Investigation of the toxic mechanism of oximes which are used in the treatment of poisoning by nerve agents

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The pyridinium aldoxime methiodide (PAM)-type oxime 4-PAO, which can reactivate AChE inhibited by alkylphosphonate and penetrate the blood-brain barrier, is expected to be effective for reactivating blocked cholinesterase in the brain. However, the toxicity of 4-PAO has not yet been investigated in detail. The present study investigated the effects of 4-PAO on the contractile activity of the rat heart using electrocardiography analysis. 4-PAO was suggested to inhibit the atrioventricular conduction system more strongly than 2-PAM. In addition, not only 4-PAO but also 2-PAM inhibited the inherent K+-EDTA-ATPase catalytic activity of myosin, indicating that oximes might interact directly with the catalytic center of the myosin molecule. No significant inhibition was observed for less than 4uM 4-PAO, 2-PAM, 4-PAPE, obidoxime and DAM, which correspond to the clinical dose of 2-PAM. Further studies are required to clarify the mechanism of 4-PAO toxicity.