Neuroprotectant Activity of Novel Water-Soluble Synthetic Neurosteroids on Organophosphate Intoxication and Status Epilepticus-Induced Long-Term Neurological Dysfunction, Neurodegeneration, and Neuroinflammation.

Organophosphates (OPs) and nerve agents are potent neurotoxic compounds that cause seizures, status epilepticus (SE), brain injury, or death. There are persistent long-term neurologic and neurodegenerative effects that manifest months to years after the initial exposure. Current antidotes are ineffective in preventing these long-term neurobehavioral and neuropathological changes. Additionally, there are few effective neuroprotectants for mitigating the long-term effects of acute OP intoxication. We have pioneered neurosteroids as novel anticonvulsants and neuroprotectants for OP intoxication and seizures. In this study, we evaluated the efficacy of two novel synthetic, water-soluble neurosteroids, valaxanolone (VX) and lysaxanolone (LX), in combating the long-term behavioral and neuropathological impairments caused by acute OP intoxication and SE. Animals were exposed to the OP nerve agent surrogate diisopropylfluorophosphate (DFP) and were treated with VX or LX in addition to midazolam at 40 minutes postexposure. The extent of neurodegeneration, along with various behavioral and memory deficits, were assessed at 3 months postexposure. VX significantly reduced deficits of aggressive behavior, anxiety, memory, and depressive-like traits in control (DFP-exposed, midazolam-treated) animals; VX also significantly prevented the DFP-induced chronic loss of NeuN(+) principal neurons and PV(+) inhibitory neurons in the hippocampus and other regions. Additionally, VX-treated animals exhibited a reduced inflammatory response with decreased GFAP(+) astrogliosis and IBA1(+) microgliosis in the hippocampus, amygdala, and other regions. Similarly, LX showed significant improvement in behavioral and memory deficits, and reduced neurodegeneration and cellular neuroinflammation. Together, these results demonstrate the neuroprotectant effects of the novel synthetic neurosteroids in mitigating the long-term neurologic dysfunction and neurodegeneration associated with OP exposure. SIGNIFICANCE STATEMENT: Survivors of nerve agents and organophosphate (OP) exposures suffer from long-term neurological deficits. Currently, there is no specific drug therapy for mitigating the impact of OP exposure. However, novel synthetic neurosteroids that activate tonic inhibition provide a viable option for treating OP intoxication. The data from this study indicates the neuroprotective effects of synthetic, water-soluble neurosteroids for attenuation of long-term neurological deficits after OP intoxication. These findings establish valaxanolone and lysaxanolone as potent and efficacious neuroprotectants suitable for injectable dosing.