Toxicity Of Tabun Research Articles

Development of versatile and potent monoquaternary reactivators of acetylcholinesterase.

To date, the only treatments developed for poisoning by organophosphorus compounds, the most toxic chemical weapons of mass destruction, have exhibited limited efficacy and versatility. The available causal antidotes are based on reactivation of the enzyme acetylcholinesterase (AChE), which is rapidly and pseudo-irreversibly inhibited by these agents. In this study, we developed a novel series of monoquaternary reactivators combining permanently charged moieties tethered to position 6- of 3-hydroxypyridine-2-aldoxime reactivating subunit. Highlighted representatives (21, 24, and 27; also coded as K1371, K1374, and K1375, respectively) that contained 1-phenylisoquinolinium, 7-amino-1-phenylisoquinolinium and 4-carbamoylpyridinium moietiesas peripheral anionic site ligands, respectively, showed efficacy superior or comparable to that of the clinically used standards. More importantly, these reactivators exhibited wide-spectrum efficacy and were minutely investigated via determination of their reactivation kinetics in parallel with molecular dynamics simulations to study their mechanisms of reactivation of the tabun-inhibited AChE conjugate. To further confirm the potential applicability of these candidates, a mouse in vivo assay was conducted. While K1375 had the lowest acute toxicity and the most suitable pharmacokinetic profile, the oxime K1374 with delayed elimination half-life was the most effective in ameliorating the signs of tabun toxicity. Moreover, both in vitro and in vivo, the versatility of the agents was substantially superior to that of clinically used standards. Their high efficacy and broad-spectrum capability make K1374 and K1375 promising candidates that should be further investigated for their potential as nerve agents and insecticide antidotes.

Reversal of Tabun Toxicity Enabled by a Triazole‐Annulated Oxime Library—Reactivators of Acetylcholinesterase

Acetylcholinesterase (AChE), an enzyme that degrades the neurotransmitter acetylcholine, when covalently inhibited by organophosphorus compounds (OPs), such as nerve agents and pesticides, can be reactivated by oximes. However, tabun remains among the most dangerous nerve agents due to the low reactivation efficacy of standard pyridinium aldoxime antidotes. Therefore, finding an optimal reactivator for prophylaxis against tabun toxicity and for post-exposure treatment is a continued challenge. In this study, we analyzed the reactivation potency of 111 novel nucleophilic oximes mostly synthesized using the CuAAC triazole ligation between alkyne and azide building blocks. We identified several oximes with significantly improved in vitro reactivating potential for tabun-inhibited human AChE, and in vivo antidotal efficacies in tabun-exposed mice. Our findings offer a significantly improved platform for further development of antidotes and scavengers directed against tabun and related phosphoramidate exposures, such as the Novichok compounds.

A comparison of the reactivating and therapeutic efficacy of two novel bispyridinium oximes (K305, K307) with the oxime K203 and trimedoxime in tabun-poisoned rats and mice

The reactivating and therapeutic efficacy of two newly developed oximes (K305, K307) was compared with the oxime K203 and trimedoxime using in vivo methods The study determining percentage of reactivation of tabun-inhibited acetylcholinesterase in the peripheral as well as central nervous system (diaphragm, brain) in tabun-poisoned rats showed that the reactivating efficacy of both newly developed oximes is lower compared to the reactivating efficacy of the oxime K203 and trimedoxime. The therapeutic efficacy of all oximes studied roughly corresponds to their reactivating efficacy. While the ability of the oxime K305 to reduce acute toxicity of tabun in mice is approaching to the therapeutic efficacy of trimedoxime, the ability of another novel bispyridinium oxime K307 to reduce acute toxicity of tabun is significantly lower compared to trimedoxime and the oxime K203. Thus, the reactivating and therapeutic efficacy of both examined newly developed oximes does not prevail the effectiveness of the oxime K203 and trimedoxime and, therefore, they are not suitable for their replacement of commonly used oximes for the treatment of acute tabun poisoning.

A comparison of the reactivating and therapeutic efficacy of two novel bispyridinium oximes (K920, K923) with the oxime K203 and trimedoxime in tabun-poisoned rats and mice

The potency of two novel oximes (K920, K923) to reactivate tabun-inhibited acetylcholinesterase and to reduce acute toxicity of tabun was compared with the oxime K203 and trimedoxime using in vivo methods. The study determining percentage of reactivation of tabun-inhibited peripheral acetylcholinesterase (diaphragm) and central acetylcholinesterase (brain) in tabun-poisoned rats showed that the reactivating efficacy of both newly developed oximes is lower than the reactivating potency of the oxime K203 and trimedoxime. The therapeutic efficacy of both newly developed oximes roughly corresponds to their weak reactivating efficacy. Their potency to reduce acute toxicity of tabun in mice was lower compared to the oxime K203 and trimedoxime. All differences in reactivating efficacy of oximes and different protective ratios were found for selected doses of oximes used in this study. Based on the results obtained, we can conclude that the reactivating and therapeutic potency of both newly developed oximes does not prevail the effectiveness of the oxime K203 and trimedoxime and, therefore, they are not suitable for their replacement of commonly used oximes for the treatment of acute tabun poisoning. The conclusion is only relevant for the experimental animals used in this study because of remarkable species differences in reactivating properties of oximes.

The evaluation of the reactivating and therapeutic efficacy of two novel oximes (K361 and K378) in comparison with the oxime K203 and trimedoxime in tabun-poisoned rats and mice

The potency of two newly developed oximes (K361 and K378) to reactivate tabun-inhibited cholinesterase and to reduce acute toxicity of tabun was compared with the oxime K203 and trimedoxime using in vivo methods. The study determining percentage of reactivation of tabun-inhibited diaphragm cholinesterase in poisoned rats showed that the reactivating efficacy of the oxime K378 is slightly lower than the reactivating potency of the oxime K203 and trimedoxime while the ability of the oxime K361 to reactivate tabun-inhibited cholinesterase is markedly lower compared with the oxime K203 and trimedoxime. In the brain, the potency of both newly developed oximes to reactivate tabun-inhibited cholinesterase was negligible. The therapeutic efficacy of both newly developed oximes roughly corresponds to their weak reactivating efficacy. Their potency to reduce acute toxicity of tabun was significantly lower compared with the oxime K203 as well as trimedoxime. In conclusion, the reactivating and therapeutic potency of both newly developed oximes does not prevail the effectiveness of the oxime K203 and trimedoxime and, therefore, they are not suitable for their replacement of commonly used oximes for the treatment of acute tabun poisoning.

The evaluation of the reactivating and therapeutic efficacy of three novel bispyridinium oximes (K454, K456, K458) in comparison with the oxime K203 and trimedoxime in tabun-poisoned rats and mice

The potency of three newly developed bispyridinium compounds (K454, K456, K458) to reactivate tabun-inhibited acetylcholinesterase and reduce tabun-induced lethal toxic effects was compared with the oxime K203 and trimedoxime using in vivo methods. The study determining percentage of reactivation of tabun-inhibited diaphragm and brain acetylcholinesterase in poisoned rats showed that the reactivating efficacy of all newly developed oximes is comparable with K203 but lower than the reactivating potency of trimedoxime in diaphragm. In the brain, their potency to reactivate tabun-inhibited acetylcholinesterase is lower compared with trimedoxime and the oxime K203. All three newly developed oximes were also found to be relatively effective in reducing lethal toxic effects in tabun-poisoned mice. Their therapeutic efficacy is consistent with the therapeutic potency of the oxime K203. On the other hand, their potency to reduce acute toxicity of tabun is significantly lower compared with trimedoxime. In conclusion, the reactivating and therapeutic potency of all three newly developed oximes does not prevail the effectiveness of the oxime K203 and trimedoxime and, therefore, they are not suitable for their replacement of commonly used oximes for the treatment of acute tabun poisoning.

Protective effect of HI-6 and trimedoxime combination in mice acutely poisoned with tabun, dichlorvos or heptenophos

The aim of this study was to compare the protective effect of two individual oximes (HI-6 and trimedoxime) with their combination in mice acutely poisoned with tabun, dichlorvos or heptenophos. Oxime HI-6 did not protect experimental animals against either dichlorvos, heptenophos or tabun. Trimedoxime was very effective against all three OPs. The ED-500 doses of trimedoxime necessary to protect 50% of animals after the simultaneous administration of OPs and oxime were 42.18, 14.97 and 32.08 μmol/kg in dichlorvos, heptenophos and tabun poisoning, respectively. Half-time of efficacy in the tabun protocol was approximately three and two times longer than in the protocol for heptenophos and dichlorvos, respectively indicating also that trimedoxime is very potent in counteracting tabun toxicity. Addition of trimedoxime significantly improved the protective effect of HI-6 in acute tabun poisoning. When dichlorvos or heptenophos were used, addition of trimedoxime generally improved the antidotal effect of HI-6, but still lower protection was obtained than in the case when trimedoxime alone was administered. The investigations of different oxime combinations have indicated that application of a mixture of two oximes represents a promising antidotal approach.

A comparison of reactivating and therapeutic efficacy of the oxime K203 and its fluorinated analog (KR-22836) with currently available oximes (obidoxime, trimedoxime, HI-6) against tabun in rats and mice

The potency of newly developed bispyridinium compound K203 and its fluorinated analog KR-22836 in reactivating tabun-inhibited acetylcholinesterase and reducing tabun-induced lethal toxic effects was compared with commonly used oximes (obidoxime, trimedoxime, the oxime HI-6) using in vivo methods. Studies determining the percentage of reactivation of tabun-inhibited blood and tissue acetylcholinesterase in rats showed that the reactivating efficacy of K203 is higher than the reactivating efficacy of its fluorinated analog KR-22836 as well as currently available oximes studied. The therapeutic efficacy of the oxime K203 and its fluorinated analog corresponds to their potency to reactivate tabun-inhibited acetylcholinesterase. According to the results, the oxime K203 is more suitable than KR-22836 for the replacement of commonly used oximes for the antidotal treatment of acute tabun poisoning due to its relatively high potency to counteract the acute toxicity of tabun.

The influence of combinations of oximes on the reactivating and therapeutic efficacy of antidotal treatment of tabun poisoning in rats and mice

The influence of the combination of oximes on the reactivating and therapeutic efficacy of antidotal treament of acute tabun poisoning was evaluated. The ability of two combinations of oximes (HI-6 + obidoxime and HI-6 + K203) to reactivate tabun-inhibited acetylcholinesterase and reduce acute toxicity of tabun was compared with the reactivating and therapeutic efficacy of antidotal treatment involving single oxime (HI-6, obidoxime, K203) using in vivo methods. Studies determining percentage of reactivation of tabun-inhibited blood and tissue acetylcholinesterase in poisoned rats showed that the reactivating efficacy of both combinations of oximes is higher than the reactivating efficacy of the most effective individual oxime in blood and diaphragm and comparable with the reactivating effects of the most effective individual oxime in brain. Moreover, both combinations of oximes were found to be slightly more efficacious in the reduction of acute lethal toxic effects in tabun-poisoned mice than the antidotal treatment involving individual oxime. A comparison of reactivating and therapeutic efficacy of individual oximes showed that the newly developed oxime K203 is slightly more effective than commonly used obidoxime and both of them are markedly more effective than the oxime HI-6. Based on the obtained data, we can conclude that the antidotal treatment involving chosen combinations of oximes brings beneficial effects for the potency of antidotal treatment to reactivate tabun-inhibited acetylcholinesterase in rats and to reduce acute toxicity of tabun in mice.

The Development of New Oximes and the Evaluation of their Reactivating, Therapeutic and Neuroprotective Efficacy Against Tabun

Tabun (O-ethyl-N,N-dimethyl phosphoramidocyanidate) belongs to highly toxic organophosphorus compounds misused as chemical warfare agents for military as well as terroristic purposes. The antidotal treatment of tabun acute poisonings still represents a serious problem and the development of new, more effective AChE reactivators to achieve the satisfactorily effective antidotal treatment of acute poisonings with tabun still represents very important goal. Since 2003, we have prepared around 200 new AChE reactivators. Their potency to reactivate tabun-inhibited acetylcholinesterase has been subsequently evaluated using our in vitro screening test. Afterwards, promising compounds were selected and kinetic parameters and reactivation constants were determined. Then, the best reactivators were subjected to the in vivo studies (toxicity test, the evaluation of therapeutic, reactivating and neuroprotective efficacy) and their potency to counteract the acute toxicity of tabun is compared to the therapeutic, reactivating and neuroprotective efficacy of commonly used oximes - obidoxime and the oxime HI-6. According to the results obtained, the newly synthesized oxime K075 showed the highest potency to reduce tabun-induced acute lethal toxicity while the therapeutic potency of obidoxime and the oxime HI-6 was significantly lower. The therapeutic efficacy of oximes studied corresponds to their reactivating efficacy in vivo as well as in vitro. The potency of all newly synthesized oximes to reactivate tabun-inhibited AChE is comparable with obidoxime with the exception of K074 that is significantly more efficacious in the brain. In addition, all newly synthesized oximes combined with atropine seem to be effective antidotes for a decrease in tabun-induced acute neurotoxicity. While the neuroprotective efficacy of obidoxime in combination with atropine is similar to the potency of newly synthesized oximes, the ability of the oxime HI-6 combined with atropine to counteract tabun-induced acute neurotoxicity is significantly lower. Due to their therapeutic, reactivating and neuroprotective efficacy, all newly synthesized oximes appear to be suitable oximes for the antidotal treatment of acute tabun poisonings.

A Comparison of the Therapeutic and Reactivating Efficacy of Newly Developed Oximes (K117, K127) and Currently Available Oximes (Obidoxime, Trimedoxime, HI-6) in Tabun-Poisoned Rats and Mice

The potency of newly developed bispyridinium compounds (K117, K127) to reactivate tabun-inhibited acetylcholinesterase and reduce tabun-induced lethal toxic effects was compared with currently available oximes (obidoxime, trimedoxime, oxime HI-6) by using in vivo methods. A study that determined the percentage of reactivation of tabun-inhibited blood and tissue acetylcholinesterase in poisoned rats showed that the reactivating efficacy of newly developed oxime K127 is comparable with obidoxime and trimedoxime in blood but lower than the reactivating potency of trimedoxime and obidoxime in the diaphragm and brain. The potency of another newly developed K117 to reactivate tabun-inhibited acetylcholinesterase is comparable with obidoxime or trimedoxime in the diaphragm, but it is significantly lower than the reactivating potency of trimedoxime and obidoxime in the blood and brain. The oxime, K127, was also found to be relatively effective in reducing lethal toxic effects in tabun-poisoned mice. Its therapeutic efficacy is consistent with the therapeutic potency of obidoxime. On the other hand, the potency of the oxime, K117, to reduce acute toxicity of tabun is significantly lower compared to trimedoxime and obidoxime. The therapeutic efficacy of K117 and K127 corresponds to their potency to reactivate tabun-inhibited acetylcholinesterase, especially in the diaphragm and brain. Contrary to obidoxime and trimedoxime, the oxime, HI-6, is not an effective oxime in the reactivation of tabun-inhibited acetycholinesterase and in reducing the lethal effects of tabun. The reactivating and therapeutic potency of both newly developed oximes does not prevail over the effectiveness of currently available obidoxime and trimedoxime and, therefore, they are not suitable for their replacement of commonly used oximes for the treatment of acute tabun poisoning.

An evaluation of therapeutic and reactivating effects of newly developed oximes (K156, K203) and commonly used oximes (obidoxime, trimedoxime, HI-6) in tabun-poisoned rats and mice

The potency of newly developed monoxime bispyridinium compounds (K156, K203) in reactivating tabun-inhibited acetylcholinesterase and reducing tabun-induced lethal toxic effects was compared with commonly used oximes (obidoxime, trimedoxime, the oxime HI-6) using in vivo methods. Studies determining percentage of reactivation of tabun-inhibited blood and tissue acetylcholinesterase in poisoned rats showed that the reactivating efficacy of newly developed oxime K203 is comparable with obidoxime and trimedoxime in blood and higher than the reactivating potency of trimedoxime and obidoxime in diaphragm and brain, where the difference in reactivating efficacy of obidoxime, trimedoxime and K203 is significant. On the other hand, the potency of newly developed K156 to reactivate tabun-inhibited acetylcholinesterase is comparable with obidoxime or trimedoxime in diaphragm and brain. It is significantly lower than the reactivating efficacy of trimedoxime and obidoxime in blood. Moreover, both newly developed oximes were found to be relatively efficacious in the reduction of lethal toxic effects in tabun-poisoned mice. Especially, the oxime K203 is able to decrease the acute toxicity of tabun nearly two times. The therapeutic efficacy of K156 and K203 corresponds to their potency to reactivate tabun-inhibited acetylcholinesterase, especially in diaphragm and brain. In contrast to obidoxime and trimedoxime, the oxime HI-6 is not effective in reactivation of tabun-inhibited acetycholinesterase and in reducing tabun lethality. While the oxime K156 does not improve the reactivating and therapeutic effectiveness of currently available obidoxime and trimedoxime, the newly developed oxime K203 is markedly more effective in reactivation of tabun-inhibited acetylcholinesterase in rats, especially in brain, and in reducing lethal toxic effects of tabun in mice and, therefore, it is suitable for the replacement of commonly used oximes for the antidotal treatment of acute tabun poisoning.

Comparison of Reactivating and Therapeutic Efficacy of Two Salts of the Oxime HI‐6 against Tabun, Soman and Cyclosarin in Rats

The reactivating and therapeutic efficacy of two salts of the oxime HI-6 (dichloride and dimethanesulphonate) against chosen nerve agents (tabun, soman and cyclosarin) was compared in rats. The potency of both salts of HI-6 to decrease the acute toxicity of tabun, soman and cyclosarin was similar in nerve agent-poisoned rats. While the potency of HI-6 dichloride and HI-6 dimethanesulphonate to counteract acute toxic effects of tabun is rather low, both salts of HI-6 were able to decrease the acute toxicity of soman two times and acute toxicity of cyclosarin more than three times. The therapeutic efficacy of both salts of the oxime HI-6 corresponds to their reactivating potency. While the reactivating efficacy of HI-6 dichloride as well as HI-6 dimethanesulphonate against tabun was negligible, their potency to reactivate soman-inhibited acetylcholinesterase and cyclosarin-inhibited acetylcholinesterase in peripheral (blood) and central (brain) compartment was relatively high. HI-6 dichloride showed a somewhat higher potency to reactivate tabun-inhibited acetylcholinesterase in brain, and soman-inhibited acetylcholinesterase in blood and brain than HI-6 dimethanesulphonate but the differences were not significant. Thus, the replacement of dichloride anion by dimethanesulphonate anion in the oxime HI-6 does not influence the therapeutic and reactivating efficacy of the oxime HI-6 against nerve agents. In addition, the higher solubility and stability of HI-6 dimethanesulphonate in comparison with HI-6 dichloride makes it possible to increase the dose and thus, the effectiveness of the oxime HI-6 in the antidotal treatment of acute nerve agent poisonings.

Toxicity of the nerve agent tabun to Daphnia magna, a new experimental species in military toxicology

Daphnia magna, a freshwater microcrustacean, is currently tested as an alternative experimental species in research dealing with nerve agents poisonings treatment. Because of this, the toxicity of the nerve agent tabun (a cyanide-group containing organophosphate) to Daphnia had to be examined by estimating the EC50 values. The immobilization of daphnids was chosen as the end-point. It was found that D. magna is sensitive to small amounts of tabun, even after 15 min exposure, and tabun toxicity increases with time. The estimated EC50 values for 15, 30, 45, and 60 min exposure were as follows: 67.39, 38.10, 26.95, and 21.9 μg l−1. In addition, the toxicity of media to which tabun was added 24 h before the start of experiments was examined. The results obtained indicate that daphnids can be used in experiments with nerve-agent intoxication treatment.

The Influence of the Time of Antidotal Treatment Administration on the Potency of Newly Developed Oximes to Counteract Acute Toxic Effects of Tabun in Mice

(1) The influence of the time of administration of antidotal treatment consisting of anticholinergic drug (atropine) and newly developed oxime (K027 or K048) on its effectiveness to eliminate tabun-induced lethal toxic effects was studied in mice. (2) The therapeutic efficacy of antidotal treatment of tabun-induced acute poisoning depends on the time of its administration regardless of the choice of the oxime. (3) Our results show that both oximes studied (K027, K048) are able to sufficiently eliminate lethal effects of tabun. Nevertheless, their efficacy significantly decreases when they were administered 5 min after tabun poisoning. (4) The findings support the hypothesis that both newly developed oximes appear to be suitable oximes to counteract acute toxicity of tabun although their ability to eliminate lethal toxic effects of tabun significantly decreases with prolonged time interval between tabun challenge and antidotal treatment administration.

The Influence of the Time of Antidotal Treatment Administration on Its Effectiveness Against Tabun-Induced Poisoning in Mice

1. The influence of the time of administration of antidotal treatment consisting of anticholinergic drug (atropine) and oxime (pralidoxime, obidoxime, HI-6 or trimedoxime) on its effectiveness to eliminate tabun-induced lethal effects was studied in mice. 2. The therapeutic efficacy of antidotal treatment of tabun-induced acute poisoning depends on the time of its administration when obidoxime or the oxime HI-6 was used as an acetylcholinesterase reactivator. 3. Pralidoxime is practically ineffective to eliminate acute toxic effects of tabun regardless of the time of its administration. 4. Our results show that trimedoxime seems to be the most effective to eliminate lethal effects of tabun. In addition, its efficacy does not decrease when it is administered 5 min after tabun poisoning. 5. The findings support the hypothesis that trimedoxime appears to be the most suitable oxime to counteract acute toxicity of tabun because of its ability to eliminate lethal effects of tabun when it is injected 5 min after tabun challenge on the contrary to other oximes tested.

Effect of Methoxime Combined with Anticholinergic, Anticonvulsant or Anti-HCN Drugs in Tabun-Poisoned Mice

The effect of methoxime combined with a) atropine, b) benactyzine, c) atropine and natrium thiosulphate, d) atropine and diazepam on antidotal treatment effectiveness was studied in tabun-poisoned mice. In addition, the influence of pretreatment consisiting of pyridostigmine, benactyzine and trihexyphenidyle (PANPAL) administered 2 hours before tabun intoxication on the treatment effectivity of methoxime combined with e) atropine or f) benactyzine was tested. The most efficacious therapeutic mixture in non-pretreated mice was methoxime, atropine and diazepam. Natrium thiosulphate did not significantly increase neither decrease the antidotal treatment efficacy in comparison with methoxime and atropine alone. Pretreatment with PANPAL significantly decreased tabun toxicity (nearly 4 times in methoxime and benactyzine combination and more than 4 times in atropine and methoxime mixture). The present study demonstrates that the tabun toxicity in mice is more effectively reduced when PANPAL prophylactically is administered than in case of treatment with methoxime and cholinergic drug alone. We established that anticholinergic drug option in the therapeutic mixture of methoxime and anticholinergic drug did not cause the difference in the antidotal treatment effectivities.

Protection against tabun toxicity in mice by prophylaxis with an enzyme hydrolyzing organophosphate esters

We demonstrate here the correlation between protection afforded by pretreatment alone with parathion hydrolase purified from Pseudomonas sp. against tabun toxicity in mice and the kinetic parameters which are assumed to determine the in vivo detoxification of tabun by the same enzyme. Results show that 15 and 22 μg of parathion hydrolase per animal conferred a protective ratio of 3.94 and 5.65 respectively, against tabun toxicity, without post-exposure treatment.

Role of Carboxylesterase in Soman, Sarin and Tabun Poisoning in Rats

TOCP (triorthocresyl phosphate) inhibits carboxylesterase (CarbE) activity in rat plasma and liver and significantly increases soman, sarin and tabun toxicity. Application of these agents after TOCP caused strong additional inhibition of CarbE and cholinesterases (ChE) in rat red blood cells, plasma, liver, brain, diaphragm and intercostal muscle. After phenobarbital pretreatment, which induced CarbE activity in plasma and liver by 80% and that of plasma ChE by 33%, acute toxicity of soman and tabun was greatly decreased, while that of sarin remained unaffected.

Acute tabun toxicity; biochemical and histochemical consequences in brain and skeletal muscles of rat

Male Sprague-Dawley rats injected s.c. with an acute non-lethal dose (200 μg/kg) of ethyl N,N- dimethylphosphoramidocyanidate (tabun) showed onset of hypercholinergic activity within 10–15 min. The maximal severity of toxicity signs was evident within 0.5–1 h and persisted for 6 h. Except for mild tremors no overt toxicity signs were evident after 24 h. Within 1 h a dramatic decline of acetylcholinesterase (AChE) activity occurred in all the brain structures ( < 3%) and skeletal muscles ( < 10% in soleus and hemi-diaphragm; and 32% in extensor digitorum longus (EDL)). No significant recovery was seen up to 48–72 h. Within 7 days rats became free of toxicity signs and AChE activity had recovered to about 40% in brain structures (except cortex, 14%) and 65–70% in skeletal muscles. Within 1 h the 16 S molecular form of AChE located at the neuromuscular junction was most severely inhibited in solues, followed by hemi-diaphragm and least in the EDL, and had fully recovered in all the muscles when examined after day 7. Muscle fiber necrosis developed within 1–3 h in soleus and hemi-diaphragm and after a delay of 24 h in EDL. The highest number of necrotic lesions in all muscles was seen at 72 h with the hemi-diahragm maximally affected and EDL the least. To determine detoxification of tabun by non-specific binding, the activity of butyrylcholinesterase (BuChE) and carboxylesterase (CarbE) was measured. The inhibition and recovery pattern of BuChE activity was quite similar to that of AChE, except that the rate of recovery was more rapid. Within 1 h the remaining activity of CarbE was 10% in plasma, about 30% in brain structures, and 79% in liver; recovery was complete within 7 days. The inhibition of BuChE and CarbE can severe as a protective mechanism against tabun toxicity by reducing the amount available for AChE inhibition. The prolonged AChE inhibition in muscle and brain may indicate storage of tabun and delayed release from non-enzymic sites. Since tabun is a cyanophosphorus compound, the toxic effects from the released cyanide (CN) could be another reason for the delayed recovery after tabun.