In vitro Characterization and Rescue of VX Metabolism in Human Liver Microsomes

VX is an organophosphate acetylcholinesterase (AChE) inhibitor and while it is one of the most toxic AChE inhibitors known the extent of metabolism in humans is not currently well understood. The known metabolism in humans is limited to the metabolite identification from a single victim of the Osaka poisoning in 1994, which allowed for the identification of several metabolic products. VX has been reported to be metabolized in vitro by paraoxonase-1 and phosphotriesterase, although their binding constants are many orders of magnitude above the LD₅₀, suggesting limited physiological relevance. Using incubation with human liver microsomes (HLM) we have now characterized the metabolism of VX and the formation of multiple metabolites, as well as identifying an FDA-approved drug (EDTA) that enhances the metabolic rate. HLM incubation alone shows a pronounced increase in the metabolism of VX as compared to buffer, suggesting that CYP-mediated metabolism of VX is occurring. We identified a biphasic decay with two distinct rates of metabolism. The enhancement of VX metabolism in multiple buffers was assessed to attempt to mitigate the effect of hydrolysis rates. The formation of VX metabolites was shown to be shifted with HLMs, suggesting a pathway enhancement over simple hydrolysis. Additionally, our investigation of hydrolysis rates in various common buffers used in biological assays discovered dramatic differences in VX stability. The new human in vitro VX metabolic data reported points to a potential in vivo treatment strategy for rescue in individuals that are poisoned though enhancement of metabolism alongside existing treatments. Significance Statement VX is a potent acetylcholinesterase inhibitor and chemical weapon. To date we do not possess a clear understanding of its metabolism in humans that would assist us in treating those exposed to it. We now describe the human liver microsomal metabolism of VX and identify EDTA which appears to enhance the rate of metabolism. This may provide a potential treatment option for human VX poisoning.