Novel approaches in prophylaxis/pretreatment and treatment of organophosphorus poisoning

Advancements in bioscavenger mediated detoxification of organophosphorus poisoning

Background

Organophosphorus compounds, widely used in agriculture and industry, pose a serious threat to human health due to their acute neurotoxicity. Although traditional interventions for organophosphate poisoning are effective, they often come with significant side effects.

Objective

This paper aims to evaluate the potential of enzymes within biological organisms as organophosphorus bioclearing agents. It analyses the technical challenges in current enzyme research, such as substrate specificity, stereoselectivity, and immunogenicity, while exploring recent advancements in the field.

Methods

A comprehensive review of literature related to detoxifying enzymes or proteins was conducted. Existing studies on organophosphorus bioclearing agents were summarised, elucidating the biological detoxification mechanisms, with a particular focus on advancements in protein engineering and novel delivery methods.

Results

Current bioclearing agents can be categorised into stoichiometric and catalytic bioclearing agents, both of which have shown some success in preventing organophosphate poisoning. Technological advancements have significantly improved various properties of bioclearing agents, yet challenges remain, particularly in substrate specificity, stereoselectivity, and immunogenicity. Future research will focus on expanding the substrate spectrum, enhancing catalytic efficiency, prolonging in vivo half-life, and developing convenient administration methods.

Conclusion

With the progression of clinical trials, bioclearing agents are expected to become widely used as a new generation of therapeutic organophosphate detoxifiers.

Penetrating the Blood-Brain Barrier for Targeted Treatment of Neurotoxicant Poisoning by Nanosustained-Released 2-PAM@VB1-MIL-101-NH2(Fe)

It is very important to establish a sustained-release pralidoxime chloride (2-PAM) drug system with brain targeting function for the treatment of neurotoxicant poisoning. Herein, Vitamin B1 (VB1), also known as thiamine, which can specifically bind to the thiamine transporter on the surface of the blood-brain barrier, was incorporated onto the surface of MIL-101-NH₂(Fe) nanoparticles with a size of ∼100 nm. Pralidoxime chloride was further loaded within the interior of the above resulted composite by soaking, and a resulting composite drug (denoted as 2-PAM@VB1-MIL-101-NH₂(Fe)) with a loading capacity of 14.8% (wt) was obtained. The results showed that the drug release rate of the composite drug was increased in PBS solution with the increase of pH (2-7.4) and a maximum drug release rate of 77.5% at pH 4. Experiments on the treatment of poisoning by gavage with the nerve agent sarin in mice combined with atropine revealed that sustained release of 2-PAM from the composite drug was achieved for more than 72 h. Sustained and stable reactivation of poisoned acetylcholinesterase (AChE) was observed with an enzyme reactivation rate of 42.7% in the ocular blood samples at 72 h. By using both zebrafish brain and mouse brain as models, we found that the composite drug could effectively cross the blood-brain barrier and restore the AChE activity in the brain of poisoned mice. The composite drug is expected to be a stable therapeutic drug with brain targeting and prolonged drug release properties for nerve agent intoxication in the middle and late stages of treatment.

1-(3-Tert-Butylphenyl)-2,2,2-Trifluoroethanone as a Potent Transition-State Analogue Slow-Binding Inhibitor of Human Acetylcholinesterase: Kinetic, MD and QM/MM Studies

Kinetic studies and molecular modeling of human acetylcholinesterase (AChE) inhibition by a fluorinated acetophenone derivative, 1-(3-tert-butylphenyl)-2,2,2-trifluoroethanone (TFK), were performed. Fast reversible inhibition of AChE by TFK is of competitive type with K_i = 5.15 nM. However, steady state of inhibition is reached slowly. Kinetic analysis showed that TFK is a slow-binding inhibitor (SBI) of type B with K_i* = 0.53 nM. Reversible binding of TFK provides a long residence time, τ = 20 min, on AChE. After binding, TFK acylates the active serine, forming an hemiketal. Then, disruption of hemiketal (deacylation) is slow. AChE recovers full activity in approximately 40 min. Molecular docking and MD simulations depicted the different steps. It was shown that TFK binds first to the peripheral anionic site. Then, subsequent slow induced-fit step enlarged the gorge, allowing tight adjustment into the catalytic active site. Modeling of interactions between TFK and AChE active site by QM/MM showed that the “isomerization” step of enzyme-inhibitor complex leads to a complex similar to substrate tetrahedral intermediate, a so-called “transition state analog”, followed by a labile covalent intermediate. SBIs of AChE show prolonged pharmacological efficacy. Thus, this fluoroalkylketone intended for neuroimaging, could be of interest in palliative therapy of Alzheimer’s disease and protection of central AChE against organophosphorus compounds.

Brain-targeted delivery of obidoxime, using aptamer-modified liposomes, for detoxification of organophosphorus compounds

Effective intracerebral delivery acetylcholinesterase (AChE) reactivator is key for the acute organophosphorus (OPs) poison treatment. However, the blood-brain barrier (BBB) restricts the transport of these drugs from blood into the brain. Herein, we developed transferrin receptor (TfR) aptamer-functionalized liposomes (Apt-LP) that could deliver AChE reactivator (obidoxime) across the BBB to act against paraoxon (POX) poisoning. The aptamer had strong affinity for TfR and was modified with 3'-inverted deoxythymidine (dT) to improve serum stability. The uptake of Apt-LP by bEnd.3 cells was significantly higher than that of non-targeting liposomes. The ability of Apt-LP to penetrate intact BBB was confirmed in in vitro BBB mice model and in vivo biodistribution studies. Treatment of POX-poisoned mice with Apt-LP-LuH-6 reactivated 18% of the brain AChE activity and prevented brain damage to some extent. Taken together, these results showed that Apt-LP may be used as a promising brain-targeted drug delivery system against OPs toxicity.

Photo-induced formation of organic nanoparticles possessing enhanced affinities for complexing nerve agent mimics

Organic nanoparticles, composed of molecular baskets, could act as nanocarriers for selective “mopping” of toxic CWAs or pesticides, after being assembled by a light stimulus.

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.

Mixed cationic liposomes for brain delivery of drugs by the intranasal route: The acetylcholinesterase reactivator 2-PAM as encapsulated drug model

New mixed cationic liposomes based on L-α-phosphatidylcholine and dihexadecylmethylhydroxyethylammonium bromide (DHDHAB) were designed to overcome the BBB crossing by using the intranasal route. Synthesis and self-assembly of DHDHAB were performed. A low critical association concentration (0.01 mM), good solubilization properties toward hydrophobic dye Orange OT and antimicrobial activity against gram-positive bacteria Staphylococcus aureus (MIC=7.8 μg mL^-1) and Bacillus cereus (MIC=7.8 μg mL^-1), low hemolytic activities against human red blood cells (less than 10%) were achieved. Conditions for preparation of cationic vesicles and mixed liposomes with excellent colloidal stability at room temperature were determined. The intranasal administration of rhodamine B-loaded cationic liposomes was shown to increase bioavailability into the brain in comparison to the intravenous injection. The cholinesterase reactivator, 2-PAM, was used as model drug for the loading in cationic liposomes. 2-PAM-loaded cationic liposomes displayed high encapsulation efficiency (∼ 90%) and hydrodynamic diameter close to 100 nm. Intranasally administered 2-PAM-loaded cationic liposomes were effective against paraoxon-induced acetylcholinesterase inhibition in the brain. 2-PAM-loaded liposomes reactivated 12 ± 1% of brain acetylcholinesterase. This promising result opens the possibility to use marketed positively charged oximes in medical countermeasures against organophosphorus poisoning for reactivation of central acetylcholinesterase by implementing a non-invasive approach, via the "nose-brain" pathway.

Water structure changes in oxime-mediated reactivation process of phosphorylated human acetylcholinesterase

The role of water in oxime-mediated reactivation of phosphylated cholinesterases (ChEs) has been asked with recurrence. To investigate oximate water structure changes in this reaction, reactivation of paraoxon-inhibited human acetylcholinesterase (AChE) was performed by the oxime asoxime (HI-6) at different pH in the presence and absence of lyotropic salts: a neutral salt (NaCl), a strong chaotropic salt (LiSCN) and strong kosmotropic salts (ammonium sulphate and phosphate HPO42-). At the same time, molecular dynamic (MD) simulations of enzyme reactivation under the same conditions were performed over 100 ns. Reactivation kinetics showed that the low concentration of chaotropic salt up to 75 mM increased the percentage of reactivation of diethylphosphorylated AChE whereas kosmotropic salts lead only to a small decrease in reactivation. This indicates that water-breaker salt induces destructuration of water molecules that are electrostricted around oximate ions. Desolvation of oximate favors nucleophilic attack on the phosphorus atom. Effects observed at high salt concentrations (>100 mM) result either from salting-out of the enzyme by kosmotropic salts (phosphate and ammonium sulphate) or denaturing action of chaotropic LiSCN. MDs simulations of diethylphosphorylated hAChE complex with HI-6 over 100 ns were performed in the presence of 100 mM (NH4)2SO4 and 50 mM LiSCN. In the presence of LiSCN, it was found that protein and water have a higher mobility, i.e. water is less organized, compared with the ammonium sulphate system. LiSCN favors protein solvation (hydrophobic hydration) and breakage of elelectrostricted water molecules around of oximate ion. As a result, more free water molecules participated to reaction steps accompanying oxime-mediated dephosphorylation.

Optimization of Cholinesterase-Based Catalytic Bioscavengers Against Organophosphorus Agents

Organophosphorus agents (OPs) are irreversible inhibitors of acetylcholinesterase (AChE). OP poisoning causes major cholinergic syndrome. Current medical counter-measures mitigate the acute effects but have limited action against OP-induced brain damage. Bioscavengers are appealing alternative therapeutic approach because they neutralize OPs in bloodstream before they reach physiological targets. First generation bioscavengers are stoichiometric bioscavengers. However, stoichiometric neutralization requires administration of huge doses of enzyme. Second generation bioscavengers are catalytic bioscavengers capable of detoxifying OPs with a turnover. High bimolecular rate constants (k_cat/K_m > 10⁶ M⁻¹min⁻¹) are required, so that low enzyme doses can be administered. Cholinesterases (ChE) are attractive candidates because OPs are hemi-substrates. Moderate OP hydrolase (OPase) activity has been observed for certain natural ChEs and for G117H-based human BChE mutants made by site-directed mutagenesis. However, before mutated ChEs can become operational catalytic bioscavengers their dephosphylation rate constant must be increased by several orders of magnitude. New strategies for converting ChEs into fast OPase are based either on combinational approaches or on computer redesign of enzyme. The keystone for rational conversion of ChEs into OPases is to understand the reaction mechanisms with OPs. In the present work we propose that efficient OP hydrolysis can be achieved by re-designing the configuration of enzyme active center residues and by creating specific routes for attack of water molecules and proton transfer. Four directions for nucleophilic attack of water on phosphorus atom were defined. Changes must lead to a novel enzyme, wherein OP hydrolysis wins over competing aging reactions. Kinetic, crystallographic, and computational data have been accumulated that describe mechanisms of reactions involving ChEs. From these studies, it appears that introducing new groups that create a stable H-bonded network susceptible to activate and orient water molecule, stabilize transition states (TS), and intermediates may determine whether dephosphylation is favored over aging. Mutations on key residues (L286, F329, F398) were considered. QM/MM calculations suggest that mutation L286H combined to other mutations favors water attack from apical position. However, the aging reaction is competing. Axial direction of water attack is not favorable to aging. QM/MM calculation shows that F329H+F398H-based multiple mutants display favorable energy barrier for fast reactivation without aging.

Nanoparticle-Delivered 2-PAM for Rat Brain Protection against Paraoxon Central Toxicity

Solid lipid nanoparticles (SLNs) are among the most promising nanocarriers to target the blood-brain barrier (BBB) for drug delivery to the central nervous system (CNS). Encapsulation of the acetylcholinesterase reactivator, pralidoxime chloride (2-PAM), in SLNs appears to be a suitable strategy for protection against poisoning by organophosphorus agents (OPs) and postexposure treatment. 2-PAM-loaded SLNs were developed for brain targeting and delivery via intravenous (iv) administration. 2-PAM-SLNs displayed a high 2-PAM encapsulation efficiency (∼90%) and loading capacity (maximum 30.8 ± 1%). Drug-loaded particles had a mean hydrodynamic diameter close to 100 nm and high negative zeta potential (-54 to -15 mV). These properties contribute to improve long-term stability of 2-PAM-SLNs when stored both at room temperature (22 °C) and at 4 °C, as well as to longer circulation time in the bloodstream compared to free 2-PAM. Paraoxon-poisoned rats (2 × LD₅₀) were treated with 2-PAM-loaded SLNs at a dose of 2-PAM of 5 mg/kg. 2-PAM-SLNs reactivated 15% of brain AChE activity. Our results confirm the potential use of SLNs loaded with positively charged oximes as a medical countermeasure both for protection against OPs poisoning and for postexposure treatment.