Organophosphorus (OPs) are a major public health problem due to their toxicity and their massive use as pesticides. Both diethylparaoxon (DEPOX) and dimethylparaxon (DMPOX) have very close physicochemical properties. In rats, they induce similar respiratory toxicity from a dynamic standpoint, those effects being different considering kinetics. We suggested that this kinetic difference may occur because of DEPOX and DMPOX blood toxicokinetics (TK) differences. To test this hypothesis, we perform a toxicokinetic/ toxicodynamic (TK/TD) study, based on measurement of free blood concentrations of these two OPs and residual activity of total blood cholinesterases related to their effects on breathing rest.

For each study, Sprague-Dawley male rats, were spread out over three groups of six animals: DEPOX and DMPOX at 50% of lethal dose (215μg/kg and 250μg/kg for DEPOX and DMPOX, respectively), and vehicle administered by subcutaneous route. For TD study, respiratory parameters were measured between 5 and 90 minutes using whole body plethysmography at rest. For TK studies, blood was collected by jugular catheter from 2.5 to 90 minutes. Free OPs blood concentrations were assayed by a HPLC-QTOF method, validated from 0.1 to 50ng/mL using Enoval™ software, and whole blood cholinesterases activity by radioenzymology. Statistics (t-test) and kinetic modeling were performed using GraphPad Prism™ and Adapt 5™ softwares, respectively.

Both toxics showed significant increase in expiratory time but regarding DMPOX, DEPOX induced faster (apparent Tmax DEPOX 15min and DMPOX 25min), stronger (Emax DEPOX 234.6±11.8% and DMPOX 160±6.3%; p=0.0005) and more long-lasting intoxication. So, DEPOX effects remained intensive during all experimental time whereas DMPOX began to decrease after 40min. Cholinesterase inhibition were similar, in kinetic and intensity, for both analogous (residual activity from 20 to 90min DEPOX 39.0±0.5% and DMPOX 39.7±0.4%; p=0.21). Blood OPs concentrations fitted with a bi-compartimental model. TK parameters supported their strong diffusion (apparent volume of distribution of 12.8L/kg and 8.4 for DEPOX and DMPOX, respectively) and short half-life (4.33 minutes and 6.93 minutes for DEPOX and DMPOX, respectively).

Even if they were close, toxicokinetic parameters suggested that DEPOX has less affinity for blood compartment than DMPOX. Earlier DEPOX toxicity would imply a more powerful inhibitory activity of this toxic. Respiratory toxicity neither seemed correlated to free blood OPs concentration, nor to blood cholinesterase inhibition: whereas similar cholinesterase TK and decreasing blood concentration profile, TD kinetic and intensity differed for both OPs. As demonstrated previously for whole blood cholinesterase activity, free blood OPs concentration, nor to blood cholinesterase inhibition: whereas similar cholinesterase TK and decreasing blood concentration profile, TD kinetic and intensity differed for both OPs. As demonstrated previously for whole blood cholinesterase activity, free blood OPs concentration would not seem relevant to monitor respiratory toxicity. Since both blood markers could not reflect tissular action of these OPs, further experimentations are necessary to explain TD difference. Concentrations of free toxics, but also of toxic adducts and metabolites, in blood, as in various tissues, during a longer experimental time would be investigated.

Despite many limitations, our study confirmed that, for an equivalent toxic dose level, kinetic and intensity of respiratory TD of DEPOX and DMPOX were different. Neither their blood kinetics nor whole blood cholinesterase inhibition seemed able to explain this difference. New hypothesis can be formulated especially variation in tissue distribution.