Soman Poisoning Research Articles

Protection of Guinea Pigs against Soman Poisoning by Pretreatment withp-Nitrophenyl Phosphoramidates

Severalp-nitrophenyl phosphoramidates with the general formula RO(NH2)P(O)OC6H4-p-NO2, in which R = CH2CH3,CH2CH2F, CH2CHF2, CH2CF3, CH2CH2CH2CH3, and CH2CH2Cl, as well as (NH2)2P(O)OC6H4-p-NO2were administered intravenously to guinea pigs as pretreatment compounds for protection against the lethal effects of 1,2,2-trimethylpropyl methylphosphonofluoridate (soman) poisoning. Administration of phosphoramidates at a dose that produces 30% acetylcholinesterase (AChE) inhibition in the blood of atropinized guinea pigs at the moment of soman poisoning increases the subcutaneous LD50 of soman up to almost fivefold depending on which compound was used. A synergistic effect with atropine was observed. Three of these compounds offered a higher degree of protection against soman poisoning than pyridostigmine. Furthermore, the surviving animals pretreated with the two phosphoramidates that provided the highest protective ratio were in a better condition at 24 hr after soman intoxication than those pretreated with pyridostigmine. Due to the limited number of compounds and their different characteristics, it appeared difficult to demonstrate a relationship between the rate of spontaneous reactivation of AChE inhibited by the pretreatment compound and the protection against soman. Nevertheless, the results indicate that a several-fold decrease in the rate of spontaneous reactivation relative to that of pyridostigmine may increase the protective ratio against soman.

Imidazolium and quinuclidinium oximes as antidotes in soman poisoning

New synthesized oximes derivatives of imidazole and quinuclidines were tested in vitro using human erythrocyte acetylcholinesterase (AChE) inhibited by soman and in vivo using soman poisoned mice. The inhibitory power of oximes (IC 50), acute toxicity (LD 50) as well as reactivating and protecitve capacities with respect to soman-inhibted AChE were tested for each of the synthesized oximes. Derivatives of imidazoles demonstrated mostly weak reactivating and protective characteristics, both in vitro and in vivo. Only BMR-3 oxime was powerful reactivator of soman inhibte human AChe (55 %). BMR-4 oxime given together with atropine sulfate, provided a good in vivo protection against 1.8 and 2.2 x LD 50 of soman. On the contrary, all tested quinuclidne compounds are protective agents against soman in vivo (PP-1, PP-2, PP-3, PP-4 protect against 2-2.5 x LD 50 of soman). Bm-1 oxime has the best protection against soman inhibition of the AChE of all tested compounds (protect against 4 LD 50 of soman). Quinuclidines in vitro activity as reactivators of soman inhibited AChE, and their protective power against soman inhibition of AChE in vitro are negligible. The results indicate that in vivo effectiveness of quinuclidine oximes against soman poisoning is not related to their reactivatin or protective potentials for AChE; their good protective effect is more likely to be related to other mechanisms of the cholinergic system.

Imidazolium and quinuclidinium oximes as antidotes in soman poisoning

Imidazolium and quinuclidinium oximes as antidotes in soman poisoning

Comparison of the efficacy of single or repeated HI-6 treatment following soman poisoning in guinea pigs and marmoset

The therapeutic efficacy of single or repeated doses of HI-6, together with atropine, against soman poisoning were compared both in guinea pigs and in marmoset monkeys. In addition, the pharmacokinetics of HI-6 were determined after single or repeated injections. Both single and repeated HI-6 injections protected guinea pigs effectively against 2 × LD 50 soman. The plasma levels of HI-6 after single HI-6 injection fitted a one-compartment elimination model. The plasma levels of HI-6 following repeated injections were in accordance with those predicted using the data obtained after single HI-6 injection. No evidence was found for any disturbance of the HI-6 elimination in guinea pigs following soman intoxication. Marmosets were intoxicated with 2 × LD 50 soman (s.c.), followed after l min by i.m. injections of atropine and HI-6. One and 2 h later, four animals received additional HI-6 injections. The pharmacokinetics of HI-6 in plasma, after single and repeated HI-6 injections were similar to those found in the guinea pig. Furthermore, repeated HI-6 injections protected effectively against soman: four out of four animals survived, in fair condition. In contrast, only one out of four animals receiving single HI-6-treatment fully recovered within a few days. Two animals died, the fourth animal survived, but had to be euthanized 3 weeks after intoxication.

The role of nitric oxide in soman-induced seizures, neuropathology, and lethality

The effects of the inhibitors of endothelial and neuronal nitric oxide (NO) synthases, N-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), respectively, and the precursor of NO, glyceryl trinitrate, on soman-induced seizures, lethality, and neuropathology were studied in rats. It was found that pretreatment of rats with L-NAME and 7-NI potentiated the severity of motor convulsions and enhanced lethality produced by soman. On the other hand, glyceryl trinitrate, administered transdermally at doses ranging from 2.5–5 mg/day 1 day before soman, decreased seizure susceptibility and lethality in soman-intoxicated animals. This was accompanied by a subsequent reduction of central neuronal damage 24 h after soman treatment. Pretreatment with glyceryl trinitrate also reversed seizure latency produced by 7-NI treatment during soman intoxication. These results indicate that neuronal NO may play a prominent role in seizures by acting as an anticonvulsant and neuroprotectant in soman intoxication.

Measurement of the oxime HI-6 after peripheral administration in tandem with neurotransmitter levels in striatal dialysates: Effects of soman intoxication

In the present study, the technique of microdialysis combined with tandem high-performance liquid chromatography was used to determine the striatal levels of HI-6 and neurotransmitters following peripheral administration of HI-6 (50 mg/kg i.m.) in conscious, freely moving rats. The results were compared with those obtained in animals given soman (135 μg/kg i.p.) 1 min before HI-6 (50 mg/kg i.m.). Principal component analysis was used to study the effects of the different treatments on neurotransmitters and signs of poisoning. In all animals given HI-6, maximum levels of HI-6 appeared in the second 20-min fraction after administration of HI-6, then gradually declined, reaching the lower limits of detection after 3 hr. There was a correlation between severity of poisoning and neurochemical changes observed; dopamine and GABA levels increased as the severity of signs of poisoning increased. These results clearly demonstrate that HI-6 can penetrate into the brain of control and soman-intoxicated animals. Tandem measurement of dopamine electrochemically and HI-6 by UV detection provides a simple method for obtaining data on HI-6 penetration into the brain in neurochemical studies of soman poisoning and its treatment.

The influence of pharmacological pretreatment on efficacy of HI-6 oxime in combination with benactyzine in soman poisoning in rats.

1. The influence of pharmacological pretreatment (pyridostigmine, benactyzine and trihexyphenidyle), designated PANPAL, on soman-induced cholinergic and stressogenic effects as well as on the efficacy of antidotal treatment (HI-6 plus obidoxime) in rats was studied. 2. PANPAL prophylaxis significantly decreased soman-induced cholinesterase inhibition in blood, brain and diaphragm as well as stressogenic effects of soman (an increase in plasma corticosterone level and liver tyrosine aminotransferase activity). 3. PANPAL pretreatment did not improve the efficacy of HI-6 in combination with benactyzine on soman-induced anticholinesterase and stressogenic effects. 4. These findings confirm that PANPAL prophylaxis can improve prognosis of soman poisoning especially by protection of cholinesterases.

Early regional changes of GFAP mRNA in rat hippocampus and dentate gyrus during somaninduced seizures

We investigated the time course of GFAP levels in the hippocampal formation during the first 24 h following soman intoxication in rats. GFAP mRNA expression was detected by in situ hybridization. Intense GFAP mRNA expression was present in the molecular layer of the dentate gyrus as early as 6 h after intoxication. This expression was comparatively lower in other dentate gyrus layers and hippocampal CA1, CA3 and CA4 areas and seemed to be related to excessive neuronal activity. Histopathological examination demonstrated that GFAP expression in dentate gyrus is not correlated with lesions. The high astrocytic reactivity in the molecular layer of the dentate gyrus is discussed in relation to the maintenance of the homeostasis of glutamate and of synaptic plasticity in this area during soman intoxication.

Treatment of organophosphate poisoning in pigs: antidote administration by a new binary autoinjector.

The therapeutic effectiveness of a new binary autoinjector containing 500 mg HI-6 and 2 mg atropine sulphate was tested in anesthetized pigs poisoned by a lethal dose of soman i.v. (9 micrograms/kg per 20 min). Pharmacokinetics and pharmacodynamics of HI-6 were studied concomitantly on administration of HI-6 alone, together with atropine sulphate, or together with atropine sulphate during soman intoxication. Cardiopulmonary parameters were monitored and serum concentrations of oxime and acetylcholinesterase (AChE) were measured in blood samples taken at intervals over a 6-h period postinjection. Five minutes after the start of soman infusion, mean AChE activity was decreased to 27 +/- 4.3% of baseline and signs of poisoning appeared. The antidotes, HI-6 and atropine sulphate, were then administered i.m. One minute after this injection there was a transient significant increase in AChE activity of 76 +/- 8.2% of baseline (p < 0.01). It then again decreased and remained suppressed throughout the experiment. Mean respiratory rate was significantly decreased (p < 0.01) to 20 +/- 3.2% of baseline after 20 min of soman infusion and remained low during the rest of the experiment. The poisoning signs were counteracted 15-20 min after antidote therapy and all pigs survived soman intoxication without ventilatory assistance. Administration of either atropine or atropine and soman had no significant effect on the pharmacokinetics of HI-6 in anesthetized pigs.

Comparison of efficacy of two oximes (HI-6 and obidoxime) in soman poisoning in rats

The action of HI-6 (1-[[[(4-aminocarbonyl)pyridiniol]methoxy]methyl]-2-[(hydroxyimino)methyl]pyridinium dichloride monohydrate) and obidoxime on soman-induced anticholinesterase and stressogenic effects was studied in rats. HI-6 significantly affected acetylcholinesterase inhibition in erythrocytes, brain and diaphragm and practically eliminated the stressogenic effects of soman, i.e. an increase in plasma corticosterone level and liver tyrosine aminotransferase activity, while obidoxime, on the other hand, had very little influence on soman-induced inhibition of acetylcholinesterase activity and the stressogenic effects of soman. These findings support a hypothesis that the effects of HI-6 are not solely due to reactivation of the enzyme. They also demonstrate its importance in the treatment of soman poisoning in rats.

Prophylactic transdermal treatment with physostigmine and scopolamine against soman intoxication in guinea-pigs.

This study was designed to evaluate the prophylactic efficacy of transdermally administered physostigmine (PHY) against soman exposure using guinea-pigs. Transdermal PHY pad (3 cm2 kg-1; 60 micrograms cm-2), containing a vehicle based on propionic acid, was applied onto the dorsal back of the animals, 24 h before exposure to the organophosphate. At the time of exposure, PHY concentrations in brain and plasma were ca. 3.6 ng g-1 and 4.1 ng ml-1, respectively. Brain and whole blood cholinesterase (ChE) activity was inhibited to 70% and 47% of the original activity, respectively. Transdermal PHY by itself protected up to 70% of the animals exposed to 1.5 LD50 of soman (100% mortality was recorded in the control group). Combining transdermal PHY with Scopoderm provided full protection against 1.5 LD50 of soman (protection of 70% against 3 LD50). When the prophylactic treatment was combined with post-exposure therapy (atropine, 10 mg kg-1; toxogonin, 10 mg kg-1) 1 min after 5 LD50 of soman, protection of 90% of the animals was achieved.

Modulation of soman-induced neuropathology with an anticonvulsant regimen.

Rat hippocampus and piriform cortex were examined for pathological changes 48 hours after exposure to a convulsant dose of soman. Animals were treated with a low dose of atropine just after soman and were then injected, after 10 or 40 minutes of seizures, with both the anticonvulsant drugs NBQX and TCP. Atropine given alone counteracted the extensive neuronal loss due to soman in both areas without prevention of neuronal suffering. Comparatively, the complete anticonvulsant regimen, given before 40 minutes of seizures, totally prevented hippocampal soman-induced neuropathology. Neurones of piriform cortex were still suffering whatever the time of injection of the drugs. This emphasizes the need for a rapid and definitive anticonvulsant treatment just after soman intoxication to block the subsequent neurotoxic effect of nerve-agent exposure.

The role of nitric oxide in the neuropathology in soman intoxication

Intoxication with organophosphorus (0P) anticholinesterase agents such as soman triggers irreversible lesions in some cerebral areas. Administration of soman at the LD 50 leads to an increased activity of NADPH-diaphorase (= NO-synthase) in the cerebral endothelial cells from the 6th hour after poisoning. This activity culminates after 24 h, whereas variations in this enzymatic activity are not easily detectable in NADPH-diaphorase positive neurons. Since soman triggers astrocytic oedema leading to a possible decrease in the local cerebral blood flow, it is likely that the induction of endothelial NO-synthase exerts an antagonistic effect, since NO is a vasodilator.

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 micrograms/kg, and 2 min later the antidotes (all i.v.): obidoxine, HLö 7, or HI 6 (30 or 100 mumol/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 micrograms/kg but was ineffective after tabun 300 micrograms/kg. However, circulatory parameters were restored almost completely in all atropine therapy groups. Therapy of tabun 300 microns/kg poisoned animals with atropine plus oxime (30 micromol/kg) improved respiration to a variable extent and restored circulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromodacryorrhea in Rats: Absence Following Soman Poisoning

Chromodacryorrhea is the secretion of so-called "bloody tears" from the harderian gland which nearly circumscribes the eye within the bony orbit. Direct-acting cholinergic agonists such as oxotremorine, carbachol, and pilocarpine caused chromodacryorrhea but nicotine did not. Atropine blocked chromodacryorrhea induced by systemic administration of direct-acting cholinergic agonists. Thus, chromodacryorrhea appears to be a muscarinic receptor-related event. Soman (pinacolyl methylphosphonofluoridate), a potent irreversible inhibitor of acetylcholinesterase which increases the synaptic concentration of the neurotransmitter acetylcholine, did not induce chromodacryorrhea in rats. Similarly, physostigmine, a tertiary, carbamate acetylcholinesterase inhibitor, did not induce chromodacryorrhea. In vivo soman-induced inhibition of harderian gland acetylcholinesterase was independent of the soman dose and the inhibition was significantly less than brain acetylcholinesterase. In vitro soman-induced inhibition of harderian gland acetylcholinesterase was not significantly different from that of diaphragm acetylcholinesterase. The lack of inhibition of acetylcholinesterase in the harderian gland does not appear to be due to a difference in sensitivity to inhibition by soman. The distribution of the various molecular forms of acetylcholinesterase between the diaphragm and harderian gland was different. There was a great deal more of the 4S form of acetylcholinesterase in the harderian gland than in the diaphragm. The lack of the following, inhibition of harderian gland acetylcholinesterase and elevation of the synaptic concentration of acetylcholine, could explain the absence of chromodacryorrhea following soman poisoning. The discrepancy between the significant soman induced inhibition of brain acetylcholinesterase and the lack of inhibition of harderian gland acetylcholinesterase allows one to speculate that there may be a very efficient scavenger of soman present in the rat harderian gland.

Huperzine A as a pretreatment candidate drug against nerve agent toxicity

Huperzine A (HUP) is a naturally-occuring, potent, reversible inhibitor of acetylcholinesterase (AChE) that crosses the blood-brain barrier. To examine its ability to protect against nerve agent poisoning, HUP was administered i.p. to mice, and the s.c. LD 50 of soman was determined at various time intervals after pretreatment. Results were compared to those obtained for animals treated with physostigmine. A protective ratio of approximately 2 was maintained for at least 6 hr after a single injection of HUP, without the need for any post-challenge drug therapy. By contrast, pretreatment with physostigmine increased the LD 50 of soman by 1.4- to 1.5-fold for only up to 90 min. The long-lasting antidotal efficacy displayed by HUP correlated with the time course of the blood-AChE inhibition. The results suggest that the protection of animals by HUP from soman poisoning was achieved by temporarily sequestering the active site region of the physiologically important AChE.

REM sleep pathways and anticholinesterase intoxication: A mechanism for nerve agent-induced, central respiratory failure

The mechanism of death following exposure to anticholinesterases, such as the highly toxic nerve agents soman and VX, and other organophosphate anticholinesterases such as the insecticide parathion, remains unclear, although evidence from nerve agent research suggests that death occurs by an atropine blockable respiratory failure mediated through mechanisms involving the central nervous system. It is proposed that REM sleep pathways, which can be triggered by acetylcholine accumulation in the pontomedullar reticular field, mediate respiratory failure through the inhibition of respiratory muscles. Cholinergic activation of REM sleep activities may also account for other physiological and behavioural effects that follow exposure to nerve agents. These include forebrain activation, which is associated with EEG desynchronization and seizures, locomotor depression with concomitant loss of righting reflex, and limb jerks and extensions. Pharmacologic evidence for atropine and clonidine protection against soman intoxication effects is entirely consistent with a scenario of cholinergic receptor activation in the pontomedullar reticular field.

Ion channel blockade by oximes and recovery of diaphragm muscle from soman poisoning in vitro

1. The actions of oximes and related compounds on the nicotinic acetylcholine receptor ion channel at the adult mouse muscle endplate were investigated by use of single-channel recording techniques. The aim of the study was to determine whether the channel-blocking properties of the compounds could contribute to their therapeutic effectiveness against soman poisoning in vitro. 2. Therapeutic effectiveness was assessed in guinea-pig phrenic nerve-hemidiaphragm preparations by measuring the degree of recovery of neuromuscular function produced by the compounds following poisoning by soman. A number of the compounds, including some which lacked the oxime group, produced a significant recovery of neuromuscular function which was unrelated to acetylcholinesterase (AChE) reactivation; this was reversed by washing off the compound, and was therefore attributed to a direct pharmacological action on the muscle. 3. Single channel recordings showed that some of the compounds blocked open nicotinic receptor ion channels in preparations of mouse muscle fibres. The compounds which showed the greatest direct pharmacological actions in diaphragms produce a very fast, flickering blockade of the channels. Several quantitative measures of channel-blocking activity correlated very well with the direct pharmacological action. Furthermore, for two compounds studied in greater detail, the direct action and channel-blocking showed similar concentration-response relationships. 4. The results of this study indicate that the direct pharmacological action of oximes and their analogues against neuromuscular blockade by soman in vitro is due to their channel-blocking activity. The direct action does not correlate well with protection against soman poisoning in vivo, however, which suggests that additional non-reactivating properties of these compounds, at sites other than the neuromuscular junction, may also be important for their therapeutic effectiveness.

Effects of anticholinergic-antiparkinsonian drugs on striatal neurotransmitter levels of rats intoxicated with soman

Antimuscarinic drugs possessing antiparkinson activity that were effective in preventing convulsions induced by the organophosphorus cholinesterase (ChE) inhibitor soman were studied for their effects on spinal cord ChE activity and striatal levels of acetylcholine (ACh) and catecholamines in soman-intoxicated rats. Either biperiden (BPR) or trihexyphenidyl (THP) was administered to rats at an anticonvulsant dose (0.125 mg/kg, IM) in the presence or absence of soman (100 μg/kg, SC). The time course (up to 2 h) for ChE activity and levels of ACh and catecholamines were measured after soman, BPR, THP, soman and BPR, or soman and THP treatment. Soman rapidly inhibited ChE activity (65–75%; 15–120 min) and increased ACh levels (35%; at 30 min). It did not affect norepinephrine or dopamine (DA), but elevated at later time points (60–120 min) levels of the DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), thus indicating increased DA turnover. BPR and THP alone reduced striatal ACh level from control, but did not affect any other neurochemical parameters studied. THP and BPR each reversed the effects of soman on DOPAC and HVA levels, but neither affected ChE activity nor ACh level induced by soman. Thus, our findings suggest that the anticonvulsant effects of BPR and THP in soman poisoning may be attributed to their earlier reported muscarinic receptor blocking properties.

Efficacy of antidotes against soman poisoning in female physostigmine-protected rats.

Female rats have been found much more sensitive to lethal effects of soman than male rats. Therefore it was of interest to examine the efficacy of different antidotes against soman poisoning in females which are usually not being used in soman poisoning studies. The effects of acetylcholinesterase (AChE) non-reactivating antidotes atropine and diazepam were analyzed in combination with physostigmine prophylaxis against supralethal doses of soman. Physostigmine prophylaxis was much more effective when supplemented by atropine and diazepam therapy, applied at the onset of the first signs of poisoning. The interval between the injection of a supralethal dose of soman and the appearance of signs of poisoning was shorter in physostigmine pretreated animals than in non-pretreated controls poisoned with the same supralethal dose of soman. The prophylactic effect of physostigmine (used at maximal dose) disappeared in about 120 min. The addition of HI-6, an AChE-reactivating oxime, to atropine + diazepam therapy further increased the survival in soman-poisoned and physostigmine-pretreated rats, yielding the highest protective ratio of 6.4. Pretreatment with physostigmine offered marked protection against inhibition of AChE by soman, as shown by enzyme activity determination in different brain regions and in diaphragm muscle. Application of HI-6 in addition to the combination of the above mentioned antidotes even preserved more AChE activity in the skeletal muscle but did not influence inhibition of the enzyme in brain.