Efficacious Oxime for Organophosphorus Poisoning : A Minireview

Oximes are well known as acetylcholinesterase reactivators and are used in organophosphorus poisoning to reactivate inhibited acetylcholinesterase. Therapeutically available oximes, namely, pralidoxime (2-PAM), obidoxime, trimedoxime and Hagedorn oxime (HI-6), have no broad-spectrum activity against structurally different kinds of organophosphorus anticholinesterases. The widely used oxime, 2-PAM, is least effective. The focus of the review is to find an oxime that is broad spectrum and superior to the presently available oximes for the treatment of organophosphorus poisoning. Numerous oxime-based reactivators have been synthesized in laboratories in Croatia, United States of America, Israel and most recently in Czech Republic. Some experimental oximes synthesized in Czech Republic and named as K-series of oximes have been found promising. Among them, K-27 and K-48 have higher or comparable efficacy to all available oximes though it is not effective against all organophosphorus (OP) nerve agents. They are also efficacious in pretreatment protocol for OP anticholinesterases. K-27 oxime is a promising candidate to replace therapeutically available oximes with respect to insecticide/pesticide organophosphorus poisoning. K27 and K48 may be candidates to replace the only approved pretreatment drug, pyridostigmine, in military combat medicine for OP nerve agent.


INTRODUCTION
Organophosphorus anticholinesterase compounds (OPCs) have a wide variety of applications, but they also constitute a serious threat with regard to occupational hazard, self-poisoning, unintentional misuse, terrorists attack and warfare.OPCs are among the most frequent agents involved in suicidal and accidental intoxication [1,2] and food poisoning [3].Acute organic insecticides poisoning is a major health problem all over the world, particularly in developing countries where organophosphates are the most common suicidal poison with high mortality and morbidity [4].They account for several hundreds of thousands of death worldwide every year and even a greater number of casualties [5].According to Eddleston et al [2] organophosphorus pesticide self-poisoning is an important chemical problem in rural regions of the developing world and kills an estimated 200,000 people every year [6].One worldwide mortality study reported mortality rates ranging from 3 -25 % [7].In 2005, 15 victims were poisoned after accidently ingesting ethion (an OPC) contaminated food in a social ceremony in Magrawa, India [7].Food contamination by organophosphates in humans mostly occurs among farmers and agriculture workers [8].A brief history of OPCs is shown in Table 1.
There are hundreds of different organophosphorus compounds (OPCs) ranging from the deadly toxic nerve agents to moderately toxic compounds used in insecticides and pesticides, but all have the same mechanism of action, that is, irreversible inhibition of acetylcholinesterase at nerve synapse.Oximes (nucleophilic agents) are acetylcholinesterase reactivators which are used as adjunct treatment in OPC poisoning to reactivate inhibited acetylcholinesterase (AChE).Unfortunately, the therapeutically available oximes are not equally effective for different kinds of OPC poisons.Therefore, there is a pressing need to develop an oxime with universal antidotal efficacy.[15].It is interesting to note that all the oximes were primarily synthesized for use against organophosphorus (OP) nerve agents such as tabun and sarin, which are liable to be used in warfare, and secondarily, the oximes have been tested and are being used to counter intentional or unintentional OP insecticide/pesticide poisoning.Among the clinically available oximes, obidoxime is considered the best for countering pesticide OP poisoning [16] while HI-6 and HLö-7 are good for OP nerve agents poisoning [17].2-PAM, is most often used by physicians worldwide but its efficacy in clinical trials is controversial and its low or null efficacy is well documented in literature [4,18,19].
Another important aspect is that oximes are not used alone but rather as an adjunct treatment to atropine sulphate.Clinically, while atropine relieves muscarinic signs and symptoms, oximes are supposed to shorten the duration of the respiratory muscle paralysis by reactivation of cholinesterase [20].According to Petroianu [21], the therapy of organophosphate poisoning is known by the catchy acronym, AFLOP, which stands for 'Atropine, Fluids, Oxygen, and Pralidoxime'.
During the past few years, some comprehensive reviews on oximes have been reported [1,14,15,22-25].Clinical opinion on the value of oximes as adjunct in the therapy of organophosphorus pesticide intoxication of human remains divided.It has been argued that oximes are unnecessary when intoxication is not severe [20].Some reported that their clinical experience with oximes is disappointing [18,[25][26] while Peter et al stated that using meta-analysis, oximes either gave null effect or possible harm [19].According to Jokanovic [27], oximes are beneficial if used properly and WHO recommendations are followed.
In fact, there are some limitations with oxime therapy.First, a particular oxime may be effective against a specific organophosphorus acetylcholinesterase inhibitor and ineffective against others.Hence, there would be limited basis for choosing one oxime over another in an unknown exposure.Secondly, AChE inhibited by OPCs undergoes a process of ageing and an aged enzyme cannot be reactivated [14].The ageing kinetics of different OPCs is different, ranging from a few minutes to many hours.Second, dosing and time of treatment also plays a vital role in successful oxime therapy.Other factors that influence oxime therapy include inhibition potency of OPCs, and its toxicokinetics, ageing kinetics of inhibited AChE, reactivating potency of oxime and its pharmacokinetics, correct dosing and evaluation for the persistent need of oxime therapy, and correct timing, i.e., whether oxime is started too late or discontinued too early, etc.
The aim of the present review is to propose an acetylcholinesterase reactivator with broad spectrum for all kinds of organophosphorus anticholinesterases.Laboratory in vivo and in vitro data and literature search suggests that there is a pressing need for a universal oxime.

Which Oxime should be therapeutically used in op poisoning?
There is no definite answer.Oximes are unfortunately not equally effective against all available organophosphorus acetylcholinesterase inhibitors.Hence, it may not be a simple choice for clinicians to choose an oxime for unknown OPC exposure.Secondly, there is a lack of published clinical trials regarding the beneficial role and efficacy of oximes particularly dose regimen, oxime choice and outcome of the treatments.There is a clear demand for an oxime with universal broad spectrum antidotal potency.The literature shows that obidoxime may be a good choice for pesticide/insecticide OPCs poisoning, better than the most widely used pralidoxime, while HI-6 is good for nerve agent poisoning.. Since no oxime has been proved or is clinically available as a universal broad spectrum agent for structurally different kinds of OP acetylcholinesterase inhibitor; numerous attempts have been made to improve the antidotal properties of the conventional mono and bis pyridinium oximes by modifying their structures.The efforts yielded the synthesis of imidazolium, quinuclidinium, pyridinium-imidazolium, pyridinium-quinuclidinium and quinuclidiniumimidazolium oximes [25] which are still at the experimental stage.The synthesis of oximes like compound by Kamil Kuca and Kamil Musilek in Czech Republic introduced series of so called K-oximes during the last few years.The oximes were basically targeted for tabun and other OP nerve agent [17] but tests were extended for pesticides induced AChE inhibition and found promising potential.More than 200 structurally different K-oximes have been synthesized since 2003 [28] but the most promising among them are K-027 [17,23,[28][29][30][31][32][33][34][35][36][37][38].
Other potentially promising reported Koximes are K-48, K-53, K-74, K-75 and K-203, etc. Structurally all the K-oximes are either asymmetrical or symmetrical bispyridinium aldoximes with changes in the position of functional aldoxime as well as in some cases changes in linker chain.Many of them were found to be better than the therapeutically available oximes but still no single oxime could be identified as a broad spectrum universal oxime for structurally different kinds of OPCs.K-27 [39] and K-48 [40] are the two promising oximes in the series of K-oximes.Experimental evidence from animal studies reveals that K-27 is a strong candidate to replace the available oximes.K-27 has been reported to be effective against many OPC compounds except the nerve agents, soman and cyclosarin [41].

In vivo data for some oximes
Table 3 shows the life-preserving capability of widely used oximes for some pesticides OP. 2-PAM and obidoxime are poorer in this regard than the experimental K oximes, K-27 and K-48.The other two therapeutically available oximes are also inferior in efficacy.These data were obtained from experiments using Wistar rats.The rats were intoxicated with different doses intraperitoneally, including supra lethal doses of experimental OPC and subsequent treatment with equitoxic doses of oximes that are half of their LD 50 .Relative risk of death was estimated by Cox regression using SPSS statistical analysis software.The higher or comparable efficacy of the two K-oximes against various OPCs have been well documented in the literature [29,[33][34][35][36][37][38][48][49][50].This makes them promising substances for the therapy of poisoning with a wide variety of OPCs.
In vitro data Kuca et al [41] tested the reactivation potency of K-27 as a potential reactivator of AChE inhibited by tabun, sarin, cyclosarin, soman, VX, Russian VX, paraoxon, methylchlorpyrifos, and dichlorvos (2,2dichlorovinyl dimethyl phosphate (DDVP).They found that K-27 reactivated AChE which had been inhibited by almost all the tested inhibitors at a level > 10 %, and this is believed to be sufficient for saving the lives of intoxicated organisms.In the case of cyclosarin-and soman-inhibited AChE, K-27 did not reach sufficient reactivation potency.The ability to protect AChE in vitro from inhibition by paraoxon, methyl paraoxon, diisopropylfluorophosphate (DFP), tabun and VX have been reported by many workers [35,[51][52][53][54][55].

CONCLUSION
Development of a universal broad spectrum oxime for OPCs poisoning is highly needed.The newer experimental K-oxime, K-27, may replace the existing conventional oximes in future for OP insecticides/pesticides poisoning because of its superior efficacy.However, it cannot be considered broad spectrum because of its inefficacy against two OP nerve poisons, soman and cyclosarin.Moreover, many other OPCs groups should be tested for efficacy.The oxime may also replace the only approved pretreatment drug, pyridostigmine, for war time protocol.

Figure 1 :
Figure 1: Structures of conventional and two promising K-oximes.

Table 1 :
A brief history of organophosphorus compounds

Table 3 :
Comparison of the relative risk of death after intoxication with different types of insecticidal/pesticidal OPCs and subsequent treatment with experimental and conventional oximes in rodent model (Note: RR of OPCs is considered as 1 where no oximes were applied) data, K-27 and K-48 were more promising than pyridostigmine when pretreatment protocol was applied with five different acute toxic organophosphorus anticholinesterases at supra lethal doses in rats.Berend et al[48]obtained similarly promising results with K-48 against OP nerve agent tabun in mice.