Physiologically Based Pharmacokinetic Model For The Marker Residue Of Cyadox In Pigs

With the development of animal husbandry, the use of drugs in animal production is more and more widely. Veterinary drug residues in animal food will endanger human healthy, ecological environment and the development of animal husbandry. The maximum residue limit (MRL) and the withdrawal time (WDT) are used to control residue of veterinary drug residue. However, the established MRL and WDT can still not effectively prevent the residues of drugs in animal derived foods because of their extra-label use in practice. In addition, the traditional residue monitoring methods may unnecessarily waste a large number of resources. Physiologically based pharmacokinetic (PBPK) model use in veterinary drugs residue monitoring can meet the shortfall of these traditional methods. Cyadox(CYA) as a feed additive with safe, antibacterial and growth-promoting effects, may be widely used in animal husbandry. Desoxycyadox (DCYA), one of its metabolits, is designated as the marker residue. The residues of CYA in animal derived foods will endanger human healthy because of its abuse. It is necessary to develop a PBPK model of CYA to control the residue of it in animal food. Therefore, a PBPK model on CYA in pigs was developed to predict the CYA and DCYA residue in tissues of pigs. And then the PBPK model of CYA was extrapolated to quinocetone (QCT).A flow-limited PBPK model and a permeability-limited PBPK model which used in estimating concentrations of CYA and DCYA after CYA administration to swine were developed. Liver was assumed to be the only metabolic site of CYA, and DCYA was assumed to be formed directly from CYA. Compartments for CYA and DCYA included blood, muscle, liver, kidney, adipose and the remaining carcass. The physiological parameters used in model were obtained from literatures. Some compound-specific parameters were also obtained from literatures directly or by use of ACSLxtreme.The plasma protein binding of CYA and DCYA were determined using an equilibrium dialysis method. Blank PBS1mL was placed in dialysis bag and incubated with drug in phosphate buffer at4℃for2,4,8,12,24,32,48,60,72h respectively. And then blank pig plasma1mL was placed in dialysis bag and incubated with drug in phosphate buffer (the concentration ranging from0.05to1.0mg/kg) at4℃. The result showed that the equilibrium of the CYA and DCYA level inside and outside the dialysis bag were achieved after48h, and the measured value were15.27±1.11%(n=9) and72.24±1.49%(n=9).Four pigs (20±5.82kg) were used to determine the renal clearance of DCYA. Pigs received a bolus injection of DCYA throng the ear vein. Blood sample (5mL) was collected via precava before and3hours after administration. Urine samples were collected during the whole experiment (a period of6hours after dosing) with their volumes recorded accurately. All samples were analyzed using the developed HPLC methods. The result showed that the measured renal clearance ranged from0.3640L/h to1.4483L/h.Four pigs (20±3.98kg) were used to determine the tissue/plasma partition coefficient. DCYA solution was infused via the ear vein at a rate of2mL/min, until the steady-state distribution of DCYA in the body was attained. The time required to achieve steady-state was determined by monitoring the plasma DCYA concentration. Subsequently, all pigs were sacrificed. Blood, liver, kidney, fat, muscle samples were collected and analyzed using developed HPLC methods. The result showed that the equilibrium of the distribution of DCYA in the body achieved at60min. The liver/plasma partition coefficient of DCYA in pigs was0.91(n=4). The kidney/plasma partition coefficient of DCYA was2.27(n=4). The muscle/plasma partition coefficient of DCYA was0.44(n=4). The fat/plasma partition coefficient of DCYA was0.18(n=4).The model was created and optimized by use of computer software ACSLxtreme. Sensitivity analysis was completed to evaluate the importance of each constant on the whole model by ways of normalized sensitivity coefficient. And then the developed flow limited model for CYA and DCYA was extrapolated to QCT. The physiological parameters of pigs were fixed. Pharmacokinetic parameters of the parent drug (such as bioavailability and absorption rate constant) were from literature.The flow limited model well predicted the residue depletion of CAY and DCYA in edible tissues. Eexperimental data (points) distributed over the model simulation (solid line). Residual values distributed nearby X coordinate axis symmetrically. Residual analysis revealed that flow limited model had good accuracy in all tissue compartments. Compared with external data sets, the model had correlations for desoxycyadox in kidney tissue of0.8987. The permeability limited model predictions of CYA concentrations in liver, muscle and fat tissue closely agreed with observed data. Experiment values of CYA and DCYA concentration in kidney were bigger than model predicted value. DCYA concentrations of liver tissue were also high-predicted by the permeability-limited model. Compared with external data sets, the permeability-limited model had correlations for desoxycyadox in kidney tissue of0.7931. Judged by the measured data and existing studies, the flow limited model was selected as the better suitable one to describe the disposition of CYA and DCYA in pig. So the flow limited model was extrapolated to QCT. QCT concentrations of liver, muscle and fat tissues at6h after last dose were high-predicted by the permeability-limited model. But the model well predicted the residue depletion of QCT in kidney tissue. And also generally good agreements were obtained between model predicted and measured tissues (liver, kidney and muscle) concentrations of DQCT.In conclusion, a flow limited PBPK model and a permeability-limited PBPK model on CYA and DCYA was developed in pigs. The flow limited model predicted the residue of CYA and DCYA in pigs well. The developed flow limited model has been extrapolated to QCT. The development of PBPK model of CYA and QCT improved the method of residue monitor, and provides reference for development PBPK model on veterinary druga and their residues markers.