In vitro reactivation potency of acetylcholinesterase reactivators--K074 and K075--to reactivate tabun-inhibited human brain cholinesterases

Synthesis and evaluation of small organic molecule as reactivator of organophosphorus inhibited acetylcholinesterase

A series of uncharged salicylaldehyde oximes were synthesized and evaluated for the reactivation of organophosphorus (OP) nerve agents simulants Diethylchlorophosphonate (DCP) & Diethylcyanophosphonate (DCNP) and pesticides (paraoxon & malaoxon) inhibited electric eel Acetylcholinesterase (AChE). The computational software Swiss ADME and molinspiration were used to unfold the probability of drug-likeness properties of the oximes derivatives. Substituted aromatic oximes with diethylamino or bromo group with free hydroxyl group ortho to oxime moiety were found efficient to regenerate the enzymatic activity in in-vitro AChE assay. The alkylation of the ortho hydroxyl group of derivatives led to the loss of reactivation potential. The derivatives with a hydroxyl group and without oxime group and vice versa did not show significant reactivation potency against tested OP toxicants. Further, we also evaluated the reactivation potential of these selected molecules on the rat brain homogenate against different OPs inhibited ChE and found maximum reactivation potency of oxime 2e. The in-vitro results were further validated by molecular docking and dynamic studies which showed that the hydroxyl group interacted with serine amino acids in the catalytic anionic site of AChE enzyme and was stable up to 200 ns consequently providing proper orientation to oxime moiety for reactivating the OP inhibited enzyme. It has thus been proved by the structure-activity relationship of oximes derivatives that hydroxyl group ortho to oxime is essential for reactivating OP inhibited electric eel AChE. Amongst the twenty-one oximes derivatives, 2e was found to be most active in regenerating the paraoxon, malaoxon, DCP and DCNP inhibited AChE enzyme.

Synthesis, Molecular Docking, BSA, and in-vitro reactivation study of imidazopyridine oxime against paraoxon inhibited acetylcholinesterase

AIM

To synthesize and evaluate the fused heterocyclic imidazopyridine oxime as a reactivator against paraoxon inhibited acetylcholinesterase.

BACKGROUND

Organophosphorus compounds (OPs) include parathion, malathion, chlorpyrifos, monocrotophos, and diazinon which are commonly used in agriculture for enhancing agricultural productivity via killing crop-damaging pests. However, people may get exposed to OPs pesticides unintentionally/intentionally via ingestion, inhalation or dermal. The current treatment regimen includes reactivator such as mono or bis-pyridinium oximes along with anticholinergic and an anticonvulsant drugs are recommended for the treatment of OP poisoning. Unfortunately, the drawback of the existing reactivator is that owing to the permanent charge present on the pyridinium makes them inefficient to cross the blood-brain barrier (BBB) and reactivate OP-inhibited central nervous system (CNS) acetylcholinesterase. Therefore, there is a need of reactivator that could cross the BBB and reactivate the OP inhibited acetylcholinesterase.

OBJECTIVE

The objectives of the study were synthesis, molecular docking, BSA binding and in-vitro estimation of oximes of various substituted imidazo [1,2-a]pyridine against paraoxon inhibited acetylcholinesterase.

METHOD

The reactivators were synthesized in three steps and characterized using various spectroscopic techniques. Molecular docking study was performed on 2WHP and 3ZLV PDB using Autodock tool. The acid dissociation constant (pKa) of oximes was calculated experimentally and drug-likeness properties of the oximes were calculated In silico using mole inspiration and Swiss ADME software. The binding of oximes with bovine serum albumin (BSA) was also investigated by UV-Vis spectrophotometer. The reactivation potential of the oximes was determined by in vitro enzymatic assay.

RESULT

in-silico study inferred that synthesized molecules fulfilled the parameters that required for a successful CNS drug candidate. Further, in-vitro enzymatic assay indicated reasonable reactivation potential of the oximes against paraoxon-inhibited AChE. The binding of oximes with bovine serum albumin (BSA) revealed static quenching of intrinsic fluorescence of BSA by oxime. The binding constant value and number of binding sites were found 0.24 mol-1 and 1 respectively.

CONCLUSION

The results of study concluded that this scaffold could be used for further designing of more efficient uncharged reactivators.

Molecular Modeling Studies on the Multistep Reactivation Process of Organophosphate-Inhibited Acetylcholinesterase and Butyrylcholinesterase

Poisoning with organophosphorus compounds used as pesticides or misused as chemical weapons remains a serious threat to human health and life. Their toxic effects result from irreversible blockade of the enzymes acetylcholinesterase and butyrylcholinesterase, which causes overstimulation of the cholinergic system and often leads to serious injury or death. Treatment of organophosphorus poisoning involves, among other strategies, the administration of oxime compounds. Oximes reactivate cholinesterases by breaking the covalent bond between the serine residue from the enzyme active site and the phosphorus atom of the organophosphorus compound. Although the general mechanism of reactivation has been known for years, the exact molecular aspects determining the efficiency and selectivity of individual oximes are still not clear. This hinders the development of new active compounds. In our research, using relatively simple and widely available molecular docking methods, we investigated the reactivation of acetyl- and butyrylcholinesterase blocked by sarin and tabun. For the selected oximes, their binding modes at each step of the reactivation process were identified. Amino acids essential for effective reactivation and those responsible for the selectivity of individual oximes against inhibited acetyl- and butyrylcholinesterase were identified. This research broadens the knowledge about cholinesterase reactivation and demonstrates the usefulness of molecular docking in the study of this process. The presented observations and methods can be used in the future to support the search for new effective reactivators.

Molecular Modeling and In Vitro Studies of a Neutral Oxime as a Potential Reactivator for Acetylcholinesterase Inhibited by Paraoxon

The present work aimed to compare the small, neutral and monoaromatic oxime, isatin-3-oxime (isatin-O), to the commercial ones, pralidoxime (2-PAM) and obidoxime, in a search for a new potential reactivator for acetylcholinesterase (AChE) inhibited by the pesticide paraoxon (AChE/POX) as well as a novel potential scaffold for further synthetic modifications. The multicriteria decision methods (MCDM) allowed the identification of the best docking poses of those molecules inside AChE/POX for further molecular dynamic (MD) studies, while Ellman's modified method enabled in vitro inhibition and reactivation assays. In corroboration with the theoretical studies, our experimental results showed that isatin-O have a reactivation potential capable of overcoming 2-PAM at the initial moments of the assay. Despite not achieving better results than obidoxime, this molecule is promising for being an active neutral oxime with capacity of crossing the blood⁻brain barrier (BBB), to reactivate AChE/POX inside the central and peripheral nervous systems. Moreover, the fact that isatin-O can also act as anticonvulsant makes this molecule a possible multipotent reactivator. Besides, the MCDM method showed to be an accurate method for the selection of the best docking poses generated in the docking studies.

A newly developed oxime K203 is the most effective reactivator of tabun-inhibited acetylcholinesterase

BACKGROUND

Based on in vitro and in vivo rat experiments, the newly developed acetylcholinesterase (AChE) reactivator, K203, appears to be much more effective in the treatment of tabun poisonings than currently fielded oximes.

METHODS

To determine if this reactivating efficacy would extend to humans, studies were conducted in vitro using human brain homogenate as the source of AChE. The efficacy of K203 was compared with commercially available oximes; pralidoxime, obidoxime and asoxime (HI-6).

RESULTS

Reactivation studies showed that K203 was the most effective reactivator with a second order kinetic constant (kr) of 2142 min- 1. M- 1, which was 51 times higher than that obtained for obidoxime (kr = 42 min- 1. M- 1). Both pralidoxime and asoxime (HI-6) failed to significantly reactivate tabun-inhibited human AChE.

DISCUSSION

According to these results and previous studies, using K203, it appears that oxime K203 is the most effective reactivator of tabun-inhibited cholinesterase in several species including humans and should be considered as a possible medical countermeasure to tabun exposure.

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.

In vitro investigation of efficacy of new reactivators on OPC inhibited rat brain acetylcholinesterase

Organophosphorus compounds (OPC) were developed as warfare nerve agents. They are also widely used as pesticides. The drug therapy of intoxication with OPC includes mainly combination of cholinesterase (ChE) reactivators and cholinolytics. There is no single ChE reactivator having an ability to reactivate sufficiently the inhibited enzyme due to the high variability of chemical structure of the inhibitors. The difficulties in reactivation of ChE activity and slight antidote effect regarding intoxication with some OPC are some of the reasons for continuous efforts to obtain new reactivators of ChE. The aim of the present study was to evaluate the efficacy of some ChE reactivators against OPC intoxication (tabun, paraoxon and dichlorvos) in in vitro experiments and to compare their activity to that known for some currently used oximes (obidoxime, HI-6, 2-PAM). Experiments were carried out using rat brain acetylcholinesterase (AChE). Reactivators showed different activity in the reactivation of rat brain AChE after dichlorvos, paraoxon and tabun inhibition. AChE was easier reactivated after paraoxon treatment. The best effect showed BT-07-4M, obidoxime, TMB-4 and BT-08 from the group of symmetric oximes, and Toxidin, BT-05 and BT-03 from asymmetric compounds. The reactivation of brain AChE inhibited with tabun demonstrated better activity of new compound BT-07-4M, TMB-4 and obidoxime from symmetric oximes, and BT-05 and BT-03 possessing asymmetric structure. All compounds showed low activity toward inhibition of AChE caused by dichlorvos. Comparison of two main structure types (symmetric/asymmetric) showed that the symmetric compounds reactivated better AChE, inhibited with this OPC, than asymmetric ones.

Minireview: does in-vitro testing of oximes help predict their in-vivo action after paraoxon exposure?

K-oximes have recently been developed in the search for efficacious broad-band reactivators of acetylcholinesterase (AChE) inhibited by organophosphorus compounds (OPC). Before clinical use, their toxicity and efficacy need to be assessed, and there is clear demand for simple in vitro tests that can predict in vivo performance. This article summarizes our in vitro data obtained for conventional and experimental oximes in human and rat blood exposed to the OPC paraoxon and correlates them with our in vivo results. The intrinsic AChE inhibitory activity of oximes, as reflected by their in vitro IC(50), is strongly correlated with their LD(50) (rat): oximes with a high IC(50) (K-27, K-48, pralidoxime and obidoxime) also show a high LD(50) and are thus relatively non-toxic, whereas oximes K-105, K-108 and K-113 have a low IC(50), a low LD(50) and are far more toxic. The IC(50) is also correlated with the in vivo capacity to protect from paraoxon-induced mortality: oximes with a higher IC(50) reduce the relative risk of death more. In contrast, the protective ability as assessed in vitro by the slope of the IC(50) shift (tanalpha), is not correlated with in vivo protection from paraoxon-induced mortality: the best in vivo protectors (K-27 and K-48) show a much lower tanalpha value (around 2) than K-110 and K-113 (tanalpha around 10), which hardly reduce the relative risk of death after paraoxon exposure. The partition coefficient logP of the individual oximes is inversely correlated with their IC(50) and with their LD(50) and is therefore an indicator of toxicity: strongly hydrophilic oximes tend to be less toxic than less hydrophilic ones. These data highlight the good predictive value of in vitro IC(50) testing for in vivo toxicity and the limited practical significance of in vitro assessment of protective potency.

Reactivation of human brain homogenate cholinesterases inhibited by Tabun using newly developed oximes K117 and K127

Newly developed acetylcholinesterase reactivators K117 [1,5-bis(4-hydroxyiminomethylpyridinium)-3-oxapentane dichloride] and K127 [(1-(4-hydroxyiminomethylpyridinium)-5-(4-carbamoylpyridinium)-3-oxapentane dibromide)] were tested for their potency to reactivate tabun-inhibited human brain cholinesterases. Pralidoxime and trimedoxime were chosen as standard reference reactivators. Human tissue was used, as that was closer on the real treatment of human beings. As a result, oxime K127 was found as the best tested reactivator according to the constant k(r), characterizing the overall reactivation process. On the contrary, the maximal reactivation ability expressed as percentage of reactivation was the best for trimedoxime. This differences were caused as a result of using the enzyme from different species. Due to this, experiments on human tissue should be conducted after in vitro and in vivo tests on animals to eliminate such important failures of promising oximes.

Efficacy of eight experimental bispyridinium oximes against paraoxon-induced mortality: comparison with the conventional oximes pralidoxime and obidoxime

Recently, several experimental K-oximes with two functional aldoxime groups have been synthesized that show excellent in vitro efficacy in protecting acetylcholinesterase (AChE) from inhibition by a broad variety of organophosphorus compounds (OPCs). However, oximes themselves are also AChE inhibitors, albeit at higher concentrations, which is a major cause of their toxicity and may be a dose-limiting factor in oxime therapy. To assess the efficacy of the experimental K-oximes in vivo, the extent of oxime-conferred protection from mortality induced by paraoxon was quantified. Rats received paraoxon in a dosage of 1, 5, or 10 mumol, and immediately thereafter intraperitoneal injections of the respective oxime at a dosage of half the LD(01). The relative risk of death (RR) over time was estimated by Cox survival analysis for treatment with experimental K-oximes (K-53, K-74, K-75, K-107, K-108, and K-113), with the clinically available oximes pralidoxime (2-PAM) and obidoxime, and with the well-characterized K-oximes K-27 and K-48, comparing results with the no-treatment group. Best protection was conferred by K-27, reducing the RR to 20% of controls (P </= 0.05), which was significantly (P <or= 0.05) better than all other tested oximes. Marked reduction in mortality was also achieved by K-48 and the three new bispyridinium oximes containing two aldoxime groups, but no xylene linker: K-48 (RR = 0.32), K-53 (RR = 0.36), K-74 (RR = 0.42), K-75 (RR = 0.35). This effect was significantly (P <or= 0.05) superior to that of all other oximes, except K-27. The remaining oximes, i.e., obidoxime (RR = 0.64), 2-PAM (RR = 0.78), K-107 (RR = 0.70), K-108 (RR = 0.77), and K-113 (RR = 0.87) reduced paraoxon-induced mortality only poorly, but significantly (P <or= 0.05). Our data show that K-27, K-48, K-53, K-74, and K-75, due to their far superior in vivo efficacy, are the most promising candidates to eventually replace the established oximes 2-PAM and obidoxime. Further studies in other species exposed to a broader spectrum of OPCs are, however, necessary before considering their use in humans.

Eight new bispyridinium oximes in comparison with the conventional oximes pralidoxime and obidoxime: in vivo efficacy to protect from diisopropylfluorophosphate toxicity

In search for more efficacious reactivators of acetylcholinesterase (AChE) inhibited by organophosphorus compounds, experimental K-oximes have been synthesized which show good in vitro efficacy. However, AChE inhibition by oximes themselves (as quantified by their intrinsic IC50) is the major cause of oxime toxicity and the dose-limiting factor. To assess K-oxime efficacy in vivo, the extent of protection from mortality induced by diisopropylfluorophosphate (DFP) was quantified by Cox survival analysis and compared with that of the clinically available oximes. Oximes were administered in an equitoxic dosage, i.e. half the LD01. Best protection was conferred by K-27, reducing the relative risk of death (RR) to 16% of control RR (P < or = 0.05), which was statistically significantly better (P < or = 0.05) than all other tested oximes, except obidoxime, K-53 and K-75. The efficacy of obidoxime (RR = 0.19), K-48 (RR = 0.28), K-53 (RR = 0.22), K-74 (RR = 0.38) and K-75 (RR = 0.29) was significantly (P < or = 0.05) better than that of 2-PAM (RR = 0.62) and K-113 (RR = 0.73). No significant protective effect was observed for K-107 and K-108. Our LD50 data show that K-107, K-108 and K-113 (which strongly inhibit AChE in vitro) are in vivo markedly more toxic than all other oximes tested and can therefore only be safely administered at a low dosage which is insufficient to protect from DFP-induced mortality. Dosage calculations based on in vitro IC50 measurements may therefore in future replace in vivo LD50 determinations, thereby reducing the number of animals required.