Success of pyridostigmine, physostigmine, eptastigmine and phosphotriesterase treatments in acute sarin intoxication.

The acute toxicity of organophosphorus (OP) compounds in mammals is due to their irreversible inhibition of acetylcholinesterase (AChE) in the nervous system, which leads to increased synaptic acetylcholine levels. The protective actions of intravenously (i.v.) administered pyridostigmine, physostigmine, eptastigmine, and an organophosphate hydrolase, phosphotriesterase, in acute sarin intoxication were studied in mice. The acute intragastric (i.g.) toxicity (LD50) of sarin with and without the pretreatments was tested by the up-and-down method. The mice received pyridostigmine (0.06 mg/kg body weight), physostigmine (0.09 mg/kg body weight), the physostigmine derivative eptastigmine (0.90 mg/kg body weight) or phosphotriesterase (104 U/g, 10.7 microg/g body weight) 10 min prior to the i.g. administration of sarin. Physostigmine was also administered with phosphotriesterase. Phosphotriesterase was the most effective antidote in sarin intoxication. The LD50 value for sarin increased 3.4-fold in mice receiving phosphotriesterase. Physostigmine was the most effective carbamate in sarin exposure. The protective ratios of physostigmine and pyridostigmine were 1.5- and 1.2-1.3-fold, respectively. Eptastigmine did not give any protection against sarin toxicity. Both the phosphotriesterase and physostigmine treatments protected the brain AChE activities measured 24 h after sarin exposure. In phosphotriesterase and physostigmine-treated mice, a 4- and 2-fold higher sarin dose, respectively, was needed to cause a 50% inhibition of brain AChE activity. Moreover, the combination of phosphotriesterase-physostigmine increased the LD50 value for sarin 4.3-fold. The animals pretreated with phosphotriesterase-ephysostigmine tolerated four times the lethal dose in control animals, furthermore their survival time was 2-3 h in comparison to 20 min in controls. In conclusion, phosphotriesterase and physostigmine were the most effective treatments against sarin intoxication. However, eptastigmine did not provide any protection against sarin toxicity.