Effective parallel evaluation of molecular design, expression and bioactivity of novel recombinant butyrylcholinesterase medical countermeasures

Current medical countermeasures (MCMs) for nerve agent poisoning have limited efficacy, and can cause serious adverse effects, prompting the requirement for new broad-spectrum therapeutics. Human plasma-derived butyrylcholinseterase (huBChE) is a promising novel bioscavenger MCM which has shown potential in animal studies, however, is economically prohibitive to manufacture at scale. This study addresses current challenges for the economical production of a bioactive and long-acting recombinant huBChE (rBChE) in mammalian cells by being the first to directly compare novel rBChE design strategies. These include co-expression of a proline rich attachment domain (PRAD) and fusion of BChE with a protein partner. Additionally, a pre-purification screening method developed in this study enables parallel comparison of the expression efficiency, activity and broad-spectrum binding to nerve agents for ten novel rBChE molecular designs. All designed rBChE demonstrated functionality to act as broad-spectrum MCMs to G, V and A series nerve agents. Expression using the ExpiCHO™ Max protocol provided greatest expression levels and activity for all constructs, with most rBChE expressing poorly in Expi293™. Fc- or hSA-fused rBChE significantly outperformed constructs designed to mimic huBChE, including PRAD-BChE, and proved an effective strategy to significantly improve enzyme activity and expression. Choice of protein partner, directionality and the addition of a linker also impacted fusion rBChE activity and expression. Overall, hSA fused rBChE provided greatest expression yield and activity, with BChE-hSA the best performing construct. The purified and characterised BChE-hSA demonstrated similar functionality to huBChE to be inhibited by GD, VX and A-234, supporting the findings of the pre-screening study and validating its capacity to assess and streamline the selection process for rBChE constructs in a cost-effective manner. Collectively, these outcomes contribute to risk mitigation in early-stage development, providing a systematic method to compare rBChE designs and a focus for future development.

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.

Biomedical Nanosystems for In Vivo Detoxification: From Passive Delivery Systems to Functional Nanodevices and Nanorobots

The problem of low efficiency of nanotherapeutic drugs challenges the creation of new alternative biomedical nanosystems known as robotic nanodevices. In addition to encapsulating properties, nanodevices can perform different biomedical functions, such as precision surgery, in vivo detection and imaging, biosensing, targeted delivery, and, more recently, detoxification of endogenous and xenobiotic compounds. Nanodevices for detoxification are aimed at removing toxic molecules from biological tissues, using a chemical- and/or enzyme-containing nanocarrier for the toxicant to diffuse inside the nanobody. This strategy is opposite to drug delivery systems that focus on encapsulating drugs and releasing them under the influence of external factors. The review describes various kinds of nanodevices intended for detoxification that differ by the type of poisoning treatment they provide, as well as the type of materials and toxicants. The final part of the review is devoted to enzyme nanosystems, an emerging area of research that provides fast and effective neutralization of toxins in vivo.

Organophosphorus poisoning in animals and enzymatic antidotes

Organophosphorus compounds (OPs) are neurotoxic molecules developed as pesticides and chemical warfare nerve agents (CWNAs). Most of them are covalent inhibitors of acetylcholinesterase (AChE), a key enzyme in nervous systems, and are therefore responsible for numerous poisonings around the world. Many animal models have been studied over the years in order to decipher the toxicity of OPs and to provide insights for therapeutic and decontamination purposes. Environmental impact on wild animal species has been analyzed to understand the consequences of OP uses in agriculture. In complement, various laboratory models, from invertebrates to aquatic organisms, rodents and primates, have been chosen to study chronic and acute toxicity as well as neurobehavioral impact, immune response, developmental disruption, and other pathological signs. Several decontamination approaches were developed to counteract the poisoning effects of OPs. Among these, enzyme-based strategies are particularly attractive as they allow efficient external decontamination without toxicity or environmental impact and may be of interest for treatment. Approaches using bioscavengers for prophylaxis, treatment, and external decontamination are emphasized and their potential is discussed in the light of toxicological observations from various animal models. The relevance of animal models, regarding their cholinergic system and the abundance of naturally protecting enzymes, is also discussed for better extrapolation of results to human.

Engineered Recombinant PON1-OPH Fusion Hybrids: Potentially Effective Catalytic Bioscavengers against Organophosphorus Nerve Agent Analogs

Organophosphorus nerve agents (OPNAs), including both G- and V-type nerve agents such as sarin, soman, tabun and VX, are extremely neurotoxic organophosphorus compounds. Catalytic bioscavengers capable of hydrolyzing OPNAs are under development because of the low protective effects and adverse side effects of chemical antidotes to OPNA poisoning. However, these bioscavengers have certain limitations for practical application, including low catalytic activity and narrow specificity. In this study, we generated a fusion-hybrid form of engineered recombinant human paraoxonase 1 (rePON1) and bacterial organophosphorus hydrolase (OPH), referred to as GV-hybrids, using a flexible linker to develop more promising catalytic bioscavengers against a broad range of OPNAs. These GV-hybrids were able to synergistically hydrolyze both G-type OPNA analogs (paraoxon: 1.7 ~ 193.7-fold, p-nitrophenyl diphenyl phosphate (PNPDPP): 2.3 ~ 33.0-fold and diisopropyl fluorophosphates (DFP): 1.4 ~ 22.8-fold) and V-type OPNA analogs (demeton-Smethyl (DSM): 1.9 ~ 34.6-fold and malathion: 1.1 ~ 4.2-fold above) better than their individual enzyme forms. Among the GV-hybrid clones, the GV7 clone showed remarkable improvements in the catalytic activity toward both G-type OPNA analogs (k_cat/K_m (10⁶ M^-1 min^-1): 59.8 ± 0.06 (paraoxon), 5.2 ± 0.02 (PNPDPP) and 47.0 ± 6.0 (DFP)) and V-type OPNA analogs (k_cat/K_m (M^-1 min^-1): 504.3 ± 48.5 (DSM) and 1324.0 ± 47.5 (malathion)). In conclusion, we developed GV-hybrid forms of rePON1 and bacterial OPH mutants as effective and suitable catalytic bioscavengers to hydrolyze a broad range of OPNA analogs.

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.

Translating the Concept of Bispecific Antibodies to Engineering Heterodimeric Phosphotriesterases with Broad Organophosphate Substrate Recognition

We have adopted the concept of bispecific antibodies, which can simultaneously block or cross-link two different biomolecular targets, to create bispecific enzymes by exploiting the homodimeric quaternary structure of bacterial phosphotriesterases (PTEs). The PTEs from Brevundimonas diminuta and Agrobacterium radiobacter, whose engineered variants can efficiently hydrolyze organophosphorus (OP) nerve agents and pesticides, respectively, have attracted considerable interest for the treatment of the corresponding intoxications. OP nerve agents and pesticides still pose a severe toxicological threat in military conflicts, including acts of terrorism, as well as in agriculture, leading to >100000 deaths per year. In principle, engineered conventional homodimeric PTEs may provoke hydrolytic inactivation of individual OPs in vivo, and their application as catalytic bioscavengers via administration into the bloodstream has been proposed. However, their narrow substrate specificity would necessitate therapeutic application of a set or mixture of different enzymes, which complicates biopharmaceutical development. We succeeded in combining subunits from both enzymes and to stabilize their heterodimerization by rationally designing electrostatic steering mutations, thus breaking the natural C2 symmetry. The resulting bispecific enzyme from two PTEs with different bacterial origin exhibits an ultrabroad OP substrate profile and allows the efficient detoxification of both nerve agents and pesticides. Our approach of combining two active sites with distinct substrate specificities within one artificial dimeric biocatalyst-retaining the size and general properties of the original enzyme without utilizing protein mixtures or much larger fusion proteins-not only should facilitate biological drug development but also may be applicable to oligomeric enzymes with other catalytic activities.

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'.

A catalytic bioscavenger with improved stability and reduced susceptibility to oxidation for treatment of acute poisoning with neurotoxic organophosphorus compounds

Organophosphorus (OP)¹ nerve agents pose a severe toxicological threat, both after dissemination in military conflicts and by terrorists. Hydrolytic enzymes, which may be administered into the blood stream of victims by injection and can decompose the circulating nerve agent into non-toxic metabolites in vivo, could offer a treatment. Indeed, for the phosphotriesterase found in the bacterium Brevundimonas diminuta (BdPTE),² engineered versions with improved catalytic efficiencies have been described; yet, their biochemical stabilities are insufficient for therapeutic use. Here, we describe the application of rational protein design to develop novel mutants of BdPTE that are less susceptible to oxidative damage. In particular, the replacement of two unpaired cysteine residues by more inert amino acids led to higher stability while maintaining high catalytic activity towards a broad spectrum of substrates, including OP pesticides and V-type nerve agents. The mutant BdPTE enzymes were produced in Escherichia coli, purified to homogeneity, and their biochemical and enzymological properties were assessed. Several candidates both revealed enhanced thermal stability and were less susceptible to oxidative stress, as demonstrated by mass spectrometry. These mutants of BdPTE may show promise for the treatment of acute intoxications by nerve agents as well as OP pesticides.

[Organophosphorus poisoning: Towards enzymatic treatments]

Organophosphorus compounds (OP) are toxic molecules developed as insecticides and chemical warfare nerve agents (CWNAs). Most OP are neurotoxic and act as nervous system disruptors by blocking cholinergic transmission. They are therefore responsible for many poisonings worldwide. OP toxicity may result either from acute or chronic exposure, and their poisoning effect were evaluated using several animal models. These latter were also used for evaluating the efficacy of antidotes. Strategies based on enzymes that can trap (stoichiometric bioscavengers) or degrade (catalytic bioscavengers) OP, were particularly studied since they allow effective decontamination, without toxicity or environmental impact. This review summarizes the results obtained in vivo with enzymes through three levels: prophylaxis, treatment and external decontamination. The efficiency of enzymatic treatments in different animal models is presented and the relevance of these models is also discussed for a better extrapolation to humans.

Detoxification of Organophosphate Poisoning Using Nanoparticle Bioscavengers

Organophosphate poisoning is highly lethal as organophosphates, which are commonly found in insecticides and nerve agents, cause irreversible phosphorylation and inactivation of acetylcholinesterase (AChE), leading to neuromuscular disorders via accumulation of acetylcholine in the body. Direct interception of organophosphates in the systemic circulation thus provides a desirable strategy in treatment of the condition. Inspired by the presence of AChE on red blood cell (RBC) membranes, we explored a biomimetic nanoparticle consisting of a polymeric core surrounded by RBC membranes to serve as an anti-organophosphate agent. Through in vitro studies, we demonstrated that the biomimetic nanoparticles retain the enzymatic activity of membrane-bound AChE and are able to bind to a model organophosphate, dichlorvos, precluding its inhibitory effect on other enzymatic substrates. In a mouse model of organophosphate poisoning, the nanoparticles were shown to improve the AChE activity in the blood and markedly improved the survival of dichlorvos-challenged mice.

Development of organophosphate hydrolase activity in a bacterial homolog of human cholinesterase

We applied a combination of rational design and directed evolution (DE) to Bacillus subtilis p-nitrobenzyl esterase (pNBE) with the goal of enhancing organophosphorus acid anhydride hydrolase (OPAAH) activity. DE started with a designed variant, pNBE A107H, carrying a histidine homologous with human butyrylcholinesterase G117H to find complementary mutations that further enhance its OPAAH activity. Five sites were selected (G105, G106, A107, A190, and A400) within a 6.7 Å radius of the nucleophilic serine Oγ. All 95 variants were screened for esterase activity with a set of five substrates: pNP-acetate, pNP-butyrate, acetylthiocholine, butyrylthiocholine, or benzoylthiocholine. A microscale assay for OPAAH activity was developed for screening DE libraries. Reductions in esterase activity were generally concomitant with enhancements in OPAAH activity. One variant, A107K, showed an unexpected 7-fold increase in its k cat/K m for benzoylthiocholine, demonstrating that it is also possible to enhance the cholinesterase activity of pNBE. Moreover, DE resulted in at least three variants with modestly enhanced OPAAH activity compared to wild type pNBE. A107H/A190C showed a 50-fold increase in paraoxonase activity and underwent a slow time- and temperature-dependent change affecting the hydrolysis of OPAA and ester substrates. Structural analysis suggests that pNBE may represent a precursor leading to human cholinesterase and carboxylesterase 1 through extension of two vestigial specificity loops; a preliminary attempt to transfer the Ω-loop of BChE into pNBE is described. Unlike butyrylcholinesterase and pNBE, introducing a G143H mutation (equivalent to G117H) did not confer detectable OP hydrolase activity on human carboxylesterase 1 (hCE1). We discuss the use of pNBE as a surrogate scaffold for the mammalian esterases, and the importance of the oxyanion-hole residues for enhancing the OPAAH activity of selected serine hydrolases.

Genotyping single-nucleotide polymorphisms of human genes involved in organophosphate detoxification by high-resolution melting

Paraoxonase-1 (PON1) and butyrylcholinesterase (BCHE) are natural bioscavengers of organophosphate acetylcholinesterase inhibitors in the human body, which can determine individual sensitivity to organophosphate toxicity. Interindividual differences in activity of PON1 (catalytic bioscavenger) and substrate specificity are strongly associated with the substitution of two amino acids: Leu/Met (L/M) at position 55 (rs854560) and Gln/Arg (Q/R) at position 192 (rs662). In the case of BCHE (stoichiometric bioscavenger) substitution, Ala/Thr (A/T) at position 539 produces the so-called "K-variant" of the enzyme (rs1803274). Threonine allele is often co-inherited with an atypical BCHE allele (rs1799807). The atypical variant of BCHE displays a lower affinity for cholinesterase inhibitors. Genotyping rs662 and rs1803274 single-nucleotide polymorphisms (SNP) by high-resolution melting (HRM) is facilitated by the nucleotide substitution A>G (G>A), which resulted in a changed number of hydrogen bonds in the PCR product and, consequently, shifted T m. In the case of rs854560, genotyping is complicated by the nucleotide substitution T>A, which has no significant effect on the T m of the PCR product. An addition of a small quantity of LL homozygote DNA into the reaction mixture before PCR discriminates the three genotypes by the melt curves due to different amounts of heteroduplexes formed in the LM and MM samples. HRM analysis can be applied for genotyping human rs854560, rs662, and rs1803274 SNPs.

Progress in the development of enzyme-based nerve agent bioscavengers

Acetylcholinesterase is the physiological target for acute toxicity of nerve agents. Attempts to protect acetylcholinesterase from phosphylation by nerve agents, is currently achieved by reversible inhibitors that transiently mask the enzyme active site. This approach either protects only peripheral acetylcholinesterase or may cause side effects. Thus, an alternative strategy consists in scavenging nerve agents in the bloodstream before they can reach acetylcholinesterase. Pre- or post-exposure administration of bioscavengers, enzymes that neutralize and detoxify organophosphorus molecules, is one of the major developments of new medical counter-measures. These enzymes act either as stoichiometric or catalytic bioscavengers. Human butyrylcholinesterase is the leading stoichiometric bioscavenger. Current efforts are devoted to its mass production with care to pharmacokinetic properties of the final product for extended lifetime. Development of specific reactivators of phosphylated butyrylcholinesterase, or variants with spontaneous reactivation activity is also envisioned for rapid in situ regeneration of the scavenger. Human paraoxonase 1 is the leading catalytic bioscavenger under development. Research efforts focus on improving its catalytic efficiency toward the most toxic isomers of nerve agents, by means of directed evolution-based strategies. Human prolidase appears to be another promising human enzyme. Other non-human efficient enzymes like bacterial phosphotriesterases or squid diisopropylfluorophosphatase are also considered though their intrinsic immunogenic properties remain challenging for use in humans. Encapsulation, PEGylation and other modifications are possible solutions to address this problem as well as that of their limited lifetime. Finally, gene therapy for in situ generation and delivery of bioscavengers is for the far future, but its proof of concept has been established.

Evolution of and perspectives on therapeutic approaches to nerve agent poisoning

After more than 70 years of considerable efforts, research on medical defense against nerve agents has come to a standstill. Major progress in medical countermeasures was achieved between the 50s and 70s with the development of anticholinergic drugs and carbamate-based pretreatment, the introduction of pyridinium oximes as antidotes, and benzodiazepines in emergency treatments. These drugs ensure good protection of the peripheral nervous system and mitigate the acute effects of exposure to lethal doses of nerve agents. However, pyridostigmine and cholinesterase reactivators currently used in the armed forces do not protect/reactivate central acetylcholinesterases. Moreover, other drugs used are not sufficiently effective in protecting the central nervous system against seizures, irreversible brain damages and long-term sequelae of nerve agent poisoning.New developments of medical counter-measures focus on: (a) detoxification of organophosphorus molecules before they react with acetylcholinesterase and other physiological targets by administration of stoichiometric or catalytic scavengers; (b) protection and reactivation of central acetylcholinesterases, and (c) improvement of neuroprotection following delayed therapy.Future developments will aim at treatment of acute and long-term effects of low level exposure to nerve agents, research on alternative routes for optimizing drug delivery, and therapies. Though gene therapy for in situ generation of bioscavengers, and cell therapy based on neural progenitor engraftment for neuronal regeneration have been successfully explored, more studies are needed before practical medical applications can be made of these new approaches.

Hyperthermophilic phosphotriesterases/lactonases for the environment and human health

In the last decades the idea to use enzymes for environmental bioremediation has been more and more proposed and, in the light of this, new solutions have been suggested and detailed studies on some classes of enzymes have been performed. In particular, our attention in the last few years has been focused on the enzymes belonging to the amidohydrolase superfamily. Several members of this superfamily are endowed with promiscuous activities. The term 'catalytic promiscuity' describes the capability of an enzyme to catalyse different chemical reactions, called secondary activities, at the active site responsible for the main activity. Recently, a new family of microbial lactonases with promiscuous phosphotriesterase activity, dubbed PTE-Like Lactonase (PLL), has been ascribed to the amidohydrolase superfamily. Among members of this family are enzymes found in the archaea Sulfolobus solfataricus and Sulfolobus acidocaldarius, which show high thermophilicity and thermal resistance. Enzymes showing phosphotriesterase activity are attractive from a biotechnological point of view because they are capable of hydrolysing the organophosphate phosphotriesters (OPs), a class of synthetic compounds employed worldwide both as insecticides and chemical warfare agents. Furthermore, from a basic point of view, studies of catalytic promiscuity offer clues to understand natural evolution of enzymes and to translate this into in vitro adaptation of enzymes to specific human needs. Thermostable enzymes able to hydrolyse OPs are considered good candidates for the set-up of efficient detoxification tools.