The association of blood cholinesterase levels with the susceptibility of animals to sarin and ethyl pyrophosphate poisoning.

An association between the blood cholinesterase (ChE) levels and the toxicity of sarin and ethyl pyrophosphate has been established. This has been demonstrated in two types of experiment. In the first, guinea-pigs were given a non-lethal dose of sarin (isopropyl methylphosphonofluoridate) which reduced the blood ChE to 20% of normal, and at intervals, as the ChE level of the blood gradually recovered, separate batches were given a second dose of the same size. A comparison was then made between the blood ChE levels immediately prior to the injection of the second dose and the mortality rate. In the second, a relatively small dose of sarin or ethyl pyrophosphate was given daily to rabbits until the enzyme value fell to a steady level. LD50 values were then determined on such groups and compared with those found in saline treated ones. In both guinea-pigs and rabbits it has been shown that a depression of blood ChE to below 40% indicated an increased toxicity of sarin and ethyl pyrophosphate to these species. The approximate quantitative relation appears to be that susceptibility is increased in the ratio 1.5 when the blood ChE is reduced by half.