Oxime-induced reactivation of acetylcholinesterase inhibited by phosphoramidates

Tuning Butyrylcholinesterase Inactivation and Reactivation by Polymer-Based Protein Engineering

Organophosphate nerve agents rapidly inhibit cholinesterases thereby destroying the ability to sustain life. Strong nucleophiles, such as oximes, have been used as therapeutic reactivators of cholinesterase-organophosphate complexes, but suffer from short half-lives and limited efficacy across the broad spectrum of organophosphate nerve agents. Cholinesterases have been used as long-lived therapeutic bioscavengers for unreacted organophosphates with limited success because they react with organophosphate nerve agents with one-to-one stoichiometries. The chemical power of nucleophilic reactivators is coupled to long-lived bioscavengers by designing and synthesizing cholinesterase-polymer-oxime conjugates using atom transfer radical polymerization and azide-alkyne "click" chemistry. Detailed kinetic studies show that butyrylcholinesterase-polymer-oxime activity is dependent on the electrostatic properties of the polymers and the amount of oxime within the conjugate. The covalent coupling of oxime-containing polymers to the surface of butyrylcholinesterase slows the rate of inactivation of paraoxon, a model nerve agent. Furthermore, when the enzyme is covalently inhibited by paraoxon, the covalently attached oxime induced inter- and intramolecular reactivation. Intramolecular reactivation will open the door to the generation of a new class of nerve agent scavengers that couple the speed and selectivity of biology to the ruggedness and simplicity of synthetic chemicals.

The Influence of Anticholinergic Drug and Oxime Selection on the Effectiveness of Antidotal Treatment Against Tabun-Induced Poisoning in Mice

1. The influence of oximes (pralidoxime, obidoxime, HI-6) and anticholinergic drugs (atropine, benactyzine, biperiden, scopolamine) on the effectiveness of antidotal treatment to eliminate tabun-induced lethal effects was studied in mice. 2. Obidoxime seems to be the most efficacious oxime for the elimination of tabun-induced lethal effects in mice, although the difference in the efficacy of obidoxime and HI-6 is not significant when they are combined with atropine. 3. Obidoxime and HI-6 when combined with centrally acting anticholinergic drugs (benactyzine, biperiden and scopolamine) seem to be more efficacious in the elimination of toxic effects of the lethal dose of tabun than their combination with atropine. 4. The findings support the hypothesis that the choice of acetylcholinesterase reactivators as well as the anticholinergic drug selection are important for the effectiveness of antidotal mixture in the case of antidotal treatment of tabun-induced acute poisoning.

Anticholinergic Drugs – Functional Antidotes for the Treatment of Tabun Intoxication

1. To study the influence of antidotes on tabun-induced neurotoxicity, the rats were injected intramuscularly with organophosphate tabun (LD50). The efficacy of choice antidotal treatment consisting of acetylcholinesterase reactivator obidoxime and one of four anticholinergic drugs (atropine, benactyzine, biperiden, scopolamine) was compared. 2. Testing of tabun-induced neurotoxicity progress was carried out using the method Functional observational battery. The experimental animals as well as controls were observed at 24 hours and 7 days following tabun or saline administration. 3. The results were compared to the condition of animals without anticholinergic drug (oxime alone) and control rats that received physiological solution instead of tabun and treatment. Antidotal treatment involving centrally acting anticholinergic drugs (benactyzine, biperiden, scopolamine) showed significantly higher neuroprotective efficacy compared to antidotal treatment containing atropine.

A Comparison of the Potency of the Oxime HLö-7 and Currently Used Oximes (HI-6, Pralidoxime, Obidoxime) to Reactivate Nerve Agent-Inhibited Rat Brain Acetylcholinesterase by in vitro Methods

1. The efficacy of the oxime HLö-7 and currently used oximes (pralidoxime, obidoxime, HI-6) to reactivate acetylcholinesterase inhibited by various nerve agents (sarin, tabun, cyclosarin, VX) was tested by in vitro methods. 2. Both H oximes (HLö-7, HI-6) were found to be more efficacious reactivators of sarin and VX-inhibited acetylcholinesterase than pralidoxime and obidoxime. On the other hand, their potency to reactivate tabun-inhibited acetylcholinesterase is very low and does not reach the reactivating efficacy of obidoxime. In the case of cyclosarin, the oxime HI-6 was only found to be able to sufficiently reactivate cyclosarin-inhibited acetylcholinesterase in vitro. 3. Thus, the oxime HLö-7 does not seem to be more efficacious reactivator of nerve agent-inhibited acetylcholinesterase than HI-6 according to in vitro evaluation of their reactivation potency and, therefore, it is not more suitable to be introduced for antidotal treatment of nerve agent-exposed people than HI-6.

The Influence of the Time of Antidotal Treatment Administration on the Potency of Newly Developed Oximes to Counteract Acute Toxic Effects of Tabun in Mice

1. The influence of the time of administration of antidotal treatment consisting of anticholinergic drug (atropine) and newly developed oxime (K027 or K048) on its effectiveness to eliminate tabun-induced lethal toxic effects was studied in mice. 2. The therapeutic efficacy of antidotal treatment of tabun-induced acute poisoning depends on the time of its administration regardless of the choice of the oxime. 3. Our results show that both oximes studied (K027, K048) are able to sufficiently eliminate lethal effects of tabun. Nevertheless, their efficacy significantly decreases when they were administered 5 min after tabun poisoning. 4. The findings support the hypothesis that both newly developed oximes appear to be suitable oximes to counteract acute toxicity of tabun although their ability to eliminate lethal toxic effects of tabun significantly decreases with prolonged time interval between tabun challenge and antidotal treatment administration.

Comparison of Effects of Different Antidotes on Tabun-Induced Cognitive Impairment in Rats Using Water Maze

In the past, scientists focused on the development of antidotes (mainly anticholinergics in combination with reactivators of inhibited acetylcholinesterase – oximes) to increase the number of surviving nerve agent-intoxicated individuals. Recently, they are interested in antidotes able not only to protect nerve agent-poisoned men from lethal toxic effects but also to improve their life quality by improvement of their central cognitive functions. In our study, the water maze was used to measure spatial working learning and memory in the case of tabun-induced cognitive impairment in albino Wistar rats. Antidotal treatment consisted of atropine alone or a combination of atropine with an oxime (obidoxime, trimedoxime or oxime HI-6). Our results suggest that atropine alone is not sufficient as a treatment for saving cognitive functions impaired by tabun. On the other hand, the addition of oxime to atropine contributes to improvement of cognitive performance in tabun-poisoned rats regardless of type of oxime.

In vivo experimental approach to treatment against tabun poisoning

Organophosphorus compounds pose a potential threat to both military and civilian populations. Since post-exposure therapy has its limitations, our research was focused on the possibility of improving pretreatment in order to limit the toxic effects of tabun. We determined the protective index of various combinations of atropine, oximes (K074, K048, and TMB-4), and pyridostigmine given to mice before tabun intoxication. Although the tested oximes showed very good therapeutic efficacy in tabun-poisoned mice, the given pretreatments improved therapy against tabun poisoning. These regimens ensured survival of all animals up to 25.2 LD(50) of tabun. Our results indicate that even pretreatment with atropine alone is sufficiently effective in enhancing the survival of mice poisoned by multiple doses of tabun, if oxime therapy follows. K048 is our oxime of choice for future research, as it shows better protective and reactivating potency.

Reactivation of immobilized acetylcholinesterase-tabun complex by pralidoxime, its isomers, and homologs

Reactivation efficacy of three homologous and three isomeric series of pralidoxime-type reactivators with aldoxime group in position 2, 3 and 4 of the heterocycle was tested in reactivation of tabun-inhibited AChE. The experiments were performed with immobilized and stabilized porcine brain AChE. The enzyme activity was measured by Ellman method. Reactivation efficacy was determined by measurement of indicator fabric coloration intensity as a measure of AChE activity. Of the studied group of nine reactivators, isomers with the functional group in position 2 were the most effective. The highest value (30 %) for reactivation of inhibited AChE was found for 2PAE after treatment for 15 min at concentration 0.5 mg/cm(3). The efficacy of the isomers decreased in the order ortho > para > meta. No marked effect on the efficacy of the reactivators was observed on prolongation of the reactivation time. The reactivators efficacy decreased with decreasing concentration of their solutions.

A comparison of the neuroprotective efficacy of newly developed oximes (K117, K127) and currently available oxime (obidoxime) in tabun-poisoned rats

The potency of newly developed bispyridinium compounds (K117, K127) to reduce tabun-induced acute neurotoxic signs and symptoms was compared with currently available oxime (obidoxime) using functional observational battery. The neuroprotective effects of atropine alone and atropine combined with one of three bispyridinium oximes (K117, K127, obidoxime) on rats poisoned with tabun at a sublethal dose (180 microg/kg i.m.; 80% of LD(50) value) were studied. Tabun-induced neurotoxicity was monitored using a functional observational battery and automatic measurement of motor activity at 24 h following tabun challenge. The results indicated that all tested oximes combined with atropine enabled tabun-poisoned rats to survive 24 h following tabun challenge while one tabun-poisoned rats died within 24 h after tabun poisoning when the rats were treated with atropine alone. Newly developed oxime K127 combined with atropine was the most effective in decreasing tabun-induced neurotoxicity in the case of sublethal poisonings among all oximes tested. Nevertheless, the differences of neuroprotective efficacy between K127 and obidoxime are not sufficient to replace obidoxime by K127 for the treatment of acute tabun poisonings.

Synthesis, characterization and inhibitory potency of two oxono and thiono analogues of phosphoramidate compounds on acetylcholinesterase

Two novel structurally related phosphoramidate compounds, 1 and 2, with likely beta-diketone system were synthesized and characterized by 1H, 13C, 31P NMR, IR spectroscopy and elemental analysis. Compound 2 exhibited a 31P NMR signal which was significantly shielded (8 ppm) relative to compound 1. Determination of human erythrocyte acetylcholinesterase (hAChE) inhibitory activity was carried out according to Ellman's modified kinetic method and the IC50 values of compounds 1 and 2 were 1.567 and 2.986 mM, respectively. The k(i) values of 1 and 2 were 1.39 to 2.65 min(-1) respectively. A comparison of the bimolecular rate constant (k(i)) and IC50 values for the irreversible inhibitors 1 and 2 revealed that the oxono analogue has greater affinity for hAChE than the thiono compound. Furthermore effects of two conventional oximes paralidoxime (A) and obidoxime (B) on reactivation of the inhibited hAChE were studied but low reactivity was shown by both the oximes.

The evaluation of the neuroprotective effects of bispyridinium oximes in tabun-poisoned rats

Tabun (O-ethyl-N,N-dimethyl phosphoramidocyanidate) belongs to the group of highly toxic organophosphorus compounds that may be used as chemical warfare agents for military as well as terrorist purposes. Tabun differs from other highly toxic organophosphates by the fact that commonly used antidotes are not able adequately to prevent tabun-induced acute toxic effects. The neuroprotective effects of four bispyridinium oximes (K075, trimedoxime, HI-6, obidoxime) in combination with atropine on rats poisoned with tabun at a sublethal dose (150 microg/kg i.m.; 80% of LD50 value) were studied. Tabun-induced neurotoxicity was monitored using a functional observational battery and automatic measurement of motor activity at 24 h and 7 d following tabun challenge. The results indicated that all tested oximes combined with atropine enabled tabun-poisoned rats to survive 7 d following challenge. Trimedoxime combined with atropine was the most effective antidote in decreasing tabun-induced neurotoxicity in the case of sublethal poisonings among all oximes tested. Due to its neuroprotective effects, trimedoxime may be considered to be more suitable oxime for the antidotal treatment of acute tabun exposure than currently used oximes (obidoxime, HI-6) and the newly synthesized oxime K075.

Structure-toxicity relationships of thiono and seleno phosphoramidate compounds: new type of acetylcholinesterase inhibitors

Novel thiono and seleno phosphoramidate compounds with the general formula (X)(Y)P(C(6)H(5))(2); (X = NMe(2) & Y = S, 1a; X = NEt(2) & Y = S, 2a; X = NMe(CH(2)Ph) & Y = S, 3a; X = NH(CH(2)Ph) & Y = S, 4a; X = NEt(CH(2)Ph) & Y = S, 5a; X = N(C(Me)(3)) (CH(2)Ph) & Y = S, 6a; X = N(CH(2)Ph)(2) & Y = S, 7a; X = NMe(2) & Y = Se, 1b; X = NEt(2) & Y = Se, 2b; X = NMe(CH(2)Ph) & Y = Se, 3b; X = NH (CH(2)Ph) & Y = Se, 4b; X = NEt(CH(2)Ph) & Y = Se, 5b; X = N(C(Me)(3))(CH(2)Ph) & Y = Se, 6b and X = N(CH(2)Ph)(2) & Y = Se, 7b) were prepared and characterized by (1)H, (31)P and( 13)C NMR and IR spectroscopy and elemental analysis. (31)P chemical shift of thiono and seleno derivatives didn't show significant different because of their little difference in electronegativity sulfur and selenium. Hydrophobic parameter of compounds was determined by measurement of octanol-water partition coefficient by shake-flask technique. Determination of human erythrocyte acetylcholinesterase (hAChE) activity was carried out according to the Ellman's modified kinetic method. IC(50 )values of the selected thiono and seleno compounds varied from 3.4 to 0.11 and 9.9 to 5.1 mM, respectively. The seleno compounds show lower affinities for hAChE relative to the thino compounds. These results demonstrate that hydrophobic and electronic factors of the organophosphorus compounds play a key role on the inhibitory potency.

Kinetic analysis of reactivation and aging of human acetylcholinesterase inhibited by different phosphoramidates

The high number of fatalities due to poisoning by organophosphorus compound-based (OP) pesticides and the availability of highly toxic OP-type chemical warfare agents (nerve agents) emphasize the necessity for an effective medical treatment. Acute OP toxicity is mainly caused by inhibition of acetylcholinesterase (AChE, EC 3.1.1.7). Reactivators (oximes) of inhibited AChE are a mainstay of treatment. However, human AChE inhibited by certain OP, e.g. the phosphoramidates tabun and fenamiphos, is rather resistant towards reactivation by oximes while AChE inhibited by others, e.g. the phosphoramidate methamidophos is easily reactivated by oximes. To get more insight into a potential structure-activity relationship human AChE was inhibited by 16 different tabun analogues and the time-dependent reactivation by 1mM obidoxime, TMB-4, MMB-4, HI 6 or HLö 7, the reactivation kinetics of obidoxime and the kinetics of aging and spontaneous reactivation were investigated. A clear structure-activity relationship of aging, spontaneous and oxime-induced reactivation kinetics could be determined with AChE inhibited by N-monoalkyl tabun analogues depending on the chain length of the N-alkyl residue. N,N-dialkyl analogues bearing ethyl and n-propyl residues were completely resistant towards reactivation while N,N-di-i-propyl tabun was highly susceptible towards reactivation by oximes. AChE inhibited by phosphonoamidate analogues of tabun, bearing a N,N-dimethyl and N,N-diethyl group, could be reactivated and had comparable reactivation kinetics with obidoxime. These results in conjunction with previous data with organophosphates and organophosphonates emphasizes the necessity for kinetic studies as basis for future work on structural analysis with human AChE and for the development of effective broad-spectrum oximes.

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

The neuroprotective effects of newly developed oximes (K074, K075) and currently available oximes (obidoxime, HI-6) in combination with atropine in rats poisoned with tabun at a sublethal dose (180 micro g/kg i.m.; 80% LD(50)) were studied. The tabun-induced neurotoxicity was monitored using a functional observational battery and an automatic measurement of motor activity. The neurotoxicity of tabun was monitored at 24h and 7 days following tabun challenge. The results indicate that all oximes studied in combination with atropine allow all tabun-poisoned rats to survive within 7 days following tabun challenge while two non-treated tabun-poisoned rats died within 2h. Both newly developed oximes combined with atropine seem to be effective antidotes for a decrease in tabun-induced neurotoxicity in the case of sublethal poisoning although they are not able to eliminate tabun-induced neurotoxicity completely. The oxime K075 showed a higher neuroprotective efficacy against tabun than K074 according to the number of eliminated tabun-induced neurotoxic signs at 24h as well as 7 days after tabun challenge. The neuroprotective efficacy of obidoxime in combination with atropine is similar to the potency of newly developed oxime K075 but the ability of the oxime HI-6 to counteract tabun-induced acute neurotoxicity is significantly lower at 24h as well as 7 days after tabun poisoning. Due to their neuroprotective effects, both newly developed oximes (especially K075) appear to be more suitable oximes for the antidotal treatment of acute tabun poisonings than the oxime HI-6.

A comparison of the efficacy of new asymmetric bispyridinium oximes (K027, K048) with currently available oximes against tabun by in vivo methods

The potency of newly developed asymmetric bispyridinium oximes (K027, K048) in reactivating tabun-inhibited acetylcholinesterase (AChE) and in eliminating tabun-induced acute toxic effects was compared with commonly used oximes (obidoxime, trimedoxime, the oxime HI-6) using in vivo methods. Studies determined the percent of reactivation of tabun-inhibited blood and tissue AChE in poisoned rats and showed that the reactivating efficacy of both newly developed oximes is comparable with obidoxime and trimedoxime, the most efficacious known reactivators of tabun-inhibited AChE. These were also found to be sufficiently efficacious in the elimination of acute lethal toxic effects in tabun-poisoned rats. The oxime HI-6, relatively efficacious against soman, did not seem to be an adequately effective oxime in reactivation of tabun-inhibited AChE and in counteracting acute lethal effects of tabun. In addition, our results confirm that the efficacy of oximes in reactivating tabun-inhibited AChE in blood, diaphragm, and brain correlates with the potency of oximes in protecting rats poisoned with supralethal doses of tabun.

Pretreatment with pyridinium oximes improves antidotal therapy against tabun poisoning

Oximes K033 [1,4-bis(2-hydroxyiminomethylpyridinium) butane dibromide] and K048 [1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium) butane dibromide] were tested as pretreatment drugs in tabun-poisoned mice followed by treatment with atropine plus K033, K048, K027 [1-(4-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium) propane dibromide], TMB-4 [1,3-bis(4-hydroxyiminomethylpyridinium) propane dibromide] and HI-6 [(1-(2-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium)-2-oxapropane dichloride)]. Oxime doses of 25% or 5% of its LD(50) were used for pretreatment 15 min before tabun-poisoning and for treatment 1 min after tabun administration to mice. The best therapeutic effect was obtained when oxime K048 (25% of its LD(50)) was used in both pretreatment and treatment with atropine. This regiment insured survival of all tested animals after the application of 10 LD(50) of tabun. In addition, since butyrylcholinesterase (BChE; EC 3.1.1.8) is considered an endogenous bioscavenger of anticholinesterase compounds and its interactions with oximes could be masked by AChE interactions, we evaluated kinetic parameters for interactions of tested oximes with native and tabun-inhibited human plasma BChE and compared them with results obtained previously for human erythrocyte acetylcholinesterase (AChE; EC 3.1.1.7). Progressive inhibition of BChE by tabun was slightly faster than that of AChE. The reactivation of tabun-inhibited BChE by oximes was very slow, and BChE binding affinity for oximes was lower than AChE's. Therefore, BChE could scavenge tabun prior to AChE inhibition, but fast oxime-assisted reactivation of tabun-inhibited AChE or protection of AChE by oxime against inhibition with tabun would not be obstructed by interaction between BChE and oximes.

In vitro potency of H oximes (HI-6, HLö-7), the oxime BI-6, and currently used oximes (pralidoxime, obidoxime, trimedoxime) to reactivate nerve agent-inhibited rat brain acetylcholinesterase

The efficacy of H oximes (HI-6, HLö-7), the oxime BI-6, and currently used oximes (pralidoxime, obidoxime, trimedoxime) to reactivate acetylcholinesterase inhibited by two nerve agents (tabun, VX agent) was tested in vitro. Both H oximes (HI-6, HLö-7) and the oxime BI-6 were found to be more efficacious reactivators of VX-inhibited acetylcholinesterase than pralidoxime and obidoxime. On the other hand, their potency to reactivate tabun-inhibited acetylcholinesterase was low and did not reach the reactivating efficacy of trimedoxime and obidoxime. Thus, none of these compounds can be considered to be a broad-spectrum reactivator of nerve agent-inhibited acetylcholinesterase in spite of high potency to reactivate acetylcholinesterase inhibited by some nerve agents. More than one oxime may be necessary for the antidotal treatment of nerve agent-exposed individuals.

The influence of oxime and anticholinergic drug selection on the potency of antidotal treatment to counteract acute toxic effects of tabun in mice

The influence of newly developed oximes, K027 [1-(4-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium) propane dibromide] and K048 [1-(4-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium) butane dibromide], or currently used oximes (pralidoxime, obidoxime, trimedoxime, HI-6) and anticholinergic drugs (atropine, benactyzine) on the ability of antidotal treatment to eliminate tabun-induced acute toxic effects was studied in mice. The therapeutical efficacy of trimedoxime and both newly developed oximes (K027, K048) is significantly higher than the potency of pralidoxime (regardless of the choice of anticholinergic drug), obidoxime (in the case of its combination with atropine) and the oxime HI-6 (in the case of its combination with benactyzine). All studied oximes with the exception of pralidoxime and the oxime HI-6, when combined with benactyzine, appear to be more efficacious in the elimination of toxic effects of the lethal dose of tabun than their combination with atropine. The findings support the hypothesis that the choice of acetylcholinesterase reactivators as well as the anticholinergic drug selection are important for the effectiveness of an antidotal mixture in the case of antidotal treatment of tabun-induced acute poisonings.

The influence of antidotal treatment of low-level tabun exposure on cognitive functions in rats using a water maze

In this study, the influence of antidotal treatment of tabun poisoning on cognitive function, in the case of low-level tabun exposure, was studied. The impairment of cognitive function was evaluated by the measurement of spatial learning and memory in rats poisoned with a sublethal dose of tabun and treated with atropine alone or in combination with newly developed oximes {K027 [1-(4-hydroxyiminomethyl- pyridinium)-3-(4-carbamoylpyridinium) propane dibromide] and K048 [1-(4-hydroxyimino- methylpyridinium)-3-(4-carbamoylpyridinium) butane dibromide]} or currently available oxime (trimedoxime), using the Morris water maze. While atropine alone caused an impairment of studied cognitive functions, the addition of an oxime to atropine contributes to the improvement of cognitive performance of treated tabun-poisoned rats regardless of the type of oxime. The differences in the ameliorative effects of oximes on atropine-induced mnemonic deficits were not significant. Therefore, each low-level nerve agent exposure should be treated by complex antidotal treatment consisting of anticholinergic drug and oxime.

A comparison of the potency of newly developed oximes (K027, K048) and commonly used oximes (obidoxime, HI-6) to counteract tabun-induced neurotoxicity in rats

The neuroprotective effects of newly developed oximes (K027, K048) and currently available oximes (obidoxime, HI-6) in combination with atropine in rats poisoned with tabun at a sublethal dose (170 microg kg(-1) i.m.; 80% of LD(50) value) were studied. The tabun-induced neurotoxicity was monitored using a functional observational battery and an automatic measurement of motor activity. The neurotoxicity of tabun was monitored at 24 h and 7 days following tabun challenge. The results indicate that the oxime HI-6 in combination with atropine was not able to protect the rats from the lethal effects of tabun. Two non-treated tabun-poisoned rats and one tabun-poisoned rat treated with atropine combined with HI-6 died within 2 h. On the other hand, all other tested oximes combined with atropine allowed all the tabun-poisoned rats to survive 7 days following tabun challenge. Both newly developed oximes combined with atropine seem to be sufficiently effective antidotes for a decrease in tabun-induced neurotoxicity in the case of sublethal poisoning although they are not able to eliminate tabun-induced neurotoxicity completely. The neuroprotective efficacy of obidoxime in combination with atropine approached the potency of newly developed oximes but the ability of the oxime HI-6 to counteract tabun-induced acute neurotoxicity was significantly lower, especially at 24 h after tabun poisoning. Due to their neuroprotective effects, both newly developed oximes appear to be suitable oximes for the antidotal treatment of acute tabun poisoning.

In vitro and in vivo evaluation of pyridinium oximes: mode of interaction with acetylcholinesterase, effect on tabun- and soman-poisoned mice and their cytotoxicity

The increased concern about terrorist use of nerve agents prompted us to search for new more effective oximes against tabun and soman poisoning. We investigated the interactions of five bispyridinium oximes: K027 [1-(4-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium) propane dibromide], K048 [1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium) butane dibromide], K033 [1,4-bis(2-hydroxyiminomethylpyridinium) butane dibromide], TMB-4 [1,3-bis(4-hydroxyiminomethylpyridinium) propane dibromide] and HI-6 [(1-(2-hydroxyiminomethylpyridinium)-3-(4-carbamoylpyridinium)-2-oxapropane dichloride)] with human erythrocyte acetylcholinesterase (AChE; E.C. 3.1.1.7) and their effects on tabun- and soman-poisoned mice. All the oximes reversibly inhibited AChE, and the enzyme-oxime dissociation constants were between 17 and 180 microM. Tabun-inhibited AChE was completely reactivated by TMB-4, K027 and K048, with the overall reactivation rate constants of 306, 376 and 673 min(-1)M(-1), respectively. The reactivation of tabun-inhibited AChE by K033 reached 50% after 24h, while HI-6 failed to reactivate any AChE at all. Soman-inhibited AChE was resistant to reactivation by 1mM oximes. All studied oximes protected AChE from phosphorylation with both soman and tabun. In vivo experiments showed that the studied oximes were relatively toxic to mice; K033 was the most toxic (LD50=33.4 mg/kg), while K027 was the least toxic (LD50=672.8 mg/kg). The best antidotal efficacy was obtained with K048, K027 and TMB-4 for tabun poisoning, and HI-6 for soman poisoning. Moreover, all tested oximes showed no cytotoxic effect on several cell lines in concentrations up to 0.8mM. The potency of the oximes K048 and K027 to protect mice from five-fold LD50 of tabun and their low toxicity make these compounds leading in the therapy of tabun poisoning. The combination of HI-6 and atropine is the therapy of choice for soman poisoning.

A COMPARISON OF THE POTENCY OF TRIMEDOXIME AND OTHER CURRENTLY AVAILABLE OXIMES TO REACTIVATE TABUN-INHIBITED ACETYLCHOLINESTERASE AND ELIMINATE ACUTE TOXIC EFFECTS OF TABUN

Tabun (O-ethyl-N,N-dimethyl phosphoramidocyanidate) belongs to highly toxic organophosphorus compounds misused as chemical warfare agents for military as well as terroristic purposes. It differs from other highly toxic organophosphates by its chemical structure and by the fact that tabun-inhibited acetylcholinesterase is extraordinarily difficult to reactivate. The potency of trimedoxime and other commonly used oximes (pralidoxime, obidoxime, the oxime HI-6) to reactivate tabun-inhibited acetylcholinesterase and to eliminate tabun-induced acute effects was evaluated using in vitro and in vivo methods. In vitro calculated kinetic parameters of reactivation of tabun-inhibited acetylcholinesterase from rat brain homogenate and in vivo determined percentage of reactivation of tabun-inhibited blood and tissue acetylcholinesterase in poisoned rats show that trimedoxime seems to be the most efficacious reactivator in the case of tabun poisonings. Trimedoxime was also found to be the most efficacious oxime in the elimination of acute lethal toxic effects in tabun-poisoned rats and mice. The oxime HI-6, so efficacious against soman, does not seem to be sufficiently effective oxime to reactivate tabun-inhibited acetylcholinesterase and to counteract acute lethal effects of tabun.

A comparison of the ability of a new bispyridinium oxime--1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium)butane dibromide and currently used oximes to reactivate nerve agent-inhibited rat brain acetylcholinesterase by in vitro methods

The efficacy of a new bispyridinium oxime 1-(4-hydroxyiminomethylpyridinium)-4-(4-carbamoylpyridinium)butane dibromide, called K048, and currently used oximes (pralidoxime, obidoxime, the oxime HI-6) to reactivate acetylcholinesterase inhibited by various nerve agents (sarin, tabun, cyclosarin, VX) was tested by in vitro methods. The new oxime K048 was found to be a more efficacious reactivator of nerve agent-inhibited acetylcholinesterase than pralidoxime (in the case of VX, tabun and cyclosarin), obidoxime (cyclosarin and tabun) and HI-6 (tabun) but it did not reach the efficacy of currently used oximes for the reactivation of acetylcholinesterase inhibited by sarin. Thus, the oxime K048 seems to be a relatively efficacious broad spectrum acetylcholinesterase reactivator and, therefore, it could be useful for the treatment of a nerve agent-exposed population if information about detection of the type of nerve agent is not available.

Specification of the Structure of Oximes Able to Reactivate Tabun-Inhibited Acetylcholinesterase

The efficacy of various oximes to reactivate acetylcholinesterase phosphorylated by tabun (O-ethyl-N,N-dimethyl phosphoramidocyanidate) was tested by in vitro and in vivo methods. The oximes commonly used for the treatment of acute poisonings with highly toxic organophosphates appeared to be almost ineffective (HI-6, pralidoxime, methoxime) or just slightly effective (obidoxime) against tabun. On the other hand, trimedoxime seemed to be a significantly more efficacious reactivator than the others in the case of tabun poisonings. In vitro, the concentration of trimedoxime corresponding to 1.0 mmol/l was able to reach 50% reactivation of tabun-inhibited brain acetylcholinesterase. Higher reactivating potency of trimedoxime in comparison with the other commonly used oximes was demonstrated by in vivo method, too. In addition, other structural analogues of trimedoxime were found to be efficacious in counteracting tabun-induced acetylcholinesterase inhibition although not as efficacious as trimedoxime itself. Some effective acetylcholinesterase reactivators were characterised by dissociation constant of enzyme-reactivator complex as well as enzyme-inhibitor-reactivator complex and by rate constant of reactivation.

Neuroprotective effects of currently used antidotes in tabun-poisoned rats

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

Review of oximes in the antidotal treatment of poisoning by organophosphorus nerve agents

The cholinesterase-inhibiting organophosphorus compounds referred to as nerve agents (soman, sarin, tabun, GF agent, and VX) are particularly toxic and are considered to be among the most dangerous chemical warfare agents. Included in antidotal medical countermeasures are oximes to reactivate the inhibited cholinesterase. Much experimental work has been done to better understand the properties of the oxime antidotal candidates including the currently available pralidoxime and obidoxime, the H oximes HI-6 and Hlö-7, and methoxime. There is no single, broad-spectrum oxime suitablefor the antidotal treatment of poisoning with all organophosphorus agents. If more than one oxime is available, the choice depends primarily on the identity of the responsible organophosphorus compound. The H oximes appear to be very promising antidotes against nerve agents because they are able to protect experimental animals from toxic effects and improve survival of animals poisoned with supralethal doses. They appear more effective against nerve agent poisoning than the currently used oximes pralidoxime and obidoxime, especially in the case of soman poisoning. On the other hand, pralidoxime and especially obidoxime seem sufficiently effective to treat poisonings with organophosphorus insecticides that have relatively less toxicity than nerve agents.

A comparison of the efficacy of pyridostigmine alone and the combination of pyridostigmine with anticholinergic drugs as pharmacological pretreatment of tabun-poisoned rats and mice

The ability of two types of pharmacological pretreatment (pyridostigmine alone or pyridostigmine in combination with two anticholinergic drugs) to increase the resistance of rats and mice against tabun and to increase the therapeutic efficacy of common antidotal treatment of tabun-poisoned rats and mice was compared. A significant decrease in the LD50 values of tabun was observed when mice as well as rats were pretreated with the prophylactic antidotal mixture consisting of pyridostigmine, benactyzine and trihexyphenidyle, designated PANPAL. Pyridostigmine-pretreated rats were also more resistant against acute lethal effects of tabun but pyridostigmine-induced resistance of rats was not so high as PANPAL-induced resistance. In addition, the pharmacological pretreatment with pyridostigmine alone was not able to protect mice against tabun-induced acute toxicity. The pharmacological pretreatment with pyridostigmine alone was able to increase the efficacy of currently used antidotal treatment (obidoxime in combination with atropine and diazepam) of tabun-induced poisoning, but PANPAL-induced increase in the efficacy of the same antidotal treatment was significantly higher than an increase induced by pyridostigmine alone. PANPAL-induced increase in the efficacy of antidotal treatment of tabun poisoning was also observed in mice. These findings confirm that PANPAL pretreatment of tabun-poisoned rats and mice seems to be much more suitable than currently used pyridostigmine alone.