Acute toxicity of organophosphorus compounds in guinea pigs is sex- and age-dependent and cannot be solely accounted for by acetylcholinesterase inhibition

Cellular responses following ex vivo lung exposure to the nerve agent VX - Potential for additional treatment targets?

Following inhalation exposure to organophosphorus nerve agents, symptoms rapidly develop and severe respiratory symptoms, such as bronchorrhea and bronchoconstriction are the leading causes of lethality. Nerve agent-induced lung injury is little investigated and the standard treatment for symptomatic relief targets the enzyme acetylcholinesterase and muscarinic acetylcholine and GABAergic receptors. In the present study, cellular responses in lung tissue during the acute (40 min) and extended phase (24 h) following severe exposure to the nerve agent VX have been investigated using an ex vivo rat precision-cut lung slice model including electrostimulation to induce a cholinergic response. Changes in protein amount, cell viability, together with, inflammatory and oxidative stress markers have been determined in both the lung tissue and incubation medium. During the acute phase, VX caused significantly increased airway contraction and decreased airway relaxation. Five micromolar of VX did not affect the sample protein levels and cell viability in lung tissue. Among seven markers of cellular responses investigated in the lung tissue, increased levels of heme oxygenase-1 and matrix metalloproteinase-9 together with decreased levels of glutathione in the incubation medium were observed in the acute phase following VX-exposure compared to electrostimulation only. No difference in cellular response was observed following VX-exposure for 24 h compared to the air control. In comparison, LPS-exposure resulted in time-dependent changes in all markers of inflammation and oxidative response. In conclusion, the present study demonstrated VX-specific patterns of oxidative responses in the lung, as well as, signs of inflammatory response and remodelling of extracellular matrix. These potential mechanisms of tissue injury should be further investigated for their potential as additional therapeutic targets during the acute phase of intoxication.

Biphasic response of human iPSC-derived neural network activity following exposure to a sarin-surrogate nerve agent

Organophosphorus nerve agents (OPNA) are hazardous environmental exposures to the civilian population and have been historically weaponized as chemical warfare agents (CWA). OPNA exposure can lead to several neurological, sensory, and motor symptoms that can manifest into chronic neurological illnesses later in life. There is still a large need for technological advancement to better understand changes in brain function following OPNA exposure. The human-relevant in vitro multi-electrode array (MEA) system, which combines the MEA technology with human stem cell technology, has the potential to monitor the acute, sub-chronic, and chronic consequences of OPNA exposure on brain activity. However, the application of this system to assess OPNA hazards and risks to human brain function remains to be investigated. In a concentration-response study, we have employed a human-relevant MEA system to monitor and detect changes in the electrical activity of engineered neural networks to increasing concentrations of the sarin surrogate 4-nitrophenyl isopropyl methylphosphonate (NIMP). We report a biphasic response in the spiking (but not bursting) activity of neurons exposed to low (i.e., 0.4 and 4 μM) versus high concentrations (i.e., 40 and 100 μM) of NIMP, which was monitored during the exposure period and up to 6 days post-exposure. Regardless of the NIMP concentration, at a network level, communication or coordination of neuronal activity decreased as early as 60 min and persisted at 24 h of NIMP exposure. Once NIMP was removed, coordinated activity was no different than control (0 μM of NIMP). Interestingly, only in the high concentration of NIMP did coordination of activity at a network level begin to decrease again at 2 days post-exposure and persisted on day 6 post-exposure. Notably, cell viability was not affected during or after NIMP exposure. Also, while the catalytic activity of AChE decreased during NIMP exposure, its activity recovered once NIMP was removed. Gene expression analysis suggests that human iPSC-derived neurons and primary human astrocytes resulted in altered genes related to the cell's interaction with the extracellular environment, its intracellular calcium signaling pathways, and inflammation, which could have contributed to how neurons communicated at a network level.

(R,S)-trihexyphenidyl, acting via a muscarinic receptor-independent mechanism, inhibits hippocampal glutamatergic and GABAergic synaptic transmissions: Potential relevance for treatment of organophosphorus intoxication

Preclinical studies have reported that, compared to the muscarinic receptor (mAChR) antagonist atropine, (R,S)-trihexyphenidyl (THP) more effectively counters the cholinergic crisis, seizures, and neuropathology triggered by organophosphorus (OP)-induced acetylcholinesterase (AChE) inhibition. The greater effectiveness of THP was attributed to its ability to block mAChRs and N-methyl-d-aspartate-type glutamatergic receptors (NMDARs) in the brain. However, THP also inhibits α7 nicotinic receptors (nAChRs). The present study examined whether THP-induced inhibition of mAChRs, α7 nAChRs, and NMDARs is required to suppress glutamatergic synaptic transmission, whose overstimulation sustains OP-induced seizures. In primary hippocampal cultures, THP (1-30 μM) suppressed the frequency of excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs, respectively) recorded from neurons in nominally Mg2+-free solution. A single sigmoidal function adequately fit the overlapping concentration-response relationships for THP-induced suppression of IPSC and EPSC frequencies yielding an IC50 of 6.3 ± 1.3 μM. Atropine (1 μM), the NMDAR antagonist d,l-2-amino-5-phosphonopentanoic acid (D,L-AP5, 50 μM), and the α7 nAChR antagonist methyllycaconitine (MLA, 10 nM) did not prevent THP-induced inhibition of synaptic transmission. THP (10 μM) did not affect the probability of transmitter release because it had no effect on the frequency of miniature IPSCs and EPSCs recorded in the presence of tetrodotoxin. Additionally, THP had no effect on the amplitudes and decay-time constants of miniature IPSCs and EPSCs; therefore, it did not affect the activity of postsynaptic GABAA and glutamate receptors. This study provides the first demonstration that THP can suppress action potential-dependent synaptic transmission via a mechanism independent of NMDAR, mAChR, and α7 nAChR inhibition.

Sex-specific acute and chronic neurotoxicity of acute diisopropylfluorophosphate (DFP)-intoxication in juvenile Sprague-Dawley rats

Preclinical efforts to improve medical countermeasures against organophosphate (OP) chemical threat agents have largely focused on adult male models. However, age and sex have been shown to influence the neurotoxicity of repeated low-level OP exposure. Therefore, to determine the influence of sex and age on outcomes associated with acute OP intoxication, postnatal day 28 Sprague-Dawley male and female rats were exposed to the OP diisopropylfluorophosphate (DFP; 3.4 mg/kg, s.c.) or an equal volume of vehicle (∼80 µL saline, s.c.) followed by atropine sulfate (0.1 mg/kg, i.m.) and pralidoxime (2-PAM; 25 mg/kg, i.m.). Seizure activity was assessed during the first 4 h post-exposure using behavioral criteria and electroencephalographic (EEG) recordings. At 1 d post-exposure, acetylcholinesterase (AChE) activity was measured in cortical tissue, and at 1, 7, and 28 d post-exposure, brains were collected for neuropathologic analyses. At 1 month post-DFP, animals were analyzed for motor ability, learning and memory, and hippocampal neurogenesis. Acute DFP intoxication triggered more severe seizure behavior in males than females, which was supported by EEG recordings. DFP caused significant neurodegeneration and persistent microglial activation in numerous brain regions of both sexes, but astrogliosis occurred earlier and was more severe in males compared to females. DFP males and females exhibited pronounced memory deficits relative to sex-matched controls. In contrast, acute DFP intoxication altered hippocampal neurogenesis in males, but not females. These findings demonstrate that acute DFP intoxication triggers seizures in juvenile rats of both sexes, but the seizure severity varies by sex. Some, but not all, chronic neurotoxic outcomes also varied by sex. The spatiotemporal patterns of neurological damage suggest that microglial activation may be a more important factor than astrogliosis or altered neurogenesis in the pathogenesis of cognitive deficits in juvenile rats acutely intoxicated with OPs.

Soman-induced toxicity, cholinesterase inhibition and neuropathology in adult male Göttingen minipigs

Animal models are essential for evaluating the toxicity of chemical warfare nerve agents (CWNAs) to extrapolate to human risk and are necessary to evaluate the efficacy of medical countermeasures. The Göttingen minipig is increasingly used for toxicological studies because it has anatomical and physiological characteristics that are similar to those of humans. Our objective was to determine whether the minipig would be a useful large animal model to evaluate the toxic effects of soman (GD). We determined the intramuscular (IM) median lethal dose (LD50) of GD in adult male Göttingen minipigs using an up-and-down dosing method. In addition to lethality estimates, we characterized the observable signs of toxicity, blood and tissue cholinesterase (ChE) activity and brain pathology following GD exposure. The 24 h LD50 of GD was estimated to be 4.7 μg/kg, with 95 % confidence limits of 3.6 and 6.3 μg/kg. As anticipated, GD inhibited ChE activity in blood and several tissues. Neurohistopathological analysis showed neurodegeneration and neuroinflammation in survivors exposed to 4.7 μg/kg of GD, including in the primary visual cortex and various thalamic nuclei. These findings suggest that the minipig will be a useful large animal model for assessing drugs to mitigate neuropathological effects of exposure to CWNAs.

Pharmacokinetics and efficacy of atropine sulfate/obidoxime chloride co-formulation against VX in a guinea pig model

Nerve agent exposure is generally treated by an antidote formulation composed of a muscarinic antagonist, atropine sulfate (ATR), and a reactivator of acetylcholinesterase (AChE) such as pralidoxime, obidoxime (OBI), methoxime, trimedoxime or HI-6 and an anticonvulsant. Organophosphates (OPs) irreversibly inhibit AChE, the enzyme responsible for termination of acetylcholine signal transduction. Inhibition of AChE leads to overstimulation of the central and peripheral nervous system with convulsive seizures, respiratory distress and death as result. The present study evaluated the efficacy and pharmacokinetics (PK) of ATR/OBI following exposure to two different VX dose levels. The PK of ATR and OBI administered either as a single drug, combined treatment but separately injected, or administered as the ATR/OBI co-formulation, was determined in plasma of naïve guinea pigs and found to be similar for all formulations. Following subcutaneous VX exposure, ATR/OBI-treated animals showed significant improvement in survival rate and progression of clinical signs compared to untreated animals. Moreover, AChE activity after VX exposure in both blood and brain tissue was significantly higher in ATR/OBI-treated animals compared to vehicle-treated control. In conclusion, ATR/OBI has been proven to be efficacious against exposure to VX and there were no PK interactions between ATR and OBI when administered as a co-formulation.

C. elegans pharyngeal pumping provides a whole organism bio-assay to investigate anti-cholinesterase intoxication and antidotes

Inhibition of acetylcholinesterase by either organophosphates or carbamates causes anti-cholinesterase poisoning. This arises through a wide range of neurotoxic effects triggered by the overstimulation of the cholinergic receptors at synapses and neuromuscular junctions. Without intervention, this poisoning can lead to profound toxic effects, including death, and the incomplete efficacy of the current treatments, particularly for oxime-insensitive agents, provokes the need to find better antidotes. Here we show how the non-parasitic nematode Caenorhabditis elegans offers an excellent tool for investigating the acetylcholinesterase intoxication. The C. elegans neuromuscular junctions show a high degree of molecular and functional conservation with the cholinergic transmission that operates in the autonomic, central and neuromuscular synapses in mammals. In fact, the anti-cholinesterase intoxication of the worm's body wall neuromuscular junction has been unprecedented in understanding molecular determinants of cholinergic function in nematodes and other organisms. We extend the use of the model organism's feeding behaviour as a tool to investigate carbamate and organophosphate mode of action. We show that inhibition of the cholinergic-dependent rhythmic pumping of the pharyngeal muscle correlates with the inhibition of the acetylcholinesterase activity caused by aldicarb, paraoxons and DFP exposure. Further, this bio-assay allows one to address oxime dependent reversal of cholinesterase inhibition in the context of whole organism recovery. Interestingly, the recovery of the pharyngeal function after such anti-cholinesterase poisoning represents a sensitive and easily quantifiable phenotype that is indicative of the spontaneous recovery or irreversible modification of the worm acetylcholinesterase after inhibition. These observations highlight the pharynx of C. elegans as a new tractable approach to explore anti-cholinesterase intoxication and recovery with the potential to resolve critical genetic determinants of these neurotoxins' mode of action.

Gestational exposures to organophosphorus insecticides: From acute poisoning to developmental neurotoxicity

For over three-quarters of a century, organophosphorus (OP) insecticides have been ubiquitously used in agricultural, residential, and commercial settings and in public health programs to mitigate insect-borne diseases. Their broad-spectrum insecticidal effectiveness is accounted for by the irreversible inhibition of acetylcholinesterase (AChE), the enzyme that catalyzes acetylcholine (ACh) hydrolysis, in the nervous system of insects. However, because AChE is evolutionarily conserved, OP insecticides are also toxic to mammals, including humans, and acute OP intoxication remains a major public health concern in countries where OP insecticide usage is poorly regulated. Environmental exposures to OP levels that are generally too low to cause marked inhibition of AChE and to trigger acute signs of intoxication, on the other hand, represent an insidious public health issue worldwide. Gestational exposures to OP insecticides are particularly concerning because of the exquisite sensitivity of the developing brain to these insecticides. The present article overviews and discusses: (i) the health effects and therapeutic management of acute OP poisoning during pregnancy, (ii) epidemiological studies examining associations between environmental OP exposures during gestation and health outcomes of offspring, (iii) preclinical evidence that OP insecticides are developmental neurotoxicants, and (iv) potential mechanisms underlying the developmental neurotoxicity of OP insecticides. Understanding how gestational exposures to different levels of OP insecticides affect pregnancy and childhood development is critical to guiding implementation of preventive measures and direct research aimed at identifying effective therapeutic interventions that can limit the negative impact of these exposures on public health.

Learning and memory retention deficits in prepubertal guinea pigs prenatally exposed to low levels of the organophosphorus insecticide malathion

High doses of malathion, an organophosphorus (OP) insecticide ubiquitously used in agriculture, residential settings, and public health programs worldwide, induce a well-defined toxidrome that results from the inhibition of acetylcholinesterase (AChE). However, prenatal exposures to malathion levels that are below the threshold for AChE inhibition have been associated with increased risks of neurodevelopmental disorders, including autism spectrum disorder with intellectual disability comorbidity. The present study tested the hypothesis that prenatal exposures to a non-AChE-inhibiting dose of malathion are causally related to sex-biased cognitive deficits later in life in a precocial species. To this end, pregnant guinea pigs were injected subcutaneously with malathion (20 mg/kg) or vehicle (peanut oil, 0.5 ml/kg) once daily between approximate gestational days 53 and 63. This malathion dose regimen caused no significant AChE inhibition in the brain or blood of dams and offspring and had no significant effect on the postnatal growth of the offspring. Around postnatal day 30, locomotor activity and habituation, a form of non-associative learning, were comparable between malathion- and peanut oil-exposed offspring. However, in the Morris water maze, malathion-exposed offspring presented significant sex-dependent spatial learning deficits in addition to memory impairments. These results are far-reaching as they indicate that: (i) malathion is a developmental neurotoxicant and (ii) AChE inhibition is not an adequate biomarker to derive safety limits of malathion exposures during gestation. Continued studies are necessary to identify the time and dose dependence of the developmental neurotoxicity of malathion and the mechanisms underlying the detrimental effects of this insecticide in the developing brain.

Age-dependent behaviors, seizure severity and neuronal damage in response to nerve agents or the organophosphate DFP in immature and adult rats

Exposure to nerve agents (NAs) and other organophosphates (OPs) can initiate seizures that rapidly progress to status epilepticus (SE). While the electrographic and neuropathological sequelae of SE evoked by NAs and OPs have been characterized in adult rodents, they have not been adequately investigated in immature animals. In this study postnatal day (PND) 14, 21 and 28 rat pups, along with PND70 animals as adult controls, were exposed to NAs (sarin, VX) or another OP (diisopropylfluorophosphate, DFP). We then evaluated behavioral and electrographic (EEG) correlates of seizure activity, and performed neuropathology using Fluoro-Jade B. Although all immature rats exhibited behaviors that are often characterized as seizures, the incidence, duration, and severity of the electrographic seizure activity were age-dependent. No (sarin and VX) or brief (DFP) EEG seizure activity was evoked in PND14 rats, while SE progressively increased in severity as a function of age in PND21, 28 and 70 animals. Fluoro-Jade B staining was observed in multiple brain regions of animals that exhibited prolonged seizure activity. Neuronal injury in PND14 animals treated with DFP was lower than in older animals and absent in rats exposed to sarin or VX. In conclusion, we found that NAs and an OP provoked robust SE and neuronal injury similar to adults in PND21 and PND28, but not in PND14, rat pups. Convulsive behaviors were often present independent of EEG seizures and were unaccompanied by neuronal damage. These differential responses should be considered when investigating medical countermeasures for NA and OP exposure in pediatric populations.

Radiolabelled soman binding to sera from Rats, Guinea Pigs and Monkeys

Soman is a highly toxic organophosphorus chemical warfare compound that binds rapidly and irreversibility to a variety of serine active enzymes, i.e., butyryl- and acetyl-cholinesterases and carboxylesterase. The in vivo toxicity of soman has been reported to vary significantly in different animal species, such as rats and guinea pigs or non-human primates. This species variation makes it difficult to identify appropriate animal models for therapeutic drug development under the US Food and Drug Administration (FDA) Animal Rule. Since species variation in soman toxicity has been correlated with species variation in serum carboxylesterase, we undertook to determine if serum from guinea pigs, rats and non-human primates bound different levels of soman in vitro in the presence of equimolar concentrations of soman. Our results demonstrated that the amount of soman bound in the serum of rats was 4 uM, but essentially null in guinea pigs or non-human primates. The results strongly correlate with the presence or absence of carboxylesterase in the serum of animals and the difference in the toxic dose of soman in various species. Our results support prior suggestions that guinea pigs and non-human primates may be better animal models for the development of antidotes under the FDA Animal Rule.

The percutaneous toxicokinetics of VX in a damaged skin porcine model and the evaluation of WoundStat™ as a topical decontaminant

This study used a damaged skin, porcine model to evaluate the in vivo efficacy of WoundStat™ for the decontamination of superficial, nerve agent-contaminated wounds. Anaesthetized animals were randomly assigned to either control (n = 7), no decontamination (n = 12) or WoundStat™ (n = 12) treatment groups. Pigs were exposed to a 5× LD₅₀ dose of neat, radiolabelled S-[2-(diisopropylamino)ethyl]-O-ethyl methyl-phosphonothioate (VX; or equivalent volume of sterile saline for the control group) via an area of superficially damaged skin on the ear. WoundStat™ was applied at 30 seconds post-exposure to assigned animals. The VX contaminant (or saline) and decontaminant remained in place for the duration of the study (up to 6 hours). Physiological parameters and signs of intoxication were recorded during the exposure period. Skin and organ samples were taken post mortem for ¹⁴ C-VX distribution analyses. Blood samples were taken periodically for toxicokinetic and whole-blood acetylcholinesterase (AChE) activity analyses. VX exposure was accompanied by a rapid decrease in AChE activity in all animals, regardless of decontamination. However, decontamination significantly improved survival rate and time and reduced the severity of signs of intoxication. In addition, the distribution of ¹⁴ C-VX in key internal organs and post mortem blood samples was significantly lower in the WoundStat™ treatment group. This study demonstrates that WoundStat™ may be a suitable medical countermeasure for increasing both survival rate and time following VX exposure. The results also suggest that AChE activity is not a useful prognostic indicator.

An Official American Thoracic Society Workshop Report: Chemical Inhalational Disasters. Biology of Lung Injury, Development of Novel Therapeutics, and Medical Preparedness

This report is based on the proceedings from the Inhalational Lung Injury Workshop jointly sponsored by the American Thoracic Society (ATS) and the National Institutes of Health (NIH) Countermeasures Against Chemical Threats (CounterACT) program on May 21, 2013, in Philadelphia, Pennsylvania. The CounterACT program facilitates research leading to the development of new and improved medical countermeasures for chemical threat agents. The workshop was initiated by the Terrorism and Inhalational Disasters Section of the Environmental, Occupational, and Population Health Assembly of the ATS. Participants included both domestic and international experts in the field, as well as representatives from U.S. governmental funding agencies. The meeting objectives were to (1) provide a forum to review the evidence supporting current standard medical therapies, (2) present updates on our understanding of the epidemiology and underlying pathophysiology of inhalational lung injuries, (3) discuss innovative investigative approaches to further delineating mechanisms of lung injury and identifying new specific therapeutic targets, (4) present promising novel medical countermeasures, (5) facilitate collaborative research efforts, and (6) identify challenges and future directions in the ongoing development, manufacture, and distribution of effective and specific medical countermeasures. Specific inhalational toxins discussed included irritants/pulmonary toxicants (chlorine gas, bromine, and phosgene), vesicants (sulfur mustard), chemical asphyxiants (cyanide), particulates (World Trade Center dust), and respirable nerve agents.

In vivo protection studies of bis-quaternary 2-(hydroxyimino)-N-(pyridin-3-yl) acetamide derivatives against sarin poisoning in mice

In vivo antidotal efficacy of new bis- quaternary 2-(hydroxyimino)- N-(pyridin-3yl) acetamide derivatives (HNK series), to counter multiples of lethal doses of nerve agent sarin (GB) and reactivation of acetylcholinesterase (AChE), was evaluated in Swiss albino mice. [Protection index PI; median lethal dose (LD50) of sarin with treatment/LD50 of sarin] was estimated, using 0.05, 0.10, and 0.20 LD50 as treatment doses of all the oximes with atropine against sarin poisoning. Dose-dependent time course study was conducted at 0.2, 0.4 and 0.8 LD50 dose of sarin for estimating maximum AChE inhibition. At optimized time (15 min), in vivo enzyme half inhibition concentration (IC50) was calculated. AChE reactivation efficacy of HNK series and pralidoxime (2-PAM) were determined by plotting shift of log IC50 doses. HNK-102 with atropine showed three fold higher PI compared to 2-PAM. In vivo IC50 of sarin for brain and serum AChE was found to be 0.87 LD50 (139.2 µg/kg) and 0.48 LD50 (77.23 µg/kg), respectively. Treatment with HNK-102 and HNK-111 (equal to their 0.20LD50) significantly reactivated sarin-intoxicated AChE ( p < 0.05) at 2× IC50 dose of sarin, compared to 2-PAM. The study revealed that HNK-102 oxime was three times more potent as antidote, for acute sarin poisoning compared to 2-PAM in vivo.

Younger rats are more susceptible to the lethal effects of sarin than adult rats: 24 h LC50 for whole-body (10 and 60 min) exposures

Chemical warfare nerve agents (CWNA) inhibit acetylcholinesterase and are among the most lethal chemicals known to man. Children are predicted to be vulnerable to CWNA exposure because of their smaller body masses, higher ventilation rates and immature central nervous systems. While a handful of studies on the effects of CWNA in younger animals have been published, exposure routes relevant to battlefield or terrorist situations (i.e. inhalation for sarin) were not used. Thus, we estimated the 24 h LC₅₀ for whole-body (10 and 60 min) exposure to sarin using a stagewise, adaptive dose design. Specifically, male and female Sprague-Dawley rats were exposed to a range of sarin concentrations (6.2-44.0 or 1.6-12.5 mg/m³) for either 10 or 60 min, respectively, at six different times during their development (postnatal day [PND] 7, 14, 21, 28, 42 and 70). For male and female rats, the lowest LC₅₀ values were observed for PND 14 and the highest LC₅₀ values for PND 28. Sex differences were observed only for PND 42 for the 10 min exposures and PND 21 and 70 for the 60 min exposures. Thus, younger rats (PND 14) were more susceptible than older rats (PND 70) to the lethal effects of whole-body exposure to sarin, while adolescent (PND 28) rats were the least susceptible and sex differences were minimal. These results underscore the importance of controlling for the age of the animal in research on the toxic effects associated with CWNA exposure.

Spatial learning impairment in prepubertal guinea pigs prenatally exposed to the organophosphorus pesticide chlorpyrifos: Toxicological implications

Exposure of the developing brain to chlorpyrifos (CPF), an organophosphorus (OP) pesticide used extensively in agriculture worldwide, has been associated with increased prevalence of cognitive deficits in children, particularly boys. The present study was designed to test the hypothesis that cognitive deficits induced by prenatal exposure to sub-acute doses of CPF can be reproduced in precocial small species. To address this hypothesis, pregnant guinea pigs were injected daily with CPF (25mg/kg,s.c.) or vehicle (peanut oil) for 10days starting on presumed gestation day (GD) 53-55. Offspring were born around GD 65, weaned on postnatal day (PND) 20, and subjected to behavioral tests starting around PND 30. On the day of birth, butyrylcholinesterase (BuChE), an OP bioscavenger used as a biomarker of OP exposures, and acetylcholinesterase (AChE), a major molecular target of OP compounds, were significantly inhibited in the blood of CPF-exposed offspring. In their brains, BuChE, but not AChE, was significantly inhibited. Prenatal CPF exposure had no significant effect on locomotor activity or on locomotor habituation, a form of non-associative memory assessed in open fields. Spatial navigation in the Morris water maze (MWM) was found to be sexually dimorphic among guinea pigs, with males outperforming females. Prenatal CPF exposure impaired spatial learning more significantly among male than female guinea pigs and, consequently, reduced the sexual dimorphism of the task. The results presented here, which strongly support the test hypothesis, reveal that the guinea pig is a valuable animal model for preclinical assessment of the developmental neurotoxicity of OP pesticides. These findings are far reaching as they lay the groundwork for future studies aimed at identifying therapeutic interventions to treat and/or prevent the neurotoxic effects of CPF in the developing brain.

Comparison of the lethal effects of chemical warfare nerve agents across multiple ages

Children may be inherently more vulnerable than adults to the lethal effects associated with chemical warfare nerve agent (CWNA) exposure because of their closer proximity to the ground, smaller body mass, higher respiratory rate, increased skin permeability and immature metabolic systems. Unfortunately, there have only been a handful of studies on the effects of CWNA in pediatric animal models, and more research is needed to confirm this hypothesis. Using a stagewise, adaptive dose design, we estimated the 24h median lethal dose for subcutaneous exposure to seven CWNA in both male and female Sprague-Dawley rats at six different developmental times. Perinatal (postnatal day [PND] 7, 14 and 21) and adult (PND 70) rats were more susceptible than pubertal (PND 28 and 42) rats to the lethal effects associated with exposure to tabun, sarin, soman and cyclosarin. Age-related differences in susceptibility were not observed in rats exposed to VM, Russian VX or VX.

The Pesticide Metabolites Paraoxon and Malaoxon Induce Cellular Death by Different Mechanisms in Cultured Human Pulmonary Cells

Organophosphorus (OP) pesticides are known to induce pulmonary toxicity in both humans and experimental animals. To elucidate the mechanism of OP-induced cytotoxicity, we examined the effects of parathion and malathion and their respective metabolites, paraoxon and malaoxon, on primary cultured human large and small airway cells. Exposure to paraoxon and malaoxon produced a dose-dependent increase in cytotoxicity following a 24-hour exposure, while treatment with parathion or malathion produced no effects at clinically relevant concentrations. Exposure to paraoxon-induced caspase activation, but malaoxon failed to induce this response. Since caspases have a major role in the regulation of apoptosis and cell death, we evaluated OP-induced cell death in the presence of a caspase inhibitor. Pharmacological caspase inhibition protected against paraoxon-induced cell death but not malaoxon-induced cell death. These data suggest that caspase activation is a key signaling element in paraoxon-induced cell death, but not malaoxon-induced cellular death in the pulmonary epithelium.

Hormone-dependence of sarin lethality in rats: Sex differences and stage of the estrous cycle

Chemical warfare nerve agents (CWNAs) are highly toxic compounds that cause a cascade of symptoms and death, if exposed casualties are left untreated. Numerous rodent models have investigated the toxicity and mechanisms of toxicity of CWNAs, but most are limited to male subjects. Given the profound physiological effects of circulating gonadal hormones in female rodents, it is possible that the daily cyclical fluctuations of these hormones affect females' sensitivity to the lethal effects of CWNAs, and previous reports that included female subjects did not control for the stage of the hormonal cycle. The aim of the current study was to determine the 24-hour median lethal dose (LD50) of the CWNA sarin in male, ovariectomized (OVEX) female, and female rats during different stages of the estrous cycle (diestrus, proestrus, and estrus). Additionally, baseline activity levels of plasma acetylcholinesterase, butyrylcholinesterase, and carboxylesterase were measured to determine differences among the groups. Results indicated that females in proestrus had a significantly higher LD50 of sarin compared to OVEX and estrous females. Although some sex differences were observed in the activity levels of plasma esterases, they were not consistent and likely not large enough to significantly affect the LD50s. These results suggest that hormonal cyclicity can influence the outcome of CWNA-related studies using female rodents, and that this variability can be minimized by controlling for the stage of the cycle. Additional research is necessary to determine the precise mechanism of the observed differences because it is unlikely to be solely explained by plasma esterase activity.

Metabolic isoenzyme shifts in cancer as potential novel therapeutic targets

The functional redundancy of metabolic enzyme expression may present a new strategy for developing targeted therapies in cancer. To satisfy the increased metabolic demand required during neoplastic transformations and proliferation, cancer cells may rely on additional isoforms of a metabolic enzyme to satisfy the increased demand for metabolic precursors, which could subsequently render cancer cells more vulnerable to isoform-specific inhibitors. In this review, we provide a survey of common isoenzyme shifts that have been reported to be important in cancer metabolism and link those to metabolic pathways that currently have drugs in various stages of development. This phenomenon suggests a potentially new therapeutic strategy for the treatment of cancer by identifying shifts in the expression of metabolic isoenzymes between cancer and normal cells. We also delineate other putative metabolic isoenzymes that could be targets for novel targeted therapies for cancer. Changes in isoenzyme expression that occur during neoplastic transformations or in response to environmental pressure in cancer cells may result in isoenzyme diversity that may subsequently render cancer cells more vulnerable to isoform-specific inhibitors due to reliance on a single isoform to perform a vital enzymatic function.

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.

Animal models that best reproduce the clinical manifestations of human intoxication with organophosphorus compounds

The translational capacity of data generated in preclinical toxicological studies is contingent upon several factors, including the appropriateness of the animal model. The primary objectives of this article are: 1) to analyze the natural history of acute and delayed signs and symptoms that develop following an acute exposure of humans to organophosphorus (OP) compounds, with an emphasis on nerve agents; 2) to identify animal models of the clinical manifestations of human exposure to OPs; and 3) to review the mechanisms that contribute to the immediate and delayed OP neurotoxicity. As discussed in this study, clinical manifestations of an acute exposure of humans to OP compounds can be faithfully reproduced in rodents and nonhuman primates. These manifestations include an acute cholinergic crisis in addition to signs of neurotoxicity that develop long after the OP exposure, particularly chronic neurologic deficits consisting of anxiety-related behavior and cognitive deficits, structural brain damage, and increased slow electroencephalographic frequencies. Because guinea pigs and nonhuman primates, like humans, have low levels of circulating carboxylesterases-the enzymes that metabolize and inactivate OP compounds-they stand out as appropriate animal models for studies of OP intoxication. These are critical points for the development of safe and effective therapeutic interventions against OP poisoning because approval of such therapies by the Food and Drug Administration is likely to rely on the Animal Efficacy Rule, which allows exclusive use of animal data as evidence of the effectiveness of a drug against pathologic conditions that cannot be ethically or feasibly tested in humans.