The Toxicity of Soman in the African Green Monkey (Chlorocebus aethiops)

Anticonvulsant effectiveness of scopolamine against soman-induced seizures in African green monkeys

Prolonged seizures are a hallmark feature of intoxication with anticholinesterase nerve agents such as soman. While benzodiazepine drugs are typically used to control these seizures, studies in both rats and guinea pigs have shown that potent, centrally acting anticholinergic drugs such as scopolamine can also terminate such seizures. The present study was performed to determine if scopolamine could produce similar anticonvulsant effects in a nonhuman primate model of soman intoxication. Adult male African green monkeys, implanted with telemetry devices to record cortical electroencephalographic activity, were pretreated with pyridostigmine (0.02 mg/kg, intramuscularly [im]) and 40 min later challenged with 15 µg/kg (im) of the nerve agent soman. One min after soman exposure the animals were treated with atropine (0.4 mg/kg, im) and the oxime 2-PAM (25.7 mg/kg, im). One min after the start of seizure activity the animals were administered scopolamine (0.01-0.1 mg/kg, im), using an up-down dosing design over successive animals. Scopolamine was highly effective in stopping soman-induced seizures with an ED₅₀ = 0.0312 mg/kg (0.021-0.047 mg/kg = 95% confidence limits). Seizure control was rapid, with all epileptiform activity stopping on average 21.7 min after scopolamine treatment. A separate pK study showed that scopolamine absorption peaked approximately 10 min after im administration and a dose of 0.032 mg/kg produced maximum plasma levels of 17.62 ng/ml. The results show that scopolamine exerts potent and rapid anticonvulsant action against soman-induced seizures and that it may serve as a valuable adjunct to current antidote treatments for nerve agent intoxication.

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.

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 role of genetic background in susceptibility to chemical warfare nerve agents across rodent and non-human primate models

Genetics likely play a role in various responses to nerve agent exposure, as genetic background plays an important role in behavioral, neurological, and physiological responses to environmental stimuli. Mouse strains or selected lines can be used to identify susceptibility based on background genetic features to nerve agent exposure. Additional genetic techniques can then be used to identify mechanisms underlying resistance and sensitivity, with the ultimate goal of developing more effective and targeted therapies. Here, we discuss the available literature on strain and selected line differences in cholinesterase activity levels and response to nerve agent-induced toxicity and seizures. We also discuss the available cholinesterase and toxicity literature across different non-human primate species. The available data suggest that robust genetic differences exist in cholinesterase activity, nerve agent-induced toxicity, and chemical-induced seizures. Available cholinesterase data suggest that acetylcholinesterase activity differs across strains, but are limited by the paucity of carboxylesterase data in strains and selected lines. Toxicity and seizures, two outcomes of nerve agent exposure, have not been fully evaluated for genetic differences, and thus further studies are required to understand baseline strain and selected line differences.

VX toxicity in the Göttingen minipig

The present experiments determined the intramuscular LD₅₀ of VX in male Göttingen minipigs at two stages of development. In pubertal animals (115 days old), the LD₅₀ of VX was indeterminate, but approximated 33.3μg/kg. However, in sexually mature animals (152 days old), the LD₅₀ was estimated to be only 17.4μg/kg. Signs of nerve agent toxicity in the Göttingen minipig were similar to those described for other species, with some notable exceptions (such as urticaria and ejaculation). Latencies to the onset of sustained convulsions were inversely related to the administered dose of VX in both ages of minipigs. Additionally, actigraphy was used to quantify the presence of tremor and convulsions and, in some cases, was useful for precisely estimating time of death. The main finding indicates that in minipigs, as in other species, even relatively small differences in age can substantially alter the toxicity of nerve agents. Additionally, actigraphy can serve as a non-invasive method of characterizing the tremors and convulsions that often accompany nerve agent intoxication.

Measurement of hematological and serum biochemical normal values of captive housed Chlorocebus aethiops sabaeus monkeys and correlation with the age

BACKGROUND

Some factors such as sex, age, and captivity conditions have a direct influence on the normal hematological and serum biochemical parameters of African green monkeys. On the other hand, reliability in reported values is in many cases limited by studied animal number (<200) and there is not report on the correlation of these parameters with the age in each sex animal group. Thus, this study sought determining normal hematological (11) and serum biochemical parameters (9) of 400 captive housed African green monkeys and also correlate them with the age of the animals.

METHODS

A total of 200 females and 200 males were grouped by the sex and age groups (1-2, 3-4, 5-6, and 7-8 years old) for measuring normal values of hematological and serum biochemical parameters and to study the correlation of these parameters with the age of the animals.

RESULTS

As key outcome, the main hematological and serum biochemical reference values of African green monkeys were determined. Significant differences (P < 0.05) were found among 95% of studied parameters between males and females. About 75% and 95% of the parameters were influenced by the age in the female and male groups, respectively. About 35% of hematological and serum biochemical parameters correlated positively (R(2) > 0.5) with the age in the female monkeys. On the contrary in the male monkeys, only 45% of parameters correlated positively with the age (R(2) > 0.5).

CONCLUSIONS

Thus, authors believe that results of this study are important for assisting researchers in the assessment of health status of captive housed African green monkeys for preclinical studies.

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.

Exposure to nerve agents: from status epilepticus to neuroinflammation, brain damage, neurogenesis and epilepsy

Epilepsy is a common neurological disorder characterized by an initial injury due to stroke, traumatic brain injury, brain infection, or febrile seizures causing status epilepticus (SE). This phenomenon precedes recurrent (secondary) seizures, the latent period (period without seizures) and downstream appearance of spontaneous recurrent seizures (SRS). Epilepsy inducers include the organophosphorous (OP) compounds modified as chemical warfare nerve agents, such as soman. SE induced by soman is a result of cholinergic system hyperactivity caused by the irreversible inhibition of acetylcholinesterase, and the subsequent increase in the amount of the neurotransmitter acetylcholine at central and peripheral sites. SE leads to profound, permanent, complex and widespread brain damage and associated cognitive and behavioral deficits, accompanied by impaired neurogenesis. Several anticonvulsant and neuroprotective strategies have been studied in order to avoid the epileptogenesis which occurs after SE caused by soman exposure. In recent studies, we showed that SRS occur post-soman exposure and neuropathology can be reduced with diazepam (DZP) and valproic acid (VPA) when administered in combination treatment. These effects are accompanied by neurogenesis seen 15 days post-exposure in the hippocampal dentate gyrus (DG). This review discusses several findings about epilepsy induced by soman exposure such as behavioral changes, EEG anomalies, neuropathology, neuroinflammation, neurogenesis, possible circuitry changes and current strategies for treatment. The soman seizure model is an important model of temporal lobe epilepsy (TLE) and comparable in certain respects with well studied models in the literature such as pilocarpine and kainic acid. All these models together allow for a greater understanding of the different mechanisms of seizure induction, propagation and options for treatment. These studies are very necessary for current military and civilian treatment regimens, against OP nerve agent exposure, which fail to prevent SE resulting in severe neuropathology and epilepsy.