Novel Genetically Modified Mouse Model to Assess Soman-Induced Toxicity and Medical Countermeasure Efficacy: Human Acetylcholinesterase Knock-in Serum Carboxylesterase Knockout Mice

Abstract

The identification of improved medical countermeasures against exposure to chemical warfare nerve agents (CWNAs), a class of organophosphorus compounds, is dependent on the choice of animal model used in preclinical studies. CWNAs bind to acetylcholinesterase and prevent the catalysis of acetylcholine, causing a plethora of peripheral and central physiologic manifestations, including seizure. Rodents are widely used to elucidate the effects of CWNA-induced seizure, albeit with a caveat: they express carboxylesterase activity in plasma. Carboxylesterase, an enzyme involved in the detoxification of some organophosphorus compounds, plays a scavenging role and decreases CWNA availability, thus exerting a protective effect. Furthermore, species-specific amino acid differences in acetylcholinesterase confound studies that use oximes or other compounds to restore its function after inhibition by CWNA. The creation of a human acetylcholinesterase knock-in/serum carboxylesterase knockout (C57BL/6-Ces1ctm1.1LocAChEtm1.1Loc/J; a.k.a KIKO) mouse may facilitate better modeling of CWNA toxicity in a small rodent species. The current studies characterize the effects of exposure to soman, a highly toxic CWNA, and evaluate the efficacy of anti-seizure drugs in this newly developed KIKO mouse model. Data demonstrate that a combination of midazolam and ketamine reduces seizure duration and severity, eliminates the development of spontaneous recurrent seizures, and protects certain brain regions from neuronal damage in a genetically modified model with human relevance to organophosphorus compound toxicity. This new animal model and the results of this study and future studies using it will enhance medical countermeasures development for both defense and homeland security purposes.

Combination of antiseizure medications phenobarbital, ketamine, and midazolam reduces soman‐induced epileptogenesis and brain pathology in rats

“…Our laboratory and others previously reported on the beneficial effect of administering the NMDAR antagonist ketamine in combination with a benzodiazepine in reducing cholinergic‐induced seizure severity, epileptogenesis, functional impairment, and brain pathology in rodent models. 8 , 9 , 10 , 12 , 13 , 30 , 31 , 32 Previous studies also showed the importance of simultaneous administration of antiseizure medications, as opposed to sequential drug administration, in reducing cholinergic‐induced seizure severity. 33 The inclusion of a third antiseizure medication, valproate, in combination with ketamine and midazolam further reduces seizure severity, epileptogenesis, and neuronal loss following soman exposure.…”

“…In the rodent models of soman-induced SE, a treatment composed of midazolam and ketamine reduced seizure severity, ameliorated impairment in the Morris water maze, and reduced neuronal loss. [10][11][12][13] However, the protection afforded by the combination of ketamine and midazolam was incomplete, suggesting a need for an additional adjunct medical countermeasure. monotherapy (midazolam or phenobarbital) in reducing cholinergic-induced toxicity.…”

Treatment of cholinergic‐induced status epilepticus with polytherapy targeting GABA and glutamate receptors

“… 22 This effect was also observed in soman exposed KIKO mice in which none of the mice treated with the dual therapy developed SRS, while all of the midazolam treated mice developed SRS. 88 In rats and Es1 −/− mice, dual therapy delayed the appearance of the first SRS but did not significantly reduce the number of SRS recorded for the first 14 days following SE exposure. However, midazolam–ketamine–valproate triple therapy not only increased the latency of the first SRS following soman exposure in rats but also significantly reduced the average number of SRS during the 2‐week period following treatment compared with midazolam monotherapy, showing the benefit of adding an additional ASM (valproate; Figure 11 ).…”

“… 87 Similar to memantine's effects of increased survival, in severe soman‐induced seizure models in Es1 −/− and KIKO mice, ketamine in combination with midazolam increases survival and reduces neurodegeneration. 22 , 88 Other report that high‐dose memantine (56 mg/kg) in combination with midazolam administered 20 minutes after soman exposure resulted in high lethality, failed to terminate SE, but reduced delta power and cell death 89 ; high doses may induce seizure. 90 , 91 , 92 Although more research combining a low dose of memantine and other ASMs (benzodiazepine and/or nonbenzodiazepine drugs and anticholinergics) may be required to explore the therapeutic potential of memantine against OPNA exposure, studies with ketamine described in Sections 2 , 4 suggest that targeting NMDA receptors to reduce the effects of cholinergic‐induced SE is a beneficial strategy.…”

Effects of Genetically Modified Animals on Human Activities and Ecosystem

“…The transfer of genes that then generate transgenic animals that mimic or are immune to a disease. Transgenic animals that are immune to a disease can be used for testing of experimental medicines [12]- [14]. A variety of models of cancer are mainly studied in mice and rats.…”