Inhibitor Report for: Tabun

Organophosphorus nerve agents (OPNAs) are highly toxic compounds inhibiting cholinergic enzymes in the central and autonomic nervous systems and neuromuscular junctions, causing severe intoxications in humans. Medical countermeasures and efficient decontamination solutions are needed to counteract the toxicity of a wide spectrum of harmful OPNAs including G, V and Novichok agents. Here, we describe the use of engineered OPNA-degrading enzymes for the degradation of various toxic agents including insecticides, a series of OPNA surrogates, as well as real chemical warfare agents (cyclosarin, sarin, soman, tabun, VX, A230, A232, A234). We demonstrate that only two enzymes can degrade most of these molecules at high concentrations (25 mM) in less than 5 min. Using surface assays adapted from NATO AEP-65 guidelines, we further show that enzyme-based solutions can decontaminate 97.6% and 99.4% of 10 gm(-)(2) of soman- and VX-contaminated surfaces, respectively. Finally, we demonstrate that these enzymes can degrade ethyl-paraoxon down to sub-inhibitory concentrations of acetylcholinesterase, confirming their efficacy from high to micromolar doses.

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

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.

Organophosphorus nerve agents (OPNAs) are highly toxic compounds inhibiting cholinergic enzymes in the central and autonomic nervous systems and neuromuscular junctions, causing severe intoxications in humans. Medical countermeasures and efficient decontamination solutions are needed to counteract the toxicity of a wide spectrum of harmful OPNAs including G, V and Novichok agents. Here, we describe the use of engineered OPNA-degrading enzymes for the degradation of various toxic agents including insecticides, a series of OPNA surrogates, as well as real chemical warfare agents (cyclosarin, sarin, soman, tabun, VX, A230, A232, A234). We demonstrate that only two enzymes can degrade most of these molecules at high concentrations (25 mM) in less than 5 min. Using surface assays adapted from NATO AEP-65 guidelines, we further show that enzyme-based solutions can decontaminate 97.6% and 99.4% of 10 gm(-)(2) of soman- and VX-contaminated surfaces, respectively. Finally, we demonstrate that these enzymes can degrade ethyl-paraoxon down to sub-inhibitory concentrations of acetylcholinesterase, confirming their efficacy from high to micromolar doses.

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

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.

Male NMRI mice were used to test the influence of orally administered prophylactic mean PANPAL on acute toxic effects of tabun and to compare its efficacy with the commonly used pyridostigmine by means of protective ratio evaluation. The effect of antidotal treatment of acute tabun poisoning which depends on the choice of anticholinergic drug or the addition of anticonvulsive drug was compared in the same way. Then the reactivating efficacy of selected oximes was tested on Wistar rats using evaluation of the percentage of tabun-inhibited acetylchollinesterase reactivation in blood, the diaphragm and in the brain of poisoned rats 30 minutes after poisoning and antidotal treatment. We also used the rats to test the influence of pharmacological prophylaxis and the choice of a suitable reactivator for tabun-induced neurotoxic symptoms with the help of functional observatory battery.The results confirm that obidoxime seems to be the most suitable acetylcholinesterase reactivator while the oxime HI-6, which is very effective against soman, is practically ineffective against tabun. Trimedoxim also appears to be a prospective acetylcholinesterase reactivator for the antidotal treatment of tabun poisoning. In order to achieve the sufficient effectiveness of antidotal treatment of acute tabun poisonings, the commonly used anticholinergic drug atropine should be replaced by a cholinolytic drug with pronounced central effects (biperiden, benactyzine, scopolamine). If atropine is used for the antidotal treatment of tabun poisoning, the addition of an anticonvulsive drug such as diazepam is suitable. If PANPAL is administered in case of tabun exposure threat, the resistance of the exposed organism as well as the efficacy of postexposure antidotal treatment are significantly increased. Unlike PANPAL, pyridostigmine is not practically effective against acute toxic effects of tabun.

Male NMRI mice were used to test the effect of single or repeated antidotal treatment with various oximes (pralidoxime, obidoxime, trimedoxime, the oxime HI-6) in combination with an anticholinergic drug atropine on the acute toxicity of organophosphorus tabun compound with the help of evaluating medial lethal (LD50) dose at a 24-hour survival of tabun-poisoned experimental animals.If some of the tested oximes in combination with atropine were administered repeatedly during acute tabun intoxication, a slight increase in the LD50 value was observed compared to a single administration when pralidoxime or obidoxime was used as an acetylcholinesterase reactivator. This means that the repeated administration of observed antidotal mixtures does not bring a significant improvement in the efficacy of antidotal treatment of acute tabun poisoning. The comparison of the therapeutic effects of tested oximes shows that trimedoxime seems to be the most suitable oxime for decreasing the acute tabun toxicity.The results confirm that no currently used oxime is sufficiently effective for eliminating acute tabun toxicity. Trimedoxime appears to be a prospective acetylcholinesterase reactivator for the antidotal treatment of tabun poisoning. The repeated administration of antidotes during acute tabun poisoning does not show a significant increase in the therapeutical efficacy of antidotal treatment of acute tabun poisoning

The treatment of poisoning by highly toxic organophosphorus compounds (nerve agents) is unsatisfactory. Until now, the efficacy of new potential antidotes has primarily been evaluated in animals. However, the extrapolation of these results to humans is hampered by species differences. Since oximes are believed to act primarily through reactivation of inhibited acetylcholinesterase (AChE) and erythrocyte AChE is regarded to be a good marker for the synaptic enzyme, the reactivating potency can be investigated with human erythrocyte AChE in vitro. The present study was undertaken to evaluate the ability of various oximes at concentrations therapeutically relevant in humans to reactivate human erythrocyte AChE inhibited by different nerve agents. Isolated human erythrocyte AChE was inhibited with soman, sarin, cyclosarin, tabun or VX for 30 min and reactivated in the absence of inhibitory activity over 5-60 min by obidoxime, pralidoxime, HI 6 or HL 7 (10 and 30 microM). The AChE activity was determined photometrically. The reactivation of human AChE by oximes was dependent on the organophosphate used. After soman, sarin, cyclosarin, or VX the reactivating potency decreased in the order HL 7 > HI 6 > obidoxime > pralidoxime. Obidoxime and pralidoxime were weak reactivators of cyclosarin-inhibited AChE. Only obidoxime and HL 7 reactivated tabun-inhibited AChE partially (20%), while pralidoxime and HI 6 were almost ineffective (5%). Therefore, HL 7 may serve as a broad-spectrum reactivator in nerve agent poisoning at doses therapeutically relevant in humans.

The reaction of human erythrocyte acetylcholinesterase (AChE) with a set of structurally related phosphoramidates was studied in order to investigate the properties of phosphorylated enzyme and the effects of 4 oximes PAM-2, TMB-4(Trimedoxime), HI-6 and BDB-106 on the reactivation of inhibited AChE. Second-order rate constant of the phosphorylation reaction of the compounds towards the active site of AChE range between 5.0 x 10(2) and 4.9 x 10(6) M-1min-1 and their inhibitory power (I50) was from 7.3 x 10(-5) to 5.7 x 10(-9) M for 20 min incubation at 37 degrees C. The oximes used were weak reactivators of inhibited AChE except for (C4H9O)(NH2)P(O)DCP (DCP, -O-2,5-dichlorphenyl group) and (C6H13O)(NH2)P(O)SCH3 where we have obtained good reactivation. Imidazole oxime BDB-106 proved to be a potent reactivator of tabun-inhibited AChE.

Organophosphorus compounds can cause two distinct toxic effects: acute, which are the consequence of acetylcholinesterase (AChE) inhibition and delayed neuropathy being inhibited by inhibition of neuropathy target esterase (NTE) with first signs (ataxia, paralysis) appearing 7-20 days after intoxication. The purpose of this study was to examine interaction of tabun with AChE and NTE and potential neuropathic effects of the compound in vivo. Tabun was more potent inhibitor reacting with more affinity with AChE than NTE of hen brain. The rate of aging of tabun-inhibited AChE was slow (t/2 = 50 hours) while it occurred very rapidly on tabun-inhibited NTE (t/2 = 6.5. min). Experiments in vivo have shown that even a high dose of tabun (12 mg/kg, 120 LD50), given with antidotes, which inhibited 67% of NTE activity did not initiate delayed neuropathic effects. It is concluded that there appears to be no risk for development of delayed neuropathy in tabun poisonings.

Male Sprague-Dawley rats when administered sc a sublethal dose of organophosphorus cholinesterase inhibitors such as the nerve agents, soman (100 micrograms/kg, sc), sarin (110 micrograms/kg, sc), tabun (200 micrograms/kg, sc), or VX (12 micrograms/kg, sc), developed seizures and severe muscle fasciculations within 15-20 min, lasting for 4-6 hr. Marked inhibition of acetylcholinesterase (AChE) and necrotic lesions in skeletal muscles such as soleus, extensor digitorum longus, and diaphragm were evident between 1-24 hr following injection. Pretreatment with memantine HCl (MEM, 18 mg/kg, sc) together with atropine sulfate (ATS, 16 mg/kg, sc), 60 min and 15 min, respectively, prior to nerve agents attenuated AChE inhibition, prevented myonecrosis, and muscle fasciculations as well as other signs of cholinergic toxicity. Pretreatment combining d-tubocurarine (d-TC, 0.075 mg/kg, sc) and ATS (16 mg/kg, sc) prevented the myonecrosis and fasciculation without protecting AChE against inhibition by these nerve agents. Neither MEM, d-TC, nor ATS in the concentration given interfered with the normal behavior of the rats. The role of d-TC and ATS interaction with presynaptic receptors regulating ACh release and MEM's role in modulating neural hyperactivity as protective mechanisms are discussed.

The neuropathic potential of acute and repeated exposures of the phosphoramidates tabun (GA) and isofenphos (IFP), of diisopropyl fluorophosphate (DFP) and paraoxon (PO) were examined in the hen with treatments for up to 90 days via intramuscular injections of the highest tolerated doses with atropine protection. Plasma acetylcholinesterase (AChE), non-specific butyrylcholinesterase (BChE) and creatine kinase (CK) activities were measured in order to monitor whether the compounds were present at biologically active concentrations. Locomotor behavior was observed and tissues from the peripheral and central nervous systems were examined for signs of organophosphate-induced delayed neuropathy (OPIDN). No behavioral or histological evidence of OPIDN was observed after treatments with GA, IFP, PO, saline or atropine sulfate. DFP-treated birds displayed locomotor and neuropathological signs of OPIDN with a no effect level (NOEL) between 25 and 50 micrograms/kg.

A pretreatment for organophosphorus (OP) anticholinesterase (e.g., soman) intoxication should prevent lethality and convulsions (CNV) at 2 LD50s and be behavioral-decrement-free when given alone. Behavioral-deficit-free pretreatment regimens (PRGs) for guinea pigs consisted of Physostigmine (0.15 mg/kg, im) and adjunct. Adjuncts [mg/kg, im] tested were akineton [0.25], aprophen [8], trihexyphenidyl [2], atropine [16], azaprophen [5], benactyzine [1.25], cogentin [4], dextromethorphan [7.5], ethopropazine [12], kemadrin [1], memantine [5], promethazine [5], scopolamine [0.08] and vontrol [2]. PRGs were given 30 min before soman (60 micrograms/kg, sc; 2 LD50s) or other OP agents. Animals were then observed and graded for signs of intoxication, including CNV at 7 time points and at 24 hr. Physostigmine alone reduced the incidence of CNV and lethality induced by 2 LD50s of soman by 42 and 60%, respectively. All of the PRGs tested abolished lethality and 12 shortened recovery time to 2 hr or less. Also, PRGs including azaprophen or atropine prevented CNV. When selected PRGs were tested against intoxication by sarin, tabun or VX, the efficacy was generally superior to that for soman. The data show that several PRGs are effective against soman intoxication in guinea pigs.

Acetylcholine esterase inhibitors block cholinergic neurotransmission. This blockade can be reversed by oximes. However, a universally effective esterase reactivator does not exist. A new H-oxime, HL 7, was tested on rat diaphragm strips. Electrically evoked contractions were blocked by di-2-propyl fluorophosphate (DFP), tabun, sarin and soman. Whereas pralidoxime, obidoxime and HI 6 reversed the blockade induced by three of these organophosphorus compounds, HL 7 restored the contractions after short blockade induced by all four organophosphorus compounds tested.

The M2 subtype of muscarinic receptor is predominant in heart, and such receptors were reported to be located in muscles as well as in presynaptic cholinergic and adrenergic nerve terminals. Muscarinic receptors of rat heart were identified by the high affinity binding of the agonist (+)-[3H]cis-methyldioxolane ([3H]CD), which has been used to label a high affinity population of M2 receptors. A single population of sites (KD 2.74 nM; Bmax of 82 fmol/mg protein) was detected and [3H]CD binding was sensitive to the M2 antagonist himbacine but much less so to pirenzepine, the M1 antagonist. These cardiac receptors had different sensitivities to NiCl2 and N-ethylmaleimide from brain muscarinic receptors, that were also labeled with [3H]CD and considered to be of the M2 subtype. Up to 70% of the [3H]CD-labeled cardiac receptors had high affinities for several organophosphate (OP) anticholinesterases. [3H]CD binding was inhibited by the nerve agents soman, VX, sarin, and tabun, with K0.5 values of 0.8, 2, 20, and 50 nM, respectively. It was also inhibited by echothiophate and paraoxon with K0.5 values of 100 and 300 nM, respectively. The apparent competitive nature of inhibition of [3H]CD binding by both sarin and paraoxon suggests that the OPs bind to the acetylcholine binding site of the muscarinic receptor. Other OP insecticides had lower potencies, inhibiting less than 50% of 5 nM [3H]CD binding by 1 microM of EPN, coumaphos, dioxathion, dichlorvos, or chlorpyriphos. There was poor correlation between the potencies of the OPs in reversibly inhibiting [3H]CD binding, and their anticholinesterase activities and toxicities. Acetylcholinesterases are the primary targets for these OP compounds because of the irreversible nature of their inhibition, which results in building of acetylcholine concentrations that activate muscarinic and nicotinic receptors and desensitize them, thereby inhibiting respiration. Nevertheless, the high affinities that cardiac muscarinic receptors have for these toxicants point to their extra vulnerability. It is suggested that the success of iv administration of the muscarinic receptor inhibitor atropine in initial therapy of poisoning by OP anticholinesterases may be related in part to the extra sensitivity of M2 receptors to certain OPs.

Using a microtiter plate spectrophotometric system, an assay procedure was developed for the following toxic organophosphorus compounds: 1,2,2-trimethylpropyl ester of methylphosphonofluoridic acid (1, soman); ethyl N,N-dimethylphosphoramidocyanidate (3, tabun); O-ethyl S-[2-[bis(1-methylethyl)amino]ethyl]- methylphosphonothiolate (4, VX); the diethyl 4-nitrophenyl ester of phosphoric acid (5, paraoxon); and bis(1-methylethyl) phosphorofluoridate (6, DFP). The procedure, based on the Ellman assay method, uses inhibition of eel acetylcholinesterase (0.01 unit per well) to carry out the determination of inhibitor concentrations for both a standard curve and the unknown samples on a single 96-well microtiter plate. On a typical plate, samples of both unknowns and standards (a minimum of six concentrations were used per standard curve) were assayed five times per sample, with three control (uninhibited) enzyme activity points included for each sample. The time required for carrying out a single plate was approx 30 min. Sensitivity for the most potent acetylcholinesterase inhibitor tested was 0.4 nM under the conditions used for a typical assay. It should be noted, however, that no attempt was made to optimize the assay procedure for sensitivity.

The LD50s and ED50s for inhibition of acetylcholinesterase (AChE) in whole mouse brain by DFP (diisopropylfluorophosphate), sarin (methylphosphonofluoridic acid 1-methyl ethyl ester), soman (methylphosphonofluoridic acid 1,2,2-trimethyl propyl ester), and tabun (dimethylphosphoramidocyanidic acid ethyl ester) were compared after iv administration. The LD50s of DFP, sarin, soman, and tabun in ICR (Institute for Cancer Research) mice were 3.40, 0.109, 0.042, and 0.287 mg/kg, respectively. The recovery of AChE activity in whole mouse brain after sub-LD50 doses of these agents was slow and did not reach control values by 14 d after iv administration. AChE activity was inhibited in a dose-dependent manner in whole mouse brain, as well as in six brain regions (cortex, hippocampus, striatum, midbrain, medulla-pons, and cerebellum). None of these brain areas appeared to be particularly sensitive to AChE inhibition. The ED50s for DFP, sarin, soman, and tabun for inhibition of AChE in whole mouse brain were approximately 19, 38, 69, and 66% of their respective LD50s. Because of the differential potencies between lethality and inhibition of AChE, it is concluded that the lethality of these agents is due to more factors than simply the inhibition of AChE within the brain.

Male Sprague-Dawley rats injected s.c. with an acute non-lethal dose (200 micrograms/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 (less than 3%) and skeletal muscles (less than 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 soleus, 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-diaphragm maximally affected and EDL the least. To determine detoxification of tabun by non-specific binding, the activity of butyrylcholinesterase (BCHE) and carboxylesterase (CarbE) was measured. The inhibition and recovery pattern of BCHE 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 BCHE and CarbE can serve 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.

The effects of low-dose administration of the organophosphate cholinesterase inhibitors, soman, sarin and tabun, on growth rates over 85 days were studied in rats. Acetylcholinesterase (AChE) activity was determined in the striatum and the remainder of the brain 24 hrs following the last exposure to these agents. Further, the cumulative mortality of daily administration of several doses of soman, sarin and tabun for 25 days was studied. The animals treated with 25 micrograms/kg of soman or sarin for 85 days demonstrated reduced growth rates which returned to control levels after 30 days. The animals which received 50 micrograms/kg of sarin also grew at reduced rates which returned to control levels after 35 days, while the tabun-treated (100 micrograms/kg) animals required 38 days to return to control growth rates. The striatal AChE activity of the soman-treated group was reduced to 36% of control while the AChE activities of the high-dose sarin-treated group were reduced to 66% of control. The striatal AChE activity of the tabun-treated group was only 13% of control. It is suggested that growth rates may be used to monitor the development of tolerance to low-dose administration of organophosphate cholinesterase inhibitors.

Acute sc toxicity of soman increased in the order, mice----rats----guinea pigs----dogs, being 12.6 times more toxic to dogs (LD50 = 0.05 mumol/kg) than to mice. It was 2.8 times more toxic than tabun to mice and 35 times more toxic to dogs. HI-6 was the least toxic and had similar toxicity values to the four animal species studied and HGG-12 the most toxic of the three oximes used. HGG-12 has shown the greatest interspecies variation (rats:dogs = 1:19.5). HI-6, HGG-12, and PAM-2 Cl (in conjunction with atropine and diazepam) revealed the best protective effect in soman-poisoned dogs, with the respective protective indices of 9, 6.3, and 3.5, followed by guinea pigs. In tabun poisoning the best, but relatively low, protective effect was found only in guinea pigs. The introduction of diazepam increased the protective effects of atropine-oxime combination in soman and tabun poisoning by 10 to 80%. We suggest that the high toxicity of soman and low toxicity of HI-6 may be anticipated in man. The inefficiency of HI-6, HGG-12, and PAM-2 Cl in tabun poisoning points either to the search of new compounds or to the use of the mixture of the oximes found to be effective against the known chemical warfare nerve agents.

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).