Chlorpyrifos PBTK/TD Model Oral And Subcutaneous Routes Of Exposure

ObjectivesChlorpyrifos (CPF) is a broad-spectrum organophosphate pesticide (OP) that is widely used in both agricultural and household applications worldwide, and the general population may be exposed to CPF via the oral, inhalation, and dermal routes. Researchers have developed a PBTK/TD model for CPF in rats and humans and validated all scenarios in a parallelogram design, however, no study has assessed the pharmacokinetics and pharmacodynamics of CPF via multiple routes in experimental rats. Therefore, the major objective of this study was to develop a multi-route PBTK/TD model to evaluate the pharmacokinetics and pharmacodynamics of CPF in rats following both oral and subcutaneous exposures to CPF. This CPF PBTK/TD model quantitatively estimates target tissue dosimetry and AChE inhibition and is a strong framework for refining a biologically based risk assessment for exposure to CPF through multiple exposure routes.Method(1) Groups of rats (5/treatment)were dosed with 50mg/kg CPF in corn oil by oral gavage. A second group of rats (5/treatment)were dosed with 50mg/kg CPF in corn oil by SC injection. A third group of rats (5/treatment)were dosed with 50mg/kg CPF by oral gavage, and 50mg/kg CPF in corn oil by SC injection. Five animals per time point were humanely anesthetized and sacrificed, and tissues were collected at 0,1,2,4,6,8,12,24h and 48h post-dosing for CPF, TCP or AChE analysis. Urine was collected continuously, and sample collections were accumulated for 0-12, 12-24,24-48h post-dosing for TCP analysis.(2)Three groups of Sprague-Dawley (SD) rats were co-administered doses (0,2.5+7.5 and 5+15 mg/kg/day) of chlorpyrifos (CPF) in corn oil over a 10-day study period by both the oral and subcutaneous (SC) routes and were then observed for 4 days post-dosing. From all of the animals that were co-exposed, urine was collected continuously, and sample collections were accumulated for 0-2,2-3,3-5,5-6,6-8,8-10,10-12, and 12-14 days for TCP analysis. Five animals per time point were humanely anesthetized and sacrificed, and tissues were collected on days 2,6,10,12,14 for CPF, TCP, or AChE analysis. Whole plasma was collected from the arteria femoralis into heparinized syringes. Both the brain and liver samples were stored at -80℃ until analysis.(3)PBTK/TD model construction:Model parameters were previously published, experimentally derived, or optimized computationally. Model structure and model implementation were developed, a local sensitivity analysis was conducted to identify the most important parameters for estimating acetylcholinesterase inhibition in blood and brain. Optimizations of model parameters were also conducted using acslXtreme using the Parameter Estimation function. The model verification was achieved by directly determining the toxicokinetic and toxicodynamic of rats with repeated CPF exposures. The rodent PBPK/PD model was extended to incorporate different doses, which will assess the pharmacokinetic (tissue CPF and metabolite levels) and pharmacodynamic (blood and brain AChE activity) after the administration (oral+SC) of CPF 20+20,10+30 and (30+10) mg/kg.Results(1)The results of toxicokinetic and toxicodynamic of rats with acute CPF exposures:the peak blood CPF concentration for oral and SC groups were observed at 2h and quantifiable at all 9 time points. CPF concentration in blood from orally administrated CPF was significantly lower than that of CPF administered SC. the peak blood TCP concentration for oral and SC groups were observed at 8h and 4h. TCP concentration in blood from orally administrated CPF was significantly higher than that of CPF administered SC. The peak blood TCP concentration was observed at 6h for co-exposed oral and SC group.TCP concentration in urine for all treatment groups were observed at 24h. TCP concentration in urine from orally administrated CPF was significantly higher than that of CPF administered SC. The maximum inhibition of plasma AChE was observed at 24h for SC group and 8h for oral group, and then plasma AchE recovered. Oral administration decreased the maximum inhibition of plasma AChE activity relative to SC administration, but the recovery following the SC administrationwas likewise prolonged. The maximum inhibition of plasma AChE was observed at 12h for both oral and SC administration. The maximum inhibition of brain AChE was observed at 24h for SC group and 8h for oral group. The overall inhibition of brain AChE for SC administration group was lower than oral administration group. The maximum inhibition of brain AChE was observed at 24h for both oral and SC administration.(2)The results of toxicokinetic and toxicodynamic of rats with repeated CPF exposures:the plasma CPF and TCP levels increased in a dose-related fashion, which were detected in all animals on days 12 and 14. Average plasma and brain AChE activity, as a percent of control activity, decreased with increasing dose and time, AChE inhibition following an oral dose of CPF recovers more quickly than it does following a SC dose. Plasma AChE inhibition was faster than brian AChE inhibition.(3)The PBTK/TD model:the sensitivity analysis show that the most important parameters for CPF concentration were absorption parameters, such as gastrointestinal(GI) and subcutaneous absorbed parameters, and for estimating acetylcholinesterase inhibition in plasma and brain were parameters for CPF-oxon Inhibition except for pharmacokinetic parameters, such as liver blood flow、partition coefficients for CPF and CPO、liver and blood metabolic constants. The multi-route PBTK/TD model behaves consistently with the general understanding of CPF toxicity, pharmacokinetics, and tissue AChE inhibition in adult rats by making correlation analysis between simulation and experimental data.It was predicted by PBTK/TD model that after given CPF 20+20, 10+30 and (30+10) mg/kg via both oral and SC routes, the concentrations of CPF and TCP in plasma increased then decreased with time in all groups. The inhibitory level of AChE activity in plasma was oral dose-dependent, while AChE activity of brain was more sensitive to CPF subcutaneous exposure.Conclusion(1)CPF was rapidly absorbed and metabolized following oral administration group relative to SC administration group. AChE activity in plasma and brain following the SC administration was likewise prolonged and the recovery slowly. The potential importance of localized metabolism following SC administration is substantial.(2)When undergoing multi-route exposure to CPF, the extent of toxicity is dependent on the dose of oral and SC. AChE activities in plasma and brain are also prolonged.(3)The PBTK/TD rat model for multiple exposures to CPF will also allow for simulations of pharmacokinetic and AChE inhibition in plasma and brain. In conclusion, this CPF PBTK/TD model is an effective framework for reconstructing exposure levels with blood and urine biomarker.