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Anticholinesterases, such as organophosphorus pesticides and warfare nerve agents, present a significant health threat. Onset of symptoms after exposure can be rapid, requiring quick-acting, efficacious therapy to mitigate the effects. The goal of the current study was to identify the safest antidote with the highest therapeutic index (TI = oxime 24-hr LD50/oxime ED50) from a panel of four oximes deemed most efficacious in a previous study. The oximes tested were pralidoxime chloride (2-PAM Cl), MMB4 DMS, HLö-7 DMS, and obidoxime Cl2. The 24-hr median lethal dose (LD50) for the four by intramuscular (IM) injection and the median effective dose (ED50) were determined. In the ED50 study, male guinea pigs clipped of hair received 2x LD50 topical challenges of undiluted Russian VX (VR), VX, or phorate oxon (PHO) and, at the onset of cholinergic signs, IM therapy of atropine (0.4 mg/kg) and varying levels of oxime. Survival was assessed at 3 hr after onset clinical signs. The 3-hr 90th percentile dose (ED90) for each oxime was compared to the guinea pig pre-hospital human-equivalent dose of 2-PAM Cl, 149 µmol/kg. The TI was calculated for each OP/oxime combination. Against VR, MMB4 DMS had a higher TI than HLö-7 DMS, whereas 2-PAM Cl and obidoxime Cl2 were ineffective. Against VX, MMB4 DMS > HLö-7 DMS > 2-PAM Cl > obidoxime Cl2. Against PHO, all performed better than 2-PAM Cl. MMB4 DMS was the most effective oxime as it was the only oxime with ED90 < 149 µmol/kg against all three topical OPs tested.

Introduction: The ability of currently available reactivators to reactivate cyclosarin is low. The aim of this study was to determine the reactivating and therapeutic efficacy of newly developed oximes (K206, K269) compared with currently available oximes against cyclosarin., Methods: Rats and mice received atropine or atropine + oxime intramuscularly (i.m.) before or after an i.m. dose of cyclosarin. Acetylcholine activity levels in blood and tissues were measured to calculate the reactivation efficacy and potency., Results and Discussion: In vivo determined percentage of reactivation of cyclosarin-inhibited blood and tissue acetylcholinesterase (AChE) in poisoned rats showed that the potency of both newly developed oximes (K206, K269) to reactivate cyclosarin-inhibited AChE is comparable with that of obidoxime in blood and diaphragm, but slightly higher than that of obidoxime in brain. Their reactivating efficacy is significantly lower compared with that of the oxime HI-6. K206 and K269 are relatively effective in reducing cyclosarin-induced lethal toxic effects in mice. Their therapeutic efficacies exceed the therapeutic potency of obidoxime but not that of HI-6., Conclusions: K206 and K269 are as effective in the reactivation of cyclosarin-inhibited AChE in rats and in the reduction of lethal toxic effects of cyclosarin in mice as obidoxime, but because their reactivating and therapeutic potency is significantly lower than that of HI-6, they are not suitable replacements for the currently available oximes for the treatment of cyclosarin poisoning.

The potency of newly developed oximes (K074, K075) and commonly used oximes (obidoxime, trimedoxime, and HI-6) to counteract tabun or cyclosarin-induced acute toxic effects was studied in mice. The therapeutic efficacy of trimedoxime and both newly developed oximes (K074, K075) was significantly higher than the potency of obidoxime and the oxime HI-6 in the case of acute tabun poisonings. On the other hand, the oxime HI-6 was significantly more efficacious than other studied oximes when mice were intoxicated with cyclosarin. The findings support the hypothesis that the therapeutic efficacy of oximes depends on the type of nerve agent. Due to their therapeutic efficacy, both newly developed K oximes can be considered to be promising oximes for the antidotal treatment of acute tabun poisonings, while the oxime HI-6 is still the most promising oxime for the treatment of acute cyclosarin poisonings due to its high potency to counteract cyclosarin-induced acute toxic effects.

It was shown that intoxications with GF-agent are rather resistant to convential oxime therapy; therefore, the development of new oximes in an effort to improve this unsatisfactory situation continues. Upon screening in vitro reactivation test for oximes, that were either newly synthesized at our department, or those that have never been tested for reactivation of GF-inhibited acetylcholinesterase (AChE), three oximes {(1,4-bis(4-hydroxyiminomethylpyridinium)butane dibromide) (K033); (1-(2-hydroxyiminomethylpyridinium)-3-(3-carbamoylpyridinium)-2-oxa-propane dichloride) (HS-6); and (1-(2-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium)-but-2-ene dibromide) (BI-6)} with the highest reactivation potency were chosen for in vivo testing in our study. 1,3-Bis(4-hydroxyiminomethylpyridinium)-2-oxa-propane dibromide) (obidoxime); (1-(2-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium)-2-oxa-propane dichloride) (HI-6); and (1,1-bis(4-hydroxyiminomethylpyridinium)-methane dibromide) (methoxime) were chosen for comparison as a standard antidotal treatment. All the oximes were applied at the same proportion of their LD50 value (5%), and because of the different acute toxicity of the oximes, the molar concentrations of their solutions for intramuscular (i.m.) administration were considerably different. The highest therapeutic ratio was achieved for therapeutic regimen consisting of HI-6 and atropine. The significantly (P < 0.05) lowest effectivity in treatment of supralethal GF-agent poisoning in comparison with all the other therapeutic regimens, was surprisingly observed for methoxime. HS-6, K033 and BI-6 as well as obidoxime were comparably effective antidotes against GF-agent intoxication and their therapeutic ratios were similar.

This report presents a non-lethal method for estimating a range of therapeutic doses of bisquaternary oximes that serve as antidotes against organophosphorus poisoning. We have estimated therapeutic oxime doses that are equivalent in their relative toxicity rather than selecting arbitrary fractions of their LD(50). Thus, toxic signs of the oximes HI-6, HLö-7, Toxogonin, AB-8 and AB-13 were monitored quantitatively in baboon monkeys and beagle dogs. Using Toxogonin as a reference oxime, a calculated unit of equivalent dose (CED) was defined as the oxime dose equal to the ratio between its minimal toxic dose (MTD) and the therapeutic ratio (TR) of Toxogonin i.e. CED = MTD/TR. Assuming that the tails of dose-response curves of toxicity for bisquaternary oximes are shallow and similar to one another, one could substitute the ED(10) for the MTD. The ED(10) values for bisquaternary oximes were estimated using the log-log model following experimental observations and quantitative scoring of toxic signs in dogs and monkeys. The MTD values then were calculated using the ED(10) values and the experimental therapeutic dose of the reference oxime Toxogonin. The following CED values were obtained for AB-8, AB-13, Toxogonin, HI-6 and HLö-7 in dogs (d) and monkeys (m): 98.7, 74.2, 30.0, 14.5 and 12.1 (d) and 281.9, 232.1, 41.7, 192.9 and 92.9 (m) micromol kg(-1), respectively. The antidotal efficacy of these oximes against poisoning by the nerve agent tabun was determined in dogs and monkeys. These dose-dependent efficacy data were obtained at 0.3 x CED, 1 x CED and 3 x CED of oximes in combination with atropine. These data provide comparative therapeutic values using oxime doses based on their relative toxicity. The highest antidotal efficacy against tabun in dogs was obtained for toxogonin, whereas HLö-7 and AB-13 were most efficacious in monkeys., (Copyright 2001 John Wiley & Sons, Ltd.)

1. The therapeutic efficacy of various oximes (pralidoxime, obidoxime, methoxime, HI-6, HLö-7, BI-6) against supralethal nerve agent poisoning (soman, sarin, cyclosin) in mice was tested. 2. New oxime BI-6, synthesized in our laboratory, is significantly more efficacious than conventional oximes but a little less efficacious than other H-oximes (HI-6, HLö-7). 3. H-oximes (HI-6, HLö-7) seem to be the most efficacious reactivators of nerve agent-inhibited acetylcholinesterase for antidotal treatment of supralethal nerve agent poisoning in mice.

Various oximes (PAM, toxogonin, TMB-4, HS-6, HI-6, HGG-12, HGG-42) combined with atropine were compared as antidotes of soman, sarin and tabun poisoning in non-fasted CD-1 male mice. TMB-4 was the most toxic oxime with an i.p. LD50 value of 80 mg/kg and HI-6 was the least toxic oxime with an i.p. LD50 of 588 mg/kg. Upon comparing ED50 values, HGG-42 was the most effective oxime versus soman and tabun poisoning whereas, HI-6 was the most effective oxime versus sarin poisoning. Further research needs to be done to explain the distinct differences in efficacy of the oximes versus poisoning by soman, sarin or tabun.

Soman is a highly toxic organophosphorus chemical warfare nerve agent which is characterized by (1) extremely rapid ageing of the phosphonylated enzyme, (2) poor reactivation of inhibited AChE due to a steric factor, (3) pronounced CNS effects, and (4) tentative direct toxic biochemical effects. By studies of Soman and its thiocholine-like analog (which yield the same type of phosphonylated enzyme), it has been established that (1) the steric factor is at least as responsible as ageing in the failure of oximes to reactivate effectively AChE inhibited by Soman, (2) that its dealkylation is catalyzed by the anionic site of the enzyme, and (3) that the velocity of reactivation and ageing of AChE is dependent on the orientation of the phosphonyl group at the enzyme surface. It has been found that PiMeP-Cl (O-pinacolyl methylphosphonochloridate) may serve as a good model in the evaluation of Soman toxicity and in the selection of adequate oximes in the treatment of its poisoning. The opposite effects of TMB-4 and HI-6, as group representatives, on PiMeP-Cl toxicity in mice (strong potentiation by TMB-4 and antagonism by HI-6) were mainly ascribed to the rates of decomposition of their corresponding O-pinacolyl methylphosphonylated products formed in vivo. They are considered to be slow with potentiators and instantaneous with antagonists, respectively. This assumption was confirmed by the finding that the most powerful oximes in Soman poisoned mice were HI-6, HGG-42 and BDB-27, which, contrary to TMB-4, possess an oxime group in position 2 and the CH2-O-CH2 linking chain. The remarkable influence of diazepam and sodium n-dipropylacetate on the survival time in Soman poisoned rats treated by atropine and bis-pyridinium oximes points to their antagonistic action at the biochemical level (decrease of elevated cGMP in CNS), not possessed by atropine. Essential antidotes in experimental treatment of Soman poisoning today are the powerful reactivators of Soman-inhibited AChE (e.g. HI-6, HGG-42 and BDB-27) and atropine. The treatment may be further improved by the use of symptomatic agents capable of counteracting biochemical changes in Soman poisoning not antagonized by atropine (e.g. diazepam), and theoretically for now, by retardation of ageing and by overcoming steric disturbances.

HI-6 was the least toxic and the most efficacious oxime examined against Soman poisoning with a high safety ratio between 26-30. Reactivation of peripheral acetylcholinesterase following Soman poisoning was more important in the beneficial therapeutic action of HI-6 than reactivation of central acetylcholinesterase. HI-6 reactivated Sarin-inhibited but not Tabun-inhibited acetylcholinesterase both peripherally and centrally. HI-6 passes the blood brain barrier as evidenced by its reactivation centrally of Sarin-inhibited acetylcholinesterase. Soman-inhibited enzyme was not aged in vivo by 30 min. In vivo diaphragm acetylcholinesterase was inhibited to a greater extent by Soman, Sarin and Tabun than intercostal muscle acetylcholinesterase. In vitro diaphragm and intercostal muscle acetylcholinesterase had similar IC50 values for Soman. HI-6 has antimuscarinic and antinicotinic activity in addition to its previously reported ganglion blocking activity (Lundy and Tremblay, 1979). These additional pharmacological actions of HI-6 may play a role in the therapeutic action of HI-6 (at the higher concentrations). The results suggest that peripheral acetylcholinesterase in the rat diaphragm is the primary lesion in Soman poisoning. The beneficial action of HI-6 in rats versus Soman poisoning is due to reactivation of diaphragm acetylcholinesterase.

The bisquaternary mono-oximes HS-6 and HI-6 may both be considered as potential therapeutic agents for soman intoxication. It has been found that HI-6 was superior to an equal dose of /S-6 in the treatment of soman intoxication in mice and rats. Not only did HI-6 protect against higher levels of soman, but also fewer "delayed deaths" were seen following HI-6. In anesthetized atropinized rats, the administration of HI-6 resulted in a longer duration of spontaneous breathing and a higher degree of recovery of neuromuscular function than did the use of HS-6. HS-6 interacted with soman to alter the rate of excretion of the oxime. This problem was not encountered when HI-6 was used.