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

Reactivators of butyrylcholinesterase inhibited by organophosphorus compounds

In this review, the current progress in the research and development of butyrylcholinesterase (BChE) reactivators is summarised and the advantages or disadvantages of these reactivators are critically discussed. Organophosphorus compounds such as nerve agents (sarin, tabun, VX) or pesticides (chlorpyrifos, diazinon) cause irreversible inhibition of acetylcholinesterase (AChE) and BChE in the human body. While AChE inhibition can be life threatening due to cholinergic overstimulation and crisis, selective BChE inhibition has presumably no adverse effects. Because BChE is mostly found in plasma, its activity is important for the scavenging of organophosphates before they can reach AChE in the central nervous system. Therefore, this enzyme in combination with its reactivator can be used as a pseudo-catalytic scavenger of organophosphates. Three structural types of BChE reactivators were found, i.e. bisquaternary salts, monoquaternary salts and uncharged compounds. Although the reviewed reactivators have certain limitations, the promising candidates for BChE reactivation were found in each structural group.

Evaluation of Anticholinesterase Activity of the Fungicides Mefentrifluconazole and Pyraclostrobin

Triazoles are compounds with various biological activities, including fungicidal action. They became popular through cholinesterase studies after the successful synthesis of the dual binding femtomolar triazole inhibitor of acetylcholinesterase (AChE, EC 3.1.1.7) by Sharpless et al. via in situ click chemistry. Here, we evaluate the anticholinesterase effect of the first isopropanol triazole fungicide mefentrifluconazole (Ravystar®), developed to overcome fungus resistance in plant disease management. Mefentrifluconazole is commercially available individually or in a binary fungicidal mixture, i.e., with pyraclostrobin (Ravycare®). Pyraclostrobin is a carbamate that contains a pyrazole ring. Carbamates are known inhibitors of cholinesterases and the carbamate rivastigmine is already in use for the treatment of Alzheimer's disease. We tested the type and potency of anticholinesterase activity of mefentrifluconazole and pyraclostrobin. Mefentrifluconazole reversibly inhibited human AChE and BChE with a seven-fold higher potency toward AChE (Ki = 101 ± 19 μM). Pyraclostrobin (50 μM) inhibited AChE and BChE progressively with rate constants of (t1/2 = 2.1 min; ki = 6.6 × 103 M-1 min-1) and (t1/2 = 1.5 min; ki = 9.2 × 103 M-1 min-1), respectively. A molecular docking study indicated key interactions between the tested fungicides and residues of the lipophilic active site of AChE and BChE. Additionally, the physicochemical properties of the tested fungicides were compared to values for CNS-active drugs to estimate the blood-brain barrier permeability. Our results can be applied in the design of new molecules with a lesser impact on humans and the environment.

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.

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.

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).

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).

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.

Use of connectivity index and simple topological parameters for estimating the inhibition potency of acetylcholinesterase

Acetylcholinesterase (AChE) has proven to be an effective drug target in the treatment of neurodegenerative diseases such as Alzheimer’s, Parkinson’s and dementia. We developed a novel QSAR regression model for estimating potency to inhibit AChE, pK_i, on a set of 75 structurally different compounds including oximes, N-hydroxyiminoacetamides, 4-aminoquinolines and flavonoids. Although the model included only three simple descriptors, the valence molecular connectivity index of the zero-order, ⁰χ^v, the number of 10-membered rings (nR10) and the number of hydroxyl groups (nOH), it yielded excellent statistics (r = 0.937, S.E. = 0.51). The stability of the model was evaluated when an initial set of 75 compounds was broadened to 165 compounds in total, with the increase of the range of pK_i (exp) from 6.0 to 10.2, yielding r = 0.882 and S.E. = 0.89. The predictive power of the model was evaluated by calculating pK_i values for 55 randomly chosen compounds (S.E._test = 0.90) from the calibration model created on other 110 compounds (S.E. = 0.89), all taken from the pool of 165 compounds.

Assessment of four organophosphorus pesticides as inhibitors of human acetylcholinesterase and butyrylcholinesterase

Toxicity of organophosphorus compounds (OPs) remains a major public health concern due to their widespread use as pesticides and the existence of nerve agents. Their common mechanism of action involves inhibition of enzymes acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) which are crucial for neurotransmission. Both chronic and acute poisoning by OPs can leave long-lasting health effects even when the patients are treated with standard medical therapy. Therefore, an increasing urgency exists to find more effective oxime reactivators for compounds which are resistant to reactivation, especially phosphoramidates. Here, we investigated in silico and in vitro interactions and kinetics of inhibition for human cholinesterases with four organophosphate pesticides-ethoprophos, fenamiphos, methamidophos and phosalone. Overall, ethoprophos and fenamiphos displayed higher potency as inhibitors for tested cholinesterases. Our results show that methamidophos-inhibited hAChE was more susceptible to reactivation than hAChE inhibited by fenamiphos by selected oximes. Molecular modelling enabled an evaluation of interactions important for specificity and selectivity of both inhibition and reactivation of cholinesterases. Two newly developed reactivators-bispyridinium triazole oxime 14A and zwitterionic oxime RS194B possess remarkable potential for further development of antidotes directed against pesticides and related phosphoramidate exposures, such as nerve agents tabun or Novichoks.

Development of a CNS-permeable reactivator for nerve agent exposure: an iterative, multi-disciplinary approach

Nerve agents have experienced a resurgence in recent times with their use against civilian targets during the attacks in Syria (2012), the poisoning of Sergei and Yulia Skripal in the United Kingdom (2018) and Alexei Navalny in Russia (2020), strongly renewing the importance of antidote development against these lethal substances. The current standard treatment against their effects relies on the use of small molecule-based oximes that can efficiently restore acetylcholinesterase (AChE) activity. Despite their efficacy in reactivating AChE, the action of drugs like 2-pralidoxime (2-PAM) is primarily limited to the peripheral nervous system (PNS) and, thus, provides no significant protection to the central nervous system (CNS). This lack of action in the CNS stems from their ionic nature that, on one end makes them very powerful reactivators and on the other renders them ineffective at crossing the Blood Brain Barrier (BBB) to reach the CNS. In this report, we describe the use of an iterative approach composed of parallel chemical and in silico syntheses, computational modeling, and a battery of detailed in vitro and in vivo assays that resulted in the identification of a promising, novel CNS-permeable oxime reactivator. Additional experiments to determine acute and chronic toxicity are ongoing.

Development of a simple QSAR model for reliable evaluation of acetylcholinesterase inhibitor potency

With the aging of the western population, more and more people are affected by the neurodegenerative Alzheimer's and Parkinson's disease. Inhibitors of acetylcholinesterase (AChE) have proven to be effective in the treatment of disease symptoms. We report the QSAR regression model for the estimation of potency of a set of 94 structurally diverse compounds (oximes, N-hydroxyiminoacetamides, 4-aminoquinolines and flavonoids) to inhibit AChE, pK_i (AChE). The model is based on three simple descriptors: the valence molecular connectivity index of the zero-order, ⁰χ^v, combined with the number of 10-membered rings (nR10) and number of hydroxyl groups in a molecule (nOH). QSAR model yielded r = 0.947, S.E. = 0.51 and S.E._cv= 0.53; the range of pK_i (exp) = 6.03. It showed its stability when the set of 94 compounds was enlarged, comprising 184 compounds in total (r = 0.886, S.E. = 0.85 and S.E._cv = 0.88; the range of pK_i (exp) = 10.21), resulting in regression parameters which were similar, although only for ⁰χ^v coefficients within the limits of S.E. (0.167(13) and 0.172(16) for the set with 94 and 184 compounds, respectively. The predictive power of the model was shown by the prediction of pK_i values for 61 randomly chosen compounds (S.E._test = 0.86) from the calibration model made on the other 123 compounds (S.E. = 0.85), all taken from the pool of 184 compounds. QSAR descriptors ⁰χ^v, nR10 and nOH were well chosen for describing the interactions of the AChE active site (amino acid interaction) with ligands through the estimation of the inhibitory potency.

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.

Structure and therapeutic uses of butyrylcholinesterase: Application in detoxification, Alzheimer's disease, and fat metabolism

Structural information of butyrylcholinesterase (BChE) and its variants associated with several diseases are discussed here. Pure human BChE has been proved safe and effective in treating organophosphorus (OPs) poisoning and has completed Phase 1 and 2 pharmacokinetic (PK) and safety studies. The introduction of specific mutations into native BChE to endow it a self-reactivating property has gained much progress in producing effective OPs hydrolases. The hydrolysis ability of native BChE on cocaine has been confirmed but was blocked to clinical application due to poor PK properties. Several BChE mutants with elevated cocaine hydrolysis activity were published, some of which have shown safety and efficiency in treating cocaine addiction of human. The increased level of BChE in progressed Alzheimer's disease patients made it a promising target to elevate acetylcholine level and attenuate cognitive status. A variety of selective BChE inhibitors with high inhibitory activity published in recent years are reviewed here. BChE could influence the weight and insulin secretion and resistance of BChE knockout (KO) mice through hydrolyzing ghrelin. The BChE-ghrelin pathway could also regulate aggressive behaviors of BChE-KO mice.

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'.