The Study Of Physiologically Based Toxicokinetic And Toxicodynamic Model Following Subcutaneous Exposure To Chlorpyrifos

Background and Objective Chlorpyrifos(CPF) is widely used in the world, and this can increase the risk of exposure among population and threaten their health, especially in the process of the production and use of chlorpyrifos, which may easily exposure through the skin surface. Physiologically based toxicokinetic and toxicodynamic model(PBTK/TD model) is a new technology in risk assessment with the advantages of predicting internal exposure dose and the level of toxic effects. Single and repeated PBTK/TD model were constructed in this study, used for assessing the dynamic changes of CPF in vivo and the toxic effects quantitatively to know the health harm levels. The model was optimized by using animal experiments data. This physiological model provided a new method for risk assessment of CPF exposure.Contents and Methods (1) Toxicokinetics and toxicodynamics of CPF in subcutaneous exposure experiment:Adult SD rats were injected subcutaneously single or 10 succession days. The dose of single exposure was 0,69.75,139.5 and 279 mg/kg, respectively. The dose of 10 succession days was 0,5,10 and 20 mg/kg, respectively. Rats in all groups were sacrificed at 3 h,6 h,12 h,24 h,48 h and 72 h in acute experiment, and at 2 d,4 d,7 d and 10 d in repeated experiment. Blood, liver, urine and cortex samples in each rat were collected, CPF concentration in blood and liver, TCP concentration in blood and urine, and activity of AChE and BuChE in blood and cortex were determined. Purpose of experiments were to study the toxicokinetic and toxicodynamic of CPF and optimize and verify the model later.(2) PBTK/TD model construction:the building steps of model were as follows. (a)Design structure of model according to CPF characteristics and physiological characteristics.(b)Build differential equation of CPF, TCP and ChE. (c)Collect parameters from literature. (d)Write a program of differential equation and parameters in acslX. Input the exposure condition and run the program. (e)Compare the output of model and results of acute experiment. Optimize the parameters until the two values approached if differences existed. (f)The optimized PBTK/TD model used for predicting dynamic changes of each biomarker in repeated condition. (g)The analysis of sensitivity and uncertainty.Results (1) The results of toxicokinetic and toxicodynamic of CPF in rats by subcutaneous exposure:serum CPF, TCP and liver CPF concentration increased first, and then decreased, the peak concentration appeared at 6 h,12-24 h and 6 h, respectively. Serum CPF and TCP in high dose group had a longer half-life than low dose group. When the exposure dose increased 1 times, the AUCo-72 of serum CPF and TCP increased nearly 1 times. Serum and cortex AChE activity decreased after exposure, the enzyme began to recover when achieved maximum inhibition. The higher the dose was, the more slowly the activity recovered. The activity was still low at 72 h. The maximum inhibition of serum and cortex AChE were 24 h and 24～48 h, respectively. Serum and cortex AChE activity were positively related (r=0.783, P<0.01), and serum AChE activity had greater inhibition (P<0.01). The maximum inhibition of serum and cortex BuChE were 24-48 h. Serum AChE and BuChE activity were positively related (r=0.779, P<0.01), and serum BuChE activity had greater inhibition (P<0.01).(2) The results of toxicokinetic and toxicodynamic of CPF in rats by repeated exposure:serum CPF, TCP and liver CPF concentration increased gradually with the daily exposure. The higher the dose was, the greater the index values were. The inhibition of serum and cortex ChE were increasing with repeated exposure.(3) PBTK/TD model:the structure of model included skin, liver, brain, diaphragm, blood, fat, rapidly perfused and slowly perfused. The acute data used for parameters optimization, the optimized model simulation showed that serum CPF and TCP concentration achieved peak at 6.7 h and 24.7 h, respectively. The maximum inhibition of serum and cortex AChE activity were 21-33 h and 23～33 h, respectively. The maximum inhibition of serum and cortex BuChE activity were 15～28 h and 27～40 h, respectively. Compared with the experiment data, the model was will fitted. Sensitivity analysis results showed that cardiac output, blood flow in liver, fat/blood partition coefficient, absorption rate etc. had great impact to serum CPF. Blood flow in fat, metabolic parameters etc. had low impact. The other parameters had very low impact. Blood flow in liver, liver/blood partition coefficient of CPFO, metabolic parameters, metabolic parameters, plasma protein binding, AChE degradation and inhibition rates in blood etc. had great impact. The Monte Carlo method used to study the uncertainty of cardiac output. The model was considered well when the simulation values of serum CPF concentration contained the experimental values when the cardiac output changed within a certain range. Finally, the data of repeated experiment used for model verification. The predictive ability of the model was nice by making correlation analysis between simulation and experiment results,.Conclusion (1) Animal experiments showed that external exposure dose had relationship with serum CPF, liver CPF, serum TCP, serum and cortex ChE. The excretion of TCP in urine can be used as a biomarker of exposure. Serum AChE activity was an effective biomarker of cortex AChE activity. Serum and cortex AChE activity were positively related, and serum AChE activity had greater inhibition.(2) The PBTK/TD model following subcutaneous exposure to chlorpyrifos in our study can accurately reflect the ADME process in tissues and organs and the changes of effective biomarker ChE. Our model provided a new technical method for risk assessment of organophosphorus pesticides.