Development of neuropathology following soman poisoning and medical countermeasures

Combination of acetylcholinesterase inhibitors and NMDA receptor antagonists increases survival rate in soman-poisoned mice

Organophosphorus nerve agents pose a global threat to both military personnel and civilian population, because of their high acute toxicity and insufficient medical countermeasures. Commonly used drugs could ameliorate the intoxication and overall medical outcomes. In this study, we tested the drugs able to alleviate the symptoms of Alzheimer's disease (donepezil, huperzine A, memantine) or Parkinson's disease (procyclidine). They were administered to mice before soman intoxication in terms of their: i) protection potential against soman toxicity and ii) influence on post-exposure therapy consisting of atropine and asoxime (also known as oxime HI-6). Their pretreatment effect was not significant, when administered alone, but in combination (acetylcholinesterase inhibitor such as denepezil or huperzine A with NMDA antagonist such as memantine or procyclidine) they lowered the soman toxicity more than twice. These combinations also positively influenced the efficacy of post-exposure treatment in a similar fashion; the combinations increased the therapeutic effectiveness of antidotal treatment. In conclusion, the most effective combination - huperzine A and procyclidine - lowered the toxicity three times and improved the post-exposure therapy efficacy more than six times. These results are unprecedented in the published literature.

A rapid in vitro assay for evaluating the effects of acetylcholinesterase inhibitors and reactivators in the rat basolateral amygdala

We established a novel brain slice assay to test the ability of acetylcholinesterase (AChE) reactivators to prevent ACh-induced M1 muscarinic acetylcholine receptor (mAChR) dependent hyperexcitability observed after exposure to the organophosphate (OP)-based AChE inhibitor and sarin surrogate 4-nitrophenyl isopropyl methylphosphonate (NIMP). Whole-cell patch clamp recordings were used to evaluate the response of pyramidal neurons in the rat basolateral amygdala (BLA) to brief (1 min) bath application of ACh (100 μM), either in control conditions, or after exposure to NIMP ± an AChE reactivator. Bath application of ACh produced atropine- and pirenzepine-sensitive inward currents in voltage clamped BLA pyramidal neurons, and increased the frequency of spontaneous EPSCs, suggesting robust activation of M1 mAChRs. Responses to ACh were increased ~3–5 fold in slices that had been preincubated in NIMP, and these effects were reversed in a concentration dependent manner by exposure to a commercially available AChE reactivator. The current work outlines a simple assay that can be used to evaluate the efficacy of both known and novel AChE reactivators in an area of the limbic system that likely contributes to seizures after acute exposure to OP-based AChE inhibitors.

The Kv7 Modulator, Retigabine, is an Efficacious Antiseizure Drug for Delayed Treatment of Organophosphate-induced Status Epilepticus

Benzodiazepines are the primary treatment option for organophosphate (OP)-induced status epilepticus (SE), but these antiseizure drugs (ASDs) lose efficacy as treatment is delayed. In the event of a mass civilian or military exposure, significant treatment delays are likely. New ASDs that combat benzodiazepine-resistant, OP-induced SE are critically needed, particularly if they can be efficacious after a long treatment delay. This study evaluated the efficacy of the Kv7 channel modulator, retigabine, as a novel therapy for OP-induced SE. Adult, male rats were exposed to soman or diisopropyl fluorophosphate (DFP) to elicit SE and monitored by electroencephalogram (EEG) recording. Retigabine was administered alone or adjunctive to midazolam (MDZ) at delays of 20- or 40-min in the soman model, and 60-min in the DFP model. Following EEG recordings, rats were euthanized and brain tissue was collected for Fluoro-Jade B (FJB) staining to quantify neuronal death. In the DFP model, MDZ + 15 mg/kg retigabine suppressed seizure activity and was neuroprotective. In the soman model, MDZ + 30 mg/kg retigabine suppressed seizures at 20- and 40-min delays. Without MDZ, 15 mg/kg retigabine provided partial antiseizure and neuroprotectant efficacy in the DFP model, while 30 mg/kg without MDZ failed to attenuate soman-induced SE. At 60 mg/kg, retigabine without MDZ strongly reduced seizure activity and neuronal degeneration against soman-induce SE. This study demonstrates the antiseizure and neuroprotective efficacy of retigabine against OP-induced SE. Our data suggest retigabine could be a useful adjunct to standard-of-care and has potential for use in the absence of MDZ.

Effects of memantine and its metabolite Mrz 2/373 on soman-induced inhibition of acetylcholinesterase in vitro

Memantine is the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, used in the treatment of Alzheimer's disease. It is also known that memantine pretreatment assured protection of skeletal muscles from poisoning with nerve agents and an interaction between memantine and AChE was proposed. In the study presented we examined interactions of memantine and its main metabolite (1-amino-3-hydroxymethyl-5-methyl adamantine, Mrz 2/373) with AChE in vitro as well as their effect on kinetics of the soman-induced AChE inhibition and aging. The results have shown that memantine and Mrz 2/373 exerted concentration-dependent inhibition of AChE, with Mrz 2/373 being a more potent inhibitor than the parent compound. Addition of soman 7.5 nmol/l induced gradual AChE inhibition that became almost complete after 20 min. Memantine (0.1, 0.5 and 1 mmol/l) and Mrz 2/373 (0.1, 0.5 and 1 mmol/l) concentration-dependently slowed down the AChE inhibition. After 30 min of incubation of AChE with soman, 5 min of aging and 20 min of reactivation by asoxime (HI-6 dichloride), AChE activity was 8.1% in control medium, 30.7% and 41.9% after addition of 1 and 10 mmol/l memantine, and 16.1% after addition of 1 mmol/l Mrz 2/373. It was concluded that it is possible that memantine and Mrz 2/373 can prevent AChE from inhibition by soman, which could, along with known memantine's neuroprotective activity, explain its potent antidotal effect in soman poisoning. The potential effect on aging of the soman-AChE complex warrants further studies.

Screening for Efficacious Anticonvulsants and Neuroprotectants in Delayed Treatment Models of Organophosphate-induced Status Epilepticus

Organophosphorus (OP) compounds are deadly chemicals that exert their intoxicating effects through the irreversible inhibition of acetylcholinesterase (AChE). In addition to an excess of peripheral ailments, OP intoxication induces status epilepticus (SE) which if left untreated may lead to permanent brain damage or death. Benzodiazepines are typically the primary therapies for OP-induced SE, but these drugs lose efficacy as treatment time is delayed. The CounterACT Neurotherapeutic Screening (CNS) Program was therefore established by the National Institutes of Health (NIH) to discover novel treatments that may be administered adjunctively with the currently approved medical countermeasures for OP-induced SE in a delayed treatment scenario. The CNS program utilizes in vivo EEG recordings and Fluoro-JadeB (FJB) histopathology in two established rat models of OP-induced SE, soman (GD) and diisopropylfluorophosphate (DFP), to evaluate the anticonvulsant and neuroprotectant efficacy of novel adjunct therapies when administered at 20 or 60 min after the induction of OP-induced SE. Here we report the results of multiple compounds that have previously shown anticonvulsant or neuroprotectant efficacy in other models of epilepsy or trauma. Drugs tested were ganaxolone, diazoxide, bumetanide, propylparaben, citicoline, MDL-28170, and chloroquine. EEG analysis revealed that ganaxolone demonstrated the most robust anticonvulsant activity, whereas all other drugs failed to attenuate ictal activity in both models of OP-induced SE. FJB staining demonstrated that none of the tested drugs had widespread neuroprotective abilities. Overall these data suggest that neurosteroids may represent the most promising anticonvulsant option for OP-induced SE out of the seven unique mechanisms tested here. Additionally, these results suggest that drugs that provide significant neuroprotection from OP-induced SE without some degree of anticonvulsant activity are elusive, which further highlights the necessity to continue screening novel adjunct treatments through the CNS program.

Novel therapeutics for treating organophosphate-induced status epilepticus co-morbidities, based on changes in calcium homeostasis

Organophosphate (OP) chemicals include pesticides such as parathion, and nerve gases such as sarin and soman and are considered major chemical threat agents. Acute OP exposure is associated with a cholinergic crisis and status epilepticus (SE). It is also known that the survivors of OP toxicity exhibit neurobehavioral deficits such as mood changes, depression, and memory impairment, and acquired epilepsy. Our research has focused on addressing the need to develop effective therapeutic agents that could be administered even after prolonged seizures and would prevent or lessen the chronic morbidity associated with OP-SE survival. We have developed rat survival models of OP pesticide metabolite paraoxon (POX) and nerve agent sarin surrogate diisopropyl fluorophosphate (DFP) induced SE that are being used to screen for medical countermeasures against an OP attack. Our research has focused on studying neuronal calcium (Ca²⁺) homeostatic mechanisms for identifying mechanisms and therapeutics for the expression of neurological morbidities associated with OP-SE survival. We have observed development of a "Ca²⁺ plateau" characterized by sustained elevations in neuronal Ca²⁺ levels in OP-SE surviving rats that coincided with the appearance of OP-SE chronic morbidities. These Ca²⁺ elevations had their origin in Ca²⁺ release from the intracellular stores such that blockade with antagonists like dantrolene, carisbamate, and levetiracetam lowered OP-SE mediated Ca²⁺ plateau and afforded significant neuroprotection. Since the Ca²⁺ plateau lasts for a prolonged period, our studies suggest that blocking it after the control of SE may represent a unique target for development of novel countermeasures to prevent long term Ca²⁺ mediated OP-SE neuropsychiatric comorbidities such as depression, anxiety, and acquired epilepsy (AE).