K033 [1,4-bis(2-hydroxyiminomethylpyridinium) butane dibromide] and K048 [1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium) butane dibromide] were tested as pretreatment drugs in tabun-poisoned mice followed by treatment with atropine plus K033, K048, K027 [1-(4-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium) propane dibromide], TMB-4 [1,3-bis(4-hydroxyiminomethylpyridinium) propane dibromide] and HI-6 [(1-(2-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium)-2-oxapropane dichloride)]. Oxime doses of 25% or 5% of its LD(50) were used for pretreatment 15 min before tabun-poisoning and for treatment 1 min after tabun administration to mice. The best therapeutic effect was obtained when oxime K048 (25% of its LD(50)) was used in both pretreatment and treatment with atropine. This regiment insured survival of all tested animals after the application of 10 LD(50) of tabun. In addition, since butyrylcholinesterase (BChE; EC 3.1.1.8) is considered an endogenous bioscavenger of anticholinesterase compounds and its interactions with oximes could be masked by AChE interactions, we evaluated kinetic parameters for interactions of tested oximes with native and tabun-inhibited human plasma BChE and compared them with results obtained previously for human erythrocyte acetylcholinesterase (AChE; EC 3.1.1.7). Progressive inhibition of BChE by tabun was slightly faster than that of AChE. The reactivation of tabun-inhibited BChE by oximes was very slow, and BChE binding affinity for oximes was lower than AChE's. Therefore, BChE could scavenge tabun prior to AChE inhibition, but fast oxime-assisted reactivation of tabun-inhibited AChE or protection of AChE by oxime against inhibition with tabun would not be obstructed by interaction between BChE and oximes.

Evaluation of monoquaternary pyridinium oximes potency to reactivate tabun-inhibited human acetylcholinesterase

Monoquaternary N-benzyl-4-hydroxyiminomethylpyridinium bromide (Py-4-H) and its analogous with diverse substituents introduced into the phenyl ring (Py-4-CH(3), Py-4-Br, Py-4-Cl and Py-4-NO(2)) were synthesized in order to examine their potency as reactivators of tabun-inhibited human erythrocyte acetylcholinesterase (AChE; EC 3.1.1.7). Within 24h, the reactivation of tabun-inhibited AChE reached 80% with Py-4-CH(3), Py-4-Br and Py-4-Cl, 40% with Py-4-NO(2), and 30% with Py-4-H. The overall reactivation rate constants were up to 5.0min(-1)M(-1). All oximes inhibited human AChE reversibly, and the inhibition potency increased in the following order Py-4-Br<Py-4-Cl<Py-4-CH(3)<Py-4-H<Py-4-NO(2). Although oximes Py-4-H and Py-4-NO(2) did not show significant reactivation ability, these oximes might be of interest as pre-treatment drugs due to their high affinity for the native AChE. Docking studies were carried out to elucidate the differences in oximes potency. The orientations of all studied oximes in the active site of human AChE have been proposed by flexible ligand docking with AutoDock 3.0. Analyses of the obtained complexes revealed the presence of numerous hydrogen bonds and close contacts between the oximes and the residues in the active site. Final docked energies predicted correctly the relative order of the inhibition potency of compounds (except in the case of Py-4-CH(3)) as well as the most probable orientation of the best reactivator, Py-4-Br, which can result in an attack on the phosphorus atom of the tabun-phosphorylated human AChE.

Crystal structures of acetylcholinesterase in complex with HI-6, Ortho-7 and obidoxime: structural basis for differences in the ability to reactivate tabun conjugates

Inhibition of acetylcholinesterase (AChE) by organophosphorus compounds (OPs) such as pesticides and nerve agents causes acute toxicity or death of the intoxicated individual. The inhibited AChE may be reactivated by certain oximes as antidotes for clinical treatment of OP-intoxications. Crystal structures of the oximes HI-6, Ortho-7 and obidoxime in complex with Mus musculus acetylcholinesterase (mAChE) reveal different roles of the peripheral anionic site (PAS) in the binding of the oximes. A limited structural change of the side chains of Trp286 and Asp74 facilitates the intercalation of the 4-carboxylamide pyridinium ring of HI-6 between the side chains of Tyr124 and Trp286. The 2-carboxyimino pyridinium ring of HI-6 is accommodated at the entrance of the catalytic site with the oximate forming a hydrogen bond to the main-chain nitrogen atom of Phe295. In contrast to HI-6, the coordination of Ortho-7 and obidoxime within the PAS is facilitated by an extended structural change of Trp286 that allows one of the carboxyimino pyridinium rings to form a cation-pi interaction with the aromatic groups of Tyr72 and Trp286. The central chain of Ortho-7 and obidoxime is loosely coordinated in the active-site gorge, whereas the second carboxyimino pyridinium ring is accommodated in the vicinity of the phenol ring of Tyr337. The structural data clearly show analogous coordination of Ortho-7 and obidoxime within the active-site gorge of AChE. Different ability to reactivate AChE inhibited by tabun is shown in end-point reactivation experiments where HI-6, Ortho-7 and obidoxime showed an efficiency of 1, 45 and 38%, respectively. The low efficiency of HI-6 and the significantly higher efficiency of Ortho-7 and obidoxime may be explained by the differential binding of the oximes in the PAS and active-site gorge of AChE.

Application of kinetic-based computer modelling to evaluate the efficacy of HI 6 in percutaneous VX poisoning

The rife use of organophosphorus compounds (OP) as pesticides and the exertion of highly toxic OP-type chemical warfare agents (nerve agents) during military conflicts and terrorist attacks in the past emphasize the necessity of the development of effective therapeutic countermeasures. Presently, standard treatment of poisoning by OP includes administration of atropine as an antimuscarinic agent and of oximes, e.g. obidoxime or pralidoxime, as reactivators of OP-inhibited acetylcholinesterase (AChE), but is considered to be rather ineffective with certain nerve agents. The evaluation of new oximes as antidotes is only possible by implementation of animal experiments for ethical reasons and therefore complicated by a limited extrapolation of animal data to humans due to marked species differences. A computer simulation based on combination of AChE kinetic data (inhibition, reactivation, aging) with OP toxicokinetics and oxime pharmacokinetics allows the calculation of AChE activities at different scenarios and may facilitate to define effective oxime concentrations and to optimize oxime dosage in OP poisoning. On the base of species-specific kinetic data this model was used to calculate AChE activities in humans and pigs after percutaneous exposure to 5 x LD50 VX and treatment with HI 6. Due to marked species differences between human and pig AChE the HI 6 dose that is necessary to cause a comparable reactivation of VX-inhibited pig AChE is conspicuously higher. Hence, designing animal experiments with the aid of computer modeling may reduce the number of animal experiments and allow a more reliable extrapolation of animal data to humans.

Analysis of inhibition, reactivation and aging kinetics of highly toxic organophosphorus compounds with human and pig acetylcholinesterase

Organophosphorus compounds (OP) are in wide spread use as pesticides and highly toxic OP may be used as chemical warfare agents (nerve agents). OP inhibit acetylcholinesterase (AChE), therefore, standard treatment includes AChE reactivators (oximes) in combination with antimuscarinic agents. In the last decades, the efficacy of oximes has been investigated in various animal models, mostly in rodents. However, extrapolating animal data to humans is problematical because of marked differences between rodents and humans concerning the toxicokinetics of nerve agents, the pharmacokinetics of antidotes and the AChE enzyme kinetics. In order to improve the understanding of species differences and to enable a more reliable extrapolation of animal data to humans a study was initiated to investigate the effect of highly toxic nerve agents, i.e. VX, Russian VX (VR) and Chinese VX (CVX), with human and pig erythrocyte AChE. Hereby, the rate constants for the inhibition of AChE by these OP (ki) and for the spontaneous dealkylation (ka) and reactivation (ks) of OP-inhibited AChE as well as for the oxime-induced reactivation of OP-inhibited AChE by the oximes obidoxime, 2-PAM, HI 6, HLö 7 and MMB-4 were determined. Compared to human AChE pig AChE showed a lower sensitivity towards the investigated OP. Furthermore, a slower spontaneous dealkylation and reactivation of pig AChE was recorded. The potency of the investigated oximes was remarkably lower with OP-inhibited pig AChE. These data may contribute to a better understanding of species differences and may provide a kinetic basis for extrapolation of data from pig experiments to humans.

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

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

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.

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

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

Development of the Bisquaternary Oxime HI-6 Toward Clinical Use in the Treatment of Organophosphate Nerve Agent Poisoning

The traditional therapeutic treatment of organophosphate cholinesterase inhibitor (nerve agents) poisoning consists of co-treatment with an antimuscarinic (atropine) and a reactivator of inhibited acetylcholinesterase (AChE), which contains a nucleophilic oxime function. Two oximes are presently widely available for clinical use, pralidoxime and obidoxime (toxogonin), but both offer little protection against important nerve agent threats. This has highlighted the real need for the development and availability of more effective oximes for human use, a search that has been going on for up to 30 years. However, despite the demonstration of more effective and safe oximes in animal experiments, no additional oximes have been licensed for human use. HI-6, (1-[[[4(aminocarbonyl)-pyridinio]methoxy]methyl]-2(hydroxyimino)pyridinium dichloride; CAS 34433-31-3) has been studied intensively and has been proved effective in a variety of species including non-human primates and appears from clinical experience to be safe in humans. These studies have led to the fielding of HI-6 for use against nerve agents by the militaries of the Czech republic, Sweden, Canada and under certain circumstances the Organisation for the Prohibition of Chemical Weapons. Nevertheless HI-6 has not been granted a license for clinical use, must be used only under restricted guidelines and is not available for civilian use as far as is known. This article will highlight those factors relating to HI-6 that pertain to the licensing of new compounds of this type, including the mechanism of action, the clinical and pre-clinical demonstration of safety and its efficacy against a variety of nerve agents particularly in non-human primates, since no relevant human population exists. This article also contains important data on the use of HI-6 in baboons, which has not been available previously. The article also discusses the possibility of successful therapy with HI-6 against poisoning in humans relative to doses used in non-human primates and relative to its ability to reactivate inhibited human AChE.

Revisiting the reactivity of oximate α-nucleophiles with electrophilic phosphorus centers. Relevance to detoxification of sarin, soman and DFP under mild conditions

Following a potentiometric determination of the relevant pKa values of the (R1R2)C=NOH functionality, the second order rate constants (k(Ox)) for reaction of a large set of oximate bases with two model organophosphorus esters, i.e. bis-(4-nitrophenyl)phenylphosphonate (BNPPP) and bis-(4-nitrophenyl)methylphosphonate (BNPMP), and three toxic compounds, i.e., sarin (GB), soman (GD) and diisopropylphosphorofluoridate (DFP), in aqueous as well as a 30 : 70 (v/v) H2O-Me2SO mixture have been measured. The corresponding Brønsted-type nucleophilicity plots of log k(Ox)vs. pKa(Ox) reveal a clear tendency of the reactivity of the oximates to suffer a saturation effect with increasing basicity in aqueous solution. In the case of BNPMP and the three toxic esters, this behaviour is reflected in a levelling off at pKa approximately 9 but a more dramatic situation prevails in the BNPPP system where the attainment of maximum reactivity at pKa approximately 9 is followed by a clear decrease in rate at higher pKa's. Interestingly, a number of data reported previously by different authors for the sarin, soman and DFP systems are found to conform rather well to the curvilinear Brønsted correlations built with our data. Based on this and previous results obtained for reactions at carbon centers, it can be concluded that the observed saturation effect is the reflection of an intrinsic property of the oximate functionality. An explanation of this behavior in terms of an especially strong requirement for desolvation of the oximates prior to nucleophilic attack which becomes more and more difficult with increasing basicity is suggested. This proposal is supported by the observed changes in pKa(Ox) and k(Ox) brought about by a transfer from H2O to a 30 : 70 H2O-Me2SO mixture. The implications of the saturation effect on the efficiency of oximates as nucleophilic catalysts for smooth decontamination are emphasized. Also discussed is the effect of basicity on the exalted (alpha-effect) reactivity of these bases.

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

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

Estimation of oxime efficacy in nerve agent poisoning: A kinetic approach

Standard treatment of poisoning by organophosphorus compounds (OP) includes the administration of an anti-muscarinic, e.g. atropine, and of an acetylcholinesterase (AChE) reactivator (oxime). Two oximes, obidoxime and pralidoxime (2-PAM), are presently commercially available, yet, these compounds are considered to be of insufficient efficacy against certain nerve agents, e.g. soman and cyclosarin. In the past decades, numerous new oximes were synthesized and tested for their antidotal efficacy. The available data indicate that two Hagedorn oximes, HI 6 and HLö 7, are promising antidotes against various nerve agents. The efficacy of antidotes against nerve agent poisoning cannot be investigated in humans for ethical reasons. Therefore, it is necessary to use surrogate parameters for the evaluation of oxime efficacy. Reactivation of inhibited AChE is considered to be the main mechanism of action of oximes. Clinical data indicate that changes in erythrocyte AChE activity correlate to neuromuscular function indicating that interactions between AChE, inhibitor and oximes can be investigated in vitro with human erythrocyte AChE. Different theoretical models were used for the evaluation of reactivating efficacy of oximes with nerve agent-inhibited human AChE and for estimating effective oxime concentrations. The calculations demonstrate the marked differences between oximes in dependence of the inhibitor and provide a basis for the estimation of the required oxime dose as well as of dosing intervals.

Diagnostic aspects of organophosphate poisoning

Organophosphate (OP)-type chemical warfare agents (nerve agents) present a constant threat to the population. Sensitive and specific methods for the detection and verification of exposure to nerve agents are required for diagnosis, therapeutic monitoring, health surveillance and forensic purposes. Determination of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activity in blood remains a mainstay for the fast initial screening but lacks sensitivity and specificity. Quantitative analysis of nerve agents and their degradation products in plasma and urine by mass spectrometric methods may prove exposure but is limited to hours or days after the incident due to the short residence time of the analytes. Investigation of protein adducts extends the time interval between exposure and sampling and may be suitable to detect low-level exposure. Definitive prove of exposure requires a spectrum of different methods, expensive and sophisticated equipment and will be limited to specialized laboratories.

High concentrations of pralidoxime are needed for the adequate reactivation of human erythrocyte acetylcholinesterase inhibited by dimethoate in vitro

Due to the current controversy about the real effectiveness of the oximes in the treatment of organophosphate poisoning, the reactivation capacity of pralidoxime has been evaluated in vitro on human erythrocyte acetylcholinesterase inhibited by dimethoate. In the in vitro model, a partial recovery of acetylcholinesterase activity was observed with concentrations from 0.066 mM pralidoxime, probably useful enough to prevent death in most cases in vivo. However, much more effectiveness was observed with concentrations up to 0.70 mM pralidoxime. Although pralidoxime should be applied as soon as possible after organophosphate exposure, the application of the antagonist can be useful even 24h after, particularly for organophosphates with biological half-life longer than one day. The protective capacity of pralidoxime after the application was reduced up to 50% in 6h and disappeared almost completely in 24h. Furthermore, the pesticide and its metabolites remained active and were able to inhibit the enzyme as soon as pralidoxime reduced its antagonist capacity. Our results in conjunction with the short half-life of pralidoxime suggest that the maintenance of higher plasmatic concentrations than the currently used should be considered in the management of severe poisoned patients, although adverse effects could be expected.

Formation and disposition of diethylphosphoryl-obidoxime, a potent anticholinesterase that is hydrolyzed by human paraoxonase (PON1)

The potential of pyridinium-4-aldoximes, such as obidoxime, to reactivate diethylphosphorylated acetylcholinesterases is not fully exploited due to the inevitable formation of phosphoryloximes (POX) with high anticholinesterase activity. Mono(diethylphosphoryl) obidoxime (DEP-obidoxime) was isolated for the first time showing remarkable stability under physiological conditions (half-life 13.5min; pH 7.1; 37 degrees C). The half-life was considerably extended to 20h at 0 degrees C, which facilitated the preparation and allowed isolation by HPLC. The structure was confirmed by mass spectrometry and the degradation pattern. DEP-obidoxime decomposed by an elimination reaction forming the intermediate nitrile that hydrolyzed mainly into the pyridone and cyanide. The intermediates were prepared and confirmed by mass spectroscopy. DEP-Obidoxime was an extremely potent inhibitor of human acetylcholinesterase approaching a second-order rate constant of 10(9)M(-1)min(-1) (pH 7.4; 37 degrees C). The nitrile and the pyridone were still good reactivators. In the presence of human plasma DEP-obidoxime was hydrolyzed into parent obidoxime. Calcium-dependence and sensitivity towards chelators, substitution pattern by other divalent cations and protein-modifying agents all pointed to human paraoxonase (hPON1) as the responsible protein with POX-hydrolase activity. Subjects, probably belonging to the homozygous (192)arginine subtype, were virtually devoid of POX-hydrolase activity while a highly purified hPON1 of the homozygous (192)glutamine subtype exhibited particularly high POX-hydrolase activity. Two parathion-poisoned patients with high and low POX-hydrolase activity responded well and poorly, respectively, to obidoxime treatment although the former patient had higher plasma paraoxon levels than the poor responder. Hence, the POX-hydrolase associated PON1 subtype may be another contributor that modulates pyridinium-4-aldoxime effectiveness.

Effect of metoclopramide and ranitidine on the inhibition of human AChE by VXin vitro

The repeated misuse of highly toxic organophosphorus-type (OP) chemical warfare agents ('nerve agents') emphasizes the necessity for the development of effective medical countermeasures. The standard treatment with atropine and acetylcholinesterase (AChE) reactivators ('oximes') is considered to be ineffective with certain nerve agents due to low oxime efficacy. Therefore, pretreatment with carbamate-type compounds, e.g. pyridostigmine, was recommended to improve antidotal efficacy. Recently, the clinically used reversible AChE inhibitors metoclopramide (MCP) and ranitidine (RAN) were shown to exhibit some protective effect against the OP pesticide paraoxon in vitro and in vivo. The present study was undertaken to investigate a potential protective effect of MCP and RAN against inhibition of human AChE by the nerve agent VX (O-ethyl S-[2-(diisopropylamino)ethyl)methylphosphonothioate). Hemoglobin-free human erythrocyte membranes were incubated with various, human relevant MCP (0.5-2 microm) and RAN (0.5-5 microm) concentrations starting 1 min before addition of VX (1-40 nm). Both compounds failed to increase VX IC(50) values. In addition, human AChE was incubated with higher than human relevant therapeutic concentrations of MCP (1 microm-1 mm) and RAN (1 microm-2.0 mm) and inhibited by 40 nm VX. At concentrations higher than 100 microm MCP and RAN caused a concentration dependent increase of residual AChE activity 15 min after addition of VX. These data indicate that MCP and RAN may be ineffective in protecting human AChE against inhibition by the nerve agent VX at human relevant doses.