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Organophosphate (OP) poisoning is a major global health issue; while compounds from this group have been used intensively over the last century, an effective antidote is still lacking. Oxime-type acetylcholinesterase (AChE) reactivators are used to reactivate the OP inhibited AChE. Pralidoxime is the only US Food and Drug Administration approved oxime for therapeutic use but its efficacy has been disappointing. Two novel oximes (K378 and K727) were investigated in silico and in vitro and compared with an experimental oxime (kamiloxime; K-27) and pralidoxime. In silico the molecular interactions between AChE and oximes were examined and binding energies were assessed. LogP (predicted log of the octanol/water partition coefficient) was estimated. In vitro the intrinsic ability of the oximes to inhibit AChE (IC 50 ) and their reactivation potency (R 50 ) when used in paraoxon inhibited human RBC-AChE was determined. Molecular docking revealed that K378 and K727 bind to the peripheral site(s) with high binding energies in contrast to the central binding of K-27 and pralidoxime. LogP values indicating that the novel compounds are significantly less hydrophilic than K-27 or pralidoxime. IC 50 of K378 and K727 were comparable (0.9 and 1 µM, respectively) but orders of magnitude lower than comparators. R 50 values revealed their inability to reactivate paraoxon inhibited AChE. It is concluded that the novel oximes K378 and K727 are unlikely to be clinically useful. The in silico and in vitro studies described allow avoidance of unnecessary in vivo animal work and contribute to the reduction of laboratory animal use.

Organophosphate (OP)-based pesticides and nerve agents are highly toxic compounds which interrupt the catalytic mechanism of acetylcholinesterase (AChE) by phosphorylating the hydroxyl moiety of serine residue. The inhibited enzyme can be reactivated by the nucleophilic action of oxime reactivators. To analyze the effect of different AChE sources on reactivation efficacy of reactivators, several in vivo studies have carried out using variety of AChE sources like pig, rat and monkey. Investigations on species differences provide a better insight for the development of new reactivators. Hence, present study was mainly targeted on comparative analysis of the reactivation of electric eel and human AChE inhibited by different OP. A series of butene-linked bis-pyridinium mono oximes which vary in functional groups present at the second pyridinium ring have been examined against sarin, VX, tabun and ethyl-paraoxon-poisoned AChE. In case of tabun-inhibited AChEs, tested oximes were better than reference oximes. For VX-poisoned human AChE, reactivator K251 (kr2;1.51 mM (-) (1 )min (-) (1)) showed good reactivation efficacy with standard oximes. Studies stipulated that butene-linked oximes consisting of different functional moieties are good reactivators and found to have better efficacy to reactivate nerve agent-inhibited human AChE in comparison to eel AChE.

The ability of two novel bispyridinium oximes K727 and K733 and currently available oximes (HI-6, obidoxime) to reactivate sarin-inhibited acetylcholinesterase and to reduce acute toxicity of sarin was evaluated. To investigate the reactivating efficacy of the oximes, the rats were administered intramuscularly with atropine and oximes in equitoxic doses corresponding to 5% of their LD50 values at 1 min after the intramuscular administration of sarin at a dose of 24 µg/kg (LD50). The activity of acetylcholinesterase was measured at 60 min after sarin poisoning. The LD50 value of sarin in non-treated and treated mice was assessed using probit-logarithmical analysis of death occurring within 24 h after intramuscular administration of sarin at five different doses. In vivo determined percentage of reactivation of sarin-inhibited rat blood, diaphragm and brain acetylcholinesterase showed that the potency of both novel oximes K727 and K733 to reactivate sarin-inhibited acetylcholinesterase roughly corresponds to the reactivating efficacy of obidoxime. On the other hand, the oxime HI-6 was found to be the most efficient reactivator of sarin-inhibited acetylcholinesterase. While the oxime HI-6 was able to reduce the acute toxicity of sarin >3 times, both novel oximes and obidoxime decreased the acute toxicity of sarin <2 times. Based on the results, we can conclude that the reactivating and therapeutic efficacy of both novel oximes K727 and K733 is significantly lower compared to the oxime HI-6 and, therefore, they are not suitable for the replacement of the oxime HI-6 for the antidotal treatment of acute sarin poisoning.

The potency of two newly developed oximes (K361 and K378) to reactivate tabun-inhibited cholinesterase and to reduce acute toxicity of tabun was compared with the oxime K203 and trimedoxime using in vivo methods. The study determining percentage of reactivation of tabun-inhibited diaphragm cholinesterase in poisoned rats showed that the reactivating efficacy of the oxime K378 is slightly lower than the reactivating potency of the oxime K203 and trimedoxime while the ability of the oxime K361 to reactivate tabun-inhibited cholinesterase is markedly lower compared with the oxime K203 and trimedoxime. In the brain, the potency of both newly developed oximes to reactivate tabun-inhibited cholinesterase was negligible. The therapeutic efficacy of both newly developed oximes roughly corresponds to their weak reactivating efficacy. Their potency to reduce acute toxicity of tabun was significantly lower compared with the oxime K203 as well as trimedoxime. In conclusion, the reactivating and therapeutic potency of both newly developed oximes does not prevail the effectiveness of the oxime K203 and trimedoxime and, therefore, they are not suitable for their replacement of commonly used oximes for the treatment of acute tabun poisoning.

The potency of three newly developed bispyridinium compounds (K454, K456, K458) to reactivate tabun-inhibited acetylcholinesterase and reduce tabun-induced lethal toxic effects was compared with the oxime K203 and trimedoxime using in vivo methods. The study determining percentage of reactivation of tabun-inhibited diaphragm and brain acetylcholinesterase in poisoned rats showed that the reactivating efficacy of all newly developed oximes is comparable with K203 but lower than the reactivating potency of trimedoxime in diaphragm. In the brain, their potency to reactivate tabun-inhibited acetylcholinesterase is lower compared with trimedoxime and the oxime K203. All three newly developed oximes were also found to be relatively effective in reducing lethal toxic effects in tabun-poisoned mice. Their therapeutic efficacy is consistent with the therapeutic potency of the oxime K203. On the other hand, their potency to reduce acute toxicity of tabun is significantly lower compared with trimedoxime. In conclusion, the reactivating and therapeutic potency of all three newly developed oximes does not prevail the effectiveness of the oxime K203 and trimedoxime and, therefore, they are not suitable for their replacement of commonly used oximes for the treatment of acute tabun poisoning.

The potency of bispyridinium acetylcholinesterase reactivator KR-22934 in reactivating tabun-inhibited acetylcholinesterase and reducing tabun-induced lethal toxic effects was compared with the oxime K203 and commonly used oximes. Studies determining percentage of reactivation of tabun-inhibited blood and tissue acetylcholinesterase in rats showed that the reactivating efficacy of KR-22934 was slightly higher than the reactivating efficacy of K203 and roughly corresponded to the reactivating efficacy of obidoxime and trimedoxime in blood and diaphragm. On the other hand, the oxime KR-22934 was not able to reactivate tabun-inhibited acetylcholinesterase in the brain. The therapeutic efficacy of all oximes studied approximately corresponded to their reactivating efficacy. Based on the results, one can conclude that the oxime KR-22934 is not suitable for the replacement of commonly used oximes for the antidotal treatment of tabun poisoning in spite of its potency to reactivate tabun-inhibited acetylcholinesterase in the peripheral compartment (blood, diaphragm).

The neuroprotective effects of newly-developed oxime K203 and currently available oximes (trimedoxime, obidoxime, HI-6) in combination with atropine in rats poisoned with soman were studied. The soman-induced neurotoxicity was monitored using a functional observational battery at 24 h following soman challenge. The results indicate that the potency of a newly-developed oxime K203 to counteract soman-induced neurotoxicity is very low and roughly corresponds to the neuroprotective efficacy of currently available oximes. Among tested oximes, the oxime HI-6 seems to be the most efficacious to counteract acute neurotoxicity of soman, although the differences in neuroprotective efficacy of chosen oximes are not significant. Thus, the oxime K203 does not provide any beneficial effect for the antidotal treatment of acute poisoning with soman and the oxime HI-6 should be still considered to be the best oxime for antidotal treatment of acute soman poisonings.

A potency of newly-developed oximes (K156, K203) and commonly used oximes (obidoxime, HI-6) to reactivate cyclosarin-inhibited acetylcholinesterase and to reduce cyclosarin-induced acute toxic effects was evaluated in this study. In vivo determined percentage of reactivation of cyclosarin-inhibited blood and tissue acetylcholinesterase in poisoned rats showed that the potency of a newly-developed oxime (K203) to reactivate cyclosarin-inhibited acetylcholinesterase and to reduce the acute lethal effects of cyclosarin, corresponding to the relatively low reactivating and therapeutic efficacy of obidoxime. The potency of another newly-developed oxime (K156) to counteract the inhibitory and acute clinical effects of cyclosarin is almost negligible. On the other hand, the oxime HI-6 is a very efficient reactivator of cyclosarin-inhibited acetylcholinesterase in the peripheral (blood, diaphragm) as well as central (brain) compartment, and it is able to reduce the acute toxicity of cyclosarin more than three times. Although the reactivating and therapeutic efficacy of the oxime K203 is higher compared to another newly-developed oxime K156, the reactivating and therapeutic potency of both newly-developed oximes is significantly lower in comparison with the oxime HI-6 and, therefore, none of them is suitable for replacement of HI-6 in the case of the treatment of cyclosarin poisoning.

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.