Potent 3-Hydroxy-2-Pyridine Aldoxime Reactivators of Organophosphate-Inhibited Cholinesterases with Predicted Blood-Brain Barrier Penetration

New Heterostilbene and Triazole Oximes as Potential CNS-Active and Cholinesterase-Targeted Therapeutics

New furan, thiophene, and triazole oximes were synthesized through several-step reaction paths to investigate their potential for the development of central nervous systems (CNS)-active and cholinesterase-targeted therapeutics in organophosphorus compound (OP) poisonings. Treating patients with acute OP poisoning is still a challenge despite the development of a large number of oxime compounds that should have the capacity to reactivate acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). The activity of these two enzymes, crucial for neurotransmission, is blocked by OP, which has the consequence of disturbing normal cholinergic nerve signal transduction in the peripheral and CNS, leading to a cholinergic crisis. The oximes in use have one or two pyridinium rings and cross the brain-blood barrier poorly due to the quaternary nitrogen. Following our recent study on 2-thienostilbene oximes, in this paper, we described the synthesis of 63 heterostilbene derivatives, of which 26 oximes were tested as inhibitors and reactivators of AChE and BChE inhibited by OP nerve agents-sarin and cyclosarin. While the majority of oximes were potent inhibitors of both enzymes in the micromolar range, we identified several oximes as BChE or AChE selective inhibitors with the potential for drug development. Furthermore, the oximes were poor reactivators of AChE; four heterocyclic derivatives reactivated cyclosarin-inhibited BChE up to 70%, and cis,trans-5 [2-((Z)-2-(5-((E)-(hydroxyimino)methyl)thiophen-2-yl)vinyl)benzonitrile] had a reactivation efficacy comparable to the standard oxime HI-6. In silico analysis and molecular docking studies, including molecular dynamics simulation, connected kinetic data to the structural features of these oximes and confirmed their productive interactions with the active site of cyclosarin-inhibited BChE. Based on inhibition and reactivation and their ADMET properties regarding lipophilicity, CNS activity, and hepatotoxicity, these compounds could be considered for further development of CNS-active reactivators in OP poisoning as well as cholinesterase-targeted therapeutics in neurodegenerative diseases such as Alzheimer's and Parkinson's.

The reactivation kinetic analysis, molecular docking, and dynamics of oximes against three V-type nerve agents inhibited four human cholinesterases

Nerve agents pose significant threats to civilian and military populations. The reactivation of acetylcholinesterase (AChE) is critical in treating acute poisoning, but there is still lacking broad-spectrum reactivators, which presents a big challenge. Therefore, insights gained from the reactivation kinetic analysis and molecular docking are essential for understanding the behavior of reactivators towards intoxicated AChE. In this research, we present a systematic determination of the reactivation kinetics of three V agents-inhibited four human ChEs [(AChE and butyrylcholinesterase (BChE)) from either native or recombinant resources, namely, red blood cell (RBC) AChE, rhAChE, hBChE, rhBChE) reactivated by five standard oximes. We unveiled the effect of native and recombinant ChEs on the reactivation kinetics of V agents ex vitro, where the reactivation kinetics characteristic of Vs-inhibited BChE was reported for the first time. In terms of the inhibition type, all of the five oxime reactivators exhibited noncompetitive inhibition. The inhibition potency of these reactivators would not lead to the difference in the reactivation kinetics between native and recombinant ChE. Despite the significant differences between the native and recombinant ChEs observed in the inhibition, aging, and spontaneous reactivation kinetics, the reactivation kinetics of V agent-inhibited ChEs by oximes were less differentiated, which were supported by the ligand docking results. We also found differences in the reactivation efficiency between five reactivators and the phosphorylated enzyme, and molecular dynamic simulations can further explain from the perspectives of conformational stability, hydrogen bonding, binding free energies, and amino acid contributions. By Poisson-Boltzmann surface area (MM-PBSA) calculations, the total binding free energy trends aligned well with the experimental kr2 values.

The risk associated with organophosphorus nerve agents: from their discovery to their unavoidable threat, current medical countermeasures and perspectives

The first organophosphorus nerve agent was discovered accidently during the development of pesticides, shortly after the first use of chemical weapons (chlorine, phosgene) on the battlefield during World War I. Despite the Chemical Weapons Convention banning these substances, they have still been employed in wars, terrorist attacks or political assassinations. Characterised by their high lethality, they target the nervous system by inhibiting the acetylcholinesterase (AChE) enzyme, preventing neurotransmission, which, if not treated rapidly, inevitably leads to serious injury or the death of the person intoxicated. The limited efficacy of current antidotes, known as AChE reactivators, pushes research towards new treatments. Numerous paths have been explored, from modifying the original pyridinium oximes to developing hybrid reactivators seeking a better affinity for the inhibited AChE. Another crucial approach resides in molecules more prone to cross the blood-brain barrier: uncharged compounds, bio-conjugated reactivators or innovative formulations. Our aim is to raise awareness on the threat and toxicity of organophosphorus nerve agents and to present the main synthetic efforts deployed since the first AChE reactivator, to tackle the task of efficiently treating victims of these chemical warfare agents.

Cholesterol Oxime Olesoxime Assessed as a Potential Ligand of Human Cholinesterases

Olesoxime, a cholesterol derivative with an oxime group, possesses the ability to cross the blood-brain barrier, and has demonstrated excellent safety and tolerability properties in clinical research. These characteristics indicate it may serve as a centrally active ligand of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), whose disruption of activity with organophosphate compounds (OP) leads to uncontrolled excitation and potentially life-threatening symptoms. To evaluate olesoxime as a binding ligand and reactivator of human AChE and BChE, we conducted in vitro kinetic studies with the active metabolite of insecticide parathion, paraoxon, and the warfare nerve agents sarin, cyclosarin, tabun, and VX. Our results showed that both enzymes possessed a binding affinity for olesoxime in the mid-micromolar range, higher than the antidotes in use (i.e., 2-PAM, HI-6, etc.). While olesoxime showed a weak ability to reactivate AChE, cyclosarin-inhibited BChE was reactivated with an overall reactivation rate constant comparable to that of standard oxime HI-6. Moreover, in combination with the oxime 2-PAM, the reactivation maximum increased by 10-30% for cyclosarin- and sarin-inhibited BChE. Molecular modeling revealed productive interactions between olesoxime and BChE, highlighting olesoxime as a potentially BChE-targeted therapy. Moreover, it might be added to OP poisoning treatment to increase the efficacy of BChE reactivation, and its cholesterol scaffold could provide a basis for the development of novel oxime antidotes.

Uncharged mono- and bisoximes: In search of a zwitterion to countermeasure organophosphorus intoxication

The current study imposes a new class of organophosphorus (OP)-inhibited cholinesterase reactivators by conceptualizing a family of asymmetric bisoximes with various reactivating scaffolds. Several novel nucleophilic warheads were investigated, putting forward 29 novel reactivating options, by evaluating their nucleophilicity and ability to directly decompose OP compounds. Adopting the so-called zwitterionic strategy, 17 mono-oxime and nine bisoxime reactivators were discovered with major emphasis on the bifunctional-moiety approach. Compounds were compared with clinically used standards and other known experimentally highlighted reactivators. Our results clearly favor the concept of asymmetric bisoximes as leading reactivators in terms of efficacy and versatility. These top-ranked compounds were characterized in detail by reactivation kinetics parameters and evaluated for potential CNS availability. The highlighted molecules 55, 57, and 58 with various reactivating warheads, surpassed the reactivating potency of pralidoxime and several notable uncharged reactivators. The versatility of lead drug candidate 55 was also inspected on OP-inhibited butyrylcholinesterase, revealing a much higher rate compared to existing clinical antidotes.

BrainPepPass: A Framework Based on Supervised Dimensionality Reduction for Predicting Blood-Brain Barrier-Penetrating Peptides

Peptides that pass through the blood-brain barrier (BBB) not only are implicated in brain-related pathologies but also are promising therapeutic tools for treating brain diseases, e.g., as shuttles carrying active medicines across the BBB. Computational prediction of BBB-penetrating peptides (B3PPs) has emerged as an interesting approach because of its ability to screen large peptide libraries in a cost-effective manner. In this study, we present BrainPepPass, a machine learning (ML) framework that utilizes supervised manifold dimensionality reduction and extreme gradient boosting (XGB) algorithms to predict natural and chemically modified B3PPs. The results indicate that the proposed tool outperforms other classifiers, with average accuracies exceeding 94% and 98% in 10-fold cross-validation and leave-one-out cross-validation (LOOCV), respectively. In addition, accuracy values ranging from 45% to 97.05% were achieved in the independent tests. The BrainPepPass tool is available in a public repository for academic use (https://github.com/ewerton-cristhian/BrainPepPass).

Noncovalent binding of carbofuran to acetylcholinesterase from Homo sapiens, Danio rerio, Apis mellifera and Caenorhabditis elegans: Homology modelling, molecular docking and dynamics, and quantum biochemistry description

Although various regulatory agencies have banned or severely restricted the use of carbofuran (CAR), recent reports indicate the presence of CAR residues in both cultivated and wild areas. This pesticide is a potent inhibitor of acetylcholinesterase (AChE), which acts by preventing the hydrolysis of acetylcholine (ACh). Given the critical role of AChE::ACh in the proper functioning of the nervous system, we thought it appropriate to investigate the binding of CAR to AChEs from Homo sapiens, Danio rerio, Apis mellifera, and Caenorhabditis elegans using homology modelling, molecular docking, molecular dynamics, and quantum biochemistry. Molecular docking and dynamics results indicated peculiar structural behavior in each AChE::CAR system. Quantum biochemistry results showed similar affinities for all complexes, confirming the description of carbofuran as a broad-spectrum pesticide, and have a limited correlation with IC50 values. We found the following decreasing affinity order of AChE species: H. sapiens > A. mellifera > C. elegans > D. rerio. The computational results suggest that CAR occupies different pockets in the AChEs studied. In addition, our results showed that CAR binds to hsAChE and ceAChE in a very similar manner: it has high affinities for the same subsites in both species and forms hydrogen bonds with residues (hsTYR124 and ceTRP107) occupying homologous positions in the peripheral site. This suggests that this nematode is a potential model to evaluate the toxicity of carbamates, even though the sequence identity between them is only 41 %. Interestingly, we also observed that the catalytic histidines of drAChE and amAChE exhibited favorable contacts with carbofuran, suggesting that the non-covalent binding of carbofuran to these proteins may promote faster carbamylation rates than the binding modes to human and worm acetylcholinesterases. Our computational results provide a better understanding of the binding mechanisms in these complexes, as well as new insights into the mechanism of carbamylation.

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.

Modeling of a near-attack conformation of oxime in phosphorylated acetylcholinesterase via a reactivation product, a phosphorylated oxime

At the present, only four antidotes are in use in therapy for poisoning by organophosphorus compounds: 2-PAM, HI-6, obidoxime and trimedoxime. Numerous compounds have been designed and synthetized to be more effective reactivators than those currently in use. Many of those new compounds fail at the enzyme level because interactions formed within the AChE active site are not favourable ones that lead to a successful reactivation. The approach in which the modeling of a phosphorylated oxime (POX), a product of successful reactivation in the AChE active site, may be a way to better understand the role of active site residues during the process of formation of the Michaelis type of complex between an enzyme and oxime. After reactivation, a change in phosphorus stereochemistry occurs leading to a different spatial arrangement of attached substituents, now including an oxime. To study interactions between the AChE oxyanion hole and a phosphorylated oxime, an S203G mutant was used to avoid the steric hindrance caused by the catalytic serine. In this way, the POX could be positioned close to the oxyanion hole. In the final step, the oxime without a phosphoester moiety was transferred into the phosphorylated AChE and molecular dynamics was used to test the stability of the near-attack conformation of the oxime near the phosphorylated serine.

Biological response and cell death signaling pathways modulated by tetrahydroisoquinoline-based aldoximes in human cells

The uncharged 3-hydroxy-2-pyridine aldoximes with protonatable tertiary amines are studied as antidotes in toxic organophosphates (OP) poisoning. Due to some of their specific structural features, we hypothesize that these compounds could exert diverse biological activity beyond their main scope of application. To examine this further, we performed an extensive cell-based assessment to determine their effects on human cells (SH-SY5Y, HEK293, HepG2, HK-2, myoblasts and myotubes) and possible mechanism of action. As our results indicated, aldoxime having a piperidine moiety did not induce significant toxicity up to 300µM within 24hours, while those with a tetrahydroisoquinoline moiety, in the same concentration range, showed time-dependent effects and stimulated mitochondria-mediated activation of the intrinsic apoptosis pathway through ERK1/2 and p38-MAPK signaling and subsequent activation of initiator caspase 9 and executive caspase 3 accompanied with DNA damage as observed already after 4hour exposure. Mitochondria and fatty acid metabolism were also likely targets of 3-hydroxy-2-pyridine aldoximes with tetrahydroisoquinoline moiety, due to increased phosphorylation of acetyl-CoA carboxylase. In silico analysis predicted kinases as their most probable target class, while pharmacophores modeling additionally predicted the inhibition of a cytochrome P450cam. Overall, if the absence of significant toxicity for piperidine bearing aldoxime highlights the potential of its further studies in medical counter-measures, the observed biological activity of aldoximes with tetrahydroisoquinoline moiety could be indicative for future design of compounds either in a negative context in OP antidotes design, or in a positive one for design of compounds for the treatment of other phenomena like cell proliferating malignancies.

Cytotoxicity-related effects of imidazolium and chlorinated bispyridinium oximes in SH-SY5Y cells

Current research has shown that several imidazolium and chlorinated bispyridinium oximes are cytotoxic and activate different mechanisms or types of cell death. To investigate this further, we analysed interactions between these oximes and acetylcholine receptors (AChRs) and how they affect several signalling pathways to find a relation between the observed toxicities and their effects on these specific targets. Chlorinated bispyridinium oximes caused time-dependent cytotoxicity by inhibiting the phosphorylation of STAT3 and AMPK without decreasing ATP and activated ERK1/2 and p38 MAPK signal cascades. Imidazolium oximes induced a time-independent and significant decrease in ATP and inhibition of the ERK1/2 signalling pathway along with phosphorylation of p38 MAPK, AMPK, and ACC. These pathways are usually triggered by a change in cellular energy status or by external signals, which suggests that oximes interact with some membrane receptors. Interestingly, in silico analysis also indicated that the highest probability of interaction for all of our oximes is with the family of G-coupled membrane receptors (GPCR). Furthermore, our experimental results showed that the tested oximes acted as acetylcholine antagonists for membrane AChRs. Even though oxime interactions with membrane receptors need further research and clarification, our findings suggest that these oximes make promising candidates for the development of specific therapies not only in the field of cholinesterase research but in other fields too, such as anticancer therapy via altering the Ca2+ flux involved in cancer progression.

Potential of Vitamin B6 Dioxime Analogues to Act as Cholinesterase Ligands

Seven pyridoxal dioxime quaternary salts (1–7) were synthesized with the aim of studying their interactions with human acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). The synthesis was achieved by the quaternization of pyridoxal monooxime with substituted 2-bromoacetophenone oximes (phenacyl bromide oximes). All compounds, prepared in good yields (43–76%) and characterized by 1D and 2D NMR spectroscopy, were evaluated as reversible inhibitors of cholinesterase and/or reactivators of enzymes inhibited by toxic organophosphorus compounds. Their potency was compared with that of their monooxime analogues and medically approved oxime HI-6. The obtained pyridoxal dioximes were relatively weak inhibitors for both enzymes (Ki = 100–400 µM). The second oxime group in the structure did not improve the binding compared to the monooxime analogues. The same was observed for reactivation of VX-, tabun-, and paraoxon-inhibited AChE and BChE, where no significant efficiency burst was noted. In silico analysis and molecular docking studies connected the kinetic data to the structural features of the tested compound, showing that the low binding affinity and reactivation efficacy may be a consequence of a bulk structure hindering important reactive groups. The tested dioximes were non-toxic to human neuroblastoma cells (SH-SY5Y) and human embryonal kidney cells (HEK293).

Choline Chloride-Based Deep Eutectic Solvents as Green Effective Medium for Quaternization Reactions

The Menshutkin reaction represents the alkylation of tertiary amines by alkyl halide where the reactants are neutral and the products, quaternary ammonium salts, are two ions with opposite signs. The most commonly used organic solvents in quaternization reactions are volatile organic solvents (VOSs), namely acetone, anhydrous benzene, dry dichloromethane (DCM), dimethylformamide (DMF) and acetonitrile (ACN). The purpose of this work was to examine eutectic solvents as a "greener" alternative to conventional solvents so that quaternization reactions take place in accordance with the principles of green chemistry. Herein, sixteen eutectic solvents were used as replacements for volatile organic ones in quaternization reactions of isonicotinamide with substituted phenacyl bromides. The reactions were carried out at 80 °C by three synthetic approaches: conventional (4-6 h), microwave (20 min) and ultrasound (3 h). Microwave-assisted organic reactions produced the highest yields, where in several reactions, the yield was almost quantitative. The most suitable eutectic solvents were based on choline chloride (ChCl) as the hydrogen bond acceptor (HBA) and glycerol, oxalic or levulinic acid as hydrogen bond donors (HBDs). The benefits of these three deep eutectic solvents (DESs) as a medium for quaternization reactions are the simplicity of their preparation for large-scale production, with inexpensive, available and nontoxic starting materials, as well as their biodegradability.

Evaluation of the Key Structural Features of Various Butyrylcholinesterase Inhibitors Using Simple Molecular Descriptors

In this study, we developed several QSAR models based on simple descriptors (such as topological and constitutional) to estimate butyrylcholinesterase (BChE) inhibition potency, pK_i (or pIC₅₀), of a set of 297 (289 after exclusion of outliers) structurally different compounds. The models were similar to the best model that we obtained previously for acetylcholinesterase AChE and were based on the valence molecular connectivity indices of second and third order (²χ^v and ³χ^v), the number of aliphatic hydroxyl groups (nOH), AlogP Ghose-Crippen octanol-water partition coeff. (logP), and O-060-atom-centred fragments (Al-O-Ar, Ar-O-Ar, R..O..R and R-O-C=X). The best models with two and three descriptors yielded r = 0.787 and S.E. = 0.89, and r = 0.827 and S.E. = 0.81, respectively. We also correlated nine scoring functions, calculated for 20 ligands whose complexes with BChE we found in the Protein Data Bank as crystal structures to pK_i (or pIC₅₀). The best correlations yielded PLP1 and PLP2 (Piecewise Linear Pairwise potential functions) with r = 0.619 and 0.689, respectively. Correlation with certain simple topological and constitutional descriptors yielded better results, e.g., ³χ^v (r = 0.730), on the same set of compounds (N = 20).

Synthesis, anticholinesterase activity, molecular docking, and molecular dynamic simulation studies of 1,3,4-oxadiazole derivatives

Two new series of 1,3,4-oxadiazoles bearing pyridine and thiazole heterocycles (4a-h and 5a-h) were synthesized (2,5-disubstituted-1,3,4-oxadiazoles). The structures of these newly synthesized compounds were confirmed by ¹ H nuclear magnetic resonance (NMR), ¹³ C NMR, high-resolution mass spectrometric and Fourier transform infrared spectroscopic methods. All these compounds were evaluated for their enzyme inhibitory activities against two cholinesterase enzymes, acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). From the studies, we identified compounds 4a, 4h, 5a, 5d, and 5e as selective AChE inhibitors, with IC₅₀ values ranging from 0.023 to 0.037 μM. Furthermore, docking studies of these compounds were performed at the active sites of their target enzymes. The molecular docking study showed that 5e possessed an ideal docking pose with interactions inside AChE.

Kinetic Processes in Enzymatic Nanoreactors for In Vivo Detoxification

Enzymatic nanoreactors are enzyme-encapsulated nanobodies that are capable of performing biosynthetic or catabolic reactions. For this paper, we focused on therapeutic enzyme nanoreactors for the neutralization of toxicants, paying special attention to the inactivation of organophosphorus compounds (OP). Therapeutic enzymes that are capable of detoxifying OPs are known as bioscavengers. The encapsulation of injectable bioscavengers by nanoparticles was first used to prevent fast clearance and the immune response to heterologous enzymes. The aim of enzyme nanoreactors is also to provide a high concentration of the reactive enzyme in stable nanocontainers. Under these conditions, the detoxification reaction takes place inside the compartment, where the enzyme concentration is much higher than in the toxicant diffusing across the nanoreactor membrane. Thus, the determination of the concentration of the encapsulated enzyme is an important issue in nanoreactor biotechnology. The implications of second-order reaction conditions, the nanoreactor’s permeability in terms of substrates, and the reaction products and their possible osmotic, viscosity, and crowding effects are also examined.

A New Class of Bi- and Trifunctional Sugar Oximes as Antidotes against Organophosphorus Poisoning

Recent events demonstrated that organophosphorus nerve agents are a serious threat for civilian and military populations. The current therapy includes a pyridinium aldoxime reactivator to restore the enzymatic activity of acetylcholinesterase located in the central nervous system and neuro-muscular junctions. One major drawback of these charged acetylcholinesterase reactivators is their poor ability to cross the blood-brain barrier. In this study, we propose to evaluate glucoconjugated oximes devoid of permanent charge as potential central nervous system reactivators. We determined their in vitro reactivation efficacy on inhibited human acetylcholinesterase, the crystal structure of two compounds in complex with the enzyme, their protective index on intoxicated mice, and their pharmacokinetics. We then evaluated their endothelial permeability coefficients with a human in vitro model. This study shed light on the structural restrains of new sugar oximes designed to reach the central nervous system through the glucose transporter located at the blood-brain barrier.

Activation of (un)regulated cell death as a new perspective for bispyridinium and imidazolium oximes

Oximes, investigated as antidotes against organophosphates (OP) poisoning, are known to display toxic effects on a cellular level, which could be explained beyond action on acetylcholinesterase as their main target. To investigate this further, we performed an in vitro cell-based evaluation of effects of two structurally diverse oxime groups at concentrations of up to 800 μM, on several cell models: skeletal muscle, kidney, liver, and neural cells. As indicated by our results, compounds with an imidazolium core induced necrosis, unregulated cell death characterized by a cell burst, increased formation of reactive oxygen species, and activation of antioxidant scavenging. On the other hand, oximes with a pyridinium core activated apoptosis through specific caspases 3, 8, and/or 9. Interestingly, some of the compounds exhibited a synergistic effect. Moreover, we generated a pharmacophore model for each oxime series and identified ligands from public databases that map to generated pharmacophores. Several interesting hits were obtained including chemotherapeutics and specific inhibitors. We were able to define the possible structural features of tested oximes triggering toxic effects: chlorine atoms in combination with but-2(E)-en-1,4-diyl linker and adding a second benzene ring with substituents such as chlorine and/or methyl on the imidazolium core. Such oximes could not be used in further OP antidote development research, but could be introduced in other research studies on new specific targets. This could undoubtedly result in an overall improved wider use of unexplored oxime database created so far in OP antidotes field of research in a completely new perspective.

Design, synthesis, characterization, in vitro and in silico evaluation of novel imidazo[2,1-b][1,3,4]thiadiazoles as highly potent acetylcholinesterase and non-classical carbonic anhydrase inhibitors

Imidazole and thiadiazole derivatives display an extensive application in pharmaceutical chemistry, and they have been investigated as bioactive molecules for medicinal chemistry purposes. Classical carbonic anhydrase (CA) inhibitors are based on sulfonamide groups, but inhibiting all CA isoforms nonspecifically, thereby causing undesired side effects, is the main drawback of these types of inhibitors. Here we reported an investigation of novel 2,6-disubstituted imidazo[2,1-b][1,3,4]thiadiazole derivatives (9a-k, 10a, and 11a) and 2,5,6-trisubstituted imidazo[2,1-b][1,3,4]thiadiazole derivatives (12a-20a) that do not possess the zinc-binding sulfonamide group for the inhibition of human carbonic anhydrase (hCA, EC 4.2.1.1) I and II isoforms and also of acetylcholinesterase (AChE, EC 3.1.1.7). Imidazo[2,1-b][1,3,4]thiadiazoles demonstrated low nanomolar inhibitory activity against hCA I, hCA II, and AChE (K_Is are in the range of 23.44-105.50 nM, 10.32-104.70 nM, and 20.52-54.06 nM, respectively). Besides, compound 9b inhibit hCA I up to 18-fold compared to acetazolamide, while compound 10a has a 5-fold selectivity towards hCA II. The synthesized compounds were also evaluated for their cytotoxic effects on the L929 mouse fibroblast cell line. Molecular docking simulations were performed to elucidate these inhibitors' potential binding modes against hCA I and II isoforms and AChE. The novel compounds reported here can represent interesting lead compounds, and the results presented here might provide further structural guidance to discover and design more potent hCA and AChE inhibitors.

Effect of Oxime Encapsulation on Acetylcholinesterase Reactivation: Pharmacokinetic Study of the Asoxime-Cucurbit[7]uril Complex in Mice Using Hydrophilic Interaction Liquid Chromatography-Mass Spectrometry

Oxime-based molecules are used for the treatment of patients to reactivate acetylcholinesterase (AChE) function after organophosphate intoxication. However, their efficacy is limited by low penetration through the blood-brain barrier and fast elimination. In this work, the cucurbit[7]uril (CB[7]) carrier was used for the encapsulation of the clinical agent asoxime to enhance brain bioavailability and the treatment window. We present a pharmacokinetic study of asoxime and the asoxime-CB[7] complex in an in vivo mouse model. Ultrahigh-performance liquid chromatography with electrospray ionization-mass spectrometry detection was developed to determine asoxime and CB[7] in biological fluids and tissues after thorough optimization of chromatographic conditions. The dihydroxypropane-silica stationary phase using hydrophilic interaction liquid chromatography conditions provided the best chromatographic performance. The final method was validated and applied for the pharmacokinetic study of mouse plasma, urine, bile, liver, kidney, and brain samples at different times after administration of asoxime and the asoxime-CB[7] complex. The results showed a greater than 3-fold increase in the area under the curve (AUC) in the brain for asoxime administered as a complex with CB[7] relative to that for the administration of asoxime alone. The effectiveness of the treatment strategy was evaluated using a reactivation study and a functional observatory battery. Protection of brain AChE activity is crucial for saving human lives or reducing the consequences of poisoning. The asoxime administered as a complex increased the brain activity by approximately 30% compared to that with atropine alone. CB[7] coadministration improved the AChE activity by 11%, which agrees with the higher asoxime AUC assessed in the pharmacokinetic study.

Non-quaternary oximes detoxify nerve agents and reactivate nerve agent-inhibited human butyrylcholinesterase

Government-sanctioned use of nerve agents (NA) has escalated dramatically in recent years. Oxime reactivators of organophosphate (OP)-inhibited acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) serve as antidotes toward poisoning by OPNAs. The oximes used as therapeutics are quaternary compounds that cannot penetrate the blood-brain barrier (BBB). There remains an urgent need for the development of next generation OPNA therapeutics. We have developed two high-throughput screening (HTS) assays using a fluorogenic NA surrogate, O-ethyl methylphosphonyl O-4-methyl-3-cyano-coumarin (EMP-MeCyC). EMP-MeCyC detoxification and EMP-BChE reactivation screening campaigns of ~155,000 small molecules resulted in the identification of 33 nucleophile candidates, including non-quaternary oximes. Four of the oximes were reactivators of both Sarin- and VX-inhibited BChE and directly detoxified Sarin. One oxime also detoxified VX. The novel reactivators included a non-quaternary pyridine amidoxime, benzamidoxime, benzaldoxime and a piperidyl-ketoxime. The VX-inhibited BChE reactivation reaction rates by these novel molecules were similar to those observed with known bis-quaternary reactivators and faster than mono-quaternary pyridinium oximes. Notably, we discovered the first ketoxime reactivator of OP-ChEs and detoxifier of OPNAs. Preliminary toxicological studies demonstrated that the newly discovered non-quaternary oximes were relatively non-toxic in mice. The discovery of unique non-quaternary oximes opens the door to the design of novel therapeutics and decontamination agents following OPNA exposure.

Butyrylcholinesterase nanodepots with enhanced prophylactic and therapeutic performance for acute organophosphorus poisoning management

Acute organophosphorus pesticide poisoning (AOPP) is a worldwide health concern that has threatened human lives for decades, which attacks acetylcholinesterase (AChE) and causes nervous system disorders. Classical treatment options are associated with short in vivo half-life and side effects. As a potential alternative, delivery of mammalian-derived butyrylcholinesterase (BChE) offers a cost-effective way to block organophosphorus attack on acetylcholinesterase, a key enzyme in the neurotransmitter cycle. Yet the use of exotic BChE as a prophylactic or therapeutic agent is compromised by short plasma residence, immune response and unfavorable biodistribution. To overcome these obstacles, BChE nanodepots (nBChE) composed of a BChE core/polymorpholine shell structure were prepared via in situ polymerization, which showed enhanced stability, prolonged plasma circulation, attenuated antigenicity and reduced accumulation in non-targeted tissues. In vivo administration of nBChE pre- or post-organophosphorus exposure in a BALB/C mouse model resulted in potent prophylactic and therapeutic efficiency. To our knowledge, this is the first systematic delivery of non-human BChE to tackle AOPP. In addition, this work also opens up a new avenue for real applications in both research and clinical settings to cope with acute intoxication-related diseases.

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.

Enantioseparation, in vitro testing, and structural characterization of triple-binding reactivators of organophosphate-inhibited cholinesterases

The enantiomers of racemic 2-hydroxyimino-N-(azidophenylpropyl)acetamide-derived triple-binding oxime reactivators were separated, and tested for inhibition and reactivation of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibited with tabun (GA), cyclosarin (GF), sarin (GB), and VX. Both enzymes showed the greatest affinity toward the methylimidazole derivative (III) of 2-hydroxyimino-N-(azidophenylpropyl)acetamide (I). The crystal structure was determined for the complex of oxime III within human BChE, confirming that all three binding groups interacted with active site residues. In the case of BChE inhibited by GF, oximes I (kr = 207 M-1 min-1) and III (kr = 213 M-1 min-1) showed better reactivation efficiency than the reference oxime 2-PAM. Finally, the key mechanistic steps in the reactivation of GF-inhibited BChE with oxime III were modeled using the PM7R6 method, stressing the importance of proton transfer from Nε of His438 to Oγ of Ser203 for achieving successful reactivation.

Organophosphorus compounds and oximes: a critical review

Organophosphorus (OP) pesticides and nerve agents still pose a threat to the population. Treatment of OP poisoning is an ongoing challenge and burden for medical services. Standard drug treatment consists of atropine and an oxime as reactivator of OP-inhibited acetylcholinesterase and is virtually unchanged since more than six decades. Established oximes, i.e. pralidoxime, obidoxime, TMB-4, HI-6 and MMB-4, are of insufficient effectiveness in some poisonings and often cover only a limited spectrum of the different nerve agents and pesticides. Moreover, the value of oximes in human OP pesticide poisoning is still disputed. Long-lasting research efforts resulted in the preparation of countless experimental oximes, and more recently non-oxime reactivators, intended to replace or supplement the established and licensed oximes. The progress of this development is slow and none of the novel compounds appears to be suitable for transfer into advanced development or into clinical use. This situation calls for a critical analysis of the value of oximes as mainstay of treatment as well as the potential and limitations of established and novel reactivators. Requirements for a straightforward identification of superior reactivators and their development to licensed drugs need to be addressed as well as options for interim solutions as a chance to improve the therapy of OP poisoning in a foreseeable time frame.

Targeting organophosphorus compounds poisoning by novel quinuclidine-3 oximes: development of butyrylcholinesterase-based bioscavengers

A library of 14 mono-oxime quinuclidinium-based compounds with alkyl or benzyl substituent were synthesized and characterized in vitro as potential antidotes for organophosphorus compounds (OP) poisoning treatment. We evaluated their potency for reversible inhibition and reactivation of OP inhibited human acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) and evaluated interactions by molecular docking studies. The reactivation was notable for both AChE and BChE inhibited by VX, cyclosarin, sarin and paraoxon, if quinuclidinium compounds contained the benzyl group attached to the quinuclidinium moiety. Out of all 14, oxime Q8 [4-bromobenzyl-3-(hydroxyimino)quinuclidinium bromide] was singled out as having the highest determined overall reactivation rate of approximately 20,000 M^-1 min^-1 for cyclosarin-inhibited BChE. Furthermore, this oxime in combination with BChE exhibited a capability to act as a bioscavenger of cyclosarin, degrading within 2 h up to 100-fold excess of cyclosarin concentration over the enzyme. Molecular modeling revealed that the position of the cyclohexyl moiety conjugated with the active site serine of BChE directs the favorable positioning of the quinuclidinium ring and the bromophenyl moiety of Q8, which makes phosphonylated-serine easily accessible for the nucleophilic displacement by the oxime group of Q8. This result presents a novel scaffold for the development of new BChE-based bioscavengers. Furthermore, a cytotoxic effect was not observed for Q8, which also makes it promising for further in vivo reactivation studies.

Oxime-mediated reactivation of organophosphate-inhibited acetylcholinesterase with emphasis on centrally-active oximes

This review provides an overview of the global research leading to the large number of compounds developed as reactivators of acetylcholinesterase inhibited by a variety of organophosphate compounds, most of which are nerve agents but also some insecticides. A number of these organophosphates are highly toxic and effective therapy by reactivators contributes to saving lives. Two major challenges for more effective therapy with reactivators are identification of a broad spectrum reactivator efficacious against a variety of organophosphate structures, and a reactivator that can cross the blood-brain barrier to protect the brain. The most effective of the reactivators developed are the nucleophilic pyridinium oximes, which bear a permanent positive charge from the quaternary nitrogen in the pyridinium ring. The permanent positive charge retards the oximes from crossing the blood-brain barrier and therefore restoration of normal cholinergic function in the brain is unlikely. A number of laboratories have developed nucleophiles, mostly oximes, that are theorized to cross the blood-brain barrier by several strategies. At the present time, no reactivator is optimally broad spectrum across the wide group of organophosphate chemistries. Some oximes, including the substituted phenoxyalkyl pyridinium oximes invented by our laboratories, have the potential to provide neuroprotection in the brain and show evidence of efficacy against both nerve agent and insecticidal chemistries, so these novel oximes have promise for future development. This article is part of the special issue entitled 'Acetylcholinesterase Inhibitors: From Bench to Bedside to Battlefield'.

Efficient detoxification of nerve agents by oxime-assisted reactivation of acetylcholinesterase mutants

The recent advancements in crystallography and kinetics studies involving reactivation mechanism of acetylcholinesterase (AChE) inhibited by nerve agents have enabled a new paradigm in the search for potent medical countermeasures in case of nerve agents exposure. Poisonings by organophosphorus compounds (OP) that lead to life-threatening toxic manifestations require immediate treatment that combines administration of anticholinergic drugs and an aldoxime as a reactivator of AChE. An alternative approach to reduce the in vivo toxicity of OP centers on the use of bioscavengers against the parent organophosphate. Our recent research showed that site-directed mutagenesis of AChE can enable aldoximes to substantially accelerate the reactivation of OP-enzyme conjugates while dramatically slowing down rates of OP-conjugate dealkylation (aging). Therefore, this review focuses on oxime-assisted catalysis by AChE mutants that provides a potential means for degradation of organophosphates in the plasma before reaching the cellular target site. This article is part of the special issue entitled 'Acetylcholinesterase Inhibitors: From Bench to Bedside to Battlefield'.

Unsubstituted Oximes as Potential Therapeutic Agents

Oximes, which are highly bioactive molecules, have versatile uses in the medical sector and have been indicated to possess biological activity. Certain oximes exist in nature in plants and animals, but they are also obtained by chemical synthesis. Oximes are known for their anti-inflammatory, antimicrobial, antioxidant and anticancer activities. Moreover, they are therapeutic agents against organophosphate (OP) poisoning. Two oximes are already commonly used in therapy. Due to these abilities, new oxime compounds have been synthesized, and their biological activity has been verified. Often, modification of carbonyl compounds into oximes leads to increased activity. Nevertheless, in some cases, oxime activity is connected to the activity of the substrate. Recent works have revealed that new oxime compounds can demonstrate such functions and thus are considered to be potential drugs for pathogenic diseases, as adjuvant therapy in various types of cancer and inflammation and as potential next-generation drugs against OP poisoning.

Pharmacokinetic Evaluation of Brain Penetrating Morpholine-3-hydroxy-2-pyridine Oxime as an Antidote for Nerve Agent Poisoning

Nerve agents, the deadliest chemical warfare agents, are potent inhibitors of acetylcholinesterase (AChE) and cause rapid cholinergic crisis with serious symptoms of poisoning. Oxime reactivators of AChE are used in medical practice in the treatment of nerve agent poisoning, but the search for novel improved reactivators with central activity is an ongoing pursuit. For numerous oximes synthesized, in vitro reactivation is a standard approach in biological evaluation with little attention given to the pharmacokinetic properties of the compounds. This study reports a comprehensive physicochemical, pharmacokinetic, and safety profiling of five lipophilic 3-hydroxy-2-pyridine aldoximes, which were recently shown to be potent AChE reactivators with a potential to be centrally active. The oxime JR595 was singled out as highly metabolically stable in human liver microsomes, noncytotoxic oxime for SH-SY5Y neuroblastoma and 1321N1 astrocytoma cell lines, and its pharmacokinetic profile was determined after intramuscular administration in mice. JR595 was rapidly absorbed into blood after 15 min with simultaneous distribution to the brain at up to about 40% of its blood concentration; however, it was eliminated from both the brain and blood within an hour. In addition, the MDCKII-MDR1 cell line assay showed that oxime JR595 was not a P-glycoprotein efflux pump substrate. Finally, the preliminary antidotal study against multiple LD₅₀ doses of VX and sarin in mice showed the potential of JR595 to provide desirable therapeutic outcomes with future improvements in its circulation time.

Rational design, synthesis, and evaluation of uncharged, "smart" bis-oxime antidotes of organophosphate-inhibited human acetylcholinesterase

Organophosphate (OP) intoxications from nerve agent and OP pesticide exposures are managed with pyridinium aldoxime-based therapies whose success rates are currently limited. The pyridinium cation hampers uptake of OPs into the central nervous system (CNS). Furthermore, it frequently binds to aromatic residues of OP-inhibited acetylcholinesterase (AChE) in orientations that are nonproductive for AChE reactivation, and the structural diversity of OPs impedes efficient reactivation. Improvements of OP antidotes need to include much better access of AChE reactivators to the CNS and optimized orientation of the antidotes' nucleophile within the AChE active-center gorge. On the basis of X-ray structures of a CNS-penetrating reactivator, monoxime RS194B, reversibly bound to native and venomous agent X (VX)-inhibited human AChE, here we created seven uncharged acetamido bis-oximes as candidate antidotes. Both oxime groups in these bis-oximes were attached to the same central, saturated heterocyclic core. Diverse protonation of the heterocyclic amines and oxime groups of the bis-oximes resulted in equilibration among up to 16 distinct ionization forms, including uncharged forms capable of diffusing into the CNS and multiple zwitterionic forms optimal for reactivation reactions. Conformationally diverse zwitterions that could act as structural antidote variants significantly improved in vitro reactivation of diverse OP-human AChE conjugates. Oxime group reorientation of one of the bis-oximes, forcing it to point into the active center for reactivation, was confirmed by X-ray structural analysis. Our findings provide detailed structure-activity properties of several CNS-directed, uncharged aliphatic bis-oximes holding promise for use as protonation-dependent, conformationally adaptive, "smart" accelerated antidotes against OP toxicity.

Recent developments on oximes to improve the blood brain barrier penetration for the treatment of organophosphorus poisoning: a review

Organophosphorus (OP) compounds are highly toxic synthetic compounds which have been used as pesticides and developed as warfare nerve agents. They represent a threat to both military and civilian populations. OP pesticides affect the nervous system and are thought to have caused at least 5 million deaths since their discovery in the 1930s. At present the treatment of OP nerve agent poisoning commonly involves the use of parenteral oximes. However, the blood brain barrier (BBB) remains a challenge in the delivery of oximes to the central nervous system (CNS). This is because almost all macromolecule drugs (including oximes) fail to pass through the BBB to reach the CNS structures. The presence of a permanent cationic charge in oximes has made these compounds inefficient in crossing the BBB. Thus, oximes are unable to reactivate acetylcholinesterase (AChE) in the CNS. Using current structural and mechanistic understanding of the BBB under both physiological and pathological conditions, it becomes possible to design delivery systems for oximes and other drugs that are able to cross the BBB effectively. This review summarises the recent strategies in the development of oximes which are capable of crossing the BBB to treat OP poisoning. Several new developments using oximes are reviewed along with their advantages and disadvantages. This review could be beneficial for future directions in the development of oxime and other drug delivery systems into the CNS.

Organophosphorus (OP) compounds are highly toxic synthetic compounds which have been used as pesticides and developed as warfare nerve agents.

Evaluation of high-affinity phenyltetrahydroisoquinoline aldoximes, linked through anti-triazoles, as reactivators of phosphylated cholinesterases

Acetylcholinesterase (AChE) is a pivotal enzyme in neurotransmission. Its inhibition leads to cholinergic crises and could ultimately result in death. A related enzyme, butyrylcholinesterase (BChE), may act in the CNS as a co-regulator in terminating nerve impulses and is a natural plasma scavenger upon exposure to organophosphate (OP) nerve agents that irreversibly inhibit both enzymes. With the aim of improving reactivation of cholinesterases phosphylated by nerve agents sarin, VX, cyclosarin, and tabun, ten phenyltetrahydroisoquinoline (PIQ) aldoximes were synthesized by Huisgen 1,3 dipolar cycloaddition between alkyne- and azide-building blocks. The PIQ moiety may serve as a peripheral site anchor positioning the aldoxime moiety at the AChE active site. In terms of evaluated dissociation inhibition constants, the aldoximes could be characterized as high-affinity ligands. Nevertheless, high binding affinity of these oximes to AChE or its phosphylated conjugates did not assure rapid and selective AChE reactivation. Rather, potential reactivators of phosphylated BChE, with its enlarged acyl pocket, were identified, especially in case of cyclosarin, where the reactivation rates of the lead reactivator was 100- and 6-times that of 2-PAM and HI-6, respectively. Nevertheless, the return of the enzyme activity was affected by the nerve agent conjugated to catalytic serine, which highlights the lack of the universality of reactivators with respect to both the target enzyme and OP structure.

Reactivity and mechanism of α-nucleophile scaffolds as catalytic organophosphate scavengers

Despite their unique benefits imparted by their structure and reactivity, certain α-nucleophile molecules remain underexplored as chemical inactivators for the topical decontamination of reactive organophosphates (OPs). Here, we present a library of thirty α-nucleophile scaffolds, each designed with either a pyridinium aldoxime (PAM) or hydroxamic acid (HA) α-nucleophile core tethered to a polar or charged scaffold for optimized physicochemical properties and reactivity. These library compounds were screened for their abilities to catalyze the hydrolysis of a model OP, paraoxon (POX), in kinetic assays. These screening experiments led to the identification of multiple lead compounds with the ability to inactivate POX two- to four-times more rapidly than Dekon 139-the active ingredient currently used for skin decontamination of OPs. Our mechanistic studies, performed under variable pH and temperature conditions suggested that the differences in the reactivity and activation energy of these compounds are fundamentally attributable to the core nucleophilicity and pKa. Following their screening and mechanistic studies, select lead compounds were further evaluated and demonstrated greater efficacy than Dekon 139 in the topical decontamination of POX in an ex vivo porcine skin model. In addition to OP reactivity, several compounds in the PAM class displayed a dual mode of activity, as they retained the ability to reactivate POX-inhibited acetylcholine esterase (AChE). In summary, this report describes a rationale for the hydrophilic scaffold design of α-nucleophiles, and it offers advanced insights into their chemical reactivity, mechanism, and practical utility as OP decontaminants.

6-Methyluracil derivatives as peripheral site ligand-hydroxamic acid conjugates: Reactivation for paraoxon-inhibited acetylcholinesterase

New uncharged conjugates of 6-methyluracil derivatives with imidazole-2-aldoxime and 1,2,4-triazole-3-hydroxamic acid units were synthesized and studied as reactivators of organophosphate-inhibited cholinesterase. Using paraoxon (POX) as a model organophosphate, it was shown that 6-methyluracil derivatives linked with hydroxamic acid are able to reactivate POX-inhibited human acetylcholinesterase (AChE) in vitro. The reactivating efficacy of one compound (5b) is lower than that of pyridinium-2-aldoxime (2-PAM). Meanwhile, unlike 2-PAM, in vivo study showed that the lead compound 5b is able: (1) to reactivate POX-inhibited AChE in the brain; (2) to decrease death of neurons and, (3) to prevent memory impairment in rat model of POX-induced neurodegeneration.

Productive reorientation of a bound oxime reactivator revealed in room temperature X-ray structures of native and VX-inhibited human acetylcholinesterase

Exposure to organophosphorus compounds (OPs) may be fatal if untreated, and a clear and present danger posed by nerve agent OPs has become palpable in recent years. OPs inactivate acetylcholinesterase (AChE) by covalently modifying its catalytic serine. Inhibited AChE cannot hydrolyze the neurotransmitter acetylcholine leading to its build-up at the cholinergic synapses and creating an acute cholinergic crisis. Current antidotes, including oxime reactivators that attack the OP-AChE conjugate to free the active enzyme, are inefficient. Better reactivators are sought, but their design is hampered by a conformationally rigid portrait of AChE extracted exclusively from 100K X-ray crystallography and scarcity of structural knowledge on human AChE (hAChE). Here, we present room temperature X-ray structures of native and VX-phosphonylated hAChE with an imidazole-based oxime reactivator, RS-170B. We discovered that inhibition with VX triggers substantial conformational changes in bound RS-170B from a "nonproductive" pose (the reactive aldoxime group points away from the VX-bound serine) in the reactivator-only complex to a "semi-productive" orientation in the VX-modified complex. This observation, supported by concurrent molecular simulations, suggested that the narrow active-site gorge of hAChE may be significantly more dynamic than previously thought, allowing RS-170B to reorient inside the gorge. Furthermore, we found that small molecules can bind in the choline-binding site hindering approach to the phosphorous of VX-bound serine. Our results provide structural and mechanistic perspectives on the reactivation of OP-inhibited hAChE and demonstrate that structural studies at physiologically relevant temperatures can deliver previously overlooked insights applicable for designing next-generation antidotes.

Assessment of ionizable, zwitterionic oximes as reactivating antidotal agents for organophosphate exposure

Since the development in the 1950's of 2-PAM (Pralidoxime), an antidote that reactivates organophosphate conjugated acetylcholinesterase in target tissues upon pesticide or nerve agent exposure, improvements in antidotal therapy have largely involved congeneric pyridinium aldoximes. Despite seminal advances in detailing the structures of the cholinesterases as the primary target site, progress with small molecule antidotes has yet to define a superior agent. Two major limitations are immediately apparent. The first is the impacted space within the active center gorge, particularly when the active center serine at its base is conjugated with an organophosphate. The reactivating nucleophile will have to negotiate the tortuous gorge terrain to access the phosphorus atom with its most nucleophilic form or ionization state, the oximate anion. A second limitation stems from the antidote crossing the blood-brain barrier sufficiently rapidly, since it is well documented that central acetylcholinesterase inhibition gives rise to cardiovascular and respiratory compromise. The associated hypoxia then leads to a sequelae of events, including poor perfusion of the brain and periphery, along with muscle fasciculation, tremors and eventually seizures. We consider both the barriers confronting and further achievements necessary to enhance efficacy of antidotes.

Butyrylcholinesterase inhibited by nerve agents is efficiently reactivated with chlorinated pyridinium oximes

Bispyridinium oximes with one (K865, K866, K867) or two (K868, K869, K870) ortho-positioned chlorine moiety, analogous to previously known K027, K048 and K203 oximes, and potent reactivators of human acetylcholinesterase (AChE) inhibited by nerve agents, were tested in the reactivation of human butyrylcholinesterase (BChE) inhibited by sarin, cyclosarin, VX, and tabun. A previously highlighted AChE reactivator, dichlorinated bispyridinium oxime with propyl linker (K868), was tested in more detail for reactivation of four nerve agent-BChE conjugates. Its BChE reactivation potency was showed to be promising when compared to the standard oximes used in medical practice, asoxime (HI-6) and pralidoxime (2-PAM), especially in case of sarin and tabun. This finding could be used in the pseudo-catalytic scavenging of the most nerve agents due to its cumulative capacity to reactivate both AChE and BChE.

Resurrection and Reactivation of Acetylcholinesterase and Butyrylcholinesterase

Organophosphorus (OP) nerve agents and pesticides present significant threats to civilian and military populations. OP compounds include the nefarious G and V chemical nerve agents, but more commonly, civilians are exposed to less toxic OP pesticides, resulting in the same negative toxicological effects and thousands of deaths on an annual basis. After decades of research, no new therapeutics have been realized since the mid-1900s. Upon phosphylation of the catalytic serine residue, a process known as inhibition, there is an accumulation of acetylcholine (ACh) in the brain synapses and neuromuscular junctions, leading to a cholinergic crisis and eventually death. Oxime nucleophiles can reactivate select OP-inhibited acetylcholinesterase (AChE). Yet, the fields of reactivation of AChE and butyrylcholinesterase encounter additional challenges as broad-spectrum reactivation of either enzyme is difficult. Additional problems include the ability to cross the blood brain barrier (BBB) and to provide therapy in the central nervous system. Yet another complication arises in a competitive reaction, known as aging, whereby OP-inhibited AChE is converted to an inactive form, which until very recently, had been impossible to reverse to an active, functional form. Evaluations of uncharged oximes and other neutral nucleophiles have been made. Non-oxime reactivators, such as aromatic general bases and Mannich bases, have been developed. The issue of aging, which generates an anionic phosphylated serine residue, has been historically recalcitrant to recovery by any therapeutic approach-that is, until earlier this year. Mannich bases not only serve as reactivators of OP-inhibited AChE, but this class of compounds can also recover activity from the aged form of AChE, a process referred to as resurrection. This review covers the modern efforts to address all of these issues and notes the complexities of therapeutic development along these different lines of research.

Counteracting tabun inhibition by reactivation by pyridinium aldoximes that interact with active center gorge mutants of acetylcholinesterase

Tabun represents the phosphoramidate class of organophosphates that are covalent inhibitors of acetylcholinesterase (AChE), an essential enzyme in neurotransmission. Currently used therapy in counteracting excessive cholinergic stimulation consists of a muscarinic antagonist (atropine) and an oxime reactivator of inhibited AChE, but the classical oximes are particularly ineffective in counteracting tabun exposure. In a recent publication (Kovarik et al., 2019), we showed that several oximes prepared by the Huisgen 1,3 dipolar cycloaddition and related precursors efficiently reactivate the tabun-AChE conjugate. Herein, we pursue the antidotal question further and examine a series of lead precursor molecules, along with triazole compounds, as reactivators of two AChE mutant enzymes. Such studies should reveal structural subtleties that reside within the architecture of the active center gorge of AChE and uncover intimate mechanisms of reactivation of alkylphosphate conjugates of AChE. The designated mutations appear to minimize steric constraints of the reactivating oximes within the impacted active center gorge. Indeed, after initial screening of the triazole oxime library and its precursors for the reactivation efficacy on Y337A and Y337A/F338A human AChE mutants, we found potentially active oxime-mutant enzyme pairs capable of degrading tabun in cycles of inhibition and reactivation. Surprisingly, the most sensitive ex vivo reactivation of mutant AChEs occurred with the alkylpyridinium aldoximes. Hence, although the use of mutant enzyme bio-scavengers in humans may be limited in practicality, bioscavenging and efficient neutralization of tabun itself or phosphoramidate mixtures of organophosphates might be achieved efficiently in vitro or ex vivo with these mutant AChE combinations.

Reversal of Tabun Toxicity Enabled by a Triazole-Annulated Oxime Library-Reactivators of Acetylcholinesterase

Acetylcholinesterase (AChE), an enzyme that degrades the neurotransmitter acetylcholine, when covalently inhibited by organophosphorus compounds (OPs), such as nerve agents and pesticides, can be reactivated by oximes. However, tabun remains among the most dangerous nerve agents due to the low reactivation efficacy of standard pyridinium aldoxime antidotes. Therefore, finding an optimal reactivator for prophylaxis against tabun toxicity and for post-exposure treatment is a continued challenge. In this study, we analyzed the reactivation potency of 111 novel nucleophilic oximes mostly synthesized using the CuAAC triazole ligation between alkyne and azide building blocks. We identified several oximes with significantly improved in vitro reactivating potential for tabun-inhibited human AChE, and in vivo antidotal efficacies in tabun-exposed mice. Our findings offer a significantly improved platform for further development of antidotes and scavengers directed against tabun and related phosphoramidate exposures, such as the Novichok compounds.

Hydrophilic scaffolds of oxime as the potent catalytic inactivator of reactive organophosphate

Despite its efficacy as a skin decontaminant of reactive organophosphates (OP), Dekon 139-a potassium salt of 2,3-butanedione monooxime (DAM)-is associated with adverse events related to percutaneous absorption largely due to its small size and lipophilicity. In order to address this physicochemical issue, we synthesized and evaluated the activity of a focused library of 14 hydrophilic oxime compounds, each designed with either a DAM or monoisonitrosoacetone (MINA) oxime tethered to a polar or charged scaffold in order to optimize the size, hydrophilicity, and oxime acidity. High-throughput colorimetric assays were performed with paraoxon (POX) as a model OP to determine the kinetics of POX inactivation by these compounds under various pH and temperature conditions. This primary screening led to the identification of 6 lead compounds, predominantly in the MINA series, which displayed superb catalytic activity by reducing the POX half-life (t_1/2) by 2-3 fold relative to Dekon 139. Our mechanistic studies show that POX inactivation by the oxime compounds occurred faster at a higher temperature and in a pH-dependent manner in which the negatively charged oximate species is ≥ 10-fold more effective than the neutral oxime species. Lastly, using one of the lead compounds, we demonstrated its promising efficacy for POX decontamination in porcine skin ex vivo, and showed its potent ability to protect acetylcholine esterase (AChE) through POX inactivation. In summary, we report the rational design and chemical biological validation of novel hydrophilic oximes which address an unmet need in therapeutic OP decontamination.

Combination delivery of two oxime-loaded lipid nanoparticles: Time-dependent additive action for prolonged rat brain protection

A novel approach for brain protection against poisoning by organophosphorus agents is developed based on the combination treatment of dual delivery of two oximes. Pralidoxime chloride (2-PAM) and a novel reactivator, 6-(5-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)pentyl)-3-hydroxy picolinaldehyde oxime (3-HPA), have been loaded in solid-lipid nanoparticles (SLNs) to offer distinct release profile and systemic half-life for both oximes. To increase the therapeutic time window of both oximes, SLNs with two different compartments were designed to load each respective drug. Oxime-loaded SLNs of hydrodynamic diameter between 100 and 160 nm and negative zeta potential (-30 to -25 mV) were stable for a period of 10 months at 4 °C. SLNs displayed longer circulation time in the bloodstream compared to free 3-HPA and free 2-PAM. Oxime-loaded SLNs were suitable for intravenous (iv) administration. Paraoxon-poisoned rats (0.8 × LD₅₀) were treated with 3-HPA-loaded SLNs and 2-PAM+3-HPA-loaded SLNs at the dose of 3-HPA and 2-PAM of 5 mg/kg. Brain AChE reactivation up to 30% was slowly achieved in 5 h after administration of 3-HPA-SLNs. For combination therapy with two oximes, a time-dependent additivity and increased reactivation up to 35% were observed.