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 Experimental Oxime K027-A Promising Protector From Organophosphate Pesticide Poisoning. A Review Comparing K027, K048, Pralidoxime, and Obidoxime

Poisoning with organophosphorus compounds (OPCs) is a major problem worldwide. Standard therapy with atropine and established oxime-type enzyme reactivators (pralidoxime, obidoxime) is unsatisfactory. In search of more efficacious broad-spectrum oximes, new bispyridinium (K-) oximes have been synthesized, with K027 being among the most promising. This review summarizes pharmacokinetic characteristics of K027, its toxicity and in vivo efficacy to protect from OPC toxicity and compares this oxime with another experimental bisquaternary asymmetric pyridinium aldoxime (K048) and two established oximes (pralidoxime, obidoxime). After intramuscular (i.m.) injection, K027 reaches maximum plasma concentration within ∼30 min; only ∼2% enter the brain. Its intrinsic cholinesterase inhibitory activity is low, making it relatively non-toxic. In vitro reactivation potency is high for ethyl-paraoxon-, methyl-paraoxon-, dichlorvos-, diisopropylfluorophosphate (DFP)- and tabun-inhibited cholinesterase. When administered in vivo after exposure to the same OPCs, K027 is comparable or more efficacious than pralidoxime and obidoxime. When given as a pretreatment before exposure to ethyl-paraoxon, methyl-paraoxon, DFP, or azinphos-methyl, it is superior to the Food and Drug Administration-approved compound pyridostigmine and comparable to physostigmine, which because of its entry into the brain may cause unwanted behavioral effects. Because of its low toxicity, K027 can be given in high dosages, making it a very efficacious oxime not only for postexposure treatment but also for prophylactic administration, especially when brain penetration is undesirable.

Oxime K027: novel low-toxic candidate for the universal reactivator of nerve agent- and pesticide-inhibited acetylcholinesterase

Oxime K027 is a low-toxic bisquaternary compound originally developed as a reactivator of acetylcholinesterase (AChE) inhibited by nerve agents. The reactivation potency of K027 has been tested as a potential reactivator of AChE inhibited by tabun, sarin, cyclosarin, soman, VX, Russian VX, paraoxon, methylchlorpyrifos, and DDVP. The results show that oxime K027 reactivated AChE inhibited by almost all tested inhibitors to more than 10%, which is believed to be enough for saving the lives of intoxicated organisms. In the case of cyclosarin- and soman-inhibited AChE, oxime K027 did not reach sufficient reactivation potency.

Pharmacokinetics and pharmacodynamics of some oximes and associated therapeutic consequences: a critical review

Undoubtedly, the use of oximes represents real progress in counteracting intoxications with organophosphates (OP), through potentiating antidotal effects of atropine. The penetration extent of these compounds through the blood-brain barrier (BBB) to significantly reactivate phosphorylated or phosphonylated acetylcholinesterase (AChE) in the brain still remains a debatable issue. Penetration of biological barriers by oximes was investigated mainly through determination of several quantitative parameters characterizing digestive absorption and BBB penetration. A weak penetration of biological barriers could be concluded from the available experimental data. The functional parameters/therapeutic effects following the penetration of oximes through BBB, more precisely the antagonism of OP-induced seizures and hypothermia, prevention of brain damage and respiratory center protection, leading to the final end-point, the survival of intoxicated organisms, are of high interest. It seems obvious that oximes are weakly penetrating the BBB, with minimal brain AChE reactivation (<5%) in important functional areas, such as the ponto-medullar. The cerebral protection achieved through administration of oximes is only partial, without major impact on the antagonism of OP-induced seizures, hypothermia and respiratory center inhibition. The antidotal effects probably result from synergic effects of other PD properties, different from the brain AChE reactivation process. Oxime structures especially designed for enhanced BBB penetration, through potentiating the hydrophobic characteristics, more often produce neurotoxic effects. Certainly, obtaining oximes with broad action spectrum (active against all OP types) would make a sense, but certainly, such a target is not achievable only through the increase in their penetrability in the brain.

Temporal effects of newly developed oximes (K027, K048) on malathion-induced acetylcholinesterase inhibition and lipid peroxidation in mouse prefrontal cortex

The potency of newly developed asymmetric bispyridinium oximes (K027, K048) in reactivating acetylcholinesterase and in eliminating oxidative stress induced by acute exposure to malathion was evaluated in mouse prefrontal cortex using in vivo methods. Malathion (1g/kg, dissolved in saline) was administered subcutaneously. The asymmetric bispyridinium oximes K027 or K048 (1/4 of LD(50), dissolved in saline, i.p.) were administered immediately after malathion and atropine sulfate (20mg/kg, dissolved in saline, i.p.). Control group received saline instead of malathion and antidotes. Acetylcholinesterase activity and biochemical parameters related to oxidative stress (glutathione levels, glutathione peroxidase and glutathione reductase activity and lipid peroxidation) were evaluated in mouse prefrontal cortex at two different time points (3 or 24 h after malathion poisoning). Malathion administration markedly inhibited cortical acetylcholinesterase activity (around 55%) at 3h after malathion challenge and such inhibition was maintained till 24 h after poisoning. Although neither atropine sulfate nor oximes were able to eliminate cortical acetylcholinesterase inhibition at 3h after malathion poisoning, K027 (in combination with atropine) completely eliminated the inhibitory effect of malathion exposure on cortical acetylcholinesterase activity at 24 h after malathion administration. K048 (in combination with atropine) significantly decreased acetylcholinesterase inhibition at 24 h after malathion poisoning. Even though glutathione levels and glutathione peroxidase and glutathione reductase activities were not affected, malathion administration markedly increased lipid peroxidation in the prefrontal cortex at 24 h after poisoning and the oxime K027 (in combination with atropine) was able to significantly decrease such phenomenon. Thus, our results clearly demonstrate that the newly developed asymmetric bispyridinium oximes K027 and K048 are able to reverse malathion-induced acetylcholinesterase inhibition in mouse prefrontal cortex. Moreover, the ameliorative effect of the oxime K027 on the increased lipid peroxidation observed at 24 h after malathion poisoning suggests a potential link between the hyperstimulation of cholinergic system and oxidative stress in the mouse prefrontal cortex after malathion exposure.

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.