Restoration of soman-blocked neuromuscular transmission in human and rat muscle by the bispyridinium non-oxime MB327 in vitro

Identification of ligands binding to MB327-PAM-1, a binding pocket relevant for resensitization of nAChRs

Desensitization of nicotinic acetylcholine receptors (nAChRs) can be induced by overstimulation with acetylcholine (ACh) caused by an insufficient degradation of ACh after poisoning with organophosphorus compounds (OPCs). Currently, there is no generally applicable treatment for OPC poisoning that directly targets the desensitized nAChR. The bispyridinium compound MB327, an allosteric modulator of nAChR, has been shown to act as a resensitizer of nAChRs, indicating that drugs binding directly to nAChRs can have beneficial effects after OPC poisoning. However, MB327 also acts as an inhibitor of nAChRs at higher concentrations and can thus not be used for OPC poisoning treatment. Consequently, novel, more potent resensitizers are required. To successfully design novel ligands, the knowledge of the binding site is of utmost importance. Recently, we performed in silico studies to identify a new potential binding site of MB327, MB327-PAM-1, for which a more affine ligand, UNC0646, has been described. In this work, we performed ligand-based screening approaches to identify novel analogs of UNC0646 to help further understand the structure-affinity relationship of this compound class. Furthermore, we used structure-based screenings and identified compounds representing four new chemotypes binding to MB327-PAM-1. One of these compounds, cycloguanil, is the active metabolite of the antimalaria drug proguanil and shows a higher affinity towards MB327-PAM-1 than MB327. Furthermore, cycloguanil can reestablish the muscle force in soman-inhibited rat muscles. These results can act as a starting point to develop more potent resensitizers of nAChR and to close the gap in the treatment after OPC poisoning.

The use of bispyridinium non-oxime analogues for the restoration of nerve agent impaired neuromuscular transmission in rat hemidiaphragms - Structure optimization

Organophosphate pesticide poisoning challenges health care systems worldwide. Furthermore, nerve agents remain a continuous threat. The treatment options for organophosphate poisoning have virtually been unchanged for decades, relying on symptomatic treatment and the use of oximes to indirectly restore neuromuscular function. Hence, compounds targeting directly nicotinic acetylcholine receptors (nAChRs) might substantially improve treatment options. The current study investigated a series of bispyridinium analogues with a trimethylene or 2,2'-diethyloxy linker in a rat hemidiaphragm model, using indirect field stimulation. Methyl- and ethyl-substituted bispyridinium analogues restored neuromuscular function up to 37 ± 17% (MB419, a 3-methyl analogue) at a stimulation frequency of 20 Hz. The bispyridinium analogues with a 2- or 3-methyl group, or a 2- or 3-ethyl group, tended towards a higher restoration of neuromuscular function than those with a 4-methyl or 4-ethyl group, respectively. The current data can be used for future studies to optimize structure-based molecular modeling of compounds targeting the nAChR.

Synthesis and biological evaluation of novel MB327 analogs as resensitizers for desensitized nicotinic acetylcholine receptors after intoxication with nerve agents

Poisoning with organophosphorus compounds, which can lead to a cholinergic crisis due to the inhibition of acetylcholinesterase and the subsequent accumulation of acetylcholine (ACh) in the synaptic cleft, is a serious problem for which treatment options are currently insufficient. Our approach to broadening the therapeutic spectrum is to use agents that interact directly with desensitized nicotinic acetylcholine receptors (nAChRs) in order to induce functional recovery after ACh overstimulation. Although MB327, one of the most prominent compounds investigated in this context, has already shown positive properties in terms of muscle force recovery, this compound is not suitable for use as a therapeutic agent due to its insufficient potency. By means of in silico studies based on our recently presented allosteric binding pocket at the nAChR, i.e. the MB327-PAM-1 binding site, three promising MB327 analogs with a 4-aminopyridinium ion partial structure (PTM0056, PTM0062, and PTM0063) were identified. In this study, we present the synthesis and biological evaluation of a series of new analogs of the aforementioned compounds with a 4-aminopyridinium ion partial structure (PTM0064-PTM0072), as well as hydroxy-substituted analogs of MB327 (PTMD90-0012 and PTMD90-0015) designed to substitute entropically unfavorable water clusters identified during molecular dynamics simulations. The compounds were characterized in terms of their binding affinity towards the aforementioned binding site by applying the UNC0642 MS Binding Assays and in terms of their muscle force reactivation in rat diaphragm myography. More potent compounds were identified compared to MB327, as some of them showed a higher affinity towards MB327-PAM-1 and also a higher recovery of neuromuscular transmission at lower compound concentrations. To improve the treatment of organophosphate poisoning, direct targeting of nAChRs with appropriate compounds is a key step, and this study is an important contribution to this research.

Screening for new ligands of the MB327-PAM-1 binding site of the nicotinic acetylcholine receptor

Intoxications with organophosphorus compounds (OPCs) effect a severe impairment of cholinergic neurotransmission that, as a result of overstimulation may lead to desensitization of nicotinic acetylcholine receptors (nAChRs) and finally to death due to respiratory paralysis. So far, therapeutics, that are capable to address and revert desensitized neuromuscular nAChRs into their resting, i.e. functional state are still missing. Still, among a class of compounds termed bispyridinium salts, which are characterized by the presence of two pyridinium subunits, constituents have been identified, that can counteract organophosphate poisoning by resensitizing desensitized nAChRs. According to comprehensive modeling studies this effect is mediated by an allosteric binding site at the nAChR termed MB327-PAM-1 site. For MB327, the most prominent representative of the bispyridinium salts and all other analogues studied so far, the affinity for the aforementioned binding site and the intrinsic activity measured in ex vivo and in in vivo experiments are distinctly too low, to meet the criteria to be fulfilled for therapeutic use. Hence, in order to identify new compounds with higher affinities for the MB327-PAM-1 binding site, as a basic requirement for an enhanced potency, two compound libraries, the ChemDiv library with 60 constituents and the Tocriscreen Plus library with 1280 members have been screened for hit compounds addressing the MB327-PAM-1 binding site, utilizing the [2H6]MB327 MS Binding Assay recently developed by us. This led to the identification of a set of 10 chemically diverse compounds, all of which exhibit an IC50 value of ≤ 10 µM (in the [2H6]MB327 MS Binding Assay), which had been defined as selection criteria. The three most affine ligands, which besides a quinazoline scaffold share similarities with regard to the substitution pattern and the nature of the substituents, are UNC0638, UNC0642 and UNC0646. With binding affinities expressed as pKi values of 6.01 ± 0.10, 5.97 ± 0.05 and 6.23 ± 0.02, respectively, these compounds exceed the binding affinity of MB327 by more than one log unit. This renders them promising starting points for the development of drugs for the treatment of organophosphorus poisoning by addressing the MB327-PAM-1 binding site of the nAChR.

Symmetrical Bispyridinium Compounds Act as Open Channel Blockers of Cation-Selective Ion Channels

Current treatments against organophosphate poisoning (OPP) do not directly address effects mediated by the overstimulation of nicotinic acetylcholine receptors (nAChR). Non-oxime bispyridinium compounds (BPC) promote acetylcholine esterase-independent recovery of organophosphate-induced paralysis. Here, we test the hypothesis that they act by positive modulatory action on nAChRs. Using two-electrode voltage clamp analysis in combination with mutagenesis and molecular docking analysis, the potency and molecular mode of action of a series of nine BPCs was investigated on human α7 and muscle-type nAChRs expressed in Xenopus laevis oocytes. The investigated BPCs inhibited α7 and/or muscle-type nAChRs with IC50 values in the high nanomolar to high micromolar range. Further analysis of the most potent analogues revealed a noncompetitive, voltage-dependent inhibition. Co-application with the α7-selective positive allosteric modulator PNU120596 and generation of α7/5HT3 receptor chimeras excluded direct interaction with the PNU120596 binding site and binding to the extracellular domain of the α7 nAChR, suggesting that they act as open channel blockers (OCBs). Molecular docking supported by mutagenesis localized the BPC binding area in the outer channel vestibule between the extracellular and transmembrane domains. Analysis of BPC action on other cation-selective channels suggests a rather nonspecific inhibition of pentameric cation channels. BPCs have been shown to ameliorate organophosphate-induced paralysis in vitro and in vivo. Our data support molecular action as OCBs at α7 and muscle-type nAChRs and suggest that their positive physiological effects are more complex than anticipated and require further investigation.

Restoration of nerve agent impaired neuromuscular transmission in rat diaphragm by bispyridinium non-oximes - Structure-activity relationships

Organophosphate (OP) poisoning is currently treated with atropine, oximes and benzodiazepines. The nicotinic signs, i.e., respiratory impairment, can only be targeted indirectly via the use of oximes as reactivators of OP-inhibited acetylcholinesterase. Hence, compounds selectively targeting nicotinic acetylcholine receptors (nAChRs) might fundamentally improve current treatment options. The bispyridinium compound MB327 has previously shown some therapeutic effect against nerve agents in vitro and in vivo. Nevertheless, compound optimization was deemed necessary, due to limitations (e.g., toxicity and efficacy). The current study investigated a series of 4-tert-butyl bispyridinium compounds and of corresponding bispyridinium compounds without substituents in a rat diaphragm model using an indirect field stimulation technique. The length of the respective linker influenced the ability of the bispyridinium compounds to restore muscle function in rat hemidiaphragms. The current data show structure-activity relationships for a series of bispyridinium compounds and provide insight for future structure-based molecular modeling.

MS Binding Assays with UNC0642 as reporter ligand for the MB327 binding site of the nicotinic acetylcholine receptor

Intoxications with organophosphorus compounds (OPCs) based chemical warfare agents and insecticides may result in a detrimental overstimulation of muscarinic and nicotinic acetylcholine receptors evolving into a cholinergic crisis leading to death due to respiratory failure. In the case of the nicotinic acetylcholine receptor (nAChR), overstimulation leads to a desensitization of the receptor, which cannot be pharmacologically treated so far. Still, compounds interacting with the MB327 binding site of the nAChR like the bispyridinium salt MB327 have been found to re-establish the functional activity of the desensitized receptor. Only recently, a series of quinazoline derivatives with UNC0642 as one of the most prominent representatives has been identified to address the MB327 binding site of the nAChR, as well. In this study, UNC0642 has been utilized as a reporter ligand to establish new Binding Assays for this target. These assays follow the concept of MS Binding Assays for which by assessing the amount of bound reporter ligand by mass spectrometry no radiolabeled material is required. According to the results of the performed MS Binding Assays comprising saturation and competition experiments it can be concluded, that UNC0642 used as a reporter ligand addresses the MB327 binding site of the Torpedo-nAChR. This is further supported by the outcome of ex vivo studies carried out with poisoned rat diaphragm muscles as well as by in silico studies predicting the binding mode of UNC0646, an analog of UNC0642 with the highest binding affinity, in the recently proposed binding site of MB327 (MB327-PAM-1). With UNC0642 addressing the MB327 binding site of the Torpedo-nAChR, this and related quinazoline derivatives represent a promising starting point for the development of novel ligands of the nAChR as antidotes for the treatment of intoxications with organophosphorus compounds. Further, the new MS Binding Assays are a potent alternative to established assays and of particular value, as they do not require the use of radiolabeled material and are based on a commercially available compound as reporter ligand, UNC0642, exhibiting one of the highest binding affinities for the MB327 binding site known so far.

Review of Possible Therapies in Treatment of Novichoks Poisoning and HAZMAT/CBRNE Approaches: State of the Art

Novichoks-organophosphorus compounds belong to the nerve agents group, constituting the fourth generation of chemical warfare agents. The tremendous toxicity of Novichoks is assumed to be several times greater than that of VX, whereas no published experimental research supports this. They were surreptitiously created during the Cold War by the Soviet Union. Novichok’s toxic action mechanism consists of the inhibition of acetylcholinesterase activity. The review includes data on treating poisoning caused by OPs which could be used as guidelines for the therapy in case of Novichok exposure and HAZMAT/CBRNE approaches. Novichoks pose a severe threat due to their toxicity; however, there is insufficient information about the identity of A-series nerve agents. Filling in the missing data gaps will accelerate progress in improving protection against Novichoks and developing optimal therapy for treating poisoning casualties. Furthermore, introducing solutions to protect medical personnel in contact with a hazardous substance increases the chances of saving casualties of HAZMAT/CBRNE incidents.

Neurotoxicity evoked by organophosphates and available countermeasures

Organophosphorus compounds (OP) are a constant problem, both in the military and in the civilian field, not only in the form of acute poisoning but also for their long-lasting consequences. No antidote has been found that satisfactorily protects against the toxic effects of organophosphates. Likewise, there is no universal cure to avert damage after poisoning. The key mechanism of organophosphate toxicity is the inhibition of acetylcholinesterase. The overstimulation of nicotinic or muscarinic receptors by accumulated acetylcholine on a synaptic cleft leads to activation of the glutamatergic system and the development of seizures. Further consequences include generation of reactive oxygen species (ROS), neuroinflammation, and the formation of various other neuropathologists. In this review, we present neuroprotection strategies which can slow down the secondary nerve cell damage and alleviate neurological and neuropsychiatric disturbance. In our opinion, there is no unequivocal approach to ensure neuroprotection, however, sooner the neurotoxicity pathway is targeted, the better the results which can be expected. It seems crucial to target the key propagation pathways, i.e., to block cholinergic and, foremostly, glutamatergic cascades. Currently, the privileged approach oriented to stimulating GABA_AR by benzodiazepines is of limited efficacy, so that antagonizing the hyperactivity of the glutamatergic system could provide an even more efficacious approach for terminating OP-induced seizures and protecting the brain from permanent damage. Encouraging results have been reported for tezampanel, an antagonist of GluK1 kainate and AMPA receptors, especially in combination with caramiphen, an anticholinergic and anti-glutamatergic agent. On the other hand, targeting ROS by antioxidants cannot or already developed neuroinflammation does not seem to be very productive as other processes are also involved.

Modelling organophosphate intoxication in C. elegans highlights nicotinic acetylcholine receptor determinants that mitigate poisoning

Organophosphate intoxication via acetylcholinesterase inhibition executes neurotoxicity via hyper stimulation of acetylcholine receptors. Here, we use the organophosphate paraoxon-ethyl to treat C. elegans and use its impact on pharyngeal pumping as a bio-assay to model poisoning through these neurotoxins. This assay provides a tractable measure of acetylcholine receptor mediated contraction of body wall muscle. Investigation of the time dependence of organophosphate treatment and the genetic determinants of the drug-induced inhibition of pumping highlight mitigating modulation of the effects of paraoxon-ethyl. We identified mutants that reduce acetylcholine receptor function protect against the consequence of intoxication by organophosphates. Data suggests that reorganization of cholinergic signalling is associated with organophosphate poisoning. This reinforces the under investigated potential of using therapeutic approaches which target a modulation of nicotinic acetylcholine receptor function to treat the poisoning effects of this important class of neurotoxins.

Supplemental treatment to atropine improves the efficacy to reverse nerve agent induced bronchoconstriction

Exposure to highly toxic organophosphorus compounds causes inhibition of the enzyme acetylcholinesterase resulting in a cholinergic toxidrome and innervation of receptors in the neuromuscular junction may cause life-threatening respiratory effects. The involvement of several receptor systems was therefore examined for their impact on bronchoconstriction using an ex vivo rat precision-cut lung slice (PCLS) model. The ability to recover airways with therapeutics following nerve agent exposure was determined by quantitative analyses of muscle contraction. PCLS exposed to nicotine resulted in a dose-dependent bronchoconstriction. The neuromuscular nicotinic antagonist tubocurarine counteracted the nicotine-induced bronchoconstriction but not the ganglion blocker mecamylamine or the common muscarinic antagonist atropine. Correspondingly, atropine demonstrated a significant airway relaxation following ACh-exposure while tubocurarine did not. Atropine, the M3 muscarinic receptor antagonist 4-DAMP, tubocurarine, the β₂-adrenergic receptor agonist formoterol, the Na⁺-channel blocker tetrodotoxin and the K_ATP-channel opener cromakalim all significantly decreased airway contractions induced by electric field stimulation. Following VX-exposure, treatment with atropine and the Ca²⁺-channel blocker magnesium sulfate resulted in significant airway relaxation. Formoterol, cromakalim and magnesium sulfate administered in combinations with atropine demonstrated an additive effect. In conclusion, the present study demonstrated improved airway function following nerve agent exposure by adjunct treatment to the standard therapy of atropine.

Influence of Experimental End Point on the Therapeutic Efficacy of Essential and Additional Antidotes in Organophosphorus Nerve Agent-Intoxicated Mice

The therapeutic efficacy of treatments for acute intoxication with highly toxic organophosphorus compounds, called nerve agents, usually involves determination of LD₅₀ values 24 h after nerve agent challenge without and with a single administration of the treatment. Herein, the LD₅₀ values of four nerve agents (sarin, soman, tabun and cyclosarin) for non-treated and treated intoxication were investigated in mice for experimental end points of 6 and 24 h. The LD₅₀ values of the nerve agents were evaluated by probit-logarithmical analysis of deaths within 6 and 24 h of i.m. challenge of the nerve agent at five different doses, using six mice per dose. The efficiency of atropine alone or atropine in combination with an oxime was practically the same at 6 and 24 h. The therapeutic efficacy of the higher dose of the antinicotinic compound MB327 was slightly higher at the 6 h end point compared to the 24 h end point for soman and tabun intoxication. A higher dose of MB327 increased the therapeutic efficacy of atropine alone for sarin, soman and tabun intoxication, and that of the standard antidotal treatment (atropine and oxime) for sarin and tabun intoxication. The therapeutic efficacy of MB327 was lower than the oxime-based antidotal treatment. To compare the 6 and 24 h end points, the influence of the experimental end point was not observed, with the exception of the higher dose of MB327. In addition, only a negligible beneficial impact of the compound MB327 was observed. Nevertheless, antinicotinics may offer an additional avenue for countering poisoning by nerve agents that are difficult to treat, and synthetic and biological studies towards the development of such novel drugs based on the core bispyridinium structure or other molecular scaffolds should continue.

Antagonistic postsynaptic and presynaptic actions of cyclohexanol on neuromuscular synaptic transmission and function

Intentional ingestion of agricultural organophosphorus insecticides is a significant public health issue in rural Asia, causing thousands of deaths annually. Some survivors develop a severe, acute or delayed myasthenic syndrome. In animal models, similar myasthenia has been associated with increasing plasma concentration of one insecticide solvent metabolite, cyclohexanol. We investigated possible mechanisms using voltage and current recordings from mouse neuromuscular junctions (NMJs) and transfected human cell lines. Cyclohexanol (10-25 mM) reduced endplate potential (EPP) amplitudes by 10-40% and enhanced depression during repetitive (2-20 Hz) stimulation by up to 60%. EPP decay was prolonged more than twofold. Miniature EPPs were attenuated by more than 50%. Cyclohexanol inhibited whole-cell currents recorded from CN21 cells expressing human postjunctional acetylcholine receptors (hnAChR) with an IC₅₀ of 3.74 mM. Cyclohexanol (10-20 mM) also caused prolonged episodes of reduced-current, multi-channel bursting in outside-out patch recordings from hnAChRs expressed in transfected HEK293T cells, reducing charge transfer by more than 50%. Molecular modelling indicated cyclohexanol binding (-6 kcal/mol) to a previously identified alcohol binding site on nicotinic AChR α-subunits. Cyclohexanol also increased quantal content of evoked transmitter release by ∼50%. In perineurial recordings, cyclohexanol selectively inhibited presynaptic K⁺ currents. Modelling indicated cyclohexanol binding (-3.8 kcal/mol) to voltage-sensitive K⁺ channels at the same site as tetraethylammonium (TEA). TEA (10 mM) blocked K⁺ channels more effectively than cyclohexanol but EPPs were more prolonged in 20 mM cyclohexanol. The results explain the pattern of neuromuscular dysfunction following ingestion of organophosphorus insecticides containing cyclohexanol precursors and suggest that cyclohexanol may facilitate investigation of mechanisms regulating synaptic strength at NMJs. KEY POINTS: Intentional ingestion of agricultural organophosphorus insecticides is a significant public health issue in rural Asia, causing thousands of deaths annually. Survivors may develop a severe myasthenic syndrome or paralysis, associated with increased plasma levels of cyclohexanol, an insecticide solvent metabolite. Analysis of synaptic transmission at neuromuscular junctions in isolated mouse skeletal muscle, using isometric tension recording and microelectrode recording of endplate voltages and currents, showed that cyclohexanol reduced postsynaptic sensitivity to acetylcholine neurotransmitter (reduced quantal size) while simultaneously enhancing evoked transmitter release (increased quantal content). Patch recording from transfected cell lines, together with molecular modelling, indicated that cyclohexanol causes selective, allosteric antagonism of postsynaptic nicotinic acetylcholine receptors and block of presynaptic K⁺ -channel function. The data provide insight into the cellular and molecular mechanisms of neuromuscular weakness following intentional ingestion of agricultural organophosphorus insecticides. Our findings also extend understanding of the effects of alcohols on synaptic transmission and homeostatic synaptic function.

Counteracting poisoning with chemical warfare nerve agents

Phosphylation of the pivotal enzyme acetylcholinesterase (AChE) by nerve agents (NAs) leads to irreversible inhibition of the enzyme and accumulation of neurotransmitter acetylcholine, which induces cholinergic crisis, that is, overstimulation of muscarinic and nicotinic membrane receptors in the central and peripheral nervous system. In severe cases, subsequent desensitisation of the receptors results in hypoxia, vasodepression, and respiratory arrest, followed by death. Prompt action is therefore critical to improve the chances of victim's survival and recovery. Standard therapy of NA poisoning generally involves administration of anticholinergic atropine and an oxime reactivator of phosphylated AChE. Anticholinesterase compounds or NA bioscavengers can also be applied to preserve native AChE from inhibition. With this review of 70 years of research we aim to present current and potential approaches to counteracting NA poisoning.

Influence of experimental end point on the therapeutic efficacy of the antinicotinic compounds MB408, MB442 and MB444 in treating nerve agent poisoned mice - a comparison with oxime-based treatment

Therapeutic efficacy of antidotal treatment of acute poisoning by nerve agents is generally assessed by the evaluation of LD₅₀ values of nerve agents over 24 h following poisoning without or with a single administration of antidotal treatment. In this study, LD₅₀ values of four nerve agents (sarin, soman, tabun and cyclosarin) for non-treated and treated poisoning were evaluated in mice for two experimental end points - 6 h and 24 h. While the efficacy of atropine or oxime-based antidotal treatment was the same regardless of the experimental end point, the therapeutic efficacy of all three newly developed bispyridinium non-oxime compounds (MB408, MB442, and MB444) was mostly slightly higher at the 6 h end point compared to the 24 h end point, although the therapeutic efficacy of MB compounds was not superior to oxime-based antidotal treatment. These results contrast with a study in guinea-pigs using a structurally-related compound, MB327, which showed a striking increase in protection at 6 h compared to 24 h. It is suggested that the disparity may be due to pharmacokinetic differences between the two animal species.

New Resensitizers for the Nicotinic Acetylcholine Receptor by Ligand-Based Pharmacophore Modeling

INTRODUCTION

Irreversible inhibition of the acetylcholinesterase upon intoxication with organophosphorus compounds leads to an accumulation of acetylcholine in the synaptic cleft and a subsequent desensitization of nicotinic acetylcholine receptors which may ultimately result in respiratory failure. A direct intervention at the nicotinic acetylcholine receptor (nAChR) was proposed as an alternative therapeutic approach to the treatment with atropine and oximes.

METHODS

The bispyridinium compound MB327 has been found to recover functional activity of nAChR thus representing a promising starting point for the development of new drugs for the treatment of organophosphate poisoning. Recent solid-supported membrane-based electrophysiological experiments have identified symmetrically substituted bispyridinium compounds e.g. MB327, MB583, and PTM0001 that are able to resensitize nAChR of Torpedo californica. In addition, six compounds have been found not to show any resensitizing potential and were thus classified as inactive. This set of active and inactive bispyridinium compounds was taken to develop a pharmacophore model and in silico screening of a virtual database of bispyridinium compounds to identify new compounds that are able to restore the functional activity of desensitized nAChR.

RESULTS

Screening of a virtual compound database of symmetrically substituted bispyridinium compounds with the derived pharmacophore yielded several promising compounds which satisfy the pharmacophore and ought to have the same or even better resensitizing effect on nAChR as the parent compound MB327.

COPD and asthma therapeutics for supportive treatment in organophosphate poisoning

Context: Nerve agents like sarin or VX have repeatedly been used in military conflicts or homicidal attacks, as seen in Syria or Malaysia 2017. Together with pesticides, nerve agents assort as organophosphorus compounds (OP), which inhibit the enzyme acetylcholinesterase. To counteract subsequent fatal symptoms due to acetylcholine (ACh) accumulation, oximes plus atropine are administered, a regimen that lacks efficacy in several cases of OP poisoning. New therapeutics are in development, but still need evaluation before clinical employment. Supportive treatment with already approved drugs presents an alternative, whereby compounds from COPD and asthma therapy are likely options. A recent pilot study by Chowdhury et al. included β2-agonist salbutamol in the treatment of OP-pesticide poisoned patients, yielding ambiguous results concerning the addition. Here, we provide experimental data for further investigations regarding the value of these drugs in OP poisoning. Methods: By video-microscopy, changes in airway area were analyzed in VX-poisoned rat precision cut lung slices (PCLS) after ACh-induced airway contraction and subsequent application of selected anticholinergics/β2-agonists. Results: Glycopyrrolate and ipratropium efficiently antagonized an ACh-induced airway contraction in VX-poisoned PCLS (EC₅₀ glycopyrrolate 15.8 nmol/L, EC₅₀ ipratropium 2.3 nmol/L). β2-agonists formoterol and salbutamol had only negligible effects when solely applied in the same setting. However, combination of formoterol or salbutamol with low dosed glycopyrrolate or atropine led to an additive effect compared to the sole application [50.6 ± 8.8% airway area increase after 10 nmol/L formoterol +1 nmol/L atropine versus 11.7 ± 9.2% (10 nmol/L formoterol) or 8.6 ± 5.9% (1 nmol/L atropine)]. Discussion: We showed antagonizing effects of anticholinergics and β2-agonists on ACh-induced airway contractions in VX-poisoned PCLS, thus providing experimental data to support a prospective comprehensive clinical study. Conclusions: Our results indicate that COPD and asthma therapeutics could be a valuable addition to the treatment of OP poisoning.

Some benefit from non-oximes MB408, MB442 and MB444 in combination with the oximes HI-6 or obidoxime and atropine in antidoting sarin or cyclosarin poisoned mice

The effect of three newly developed bispyridinium non-oxime compounds (MB408, MB442, and MB444) on the therapeutic efficacy of a standard antidotal treatment (atropine in combination with the oxime HI-6 or obidoxime) of acute poisoning by two nerve agents (sarin and cyclosarin) in mice was studied. The therapeutic efficacy of atropine in combination with an oxime with or without one of the bispyridinium non-oximes was evaluated by determination of the 24 h LD₅₀ values of the nerve agents studied and by measurement of the survival time after supralethal poisoning. Addition of all tested non-oximes increased the therapeutic efficacy of atropine in combination with an oxime against sarin poisoning; however, the differences were not significant. The non-oximes also positively influenced the number of surviving mice 6 h after supralethal poisoning with sarin. In the case of cyclosarin, they were also slightly beneficial in the treatment of acute poisoning. The higher dose of MB444 was able to significantly increase the therapeutic efficacy of standard antidotal treatment of poisoning with cyclosarin. The benefit of each bispyridinium non-oxime compound itself was obviously dose-dependent. In summary, the addition of MB compounds to the standard antidotal treatment of acute nerve agent poisoning was beneficial for the antidotal treatment of sarin or cyclosarin poisoning, although their benefit at 24 h after poisoning was not significant, with the exception of the higher dose of MB444 against cyclosarin.

Human small bowel as a useful tool to investigate smooth muscle effects of potential therapeutics in organophosphate poisoning

Isolated organs proofed to be a robust tool to study effects of (potential) therapeutics in organophosphate poisoning. Small bowel samples have been successfully used to reveal smooth muscle relaxing effects. In the present study, the effects of obidoxime, TMB-4, HI-6 and MB 327 were investigated on human small bowel tissue and compared with rat data. Hereby, the substances were tested in at least seven different concentrations in the jejunum or ileum both pre-contracted with carbamoylcholine. Additionally, the cholinesterase activity of native tissue was determined. Human small intestine specimens showed classical dose response-curves, similar to rat tissue, with MB 327 exerting the most potent smooth muscle relaxant effect in both species (human EC₅₀=0.7×10^-5M and rat EC₅₀=0.7×10^-5M). The AChE activity for human and rat samples did not differ significantly (rat jejunum=1351±166 mU/mg wet weight; rat ileum=1078±123 mU/mg wet weight; human jejunum=1030±258 mU/mg wet weight; human ileum=1293±243 mU/mg wet weight). Summarizing, our isolated small bowel setup seems to be a solid tool to investigate the effects of (potential) therapeutics on pre-contracted smooth muscle, with data being transferable between rat and humans.

Counteracting desensitization of human α7-nicotinic acetylcholine receptors with bispyridinium compounds as an approach against organophosphorus poisoning

Irreversible inhibition of acetylcholinesterase (AChE) resulting in accumulation of acetylcholine and overstimulation of muscarinic and nicotinic receptors accounts for the acute toxicity of organophosphorus compounds (OP). Accordingly, the mainstay pharmacotherapy against poisoning by OP comprises the competitive muscarinic acetylcholine receptor antagonist atropine to treat muscarinic effects and, in addition, oximes to reactivate inhibited AChE. A therapeutic gap still remains in the treatment of desensitized nicotinic acetylcholine receptors following OP exposure. Hereby, nicotinic effects result in paralysis of the central and peripheral respiratory system if untreated. Thus, these receptors pose an essential target for therapeutic indication to address these life-threatening nicotinic symptoms of the cholinergic crisis. Identification of ligands regulating dynamic transitions between functional states by binding to modulatory sites appears to be a promising strategy for therapeutic intervention. In this patch clamp study, the ability of differently substituted bispyridinium non-oximes to "resensitize" i.e. to recover the activity of desensitized human homomeric α7-type nAChRs stably transfected in CHO cells was investigated and compared to the already described α7-specific positive allosteric modulator PNU-120596. The structures of these bispyridinium analogues were based on the lead structure of the tert-butyl-substituted bispyridinium propane MB327, which has been shown to have a positive therapeutic effect due to a non-competitive antagonistic action at muscle-type nAChRs in vivo and has been found to have a positive allosteric activity at neuronal receptors in vitro. Prior to test compounds, desensitization of hα7-nAChRs was verified by applying an excess of nicotine revealing activation at low, and desensitization at high concentrations. Thereby, desensitization could be reduced by modulation with PNU-120596. Desensitization was further verified by dose-response profiles of agonists, carbamoylcholine and epibatidine in the absence and presence of PNU-120596. Although less pronounced than PNU-120596 and the lead structure MB327, bispyridinium compounds, particularly those substituted at position 3 and 4, resensitized the nicotine desensitized hα7-nAChRs in a concentration-dependent manner and prolonged the mean channel open time. In summary, identification of more potent compounds able to restore nAChR function in OP intoxication is needed for development of a putative efficient antidote.

Development of MS Binding Assays targeting the binding site of MB327 at the nicotinic acetylcholine receptor

The bispyridinium compound MB327 has been shown previously to have a positive pharmacological effect against poisoning with organophosphorous compounds (OPCs). The mechanism by which it exerts its therapeutic effect seems to be directly mediated by the nicotinic acetylcholine receptor (nAChR). In the present study, the development of mass spectrometry based binding assays (MS Binding Assays) for characterization of the binding site of MB327 at the nAChR from Torpedo californica is described. MS Binding Assays follow the principle of radioligand binding assays, but do not, in contrast to the latter, require a radiolabeled reporter ligand, as the readout is in this case based on mass spectrometric detection. For [²H₆]MB327, a deuterated MB327 analogue employed as reporter ligand in the MS Binding Assays, an LC-ESI-MS/MS method was established allowing for its fast and reliable quantification in samples resulting from binding experiments. Using centrifugation for separation of non-bound [²H₆]MB327 from target-bound [²H₆]MB327 in saturation and autocompetition experiments (employing native MB327 as competitor) enabled reliable determination of specific binding. In this way, the affinities for [²H₆]MB327 (K_d=15.5±0.9μmolL^-1) and for MB327 (K_i=18.3±2.6μmolL^-1) towards the nAChR could be determined for the first time. The almost exactly matching affinities for MB327 and [²H₆]MB327 obtained in the MS Binding Assays are in agreement with potencies previously found in functional studies. In summary, our results demonstrate that the established MS Binding Assays represent a promising tool for affinity determination of test compounds towards the binding site of MB327 at the nAChR.

Evaluation of the Influence of Three Newly Developed Bispyridinium Anti-nicotinic Compounds (MB408, MB442, MB444) on the Efficacy of Antidotal Treatment of Nerve Agent Poisoning in Mice

The influence of three newly developed bispyridinium antinicotinic compounds (the non-oximes MB408, MB442 and MB444) on the therapeutic efficacy of a standard antidotal treatment (atropine in combination with an oxime) of acute poisoning by the organophosphorus nerve agents tabun and soman was studied in mice. The therapeutic efficacy of atropine in combination with an oxime with or without one of the bispyridinium non-oximes was evaluated by determination of the LD₅₀ values of the nerve agents and measurement of the survival time after supralethal poisoning. Addition of all the tested non-oximes increased significantly the therapeutic efficacy of atropine in combination with an oxime against tabun poisoning. They also positively influenced the number of surviving mice 6 hr after supralethal poisoning with tabun. However, they were only slightly effective for the treatment of soman poisoning. The benefit of the tested bispyridinium non-oximes was dose-dependent. To conclude, the addition of bispyridinium non-oximes to the standard antidotal treatment of acute poisoning with tabun was beneficial regardless of the chosen non-oxime, but only slightly beneficial in the case of soman poisoning.

Oximes in organophosphate poisoning: 60 years of hope and despair

The high number of annual fatalities following suicidal poisoning by organophosphorus (OP) pesticides and the recent homicidal use of the chemical warfare nerve agent sarin against civilian population in Syria underlines the continuous threat by these highly toxic agents. The need for an effective treatment of OP poisoning resulted in the implementation of a combination therapy with the muscarinic receptor antagonist atropine and an oxime for the reactivation of OP-inhibited acetylcholinesterase (AChE). Since the invention of the first clinically used oxime pralidoxime (2-PAM) in the 1950s ongoing research attempted to identify more effective oximes. In fact, several thousand oximes were synthesized in the past six decades. These include charged and non-charged compounds, mono- and bispyridinium oximes, asymmetric oximes, oximes with different substitutes and more recently non-oxime reactivators. Multiple in vitro and in vivo studies investigated the potential of oximes to reactivate OP-inhibited AChE and to reverse OP-induced cholinergic signs. Depending on the experimental model, the investigated species and the tested OP largely variable results were obtained by different laboratories. These findings and the inconsistent effectiveness of oximes in the treatment of OP-pesticide poisoned patients led to a continuous discussion on the value of oximes. In order to provide a forward-looking evaluation of the significance of oximes in OP poisoning multiple aspects, including intrinsic toxicity, in vitro reactivation potency, efficacy and pharmacokinetics, as well as the impact of the causative OP have to be considered. The different influencing factors in order to define the benefit and limitations of oximes in OP poisoning will be discussed.

Reversed-phase ion-pair chromatography-diode array detection of the bispyridinium compound MB327: plasma analysis of a potential novel antidote for the treatment of organophosphorus poisoning

In the case of poisoning by organophosphorus nerve agents or pesticides, there is still a lack of pharmacological treatment of the cholinergic crisis selectively targeting the nicotinic acetylcholine receptor. Recently, the compound MB327 was identified as a potential novel lead structure to close this gap, thus demanding a quantitative assay for initial pharmacokinetic (PK) studies. MB327 is a salt consisting of the dicationic bispyridinium compound (BPC) 1,1´-(propane-1,3-diyl)bis(4-tert-butylpyridinium) and two iodide counter ions. Due to the permanent positive charge of the BPC, an isocratic reversed-phase ion-pair chromatographic separation (RPIPC) was developed using heptanesulfonic acid as ion-pairing reagent and 45% v/v methanol as organic modifier (1 mL/min). Selective UV-detection (230 nm) was done by a diode array detector (DAD) for reliable, rugged, precise (RSD < 7%) and accurate (96-104%) quantitative analysis of 50 μL swine plasma (linear range 1-1000 µg BPC/mL plasma, lower limit of quantification 2 µg/mL). During method validation, diverse parameters essential for the chromatographic process were investigated to generate van´t Hoff, van Deemter and width plots allowing calculation of thermodynamic data like the distribution constant K (5.7 ± 0.3), change in enthalpy, ΔH(0) : -23.66 kJ/mol, and entropy, ΔS(0) : -65 J/(mol*K). In addition, RPIPC-DAD analysis enabled calculation of molar absorptivities of the BPC, ε230 : 17 400 ± 1100 L/(mol*cm), and iodide, ε230 : 9900 ± 400 L/(mol*cm), which determination was hampered by interference with each other in conventional cuvette UV-spectrophotometric measurements. Finally, the RPIPC-DAD procedure was applied to samples from an in vivo study of swine.

Drug development for the management of organophosphorus poisoning

INTRODUCTION

The continuous application of organophosphate pesticides in developing countries, in addition to the remaining stock piles of chemical warfare nerve agents and their possible use is a significant threat to the public. Yet, today's options for a treatment of organophosphorus poisonings are still inadequate.

AREAS COVERED

This article provides a concise overview of current and future research trying to improve both prophylaxis and treatment of organophosphorus intoxications. The authors provide a summary of current oxime therapy and highlight several new concepts to overcome existing gaps. This overview of therapeutic options is accompanied by two sections on cyclodextrins, related compounds and bioscavengers, which may be used for either prophylaxis or treatment. For both groups, the authors review current drug design and screening approaches, the resulting developments and future challenges.

EXPERT OPINION

While the search for one multipotent oxime has been a fruitless endeavor, combination of multiple oximes with complemental and systemic reactivity appears as a valuable concept. Development of potential scavengers, be it cyclodextrins or bioscavengers, is still hampered by insufficient efficacy of these compounds. Future strategies will aim at improving their catalytic efficacy while minimizing immunogenicity.

Limitations and challenges in treatment of acute chemical warfare agent poisoning

Recent news from Syria on a possible use of chemical warfare agents made the headlines. Furthermore, the motivation of terrorists to cause maximal harm shifts these agents into the public focus. For incidents with mass casualties appropriate medical countermeasures must be available. At present, the most important threats arise from nerve agents and sulfur mustard. At first, self-protection and protection of medical units from contamination is of utmost importance. Volatile nerve agent exposure, e.g. sarin, results in fast development of cholinergic crisis. Immediate clinical diagnosis can be confirmed on-site by assessment of acetylcholinesterase activity. Treatment with autoinjectors that are filled with 2mg atropine and an oxime (at present obidoxime, pralidoxime, TMB-4 or HI-6) are not effective against all nerve agents. A more aggressive atropinisation has to be considered and more effective oximes (if possible with a broad spectrum or a combination of different oximes) as well as alternative strategies to cope with high acetylcholine levels at synaptic sites should be developed. A further gap exists for the treatment of patients with sustained cholinergic crisis that has to be expected after exposure to persistent nerve agents, e.g. VX. The requirement for long-lasting artificial ventilation can be reduced with an oxime therapy that is optimized by using the cholinesterase status for guidance or by measures (e.g. scavengers) that are able to reduce the poison load substantially in the patients. For sulfur mustard poisoning no specific antidote is available until now. Symptomatic measures as used for treatment of burns are recommended together with surgical or laser debridement. Thus, huge amounts of resources are expected to be consumed as wound healing is impaired. Possible depots of sulfur mustard in tissues may aggravate the situation. More basic knowledge is necessary to improve substantially therapeutic options. The use of stem cells may provide a new and promising option.

Effect of MB327 and oximes on rat intestinal smooth muscle function

Organophosphorous compounds (OP) are highly toxic compounds. Great efforts have been undertaken in the last decades to develop new reactivators of OP-inhibited acetylcholinesterase. So far, a broad-spectrum oxime bearing efficacy against all OP is still missing and alternative approaches are presently under investigation. Previous experiments demonstrated that the bispyridinium non-oxime MB327 was able to improve OP-impaired muscle force in human, rat and guinea pig respiratory muscles and to increase survival in soman, sarin and tabun poisoned guinea pigs. Recent studies indicate that MB327 exhibits a high affinity to muscarinic acetylcholine receptors but up to now, only scarce information is available on the effects of MB327 in isolated organs. Now, the antimuscarinic effect of MB327 was compared to that of established oximes and atropine in a rat jejunum smooth muscle model. MB327 showed a fully reversible smooth muscle relaxing effect at lower concentrations (EC₅₀≈ 6 μM) than all tested oximes. In fact, MB327 exhibited an antimuscarinic smooth muscle relaxing effect at concentrations which were shown to improve OP-impaired skeletal muscle force. Hence, it may be assumed that the antimuscarinic potency of MB327 may contribute to its therapeutic effect in OP poisoning.

The summary on non-reactivation cholinergic properties of oxime reactivators: the interaction with muscarinic and nicotinic receptors

Organophosphorus inhibitors (OP) of acetylcholinesterase (AChE) represent a group of highly toxic compounds. The treatment of OP intoxication is, however, insufficiently ensured. Currently, two main categories of drugs-anticholinergics and oxime reactivators- are employed as antidotes. Oximes have been reported to act at several levels of the cholinergic transmission, and among the non-reactivation effects, the interaction with cholinergic receptors stands out. This review addresses issues correlated with non-reactivating effects of oxime reactivators with a special focus on the muscarinic and nicotinic receptors, but involvement of other cholinergic structures such as AChE and choline uptake carriers are discussed too. It can be concluded that the oxime reactivators show a variation in their antagonistic effect on the muscarinic and nicotinic receptors, which is likely to be of significance in the treatment of OP poisoning. In vitro data reported oximes to exert higher efficacy on the muscarinic M2 subtype than on the AChE. However, this effect seemed to be subtype specific since the antagonistic M3 effect was lower. Also, and importantly, the antimuscarinic effect was larger than that on nicotinic receptors. Even though atropine showed a much higher muscarinic antagonism, it is supposed that non-reactivation properties of oxime reactivators play a significant role in the treatment of OP poisoning.