Abstracts

Rationale: Organophosphorus nerve agents (OPNA) are irreversible inhibitors of acetylcholinesterase that pose a serious threat to public health because of their use as chemical weapons. Exposure to high doses of OPNA can dramatically potentiate cholinergic synaptic activity and cause status epilepticus (SE). Current standard of care for OPNA exposure involves treatment with cholinergic antagonists, oxime cholinesterase reactivators, and benzodiazepines. However, data from pre-clinical models suggest that OPNA-induced SE rapidly becomes refractory to benzodiazepines. Neuroactive steroids (NAS), such as allopregnanolone, retain anticonvulsant activity in rodent models of benzodiazepine-resistant SE, perhaps because they modulate a broader variety of GABAA receptor subtypes. SGE-516 is a novel, next generation NAS and a potent and selective GABAA receptor positive allosteric modulator (Martinez Botella et al., 2015) with demonstrated anticonvulsant activity in acute seizure animal models. We hypothesized that SGE-516 would reduce OPNA-induced SE in the soman intoxication model even after a significant treatment delay, a time point when benzodiazepines lose activity in similar models (Reddy and Reddy, 2015). Methods: Adult male Sprague Dawley rats (n = 34) implanted with EEG electrodes were placed in individual recording chambers and ?-30 minutes of baseline EEG data were recorded. Thirty minutes prior to soman exposure, rats were administered the oxime cholinesterase reactivator HI-6 (125 mg/kg, IP). One minute after exposure to soman (1.6 x LD50, SC), rats were administered atropine methyl nitrate (2 mg/kg, IM). At either 20 minutes or 40 minutes after SE onset, animals were treated with atropine sulfate (0.45 mg/kg) admixed with pralidoxime (25 mg/kg, IM), midazolam (1.8 mg/kg, IM) and either saline or SGE-516 (5.6, 7.5, or 10 mg/kg, IP). EEG was recorded for ?-4 hours, after which a subset of rats were perfused for histology. EEG data were analyzed using custom Matlab code to calculate the probability of observing aberrant spike events as a surrogate endpoint for electrographic seizure activity. Neuronal cell death was evaluated by counting Fluorojade B (FJB)-positive cells in fixed, slide mounted brain slices (1 coronal section at 3.24 mm posterior to bregma was evaluated for each rat). Cell counting was performed by a treatment-blinded technician. Results: All three doses of SGE-516 significantly reduced the mean spike probability compared to control when administered 20 minutes after SE onset (P = 0.0003, one-way ANOVA with Dunnett's multiple comparisons test). When 10 mg/kg SGE-516 was administered 40 minutes after SE onset, mean spike probability was still significantly reduced compared to control (P = 0.0245, student's t-test). The average number of FJB+ cells from rats treated with SGE-516 20 minutes after SE onset were significantly lower than control-injected rats (P < 0.0001, one-way ANOVA with Dunnett's post-hoc test), but there was no difference in number of FJB+ cells between treatment groups. The average number of FJB+ cells from rats treated 40 minutes after SE onset with 10 mg/kg SGE-516 was also significantly lower than that from control-injected rats (P = 0.0002, student's t-test). Conclusions: Administration of SGE-516, a next generation NAS, significantly reduced soman-induced SE and neuronal cell death in rats at all doses and time points tested. These data suggest synthetic NASs that positively modulate both synaptic and extrasynaptic GABAA receptors may be good candidates for further study in the treatment of OPNA-induced SE. Funding: NA