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

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

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.

Effects of oximes on muscle force and acetylcholinesterase activity in isolated mouse hemidiaphragms exposed to paraoxon

Toxicity of organophosphates (OP) is caused by inhibition of acetylcholinesterase (AChE), resulting in accumulation of acetylcholine. While cholinolytics such as atropine are able to counteract muscarinic symptoms, they are unable to restore the impaired neuromuscular transmission (NMT). Here, oximes as potential reactivators of inhibited AChE may be effective. Until now, no unequivocal relation between oxime-induced increase in muscle force and reactivation has been demonstrated. To address this issue the isolated circumfused mouse hemidiaphragm was used as an experimental model. The muscle force generation upon tetanic stimuli was recorded during AChE inhibition by 1 microM paraoxon and after a wash-out period in the presence of obidoxime, pralidoxime and the experimental oximes HI 6, and HLö 7, 10 microM each. At the end of the experiments AChE activity was determined in the diaphragm homogenates by a radiometric assay. At 50-Hz stimulation, recovery was complete with obidoxime, nearly complete with HLö 7 but incomplete with HI 6 and pralidoxime. Only with obidoxime a significant increase in AChE activity was found. An increase of AChE to 10% of normal was sufficient to allow normal muscle force generation. When paraoxon was still present, obidoxime and HLö 7 were effective at 0.1 microM paraoxon, but failed so at paraoxon >1 microM. The data show different effectiveness of the oximes investigated in reactivation of muscle AChE and recovery of NMT after inhibition by paraoxon. Although an increase in muscle force by the oximes was accompanied by a measurable increase in AChE activity only in the case of obidoxime, the plot of muscle force against AChE activity as well as lacking evidence for a direct effect and adaptive processes indicate that reactivation of the enzyme is the main mechanism of NMT recovery. In agreement, in presence of AChE inhibitory concentrations of paraoxon during reactivation a reduced effectiveness of oximes was found.

[A comparison of the efficacy of the reactivators of acetylcholinesterase inhibited with tabun]

The nerve agent tabun inhibits acetylcholinesterase (AChE; EC 3.1.1.7) by the formation of a covalent bond with the enzyme. Afterwards, AChE is not able to fulfil its role in the organism and subsequently cholinergic crisis occurs. AChE reactivators (pralidoxime, obidoxime and HI-6) as causal antidotes are used for the cleavage of the bond between the enzyme and nerve agent. Unfortunately, their potency for reactivation of tabun-inhibited AChE is poor. The aim of the study was to choose the most potent reactivator of tabun-inhibited AChE. We have tested eight AChE reactivators--pralidoxime, obidoxime, trimedoxime, HI-6, methoxime, Hlö-7 and our newly synthesized oximes K027 and K048. All reactivators were tested using our standard in vitro reactivation test (pH 8, 25 degrees C, time of inhibition by the nerve agent 30 minutes, time of reactivation by AChE reactivator 10 minutes). According to our results, only trimedoxime was able to achieve 50% reactivation potency. However, this relatively high potency was achieved at high oxime concentration (10(-2) M). At a lower concentration of 10(-4) M (the probably attainable concentration in vivo), four AChE reactivators (trimedoxime, obidoxime, K027, and K048) were able to reactivate AChE inhibited by tabun reaching from 10 to 18%.

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

Six unsymmetrical bis-quaternary monooximes viz. dibromides of 1-(4-hydroxyiminomethyl pyridinium)-3-(3/4-carbamoyl pyridinium)propane, 1-(4-hydroxyiminomethyl pyridinium)-4-(3/4-carbamoyl pyridinium) butane, 1-(4-hydroxyiminomethyl pyridinium)-5-(3/4-carbamoyl pyridinium)pentane were synthesized and characterized by spectral data. Their ability to reactivate tetraethyl pyrophosphate inhibited mouse total brain cholinesterase was investigated and compared with 2-pyridine aldoxime chloride (2-PAM). All the compounds were found to be more effective acetylcholinesterase reactivators when compared with the conventional oxime, 2-PAM, except the compound (5a) with pentylene bridge and carbamoyl group present at fourth position. The bis-pyridinium monooximes with 3-carbamoyl group were more potent reactivators than the corresponding 4-carbamoyl compounds and bis-oximes tested.

Pre-junctional effects of oximes on [3H]-acetylcholine release in rat hippocampal slices during soman intoxication

In this study, the non-reactivating effects of oximes in the hippocampus of the rat are investigated. The potassium (51 mM) evoked release of [(3)H]-acetylcholine and the liberation of [(3)H]-choline were determined in hippocampal slices following in vitro exposure to soman and five oximes (toxogonin, HI-6, HLö-7, P2S and 2-PAM) in separate experiments by superfusion. In the absence of soman, toxogonin and HLö-7 in particular induced a concentration dependent significant increase in the evoked release of [(3)H]-acetylcholine. There was also a significant effect of HI-6, but the effect was much smaller. Two pralidoxime salts, P2S (methanesulfonate salt) and 2-PAM (methiodide salt), had similar but lower effects that were only observed at relatively high concentrations. Experiments performed following complete inhibition of the acetylcholinesterase activity by soman (1.0 microM) showed that HI-6 and HLö-7 induced a significant decrease in the potassium-evoked release of [(3)H]-acetylcholine, while the liberation of [(3)H]-choline increased. Toxogonin, P2S and 2-PAM did not reduce significantly the evoked release of [(3)H]-acetylcholine. Only limited reactivation of the acetylcholinesterase activity was observed in superfusion experiments with toxogonin, HI-6, P2S and 2-PAM following exposure of hippocampal slices to soman. However, HLö-7 was proved to be relatively more effective in reactivating the acetylcholinesterase activity at high concentrations (50 and 200 microM). The acetylcholinesterase activity was reactivated to approximately 12% and 40% of control, respectively. It is concluded that HI-6 and HLö-7 have important non-acetylcholinesterase reactivating properties following soman poisoning, as may be seen by the significant reduction in the evoked release of [(3)H]-acetylcholine effected by these oximes. HLö-7 is of particular interest in view of its ability to additionally improve reactivation of the acetylcholinesterase activity.

Biochemical and clinical profile after organophosphorus poisoning--a placebo-controlled trial using pralidoxime

BACKGROUND

Organophosphorus (OP) compounds are the most common suicidal poison in developing countries and mortality continues to be high.

METHODS

A study was done to see butyryl cholinesterase (BuChE) profile after OP poisoning in pralidoxime (P2AM) and placebo treated cases. Highest recommended dose of P2AM was used to study the reactivation of cholinesterase. Clinical outcomes like, correlation of BuChE and severity of poisoning, mortality and complications like Type I and II paralysis, need for ventilation and ICU stay were also studied.

RESULTS

Twenty one cases of moderate and severe poisoning with OP compounds were included in the study. Mean BuChE levels came up gradually over 6-7 days, some taking up to two weeks. There was no. difference between the treatment and placebo groups. BuChE levels did not correlate with severity of poisoning nor did it correlate with Type I or II paralysis, need for ventilation, ICU stay or mortality.

CONCLUSIONS

Treatment with P2AM does not make any difference in BuChE reactivation or complications of moderate and severe OP poisoning. We have not been using P2AM for OP poisoning in our medical ICU with good patient outcomes.

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

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

Protection and inflammatory markers following exposure of guinea pigs to sarin vapour: comparative efficacy of three oximes

The purpose of the present study was to compare the antidotal efficacy and the combined effects on inflammatory markers of three oximes--toxogonine, TMB4 and 2-PAM--in combination with anticholinergic drugs following exposure to sarin vapour by inhalation. Guinea pigs restrained in plethysmographs were exposed to various doses of sarin vapour (in the range of 1.4-4.4LD50). The antidotal mixture was injected immediately (5-20 s) following exposure (3 mg kg(-1) atropine and 1 mg kg(-1) benactyzine in combination with 6 mg kg(-1) toxogonine, 2 mg kg(-1) TMB4 or 12 mg kg(-1) 2-PAM). Bronchoalveolar lavage (BAL) samples were taken from surviving animals 24 h after exposure to determine the levels of inflammatory markers. A differential cell count was performed in BAL samples on Giemsa-stained slides. The inflammatory markers--histamine and prostaglandins (PGE)--were measured in BAL using radioimmunoassay (RIA) techniques. The survival rate in the various treatment groups and analysis of BAL samples showed that: (i) Toxogonine, TMB4 and 2-PAM, without pyridostigmine pretreatment, at doses that were proportional to their doses in the respective auto-injectors, exhibited similar antidotal efficacy against sarin exposure. (ii) The results demonstrated that a centrally acting anticholinergic drug is essential in the antidotal mixture to ensure survival. (iii) Histamine release and eosinophilia following sarin inhalation might require additional intervention, aimed at reducing the symptoms of allergic reaction and possibly expediting recovery.

In vitro inhibitory effect of aflatoxin B1 on acetylcholinesterase activity in mouse brain

Growing concern on the problem of mycotoxins in the alimentary chain underlines the need to investigate the mechanisms explaining the cholinergic effects of aflatoxin B(1) (AFB(1)). We examined the effect of AFB(1), a mycotoxin produced by Aspergillus flavus, on mouse brain acetylcholinesterase (AChE) and specifically on its molecular isoforms (G(1) and G(4)) after in vitro exposure. AFB(1) (from 10(-9) to 10(-4)M), inhibited mouse brain AChE activity (IC(50) = 31.6 x 10(-6)M) and its G(1) and G(4) molecular isoforms in a dose-dependent manner. Michaelis-Menten parameters indicate that the K(m) value increased from 55.2 to 232.2% whereas V(max) decreased by 46.2-75.1%. The direct, the Lineweaver-Burk and the secondary plots indicated a non-competitive-mixed type antagonism, induced when the inhibitor binds to the free enzyme and to the enzyme-substrate complex. AFB(1)-inhibited AChE was partially reactivated by pyridine 2-aldoxime (2-PAM) (10(-4)M) but the AChE-inhibiting time courses of AFB(1) (10(-4)M) and diisopropylfluorophosphate (DFP) (2 x 10(-7)M) differed. Overall these data suggest that AFB(1) non-competitively inhibits mouse brain AChE by blocking access of the substrate to the active site or by inducing a defective conformational change in the enzyme through non-covalent binding interacting with the AChE peripheral binding site, or through both mechanisms.

Effective bisquaternary reactivators of tabun-inhibited AChE

Two cholinesterase reactivators (K074 and K075) were synthesized and their reactivation efficacy against tabun-inhibited acetylcholinesterase of the rat brain was tested in vitro. Comparing this efficacy showed that commonly used oximes (pralidoxim, obidoxime and HI-6) were practically without reactivation potency. On the other hand, oximes K074, K075 and trimedoxime were satisfactorily effective. Moreover, K-oximes reactivated tabun-inhibited AChE at lower concentration (10(-4) and 10(-3) m) in comparison with trimedoxime (10(-3) and 10(-2) m). Thus, K-oximes can be considered as the most effective reactivators of tabun-inhibited AChE at present.

A comparison of the potency of the oxime HLö-7 and currently used oximes (HI-6, pralidoxime, obidoxime) to reactivate nerve agent-inhibited rat brain acetylcholinesterase by in vitro methods

(1) The efficacy of the oxime HLö7 and currently used oximes (pralidoxime, obidoxime, HI-6) to reactivate acetylcholinesterase inhibited by various nerve agents (sarin, tabun, cyclosarin, VX) was tested by in vitro methods. (2) Both H oximes (HLö-7, HI-6) were found to be more efficacious reactivators of sarin and VX-inhibited acetylcholinesterase than pralidoxime and obidoxime. On the other hand, their potency to reactivate tabun-inhibited acetylcholinesterase is very low and does not reach the reactivating efficacy of obidoxime. In the case of cyclosarin, the oxime HI-6 was only found to be able to sufficiently reactivate cyclosarin-inhibited acetylcholinesterase in vitro. (3) Thus, the oxime HLö-7 does not seem to be more efficacious reactivator of nerve agent-inhibited acetylcholinesterase than HI-6 according to in vitro evaluation of their reactivation potency and, therefore, it is not more suitable to be introduced for antidotal treatment of nerve agent-exposed people than HI-6.

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.

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

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

Five oximes (K-27, K-33, K-48, BI-6 and methoxime) in comparison with pralidoxime:in vitro reactivation of red blood cell acetylcholinesterase inhibitied by paraoxon

Oximes are cholinesterase reactivators of use in poisoning with organophosphorus compounds. Pralidoxime (PRX) is used clinically as an adjunct to atropine in such exposure. Clinical experience with PRX (and other oximes) is, however, disappointing and routine use has been questioned. In addition it is known that oximes are not equally effective against all existing organophosphorus compounds. There is a clear demand for 'broad spectrum' cholinesterase reactivators with a higher efficacy than PRX. Over the years new reactivators of cholinesterase of potential clinical utility have been developed. Their chemical structures were derived from those of existing esterase reactivators, especially pralidoxime, obidoxime and HI-6. The purpose of the study was to quantify in vitro the extent of oxime (pralidoxime, K-27, K-33, K-48, methoxime and BI-6) conferred protection, using paraoxon as an inhibitor. Paraoxon (POX), the active metabolite of parathion (O,O-diethyl-O-p-nitro-phenyl phosphorothioate) is a non-neuropathic organophosphate. Red blood cell (RBC) acetylcholinesterase (AChE) activities in whole blood were measured photometrically in the presence of different POX concentrations and the IC50 was calculated. Determinations were repeated in the presence of increasing oxime concentrations. The IC50 of POX increases with the oxime concentration in a linear manner. The calculated IC50 values were plotted against the oxime concentrations to obtain an IC50 shift curve. The slope of the shift curve (tg alpha) was used to quantify the magnitude of the protective effect (nm IC50 increase per microm reactivator). Based on our determinations the new K series of reactivators is far superior to pralidoxime, methoxime and BI-6, K-27 being the outstanding compound with a tg alpha value of 3.7 (nm IC50 increase per microm reactivator) which is approximately 13 times the reactivator ability of PRX. In general there is an (expected) inverse relationship between the binding constant K and the slope of the IC50 shift curve (tg alpha) for all oximes examined. K-27 (the most protective substance judging by the tg alpha) has the lowest K value (highest affinity). In vivo testing of the new oximes as an organophosphate protective agent is necessary.

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

Cyclohexyl methylphosphonofluoridate (cyclosarin, cyclosin, GF) is a highly toxic organophosphate, which is resistant to conventional oxime therapy. To gain insight into the reactivation kinetics, rat brain acetylcholinesterase (AChE) was inhibited in vitro by cyclosarin (pH 8.0, 25 degrees C) and reactivated with 22 different pyridiniumoximes. Three compounds were shown to be superior to the other oximes: 4-carbamoyl-4'-[(hydroxyimino)methyl]-1,1'-(oxydimethylene)dipyridin-1-ium dichloride (HS-6), 4'-carbamoyl-2-[(hydroxyimino)methyl]-1,1'-(oxydimethylene)dipyridin-1-ium dichloride (HI-6), and 4'-carbamoyl-2-[(hydroxyimino)-methyl]-1,1'-(but-2-ene-1,4-diyl)dipyridin-1-ium dichloride (BI-6).

The Application of the Fluoride Reactivation Process to the Detection of Sarin and Soman Nerve Agent Exposures in Biological Samples

The fluoride reactivation process was evaluated for measuring the level of sarin or soman nerve agents reactivated from substrates in plasma and tissue from in vivo exposed guinea pigs (Cava porcellus), in blood from in vivo exposed rhesus monkeys (Macaca mulatta), and in spiked human plasma and purified human albumin. Guinea pig exposures ranged from 0.05 to 44 LD50, and reactivated nerve agent levels ranged from 1.0 ng/mL in plasma obtained from 0.05 LD50 sarin-exposed guinea pigs to an average of 147 ng/g in kidney tissue obtained from two 2.0 LD50 soman-exposed guinea pigs. Positive dose-response relationships were observed in all low-level, 0.05 to 0.4 LD50, exposure studies. An average value of 2.4 ng/mL for reactivated soman was determined in plasma obtained from two rhesus monkeys three days after a 2 LD50 exposure. Of the five types of guinea pig tissue studied, plasma, heart, liver, kidney and lung, the lung and kidney tissue yielded the highest amounts of reactivated agent. In similar tissue and with similar exposure procedures, reactivated soman levels were greater than reactivated sarin levels. Levels of reactivated agents decreased rapidly with time while the guinea pig was alive, but decreased much more slowly after death. This latter chemical stability should facilitate forensic retrospective identification. The high level of reactivated agents in guinea pig samples led to the hypothesis that the principal source of reactivated agent came from the agent-carboxylesterase adduct. However, there could be contributions from adducts of the cholinesterases, albumin and fibrous tissue, as well. Quantitative analysis was performed with a GC-MS system using selected ion monitoring of the 99 and 125 ions for sarin and the 99 and 126 ions for soman. Detection levels were as low as 0.5 ng/mL. The assay was precise and easy to perform, and has potential for exposure analysis from organophosphate nerve agents and pesticides in other animal species.

A comparison of the efficacy of a bispyridinium oxime--1,4-bis-(2-hydroxyiminomethylpyridinium) butane dibromide and currently used oximes to reactivate sarin, tabun or cyclosarin-inhibited acetylcholinesterase by in vitro methods

The efficacy of a bispyridinium oxime 1,4-bis(2-hydroxyiminomethylpyridinium) butane dibromide, called K033, and of currently used oximes (pralidoxime, obidoxime, oxime HI-6), to reactivate acetylcholinesterase inhibited by various nerve agents (sarin, tabun cyclosarin) was tested by in vitro methods. The new oxime K033 was found to be a more efficacious reactivator of sarin or cyclosarin-inhibited acetylcholinesterase than pralidoxime and obidoxime but it did not reach the efficacy of oxime HI-6 in the case of the inhibition of acetylcholinesterase by sarin or cyclosarin. On the other hand, oxime K033 was more efficacious than oxime HI-6 in reactivating tabun-inhibited acetylcholinesterase. Thus, oxime K033 seems to be a relatively efficacious broad spectrum acetylcholinesterase reactivator and, therefore, could be useful if no information about the type of nerve agent used was available.

A comparison of the ability of a new bispyridinium oxime--1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium)butane dibromide and currently used oximes to reactivate nerve agent-inhibited rat brain acetylcholinesterase by in vitro methods

The efficacy of a new bispyridinium oxime 1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium)butane dibromide, called K048, and currently used oximes (pralidoxime, obidoxime, the oxime HI-6) to reactivate acetylcholinesterase inhibited by various nerve agents (sarin, tabun, cyclosarin, VX) was tested by in vitro methods. The new oxime K048 was found to be a more efficacious reactivator of nerve agent-inhibited acetylcholinesterase than pralidoxime (in the case of VX, tabun and cyclosarin), obidoxime (cyclosarin and tabun) and HI-6 (tabun) but it did not reach the efficacy of currently used oximes for the reactivation of acetylcholinesterase inhibited by sarin. Thus, the oxime K048 seems to be a relatively efficacious broad spectrum acetylcholinesterase reactivator and, therefore, it could be useful for the treatment of a nerve agent-exposed population if information about detection of the type of nerve agent is not available.

In vivo metoclopramide protection of cholinesterase from paraoxon inhibition: direct comparison with pralidoxime in subchronic low-dose exposure

The benzamide compound metoclopramide (MCP) protects against cholinesterase inhibition by paraoxon (POX) both in vitro and in vivo. This study evaluates MCP-conferred protection of enzyme activity head to head against the therapeutic gold standard pralidoxime (PRX). Six groups of rats were used. All substances were applied i.p. daily for 5 days, followed by a 2-day rest. The 7-day cycle was repeated eight times. Group 1 received 100 nM POX, group 2 received 50 micro M MCP, group 3 received 100 nM POX + 50 micro M MCP, group 4 received 50 micro M PRX, group 5 received 100 nM POX + 50 micro M PRX and group 6 received saline. Red blood cell acetylcholinesterase (RBC-AChE) measurements were performed at baseline and on day 5 of each 7-day cycle. The sums of enzyme activities over time (weekly values expressed as % of baseline of 100%) were compared using the Mann-Whitney rank order test. A Bonferroni correction of 4 for multiple comparisons was applied. Paraoxon significantly reduced enzyme activities when compared with saline (Sigma = 535 +/- 25 vs 902 +/- 42). Metoclopramide conferred statistically significant in vivo protection from inhibition of RBC-AChE by POX (Sigma = 640 +/- 58). The extent of protection was significantly less than that conferred by the gold standard PRX (Sigma = 765 +/- 57). Metoclopramide, in addition to being less effective as an RBC-AChE protective agent, also caused a failure to thrive in the POX+MCP-exposed rats, as evidenced by the changes in body weight.

Effect of organophosphorus hydrolysing enzymes on obidoxime-induced reactivation of organophosphate-inhibited human acetylcholinesterase

The reactivation of organophosphate (OP)-inhibited acetylcholinesterase (AChE) by oximes results inevitably in the formation of highly reactive phosphyloximes (POX), which may re-inhibit the enzyme. An impairment of net reactivation by stable POX was found with 4-pyridinium aldoximes, e.g. obidoxime, and a variety of OP compounds. In this study the effect of organophosphorus hydrolase (OPH), organophosphorus acid anhydrolase (OPAA) and diisopropylfluorophosphatase (DFPase) on obidoxime-induced reactivation of human acetylcholinesterase (AChE) inhibited by different OPs was investigated. Reactivation of paraoxon-, sarin-, soman- and VX-inhibited AChE by obidoxime was impaired by POX-induced re-inhibition whereas no deviation of pseudo first-order kinetics was observed with tabun, cyclosarin and VR. OPH prevented (paraoxon) or markedly reduced the POX-induced re-inhibition (VX, sarin, soman), whereas OPAA and DFPase were without effect. Additional experiments with sarin-inhibited AChE indicate that the POX hydrolysis by OPH was concentration-dependent. The activity of OP-inhibited AChE was not affected by OPH in the absence of obidoxime. In conclusion, OPH may be a valuable contribution to the therapeutic regimen against OP poisoning by accelerating the degradation of both the parent compound, OP, and the reaction product, POX.

In vitro reactivation of tabun-inhibited acetylcholinesterase using new oximes--K027, K005, K033 and K048

Four new AChE oximes for reactivation of acetylcholinesterase inhibited with tabun - K027 [1-(4-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium) propane dibromide], K005 [1,3-bis(2-hydroxyiminomethylpyridinium) propane dibromide], K033 [1,4-bis(2-hydroxyiminomethylpyridinium) butane dibromide] and K048 [1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium) butane dibromide] were prepared. Their efficacies to reactivate tabun-inhibited acetylcholinesterase were studied and compared with the currently used acetylcholinesterase reactivators (pralidoxime, obidoxime and HI-6). Reactivator K048 seems to be promising reactivator of tabun-inhibited AChE. Its reactivation potency is significantly higher than the efficacy of HI-6 and pralidoxime, and comparable with the potency of the obidoxime at human relevant doses.

Mechanism of organophosphates (nerve gases and pesticides) and antidotes: electron transfer and oxidative stress

Evidence indicates that nerve gas toxins operate in ways in addition to inhibition of acetylcholine esterase. Alternative bioactivities are discussed with focus on electron transfer. The main class, including pralidoxime (2-PAM), incorporates conjugated iminium and oxime moieties that are electron affinic. Various physiological properties of iminium and oxime species are reviewed. The organophosphates encompass both nerve gases and insecticides, possessing similar properties, but different activities. Toxic manifestations are apparently due, in part, to oxidative stress. Alkylation of DNA takes place which may lead to generation of reactive oxygen species. Structure-activity relationships are examined, including reduction potentials and the captodative effect.