Efficacy of the antinicotinic compound MB327 against soman poisoning – Importance of experimental end point

The synthesis and influence of the novel bispyridinium compound LB1 on the effectiveness of the standard antidotal treatment of organophosphorus nerve agent intoxicated mice and some structure-activity considerations

The design of MB327, a bispyridinium compound that ameliorates the nicotinic effects of acute organophosphorus nerve agent (NA) intoxication, followed an observation made by the German pharmacologist Klaus Schoene in the 1970s, who noted therapeutic activity in bispyridinium molecules missing the usual oxime group, CHNOH. Some of these compounds protected mice against soman. One structurally related to obidoxime called HY10 had this action. Its oxime moieties were capped by tert-butyl groups: CH=NOtBu. We modified HY10 by changing the bridge between the pyridinium units from a dimethylene ether to a trimethylene group (CH2OCH2 → CH2CH2CH2) and prepared a novel relative of trimedoxime, called LB1, whose synthesis and stereochemistry are described. Unlike obidoxime or trimedoxime, LB1 because of its capped oxime groups, cannot directly reactivate NA inhibited acetylcholinesterase. Its antidotal activity in mice is now reported. The therapeutic efficacy of LB1, atropine alone, atropine with LB1, atropine with an oxime (HI-6, obidoxime or trimedoxime), and atropine with an oxime and LB1, was studied by determining the LD50 values of the NAs soman, sarin, or tabun in mice treated with these compounds or mixtures. LB1 exceeded MB327 in toxicity and its activity was insufficient for a useful addition to the current standard antidotal treatment (protective ratio data are compared to those of MB327). Although this study produced largely negative biological results, the therapeutically beneficial mechanism of the effective bispyridinium non-oxime analogues is unclear, and has been demonstrated only in vivo. The present study points out directions in structural optimisation unlikely to yield the desired therapeutic outcomes and provides a literature review that could promote creative thinking for the design of widely-desirable non-oxime therapeutics for anticholinesterase inhibitors.

Structure-activity studies of bispyridinium antinicotinics to select candidates to treat soman intoxication as part of a combined therapy

The standard treatment of atropine and oximes is insufficiently effective against all organophosphorus nerve agents. Bispyridinium non-oxime nicotinic antagonists are promising components to add to treatments. One of these, MB327, improves the survival of guinea-pigs after intoxication with tabun, sarin or soman. We extend our previous study of unsubstituted bispyridinium non-oximes with C1 to C10 alkane linkers to analogues having 4-tert-butylpyridinium rings and the same linker range. We report their effects on nicotinic-mediated calcium responses in muscle-derived (CN21) cells where nicotinic responses were inhibited in a concentration-dependent manner. A clear structure-activity relationship resulted: the inhibitory potency increased as the linker lengthened. Previous data showed the inhibition of human acetylcholinesterase in vitro increased similarly and that in general the toxicity to mice increased accordingly. However, the shorter analogues MB327 (4-tert-butyl C3) and MB442 (unsubstituted C5) compared favourably in toxicity to some oximes used to treat nerve agent poisoning. Like MB327, the non-oxime MB442, selected by the process described, improved the survival of guinea-pigs intoxicated with soman when combined with hyoscine and physostigmine or atropine and avizafone. Our research has now afforded two compounds able to protect guinea-pigs against nerve agent toxicity through a mechanism not previously exploited deliberately for this purpose.

A molecular docking-based comparative assessment of various anticholinergic drugs as antidotes to different nerve agent poisoning

Nerve agent poisoning is still a threat to civilization. Nerve agents function by binding with the enzyme acetylcholinesterase irreversibly. Accumulation of acetylcholine in the synapse causes over-stimulation of muscarinic and nicotinic acetylcholinergic receptors. Thus miosis, glandular hyper secretion, bronchoconstriction, vomiting, diarrhea and bradycardia occurs (by M1-M5 receptors stimulation); whereas convulsion and seizures occur due to the nicotinic receptors. Atropine is a non-selective muscarinic antagonists but no nicotinic antagonists are known. Seizures are controlled by diazepam. Enzyme aging occurs without treatment which causes the enzyme resistant to oxime therapy. Though numerous wet-lab based works has carried out, however, recent time there is an over-growing trend to make comparative assessment of drugs and toxicants. Here we made a molecular docking based comparative assessments between nerve agents toxicity and efficacy of different drugs to prevent this toxicity. Our results suggest that VX is the most harmful organophosphate nerve agents and HI-6 is the best drug followed by Obidoxime and Pralidoxime to free acetylcholinestarase. Docking results correspond the data trend of different in vivo experiments for the assessment of severity of different nerve agents and/or effectiveness of different antidote drugs. Our study reinforces the utility of pretreatment of the enzyme with a carbamic acid derivative like Pyridostigmine bromide which inhibits the enzyme reversibly to a smaller extent and thus, prevent the enzyme from aging and the nerve agent binding.Communicated by Ramaswamy H. Sarma.

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.

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.

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.

3-Quinuclidinyl-α-methoxydiphenylacetate: A multi-targeted ligand with antimuscarinic and antinicotinic effects designed for the treatment of anticholinesterase poisoning

Racemic 3-quinuclidinyl-α-methoxydiphenylacetate (MB266) was synthesised. Its activity at muscarinic acetylcholine receptors (mAChRs), and muscle and neuronal nicotinic acetylcholine receptors (nAChRs), was compared to that of atropine and racemic 3-quinucidinyl benzilate (QNB) using a functional assay based on agonist-induced elevation of intracellular calcium ion concentration in CN21, Chinese Hamster Ovary (CHO) and SHSY5Y human cell lines. MB266 acted as an antagonist at acetylcholine receptors, displaying 18-fold selectivity for mAChR versus nAChR (compared to the 15,200-fold selectivity observed for QNB). Thus O-methylation of QNB reduced the affinity for mAChR antagonism and increased the relative potency at both muscle and neuronal nAChRs. Despite MB266 having a pharmacological profile potentially useful for the treatment of anticholinesterase poisoning, its administration did not improve the neuromuscular function in a soman-poisoned guinea-pig diaphragm preparation pretreated with the organophosphorus nerve agent soman. Consideration should be given to exploring the potential of MB266 for possible anticonvulsant action in vitro as part of a multi-targeted ligand approach.

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.