Abstracts

Rationale: The discovery of therapeutic agents for the treatment of chemical warfare nerve agent (CWNA) exposure relies on the use of animal models that closely model human organophosphate (OP) toxicity. The carboxylesterase knockout (ES1 -/-) mouse specifically lacks plasma carboxylesterase activity, similar to humans, which eliminates the confounding effect of an enzyme whose activity is a key determinant of the severity of OP toxicity. Delayed midazolam treatment is a probable scenario in the event of a mass casualty situation. We evaluated in male and female ES1 -/- mice the dose-response effects of delayed midazolam treatment of seizure induced by soman (GD). Methods: Male and female ES1 -/- mice were implanted with telemetry transmitters to record EEG, temperature and activity. On the day of GD exposure, mice received 82 mg/kg (4 LD₅₀) of soman (sc), followed by an admix administration (ip) of atropine sulfate (4 mg/kg) and the oxime HI-6 (50 mg/kg) to increase survival or saline. Rodents were treated with the anticonvulsant midazolam (1, 3, 9 mg/kg, ip) 40 min after seizure onset and monitored for signs of toxicity. Electroencephalographic (EEG) activity was monitored continuously for two weeks after exposure to identify the development of status epilepticus (SE) and spontaneous recurrent seizures (SRS) and to evaluate changes in power bands. Results: Delayed treatment with midazolam dose-dependently increased survival.  The lowest midazolam dose (1 mg/kg, ip) resulted in poor survival in males and no survival in females. Although midazolam increased survival, delayed midazolam was ineffective at reducing the severity of behavioral seizures (Racine score). Mice exposed to 4 LD₅₀ developed prolonged SE, displayed an increase in the EEG power in the delta and theta band and a reduction in gamma band after SE and developed spontaneous recurrent seizures (SRS) within 1-2 weeks of exposure; midazolam was unable to prevent these effects. Conclusions: This study further demonstrates that when treatment with midazolam is delayed, as is anticipated in mass casualty scenario, midazolam increases survival but does not prevent the long-term effects of soman exposure.  The ES1 -/- mouse model may be a useful model to screen anti-epileptic drugs for efficacy as adjunct to midazolam in reducing the deleterious effects of CWNA exposure. In addition, changes in power band may be a useful tool in evaluating drug efficacy against CWNA exposure. Funding: Supported by the CounterACT Program, National Institutes of Health Office of the Director (NIH OD), and the National Institute of Neurological Disorders and Stroke (NINDS), Grant Number1R21NS103820-01. The views expressed in this manuscript are those of the authors and do not reflect the official policy of the Department of the Army, Department of Defense or the US Government. The experimental protocol was approved by the Animal Care and Use Committees at the US Army Medical Research Institute of Chemical Defense (USAMRICD), and all procedures were conducted in accordance with the principles stated in the Guide for the Care and Use of Laboratory Animals and the Animal Welfare Act of 1966 (P.L. 89-544), as amended. Erica Kundrick was supported in part by an appointment to the Research Participation Program for the U.S. Army Medical Research and Materiel Command administered by the Oak Ridge Institute for Science and Education through an agreement between the U.S. Department of Energy and U.S. Army Medical Research and Materiel Command.