Pharmacokinetics and efficacies of obidoxime and atropine in paraoxon poisoning.

The efficacies (ED sub 50) of obidoxime and atropine against paraoxon poisoning in mice were determined by administering the antidotes 5, 20, 40 and 60 min before administration of the organophosphate. With increasing time intervals t between the administration of the antidote and paraoxon, the dose of antidote (ED sub 50), that reduced the lethality of 2 LD sub 50 of paraoxon to 50% increased. The standardized log ED sub 50/t plot was linear and yielded the "efficacy half-life". In addition, the blood concentrations c of the antidotes were measured, resulting in a linear log c/t plot. The "efficacy half-life" was approximately twice the half-life of the antidote in blood. The possible reasons for this phenomenon are discussed. (IUCT)