Abstracts

This is a Late Breaking abstract

Rationale: Soman (GD) is an organophosphorous nerve agent (OPNA) that irreversibly inhibits acetylcholinesterase. The currently fielded countermeasures against OPNA exposure include a muscarinic antagonist (i.e., atropine), an oxime to reactivate acetylcholinesterase and a benzodiazepine as an anti-seizure medication (ASM). However, when treatment with benzodiazepine is delayed, GABA receptors are internalized and pharmacoresistent status epilepticus can develop, signaling the need for a multi-targeted approach to better protect against the toxic effects of OPNA exposure. Our laboratory previously reported the protective potential of neurosteroids (allopregnanolone, pregananolone) as adjuncts to diazepam as well as a triple therapy regimen consisting of midazolam, ketamine and a third ASM (valproate, phenobarbital). We hypothesized that targeting GABA receptors at a non-benzodiazepine site would have added benefit. The current study reports on the effectiveness of allopregnanolone in combination with ketamine and midazolam against a model of refractory status epilepticus in GD-exposed rats.

Methods: Adult male rats were anesthetized and implanted subcutaneously with telemetry devices for continuous recording of electroencephalographic activity. After surgical recovery, subjects were exposed to a seizure-inducing dose of GD and treated with atropine sulfate and an oxime 1 min later, and with midazolam, midazolam-allopregnanolone or midazolam-allopregnanolone-ketamine 40 min after seizure onset. Rats were euthanized 2 weeks after exposure, and brains were collected and processed to assess neurodegeneration and microglial activation and compared with control (no GD) rats.

Results: Groups exposed to GD showed extensive neuronal loss and microglial activation in comparison to control animals. However, microglial activation was reduced in the piriform, hippocampus and thalamus in rats that received midazolam-allopregnanolone-ketamine triple therapy compared to rats that received midazolam monotherapy. Neuronal loss was reduced in the medial and lateral thalamus in subjects that received triple therapy and reduced in the amygdala in subjects that received MDZ/ALLO dual therapy compared to midazolam monotherapy. A lower percentage of rats that were treated with MDZ/ALLO/KET developed spontaneous recurrent seizures and had a lower number of recurrent seizures after exposure.

Conclusions: While the currently fielded treatment against OPNAs can increase the likelihood of survival in the event of exposure, the risk of developing refractory status epilepticus increases if treatment is delayed, resulting in neurotoxic effects such as self-sustaining seizure activity and widespread severe brain damage. Compared to benzodiazepine monotherapy, polytherapy that additionally targets glutamatergic excitotoxicity and GABA inhibition through non-benzodiazepine sites presents some level of protection in reducing seizure severity, epileptogenesis, and brain damage, showing the potential and possible necessity of combining multiple ASMs to treat refractory status epilepticus.

Funding: This research was supported by NINDS [U01 NS074926] and by an interagency agreement (AOD21005-001 00000) between NIAID/NIH and the USAMRICD.