Pharmacokinetics and toxicodynamics of pralidoxime effects on paraoxon-induced respiratory toxicity

Empirical studies suggest that the antidotal effect of pralidoxime depends on plasma concentrations with therapeutic effects associated with concentrations above 4 mg/l. The purpose of this study was to determine the pharmacokinetic-toxicodynamic (PK-TD) relationships for the antidotal effect of pralidoxime on paraoxon-induced toxicity in rats. Diethylparaoxon inactivation of whole-blood cholinesterase activity was studied both in vitro and in male Sprague-Dawley rats. Toxin-induced respiratory effects were measured via whole-body plethysmography in control and pralidoxime-treated animals (50 mg/kg im injection). In the in vitro analysis, cholinesterase reactivation by pralidoxime in blood-poisoned diethylparaoxon (10nM) was proportional to the logarithm of drug concentrations. A mechanism-based TD model was developed, which well described the inhibition of cholinesterases by diethylparaoxon and reactivation with pralidoxime. The in vitro pralidoxime EC(50) was estimated to be 4.67 mg/l. Animals exposed to diethylparaoxon exhibited a decrease in respiratory rate and an increase in expiratory time, and pralidoxime treatment resulted in a rapid complete but transient (< 30 min) correction in respiratory toxicity. In contrast, there was a fast and total reactivation of blood cholinesterase activity over the 210-min study period. The in vitro TD model was extended to capture the time-course of in vivo pralidoxime antidotal effects, which explained the complex relationship between drug exposure and pharmacological response profile. This study provides insights into the role of oxime-rescue of paraoxon-induced toxicity, and the final PK-TD model might prove useful in optimizing the design and development of such therapy.