Two medical therapies very effective shortly after high levels of soman poisoning in rats,but only one with universal utility

A treatment regimen consisting of HI-6, scopolamine, and physostigmine (termed the physostigmine regimen) has been based on the serendipitous discovery that it exerts powerful antidotal effects against high levels of soman poisoning if it is administered 1 min after exposure. A medical therapy with corresponding efficacy, but without the time limitation of the latter regimen, has been developed through studies of microinfusions of anticonvulsants into seizure controlling sites in the forebrain of rats. From these studies procyclidine emerged as the most potent anticonvulsant, and its potency was further enhanced when being combined with the antiepileptic levetiracetam during systemic administration. In the present study, the capacity of HI-6, levetiracetam, and procyclidine (termed the procyclidine regimen) was tested against that of the physostigmine regimen. The results showed that both regimens were very effective against supralethal doses of soman (3, 4, 5 × LD₅₀) when given 1 and 5 min after intoxication. When the treatments were administered 10 and 14 or 20 and 24 min after soman exposure, only the procyclidine regimen was able to terminate seizures and preserve lives. When used as prophylactic therapies, both regimens protected equally well against seizures, but only the procyclidine regimen provided neuroprotection. The procyclidine regimen has apparently capacities to serve as a universal therapy against soman intoxication in rats.     

Neuroprotective Effects of Currently Used Antidotes in Tabun‐Poisoned Rats

Abstract: The neuroprotective effects of antidotes (atropine, pralidoxime/atropine, obidoxime/atropine and HI‐6/atropine mixtures) on rats poisoned with tabun at a lethal dose (220 μg/kg intramuscularly; 100% of LD50 value) were studied. The tabun‐induced neurotoxicity was monitored using a functional observational battery and an automatic measurement of motor activity. The neurotoxicity of tabun was monitored at 24 hr and 7 days after tabun challenge. The results indicate that atropine alone is not able to protect the rats from the lethal effects of tabun. Three non‐treated tabun‐poisoned rats and one tabun‐poisoned rat treated with atropine alone died within 24 hr. On the other hand, atropine combined with all tested oximes allows all tabun‐poisoned rats to survive at least 7 days following tabun challenge. Obidoxime combined with atropine seems to be the most effective antidotal treatment for the elimination of tabun‐induced neurotoxicity in the case of lethal poisoning among tested antidotal mixtures. The antidotal mixture consisting of atropine and HI‐6 is significantly less effective than the combination of atropine with obidoxime in the elimination of tabun‐induced neurotoxicity in rats at 24 hr following tabun challenge. Pralidoxime in combination with atropine appears to be practically ineffective to decrease tabun‐induced neurotoxicity at 24 hours as well as 7 days following tabun poisoning. Due to its neuroprotective effects, obidoxime seems to be the most effective and most suitable oxime for the antidotal treatment of acute tabun exposure among currently used oximes. Thus, the replacement of obidoxime by a more effective acetylcholinesterase reactivator for soman poisoning, the oxime HI‐6, can to a small extent diminish the neuroprotective efficacy of antidotal treatment in the case of acute tabun poisonings.     

The Evaluation of Prophylactic Efficacy of Newly Developed Reversible Inhibitors of Acetylcholinesterase in Soman‐Poisoned Mice – A Comparison with Commonly Used Pyridostigmine

The ability of four newly developed reversible inhibitors of acetylcholinesterase (PC‐37, PC‐48, JaKo 39, JaKo 40) and currently available carbamate pyridostigmine to increase the resistance of mice against soman and the efficacy of antidotal treatment of soman‐poisoned mice was evaluated and compared. No reversible inhibitor of acetylcholinesterase studied was able to decrease the LD₅₀ value of soman in mice. Thus, the pharmacological pre‐treatment with pyridostigmine or newly synthesized inhibitors of acetylcholinesterase was not able to significantly protect mice against soman‐induced lethal acute toxicity. In addition, neither pyridostigmine nor new reversible inhibitors of acetylcholinesterase was able to increase the efficacy of antidotal treatment (the oxime HI‐6 in combination with atropine) of soman‐poisoned mice. These findings demonstrate that pharmacological pre‐treatment of soman‐poisoned mice with tested reversible inhibitors of acetylcholinesterase is not promising.     

Evaluation of the Neuroprotective Efficacy of Newly Developed Oximes (K206, K269) and Currently Available Oximes (Obidoxime,HI‐6) in Cyclosarin‐Poisoned Rats

Abstract: The neuroprotective effects of newly developed oximes (K206, K269) and currently available oximes (obidoxime, HI‐6) in combination with atropine in rats poisoned with cyclosarin were studied. The cyclosarin‐induced neurotoxicity was monitored using a functional observational battery at 24 hr following cyclosarin challenge. The results indicate that a newly developed oxime K206 is able to counteract cyclosarin‐induced neurotoxicity while the neuroprotective potency of another newly developed oxime (K269) is negligible. The neuroprotective efficacy of K206 is markedly higher than commonly used obidoxime; nevertheless, its potency to eliminate cyclosarin‐induced neurotoxicity is slightly lower compared to the oxime HI‐6. Thus, a newly developed oxime K206 seems to be a better oxime for the antidotal treatment of cyclosarin poisonings than obidoxime due to higher neuroprotective potency although the oxime HI‐6 is still the most suitable oxime for the antidotal treatment of acute poisonings with cyclosarin.     

Neuroprotective effects of currently used antidotes in soman-poisoned rats

The neuroprotective effects of antidotes (atropine, obidoxime, obidoxime/atropine mixture) on rats poisoned with soman at a sublethal dose (54 microg/kg, im, 80% of LD(50) value) were studied. The soman-induced neurotoxicity was monitored using a functional observational battery (FOB) and an automatic measurement of motor activity. The neurotoxicity of soman was monitored at 24 h and 7 days following soman challenge. The results indicate that obidoxime alone is not able to protect the rats from the lethal effects of soman. Three soman-poisoned rats treated with obidoxime alone died within 24 h. On the other hand, atropine alone or combined with obidoxime allows all soman-poisoned rats to survive within 7 days following soman challenge. Atropine alone and combined with obidoxime seems to be relatively effective antidotal treatment for the elimination of soman-induced neurotoxicity in the case of sublethal poisonings, although the antidotal mixture is significantly less effective than atropine alone because obidoxime can counteract the beneficial effects of atropine. Obidoxime appears to be practically ineffective to diminish soman-induced neurotoxicity. The neuroprotective effects of antidotal mixture consisting of atropine and obidoxime depend on the antimuscarinic effects of atropine only. Thus, the replacement of obidoxime by more effective acetylcholinesterase (AChE) reactivators is necessary to increase the neuroprotective efficacy of antidotal treatment in the case of soman poisonings.     

Evaluation of the Influence of Three Newly Developed Bispyridinium Anti‐nicotinic Compounds (MB408, MB442, MB444) on the Efficacy of Antidotal Treatment of Nerve Agent Poisoning in Mice

The influence of three newly developed bispyridinium antinicotinic compounds (the non‐oximes MB408, MB442 and MB444) on the therapeutic efficacy of a standard antidotal treatment (atropine in combination with an oxime) of acute poisoning by the organophosphorus nerve agents tabun and soman was studied in mice. The therapeutic efficacy of atropine in combination with an oxime with or without one of the bispyridinium non‐oximes was evaluated by determination of the LD₅₀ values of the nerve agents and measurement of the survival time after supralethal poisoning. Addition of all the tested non‐oximes increased significantly the therapeutic efficacy of atropine in combination with an oxime against tabun poisoning. They also positively influenced the number of surviving mice 6 hr after supralethal poisoning with tabun. However, they were only slightly effective for the treatment of soman poisoning. The benefit of the tested bispyridinium non‐oximes was dose‐dependent. To conclude, the addition of bispyridinium non‐oximes to the standard antidotal treatment of acute poisoning with tabun was beneficial regardless of the chosen non‐oxime, but only slightly beneficial in the case of soman poisoning.     

A Comparison of the Potency of a Novel Bispyridinium Oxime K203 and currently available Oximes (Obidoxime,HI‐6) to Counteract the Acute Neurotoxicity of Sarin in Rats

The neuroprotective effects of a newly developed oxime K203 and currently available oximes (obidoxime, HI‐6) in combination with atropine in rats poisoned with sarin were studied. The sarin‐induced neurotoxicity was monitored using a functional observatory battery at 2 hr after sarin challenge. The results indicate that the potency of a novel bispyridinium oxime K203 to counteract sarin‐induced neurotoxicity is relatively low and roughly corresponds to the neuroprotective efficacy of obidoxime. Among tested oximes, the oxime HI‐6 seems to be significanlty more efficacious to counteract acute neurotoxicity of sarin than commonly used obidoxime and a newly developed oxime K203. Thus, the oxime K203 does not provide any beneficial effect for the antidotal treatment of acute poisoning with sarin in comparison with the oxime HI‐6 that should be considered to be the best oxime for antidotal treatment of acute sarin poisonings.     

Minimum effective drug concentrations of a transdermal patch system containing procyclidine and physostigmine for prophylaxis against soman poisoning in rhesus monkeys

A transdermal patch system containing procyclidine, an N-methyl-d-aspartate receptor antagonist possessing anticholinergic action, and physostigmine, a reversible cholinesterase inhibitor, was developed, and its prophylactic efficacy against soman intoxication was investigated. Male rhesus monkeys were shaved on the dorsal area, attached with a matrix-type patch with various sizes (2 × 2 to 7 × 7 cm) for 24 or 72 h, and challenged with 2 × LD₅₀ doses (13 μg/kg) of soman. The smallest patch size for the protection against lethality induced by soman intoxication was 3 × 3 cm, resulting in blood procyclidine concentration of 10.8 ng/ml, blood physostigmine concentration of 0.54 ng/ml, which are much lower concentrations than maximum sign-free doses, and blood cholinesterase inhibition of 42%. The drug concentrations and enzyme inhibition rate corresponding to a diverging point of survivability were presumably estimated to be around 7 ng/ml for procyclidine, 0.35 ng/ml for physostigmine, and 37% of enzyme inhibition. Separately, in combination with the patch treatment, the post treatment consisting of atropine (0.5 mg/kg) plus 1-[([4-(aminocarbonyl)pyridinio]methoxy)methyl]-2-[(hydroxyimino)methyl]pyridinium (HI-6, 50 mg/kg) exerted protection against 5 × LD₅₀ challenge of soman, which means the posttreatment remarkably augmented the efficacy of the patch. Additionally, it was found that brain injuries induced by soman toxicity were effectively prevented by the patch treatment according to histopathological examinations. These results suggest that the patch system could be an effective alternative for diazepam, an anticonvulsant, and the current pyridostigmine pretreatment, and especially in combination with atropine plus HI-6, could be a choice for quality survival from nerve-agent poisoning.     

Protection by a transdermal patch containing physostigmine and procyclidine of soman poisoning in dogs

The prophylactic efficacy of a combinational patch system containing physostigmine and procyclidine against soman intoxication was evaluated using dogs. Female beagle dogs (body weights 9-10 kg) were shaved on the abdominal side, attached with a matrix-type patch (7x7 cm) containing 1.5% of physostigmine plus 6% procyclidine for 2 days, and challenged with subcutaneous injection of serial doses (2-10 LD50) of soman. Separately, in combination with the patch attachment, atropine (2 mg/dog) plus 2-pralidoxime (600 mg/dog) or atropine plus 1-[([4-(aminocarbonyl)pyridinio]methoxy)methyl]-2-[(hydroxyimino)methyl]pyridinium (HI-6, 500 mg/dog) were injected intramuscularly 1 min after soman poisoning. The LD50 value of soman was determined to be 9.1 microg/kg, and high doses (> or = 1.4 LD50) of soman induced salivation, emesis, defecation and diarrhea, tremors and seizures, and recumbency of dogs, leading to 100% mortality in 24 h. The prophylactic patch, which led to mean 18.5-18.8% inhibition of blood cholinesterase activity by physostigmine and mean 7.9-8.3 ng/ml of blood concentration of procyclidine, exerted a high protection ratio (4.7 LD50), in comparison with relatively-low effects of traditional antidotes, atropine plus 2-pralidoxime (2.5 LD50) and atropine plus HI-6 (2.7 LD50). Noteworthy, a synergistic increase in the protection ratio was achieved by the combination of the patch with atropine plus HI-6 (9 LD50), but not with atropine plus 2-pralidoxime (5 LD50). In addition, the patch system markedly attenuated the cholinergic signs and seizures induced by soman, especially when combined with atropine plus HI-6, leading to elimination of brain injuries and physical incapacitation up to 6 LD50 of soman poisoning. Taken together, it is suggested that the patch system containing physostigmine and procyclidine, especially in combination with atropine and HI-6, could be a choice for the quality survival from nerve-agent poisoning.     

Evaluation of antidotal effects of adamantyl derivative Tamorf in soman poisoning

The nerve agent soman, a powerful inhibitor of acetylcholinesterase (AChE; EC 3.1.1.7), causes an array of toxic effects in the central nervous system. Adamantyl tenocyclidine derivative Tamorf (1-[2-(2-thienyl)-2-adamantyl] morpholine), a compound with potential activity at the N-methyl-D-aspartate (NMDA) receptors and with neuroprotective properties, is effective against convulsions and brain lesions related to soman poisoning. The objective of this study was to evaluate the antidotal potency of Tamorf (2.5 mg kg(-1)), which was tested alone as a pretreatment or in combination with atropine (10.0 mg kg(-1)) as a therapy in rats poisoned with two different sub-lethal doses of soman (1/4 and 1/2 of LD50). The effect of Tamorf was compared with carbamate physostigmine (0.1 mg kg(-1)). The study also determined the possible genotoxic effects of Tamorf and physostigmine, especially primary DNA damage in white blood cells, liver and brain tissue. Tamorf administered 5 min before poisoning stopped soman-induced seizures, was successful against sub-lethal doses of soman and protected AChE activity in the brain (P = 0.0014, P = 0.0019), and in plasma (P = 0.0464, P = 0.0405). Compared with Tamorf, physostigmine was slightly effective in the elimination of soman-induced poisoning in rats. The pharmacological effect of Tamorf and atropine was less effective as therapy, but did not increase soman toxicity (P > 0.05 for all interactions). The results obtained indicate that Tamorf and physostigmine are not genotoxic to rats in the concentrations tested. Treatment with Tamorf seems to be a good alternative for current pretreatment in soman poisoning. Its antidotal mechanism is complex and is based on combined biochemical and receptor properties.     

The Efficacy of Monopyridinium (2-PAAM, 2-PAEM) and Bispyridinium (Obidoxime,HI-6) Oximes against Mevinphos in Mice

Abstract: The efficacy of two new monopyridinium oximes (2-PAAM, 2-PAEM) and two bispyridinium oximes (obidoxime. HI-6) was tested in combination with atropine sulphate against acute poisoning with the organophosphorus insecticide mevinphos in mice. When mice were treated two min. after mevinphos poisoning, no significant differences in the therapeutic effect of tested oximes were observed. The oximes increased the 24 hr LD₅₀ values of mevinphos about two times in comparison with the 24 hr LD₅₀ values of mevinphos in mice protected with atropine sulphate alone and more than three times in comparison with non-treated intoxicated animals. On the other hand, both monopyridinium oximes were significantly more efficacious than HI-6 and as efficacious as obidoxime when they were administered 30 sec. after mevinphos poisoning. Both monopyridinium oximes and obidoxime increased the 24 hr values of mevinphos almost three times in comparison with the 24 hr values of mevinphos in mice protected with atropine sulphate alone and about twenty-five times in comparison with non-treated intoxicated animals, while the oxime HI-6 less than two times in comparison with the 24 hr values of mevinphos in mice protected with atropine sulphate alone and about fifteen times in comparison with non-treated intoxicated animals. (change-para-here) Use of new monopyridinium oximes seems to be the improvement in the antidotal treatment of poisoning with organophosphorus insecticide mevinphos in comparison with HI-6 but not in comparison with obidoxime when oximes are used in equimolar doses.     

A Comparison of the Potency of Newly Developed Oximes (K074, K075) and Currently Available Oximes (Obidoxime,HI-6) to Counteract Soman-Induced Neurotoxicity in Rats

The neuroprotective effects of newly developed oximes (K074, K075) and currently available oximes (obidoxime, HI-6) in combination with atropine in rats poisoned with soman were studied. The soman-induced neurotoxicity was monitored using a functional observational battery at 24 h and 7 days after soman challenge. The results indicate that the oxime HI-6 combined with atropine seems to be an effective antidote for a decrease in soman-induced neurotoxicity, whereas the ability of both newly developed oximes (K074, K075) as well as obidoxime to counteract soman-induced acute neurotoxicity is negligible. Due to the absence of their neuroprotective potency, both newly developed oximes are not suitable oximes for antidotal treatment after exposure to soman. The oxime HI-6 is still the best acetylcholinesterase reactivator for the antidotal treatment of acute poisonings with soman.     

Studies on the role of central catecholaminergic mechanisms in the antidotal effect of the oxime HI 6 in soman poisoned mice

The effects of atropine and the oxime HI 6 on running performance, brain and plasma cholinesterase activity and brain catecholamines were investigated in mice intoxicated with sublethal doses of soman (100 g/kg s.c.). The running time on a rotating mash wire drum (total running time 60 min) after injection of soman was reduced to 17.2 min. Treatment with atropine (10 mg/kg i.p.) or HI 6 (55 mg/kg i.p.) improved the running peformance to 48.2 and 44.8 min, respectively. Cholinesterase activity was decreased in soman poisoned mice to 47.3% in plasma and 43.5% in brain. Therapy with the oxime HI 6 resulted in a reactivation of soman-inhibited peripheral cholinesterase to 76.6%, but failed to reactivate central cholinesterase. Dopamine levels in mice brain were elevated in soman poisoning by 23.2%, whereas noradrenaline levels remained unchanged. The increase in brain dopamine levels was antagonized by atropine as well as by HI 6. The results of this study lead to the speculation that central dopaminergic mechanisms may be involved in soman toxicity as well as in the antidotal action of atropine and the mainly peripherally acting oxime HI 6.     

A comparison of the potency of newly developed oximes (K074, K075) and currently available oximes (obidoxime, HI-6) to counteract soman-induced neurotoxicity in rats

The neuroprotective effects of newly developed oximes (K074, K075) and currently available oximes (obidoxime, HI-6) in combination with atropine in rats poisoned with soman were studied. The soman-induced neurotoxicity was monitored using a functional observational battery at 24 h and 7 days after soman challenge. The results indicate that the oxime HI-6 combined with atropine seems to be an effective antidote for a decrease in soman-induced neurotoxicity, whereas the ability of both newly developed oximes (K074, K075) as well as obidoxime to counteract soman-induced acute neurotoxicity is negligible. Due to the absence of their neuroprotective potency, both newly developed oximes are not suitable oximes for antidotal treatment after exposure to soman. The oxime HI-6 is still the best acetylcholinesterase reactivator for the antidotal treatment of acute poisonings with soman.     

HI-6, an oxime which is an effective antidote of soman poisoning: A structure-activity study

In contrast to other organophosphates, soman poisoning is resistant to atropine (AT) + oxime therapy. However, new bispyridinium-type oximes + AT are effective antidotes of soman poisoning. In structure-activity studies, T 4925 (1,1′-[oxybis(methylene)]bispyridinium dichloride) had an LD₅₀ of 178 mg/kg and HS-14 (1-[[[pyridinio]methoxy]methyl]-2[(hydroxyiminio)methyl]pyridinium dichloride) had an LD₅₀ of 130 mg/kg. DL-10 (1-[[[3-(aminocarbonyl)pyridinio]methoxy]methyl]pyridinium dichloride) and DL-11 (1-[[[4-(aminocarbonyl)pyridinio]methoxy]methyl]pyridinium dichloride) had LD₅₀'s of 326 and 715 mg/kg, respectively, whereas HS-6 (Reg. No. 22625-23-6) and HI-6 (Reg. No. 34433-31-3) had LD₅₀s of 316 and 514 mg/kg (ip), respectively. T 4925, HS-14, DL-10, and DL-11 at an LD₁ dose + AT (17.4 mg/kg) were relatively ineffective against soman (540 μg/kg; sc) compared to HI-6 and HS-6. Prophylactic ED₅₀s for HI-6 and HS-6 vs soman were 17 and 110 mg/kg, producing safety ratios of 30 and 2.9, respectively. The therapeutic ED₅₀ for HI-6 was 20 mg/kg. Brain cholinesterase (ChE) in mice surviving soman (540 μg/kg; sc) + HI-6 (94 mg/kg) + AT was 0% of control activity at 1 and 2 hr, 5% at 24 hr, and 15% 48 hr later. Mice which died postsoman (540 μg/kg; sc) had 20% brain ChE activity. These results show that HI-6 was one of the least toxic and most efficacious compounds studied and suggest brain ChE inhibition was not the primary lesion in soman poisoning.     

Neuroprotective effects of currently used antidotes in tabun-poisoned rats

The neuroprotective effects of antidotes (atropine, pralidoxime/atropine, obidoxime/atropine and HI-6/atropine mixtures) on rats poisoned with tabun at a lethal dose (220 microg/kg intramuscularly; 100% of LD50 value) were studied. The tabun-induced neurotoxicity was monitored using a functional observational battery and an automatic measurement of motor activity. The neurotoxicity of tabun was monitored at 24 hr and 7 days after tabun challenge. The results indicate that atropine alone is not able to protect the rats from the lethal effects of tabun. Three non-treated tabun-poisoned rats and one tabun-poisoned rat treated with atropine alone died within 24 hr. On the other hand, atropine combined with all tested oximes allows all tabun-poisoned rats to survive at least 7 days following tabun challenge. Obidoxime combined with atropine seems to be the most effective antidotal treatment for the elimination of tabun-induced neurotoxicity in the case of lethal poisoning among tested antidotal mixtures. The antidotal mixture consisting of atropine and HI-6 is significantly less effective than the combination of atropine with obidoxime in the elimination of tabun-induced neurotoxicity in rats at 24 hr following tabun challenge. Pralidoxime in combination with atropine appears to be practically ineffective to decrease tabun-induced neurotoxicity at 24 hours as well as 7 days following tabun poisoning. Due to its neuroprotective effects, obidoxime seems to be the most effective and most suitable oxime for the antidotal treatment of acute tabun exposure among currently used oximes. Thus, the replacement of obidoxime by a more effective acetylcholinesterase reactivator for soman poisoning, the oxime HI-6, can to a small extent diminish the neuroprotective efficacy of antidotal treatment in the case of acute tabun poisonings.     

Protection of guinea pigs against soman poisoning with ferrocene carbamate

The protective effect of ferrocene carbamate pretreatment against soman poisoning was studied in guinea pigs. At doses corresponding to 1/20 x and 1/10 × LD₅₀ of this carbamate a 20% and 45% decrease of the acetylcholinesterase in blood and brain, respectively, was obtained. In combination with additional pretreatment, diazepam, and therapy, HI-6 and atropine, the protective ratios (LD₅₀ of soman in treated animals/LD₅₀ of soman in untreated animals) were around 20 and 40, respectively. Animals pretreated with the high dose of the ferrocene carbamate that survived 10 x and 15xLD_50s of soman showed no remaining signs of poisoning after 24 h. Thus, the ferrocene carbamate afforded a better protection against soman than physostigmine. The explanation for this could be due to the properties of the ferrocene carbamate, not correlated to its cholinesterase inhibiting activity. This hypothesis is discussed.     

Evaluation of reactivation test in anaesthetized dogs with experimental intoxication with nerve agents

Following repeated antidotal treatment of anaesthetized dogs (1 min with atropine, 10 min with atropine and obidoxime, 60 min with atropine and obidoxime) after the intoxication with soman, sarin and VX (1 x LD50, i.m.), the blood cholinesterases (erythrocyte, whole blood, plasma) were monitored and their reactivatability (whole blood) was determined. During this treatment, the activities of erythrocyte acetylcholinesterase (AChE), plasma butyrylcholinesterase (BuChE) and whole blood cholinesterases were monitored. Atropine and obidoxime did not affect cholinesterase activities in control animals, whereas administration of obidoxime to dogs intoxicated with nerve agent caused an increase in the cholinesterase activities. The sensitivity of cholinesterases decreased in the order erythrocyte AChE > whole blood cholinesterases > plasma BuChE, respectively. Following sarin intoxication, blood cholinesterases were increased after the obidoxime administration. Intoxication with VX showed a similar picture but reactivation after the obidoxime administration was greater. In soman intoxication, the picture of cholinesterase changes was similar during the first 30 min of treatment. Then the increase in AChE activity following obidoxime administration was not as high as in the case of sarin and VX intoxication. Thus, the reactivation efficacy of obidoxime during nerve agent intoxication indicates that its repeated administration could be easily monitored using the reactivation test.     

Delayed neuropathy by the organophosphorus nerve agents soman and tabun

The organophosphorus nerve agents soman and tabun were tested in the hen at doses 120–150 times higher than their acute LD₅₀, as it was assumed that these doses would produce delayed neuropathy. The animals were protected against the acute lethal effect of these agents by pretreatment with atropine, physostigmine, diazepam, and the oxime HI-6 or obidoxime.The surviving animals were followed for 30 days and the occurrence of delayed neuropathy was clinically diagnosed. Soman produced severe delayed neuropathy at a dose of 1.5 mg/kg, a dose which produced acute lethality in five animals out of six. Tabun elicited very mild neuropathic symptoms in one animal out of two at a dose of 6 mg/kg given on 2 consecutive days. Delayed neuropathy was not seen in the hens that survived the acute toxicity of a single dose of tabun, 12 mg/kg (three out of six) or 15 mg/kg (two out of six).     

Treatment of tabun poisoned guinea-pigs with atropine,HLö 7 or HI 6: effect on respiratory and circulatory function

The oxime HI 6 (in combination with atropine) is considered to be an effective antidote in soman intoxication but was shown to be less effective in tabun poisoning. In contrast to HI 6, first in vitro studies with HLö 7 demonstrated a reasonable reactivating potency at acetylcholinesterase (AChE) inhibited by soman and tabun. Therefore, the therapeutic efficacy of HLö 7, HI 6 and obidoxime (with and without atropine) was compared in tabun poisoned guinea-pigs. In addition, the therapeutic effect of atropine in guinea-pigs poisoned by various doses of tabun was investigated. Female Pirbright-white guinea-pigs were anaesthetized with urethane (1.8 g/kg) and the carotid artery, jugular vein and trachea were cannulated. After baseline measurements the animals received tabun, 60, 180 or 300 μg/kg, and 2 min later the antidotes (all i.v.): obidoxime, HLö 7, or HI 6 (30 or 100 μmol/kg, each) or atropine 10 mg/kg or a combination of atropine and one of the oximes. Respiratory and circulatory parameters were recorded for 60 min or until the death of the animal. Erythrocyte, brain and diaphragm AChE activity was determined in every animal after the experiment. Poisoning by tabun resulted in a rapid deterioration of respiratory function and respiratory arrest within 5 min. Atropine treatment was very effective in improving the respiratory function after tabun 60 μg/kg but was ineffective after tabun 300 μg/kg. However, circulatory parameters were restored almost completely in all atropine therapy groups. Therapy of tabun 300 μg/kg poisoned animals with atropine plus oxime (30 μmol/kg) improved respiration to a variable extent and restored circulation. The efficacy decreased in the order obidoxime>HLö 7> >HI 6. Use of oximes 100 μmol/kg did not further increase the therapeutic effect. Oximes alone were completely ineffective. The considerable therapeutic efficacy of atropine and oximes was not accompanied by a reactivation of diaphragm or brain AChE. Erythrocyte AChE was partially reactivated by obidoxime. Tabun primarily impaired central respiratory control but peripheral neuromuscular block developed already at low tabun doses. Atropine was very effective in restoring circulation but respiration was improved only after low doses of tabun. The results of this investigation demonstrate a considerable effect of atropine plus obidoxime or HLö 7 in improving respiration and circulation after high dose tabun. This effect was not accompanied by a noticeable AChE reactivation, indicating the involvement of some other “direct” mechanisms. HLö 7 has to be considered as a broad-spectrum antidote, being superior to obidoxime or HI 6.