Antidotal action of the oxime HS6 at the soman poisoned neuromuscular junction of the rat and guinea-pig

Protection against nerve agent poisoning by a noncompetitive nicotinic antagonist

The acute toxicity of organophosphorus (OP) nerve agents arises from accumulation of acetylcholine (ACh) and overstimulation of ACh receptors. The mainstay of current pharmacotherapy is the competitive muscarinic antagonist, atropine. Nicotinic antagonists have not been used due to the difficulties of administering a dose of a competitive neuromuscular blocker sufficient to antagonise the effects of excessive ACh, but not so much that it paralyses the muscles. An alternative approach would be to use a noncompetitive antagonist whose effects would not be overcome by increasing ACh concentrations. This study demonstrates that the compound 1,1'-(propane-1,3-diyl)bis(4-tert-butylpyridinium), which blocks open nicotinic ion channels noncompetitively, is able to reverse the neuromuscular paralysis after nerve agent poisoning in vitro and to protect guinea pigs against poisoning by nerve agents when used as part of a therapeutic drug combination including a muscarinic antagonist. In contrast to the oxime HI-6, this compound was equally effective in protecting against poisoning by sarin or tabun. Further studies should identify more effective compounds with this action and optimise doses for protection against nerve agent poisoning in vivo.

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

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

In vitro effects of toxogonin, HI-6 and HLö-7 on the release of [3H]acetylcholine from peripheral cholinergic nerves in rat airway smooth muscle

The purpose of this work was to evaluate the possible non-reactivating effects of toxogonin (1,1'[oxybis(methylene)]bis[4-[hydroxyimino) methyl]pyridinium]-dichloride), HI-6 (1-[[[(4-aminocarbonyl)pyridinio] methoxy]methyl]-2-[(hydroxyimino)methyl]pyridinium-dichloride) and HLö-7 (pyridinium, 1-[[[4-(aminocarbonyl)pyridino]methoxy] methyl]-2,4-bis-[(hydroxyimino)methyl]diiodide) on the release of acetylcholine from cholinergic nerves. The oximes have been tested in our rat bronchial smooth muscle model, with respect to the effects of oximes on the K+ (51 mM)-evoked release of [3H]acetylcholine in the presence and absence of soman (1.0 microM). Toxogonin (100 microM) had no effect on the K(+)-evoked release of [3H]acetylcholine in the presence or absence of soman (1.0 microM). Similar results were found for HI-6 (100 microM). In contrast, HLö-7 (100 microM) enhanced the K(+)-evoked release of [3H]acetylcholine in the absence of soman. In the presence of soman HLö-7 did not alter the release of [3H]acetylcholine induced by K+ stimulation. The potentiating effect of HLö-7 on the release of [3H]acetylcholine could be blocked by the L-, N- and P-Ca2+ channel blockers verapamil (0.1 and 1.0 microM), omega-conotoxin GVIA (1.0 microM) and omega-agatoxin IV-A (0.2 microM), respectively. Muscarinic receptor antagonists (atropine (10 microM), pirenzepine (M1) (1.0 microM) and methoctramine (M2) (1.0 microM) had no effects on the HLö-7 (100 microM)-enhanced release of [3H]acetylcholine. Protein kinase inhibitors (H-7 (20 microM), calphostin C (1.0 microM) and KN-62 (10 microM) inhibited the HLö-7 (100 microM)-enhanced K(+)-evoked release of [3H]acetylcholine. The results showed that only HLö-7 had a direct enhancing effect on the release of acetylcholine through activation or opening of Ca2+ channels and a subsequent protein phosphorylation in the nerve terminal.

Effect of atropine and bispyridinium oximes on respiratory and circulatory function in guinea-pigs poisoned by sarin

During the past decade the oxime HI 6(1-[[[4-(aminocarbonyl)pyridinio]methoxy]methyl]-2- [(hydroxyimino)methyl] pyridinium dichloride) was shown to improve survival in nerve agent poisoning (in combination with atropine). Recent studies indicate, that HLö 7 (1-[[[4-(aminocarbonyl)pyridinio]methoxy]methyl]-2,4-bis [(hydroxyimino)methyl] pyridinium diiodide or dimethanesulfonate) is also an effective antidote in nerve agent poisoning but, with both oximes, data on restoration of respiration and circulation are scarce. The ability of HLö 7 or HI 6 with atropine to improve the respiratory and circulatory function in sarin-poisoned guinea-pigs was therefore investigated. Female Dunkin-Hartley guinea-pigs were anaesthetised with urethane (1.8 g/kg) and the arteria carotis, vena jugularis and trachea were cannulated. After baseline measurements the animals received 100 or 200 micrograms/kg sarin, and 2 min later the antidotes (all i.v.): 10 mg/kg atropine sulfate or a combination of atropine and HLö 7 or HI 6 (30 mumol/kg, each). Respiratory and circulatory parameters were recorded for the whole experimental period of 60 min or until the death of the animal. Brain and diaphragm acetylcholinesterase (AChE) activity was determined in each animal after the experiment. Poisoning by sarin resulted in a rapid respiratory arrest within 5 min. Atropine treatment was only partially effective in improving respiration after 100 micrograms/kg sarin but was ineffective after 200 micrograms/kg sarin. Therapy of sarin-poisoned animals with atropine plus oxime further improved respiration to various extents, restored circulation and increased survival time, HLö 7 being more effective than HI 6. Diaphragm and brain AChE were reactivated by HLö 7 and, to a minor extent, by HI 6. The results of this investigation suggest, that at equimolar doses (30 mumol/kg) the new bispyridinium dioxime HLö 7 has a higher therapeutic efficacy in sarin-poisoned guinea-pigs when compared to HI 6 (both in combination with atropine).

Search for a therapy against soman-intoxication

This mini-review mainly describes a part of the pharmacological research carried out in our laboratory during the past decades, aimed at finding a therapy against intoxication by cholinesterase-inhibiting organophosphates, in particular against the nerve agent soman. In particular soman, because this is one of the nerve agents that consistently appears to be very resistant to treatment. Various experimental approaches are described. Yet, even after all these years of research an adequate (pre)treatment against poisoning by soman is still not available.

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)

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.

Atropine and oxime treatment in lethal soman poisoning of anaesthetized guinea-pigs: HLö 7 dimethanesulfonate versus HI 6 dichloride

The oxime HI 6 is considered to be effective in soman poisoning and less effective in tabun poisoning. Recently, HLö 7 was shown to reactivate acetylcholinesterase (AChE) inhibited by soman and tabun. Therefore, the efficacy of HLö 7 and HI 6 was compared in soman poisoned guinea-pigs. Female Pirbright-white guinea-pigs were anaesthetized with urethane (1.8 g/kg) and the a. carotis, v. jugularis and trachea were cannulated. After base line measurements soman 0.08 mg/kg (= 5 x LD50) or 0.16 mg/kg (= 10 x LD50) was injected intravenously, 2 min. later the antidotes were applied intravenously: HLö 7 0.03 or 0.1 mmol/kg, HI 6 0.03 or 0.1 mmol/kg, atropine 10 mg/kg, or a combination of atropine and an oxime. Respiratory and circulatory parameters were recorded for 60 min. or until the death of the animal. The injection of 5 x LD50 soman resulted in a rapid respiratory arrest followed by circulatory failure in the soman and soman plus oxime groups (survival time about 7 min). Atropine restored the circulatory parameters to base line but was unable to provide a sufficient respiratory function (survival time 26 min.). The combination therapy with atropine plus HLö 7 or HI 6 improved the respiration sufficiently, restored the circulation completely, and prolonged the survival time to about 50 min. Atropine treatment was insufficient in animals poisoned with 10 x LD50 soman. The combination of atropine and HLö 7 or HI 6 improved respiration, circulation, and survival time to various extent. Despite of the striking therapeutic effect no reactivation of erythrocyte AChE by the antidotes was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Protection against the effects of anticholinesterases on the latencies of action potentials in mouse skeletal muscles

1. Adult male albino mice were injected subcutaneously with an organophosphorous anticholinesterase to initiate excessive variability in the latency of indirectly elicited muscle action potentials (jitter) when assessed 5 days later. 2. Pretreatment of the mice with a single dose of pyridostigmine prevented the development of jitter after subsequent dosing with an organophosphate. 3. Treatment with one dose of pralidoxime (2PAM) prevented the development of jitter if given less than 1 h after treatment with ecothiopate, a reactivatable inhibitor of cholinesterase. Similar treatment with 2PAM after a non-reactivatable inhibitor did not prevent the development of jitter. The repeated administration of 2PAM over 12 h did ameliorate jitter. 4. Pretreatment of mice orally with alpha-tocopherol and N-acetylcysteine, known to prevent ecothiopate-induced myopathy, did not prevent the development of jitter after ecothiopate. 5. It is concluded that the development of jitter was a consequence of the inhibition of acetylcholinesterase, and although jitter did not develop acutely, the potential for the full development of jitter was achieved about 1 h after intoxication with ecothiopate. The development of jitter did not involve the generation of free radicals. Reduction of the early effects of intoxication with anticholinesterases by pyridostigmine or 2PAM prevented the development of jitter.

Therapeutic efficacy of HI-6 in soman-poisoned marmoset monkeys

The therapeutic efficacy of the oxime HI-6 against intoxication with the irreversible cholinesterase (ChE) inhibitor soman was tested in marmoset monkeys. Five out of six marmosets, intoxicated with 5 x LD50 soman and treated immediately with diazepam (0.2 mg.kg-1 iv) and 15 sec later with atropine (0.5 mg.kg-1 im) and HI-6 (50 mg.kg-1 im), survived for more than 24 hr. One of these animals died after 4 days. In the HI-6-treated marmosets blood ChE activity was inhibited at a rate slower than that in three animals treated similarly but with saline instead of HI-6. The latter marmosets died within 8 min after soman. HI-6 achieved its plasma peak 5 min after injection and was eliminated with a t1/2 of about 40 min. In a second experiment similarly treated marmosets were euthanized at 5 min (three saline-treated animals) or at 10 min (three HI-6-treated animals) after the soman intoxication to enable the determination of acetylcholinesterase (AChE) activities in diaphragm and brain tissue. In addition, in these animals blood AChE and butyrylcholine esterase (BuChE) activities were determined. Low AChE activities were encountered in diaphragms and brains. These levels were not significantly different between saline- and HI-6-treated marmosets. In vitro treatment with HI-6 at 40 min after soman still led to an increase of the AChE activity, which was significant in diaphragm, suggesting that postmortem AChE inhibition had occurred. The ratio of AChE to BuChE in blood was significantly enhanced in HI-6-treated animals, indicating that HI-6 preferentially reactivated AChE. It is concluded that (i) HI-6 is an effective treatment against soman poisoning in marmosets and (ii) AChE reactivation or protection by HI-6 contributed to the survival of the animals.

Therapy of organophosphate poisoning in the rat by direct effects of oximes unrelated to ChE reactivation

Isolated rat diaphragm preparations treated with soman or with the irreversible and oxime resistant cholinesterase (ChE) inhibitor S27 showed a considerable recovery of neuromuscular transmission (NMT) during incubation with the (bis)pyridinium oximes HI-6, HGG-12, P2S and obidoxime. In the soman-treated preparations this NMT recovery was predominantly caused by reactivation of acetylcholinesterase (AChE) but in the S27-treated preparations it was caused by a direct (pharmacological) effect unrelated to enzyme reactivation. Atropinized rats were artificially ventilated after injection with 3 x LD50 soman for 3 h and then treated with HI-6, i.e. at a time when oxime reactivation of soman inhibited ChE is no longer possible. Nevertheless, these rats started to breathe spontaneously and 50-60% survived more than 24 h, whereas all control animals (saline instead of HI-6) died within 10 min after artificial ventilation was terminated. In such animals no significant reactivation of ChE activity at various time intervals following HI-6 treatment was found, either in the diaphragms or in the brains. There was a significant amount of NMT (50%) in vitro in diaphragms obtained from these animals. This NMT did not improve in vitro in the presence of HI-6 and was not inhibited by soman administered to the medium. It is concluded that in this case the NMT found was based on synaptic adaptation to the continued inhibition of ChE and that the survival of the animals might be due to a combination of this synaptic adaptation and central direct effects of HI-6.

On the mechanism whereby HI-6 improves neuromuscular function after oxime-resistant acetylcholinesterase inhibition and subsequent impairment of neuromuscular transmission

Experiments were performed to elucidate the mechanism of action by which the oxime HI-6 causes a recovery of neuromuscular function after oxime-resistant inhibition of acetylcholinesterase by the organophosphate S27. In the presence of HI-6 (1-3 mM), the ability of isolated rat diaphragm muscle strips to sustain tetanic contractions after inhibition by S27 was markedly improved, as was the electrophysiological response to indirect tetanic stimulation. At lower concentrations (0-1 mM), HI-6 reduced the amplitude of the miniature endplate potentials and their decay time constant in a dose-dependent manner without having any effect on the resting membrane potential. In addition, HI-6 dose dependently increased the quantal content. It is likely that these post- and presynaptic effects of HI-6 are responsible for the improvement of muscle contractions after inhibition of acetylcholinesterase and they could well be of value in the therapy of organophosphate poisoning.

The reactivation by oximes of phosphonylated acetylcholinesterase: the possible erroneous interpretation of reactivating potency

A comparative study of the reactivation by two oximes of acetylcholinesterase inhibited by several organophosphates has been made, with particular reference to the dependence of the degree of reactivation produced by an oxime (reactivating potency) upon the concentration of inhibited enzyme. In the case of one inhibitor it is demonstrated that the relative reactivating potency of the two oximes can be reversed by a change in experimental conditions. It is concluded that the measurement of the reactivation produced by two or more oximes, particularly when carried out under standardized conditions, is of little value in determining their relative reactivating potencies, and of negligible value in predicting their likely therapeutic effectiveness against organophosphate poisoning.

The reversal by oximes and their de oximinomethyl analogues of neuromuscular block produced by soman

A series of oximes and related compounds were assessed for their ability to restore soman-induced neuromuscular block in the isolated diaphragm preparation of the rat, guinea-pig and marmoset. In the rat the bispyridinium oximes HS6, HI6 and HS14 were superior to P2S and all other compounds tested. Conversely, in the guinea-pig, most of the compounds tested produced a good reversal of neuromuscular block. In a limited number of experiments in the marmoset, only a partial reversal of neuromuscular block, was obtained with the oximes HI6 and HS6. The restoration of neuromuscular block was due to one or more of the following factors: (i) enzyme reactivation (ii) direct action (iii) adaptation. The results of this study suggest that both the acetylcholine receptor and the rate of 'ageing' of soman inhibited AChE are different in these three species.

The efficacy of bispyridinium derivatives in the treatment of organophosphonate poisoning in the guinea-pig

The efficacy of a number of bispyridinium compounds, including both oxime and non-oxime derivatives, has been determined against poisoning by sarin, soman, tabun and VX in guinea-pigs receiving various supporting treatments. In conjunction with atropine therapy only, the oximes were effective against sarin and VX poisoning and of them only the 4-substituted oximes were beneficial against tabun poisoning. None of the compounds was effective against poisoning by soman. When the supporting drug treatment consisted of pyridostigmine pretreatment and therapy with atropine and diazepam (this treatment itself gave considerable protection against organophosphate poisoning) both the non-oxime and oxime derivatives increased the protection against all four agents although obidoxime and TMB-4 were not beneficial against soman poisoning. The results are discussed in relation to published studies in which these compounds have been found to be beneficial against soman poisoning in atropine-treated rats and mice. It is suggested that the guinea-pig is a better model for predicting the efficacy of treatments for organophosphate poisoning in primate species.

HI6 as an antidote to soman poisoning in rhesus monkey respiratory muscles in-vitro

This study was carried out to evaluate the rhesus monkey as a model for man with respect to oxime-induced acetylcholinesterase reactivation as a mechanism contributing to restoration of soman poisoned neuromuscular function. In-vitro neuromuscular blockade in intercostal and diaphragm muscle strips of rhesus monkeys was induced by exposing them for 2.5 or 15 min to the cholinesterase inhibitor soman. Subsequent treatment with the oxime HI6 produced only a partial reversal of this blockade. Only a minor part of the recovery obtained could be attributed to enzyme reactivation and suggested that this species responded in a manner closer to that reported in man than other animal species studied.

HI-6: reactivation of central and peripheral acetylcholinesterase following inhibition by soman, sarin and tabun in vivo in the rat

HI-6, ([[[(4-aminocarbonyl)pyridino]methoxy]methyl]-2-] (hydroxyimino)methyl]-pyridinium dichloride), is an oxime which, when combined with atropine, is an extremely effective therapy against organophosphate poisoning. It was found that, following soman (287 micrograms/kg) poisoning, HI-6 reactivated acetylcholinesterase in the diaphragm and intercostal muscles but not in the brain. At a lower dose of soman (110 micrograms/kg), HI-6 reactivated sarin-inhibited acetylcholinesterase in the brain and in the respiratory musculature but did not reactivate tabun-inhibited acetylcholinesterase. It was also found that soman produced a differential inhibition of diaphragm and intercostal muscle acetylcholinesterase in vivo, whereas the in vitro I50 for soman was the same in both areas. HI-6 was capable of reactivating soman-inhibited acetylcholinesterase when administered up to 30 min post-soman, indicating that the rate of aging of the soman-acetylcholinesterase complex is slower than previously reported. The above results suggest that, in severe soman poisoning, the primary lesion occurs in peripheral acetylcholinesterase in the respiratory musculature (specifically the diaphragm).

The efficacy of some bis-pyridinium oximes as antidotes to soman in isolated muscles of several species including man

Previous results had shown that bis-pyridinium oximes, particularly HI-6 are quite effective therapeutically in soman-poisoned rats and mice in vivo and in the rat diaphragm preparation in vitro. The aim of the present study was to investigate the efficacy of bis-pyridinium oximes on soman-inhibited neuromuscular transmission in muscle preparations from several species including man. The muscles tested were preparations of rat diaphragm and intercostal muscle, guinea-pig diaphragm, dog external intercostal muscle and human external interscotal muscle. These muscles were stimulated indirectly with field stimulation. With a few exceptions the preparations were exposed to soman for 2.5 or 15 min. In some cases different exposure times were employed or the organophosphate sarin was administered instead of its analogue soman. After the degree of inhibition of neuromuscular transmission had been established, oximes were added to the bath fluid. After washout 15 min later, recovery of neuromuscular transmission was tested. Subsequently, a second dose of soman was administered to investigate whether the recovery observed had been caused by cholinesterase reactivation. The results of these experiments indicate that the oximes tested, mostly HI-6, were quite effective as soman antidotes in muscle preparations of rats, guinea-pigs and dogs. In the human preparation while these oximes were quite effective after sarin intoxication they were essentially without effect against soman.

Analysis of oxime-induced neuromuscular recovery in guinea pig, rat and man following soman poisoning in vitro

The bispyridinium oximes HS6 and HI6 were tested in vitro for their ability to restore neuromuscular function in soman-poisoned tissue, using diaphragm and intercostal muscle of the rat and guinea pig and intercostal muscle of man. It was found that the oxime-mediated recovery of function in both tissues of the rat and guinea pig was composed of direct oxime actions, AChE reactivation and adaptation. In the human intercostal muscle, however, only adaptation was observed. These findings might suggest that HS6 and HI6 may have only limited value in the treatment of soman poisoning in man. However, recovery of function in rodent tissues was consistently greater in the diaphragm than in the intercostal muscle and, since human diaphragm tissue was not included in this study the therapeutic efficacy of these oximes in this tissue remains unknown.