Efficacy and dosing of antidotes applied to Daphnia intoxicated by nerve agent tabun

A new assay with Daphnia, which can be used as a time, cost, and human effort-saving tool in the development of effective antidotes against organophosphate intoxications, is presented. Five concentrations of atropine (antimuscarinic anticholinergics) as well as a reactivator (trimedoxime) were tested to define the optimal dosage. Various reactivators (trimedoxime, obidoxime) were used to examine difference in effectivity of treatments. The most effective dose of trimedoxime corresponded to the 75% of its EC50(24) value, i.e. 77.85 mg l−1. The most effective dose of atropine corresponded to the 25% of its EC50(24) value, i.e. 104.70 mg l−1. The most effective treatment was a combined atropine-obidoxime treatment, followed by the combined atropine-trimedoxime treatment, the atropine only and the obidoxime only treatments. The efficacy of the trimedoxime only treatment was doubtful. The surprisingly high efficacy of obidoxime in the obidoxime only treatment indicates that some oximes might act in daphnids not just as reactivators but also by some other mechanisms.