A physiologically based pharmacokinetic model for quinoxaline‐2‐carboxylic acid in rats,extrapolation to pigs

A multi‐compartment physiologically based pharmacokinetic (PBPK) model to describe the disposition of cyadox (CYX) and its metabolite quinoxaline‐2‐carboxylic acid (QCA) after a single oral administration was developed in rats (200 mg/kg b.w. of CYX). Considering interspecies differences in physiology and physiochemistry, the model efficiency was validated by pharmacokinetic data set in swine. The model included six compartments that were blood, muscle, liver, kidney, adipose, and a combined compartment for the rest of tissues. The model was parameterized using rat plasma and tissue concentration data that were generated from this study. Model simulations were achieved using a commercially available software program (ACSLX_Libero version 3.0.2.1). Results supported the validity of the model with simulated tissue concentrations within the range of the observations. The correlation coefficients of the predicted and experimentally determined values for plasma, liver, kidney, adipose, and muscles in rats were 0.98, 0.98, 0.98, 0.99, and 0.95, respectively. The rat model parameters were then extrapolated to pigs to estimate QCA disposition in tissues and validated by tissue concentration of QCA in swine. The correlation coefficients between the predicted and observed values were over 0.90. This model could provide a foundation for developing more reliable pig models once more data are available.     

Development of a physiologically based pharmacokinetic model to predict tulathromycin distribution in goats

Physiologically based pharmacokinetic (PBPK) models, which incorporate species- and chemical-specific parameters, could be useful tools for extrapolating withdrawal times for drugs across species and doses. The objective of this research was to develop a PBPK model for goats to simulate the pharmacokinetics of tulathromycin, a macrolide antibiotic effective for treating respiratory infections. Model compartments included plasma, lung, liver, muscle, adipose tissue, kidney, and remaining poorly and richly perfused tissues. Tulathromycin was assumed to be 50% protein bound in plasma with first-order clearance. Literature values were compiled for physiological parameters, partition coefficients were estimated from tissue:plasma ratios of AUC, and the remaining model parameters were estimated by comparison against the experimental data. Three separate model structures were compared with plasma and tissue concentrations of tulathromycin in market age goats administered 2.5 mg/kg tulathromycin subcutaneously. The best simulation was achieved with a diffusion-limited PBPK model and absorption from a two-compartment injection site, which allowed for low persistent concentrations at the injection site and slower depletion in the tissues than the plasma as observed with the experimental data. The model with age-appropriate physiological parameters also predicted plasma concentrations in juvenile goats administered tulathromycin subcutaneously. The developed model and compilation of physiological parameters for goats provide initial tools that can be used as a basis for predicting withdrawal times of drugs in this minor species.     

A physiologically based pharmacokinetic model linking plasma protein binding interactions with drug disposition

Combination drug therapy increases the chance for an adverse drug reactions due to drug-drug interactions. Altered disposition for sulfamethazine (SMZ) when concurrently administered with flunixin meglumine (FLU) in swine could lead to increased tissue residues. There is a need for a pharmacokinetic modeling technique that can predict the consequences of possible drug interactions. A physiologically based pharmacokinetic model was developed that links plasma protein binding interactions to drug disposition for SMZ and FLU in swine. The model predicted a sustained decrease in total drug and a temporary increase in free drug concentration. An in vivo study confirmed the presence of a drug interaction. Neither the model nor the in vivo study revealed clinically significant changes that alter tissue disposition. This novel linkage approach has use in the prediction of the clinical impact of plasma protein binding interactions. Ultimately it could be used in the design of dosing regimens and in the protection of the food supply through prediction and minimization of tissue residues.     

Sulfamethazine residues in swine

Two possible causes of violative sulfonamide residues in swine were studied. To determine if sulfamethazine accumulated in the tissues of swine when the drug was administered in feed, the rates of plasma drug disappearance following a single oral dose and continuous feeding of the drug were compared. The rate of plasma drug disappearance was not significantly different (α= 0.05) when the two methods of drug dosing were compared. When feed containing 2 μg sulfamethazine/gm was fed to swine during a 7—day period preceding slaughter, the animal's liver contained violative residues. Violative concentrations of sulfamethazine were detected in the livers, kidneys, and skeletal muscle of swine which consumed feed containing 8 μg sulfamethazine/gm.     

Factors that affect drug disposition in food-producing animals during maturation.

Drugs administered to neonatal food-producing animals (cattle, sheep, goats, swine) may exhibit significantly different pharmacokinetic/disposition characteristics than they do in adult animals of the same species. Undesirable consequences such as suboptimum therapeutic concentrations, toxic effects, and violative tissue residues may result if adult dosage regimens are employed in young animals. Using selected drugs as examples, this paper reviews factors that contribute to differences in drug disposition in newborn vs adult animals. Immaturity of mechanisms involved in drug absorption, especially from gastrointestinal and parenteral sites of administration, and of drug distribution to sites such as plasma proteins, adipose tissue, and fluid compartments are considered. The role of developmental changes in drug biotransformation in the liver and other tissues and the maturation of excretory mechanisms, primarily from the kidney, in the increased rate of drug clearance during maturation is described. Pharmacokinetic studies with specific drugs in the target species are an important approach to establishing rational drug use in immature food-producing animals.     

Elimination of sulfamethazine from edible tissues, blood, urine, and feces of turkey poults.

Sulfamethazine was administered to 8- to 10-week-old turkey poults intravenously (IV) at the dose level of 71.5 mg/kg of body weight, orally at the dose level of 143 mg/kg of body weight, or in the drinking water at the concentration of 0.1% over a 6-day period. The concentrations of free sulfamethazine in blood, muscle, skin, kidney, and liver were determined and semilogarithmic plots of concentration vs time for the various tissues indicated that the curve had a linear portion within the first 72-hour period of drug withdrawal. The rates of disappearance of sulfamethazine from the various tissues were proportional to the concentration in the tissues. After 72 hours of withdrawal and for as long as 14 days, sulfamethazine concentrations in kidney, liver, and skin of turkeys given the drug in the drinking water fluctuated between 0.1 and 0.4 ppm. Only 8.6% of the oral dose (143 mg/kg) and 16.5 to 17% of the IV dose (71.5 mg/kg) were recovered in urine and feces as the parent compound during the initial 72-hour period.     

Elimination half-lives of sulphadimethoxine and its N4-acetyl metabolite in tissues of laying hens

The depletion rates of sulphadimethoxine (SDM) and its metabolite N4-acetylsulphadimethoxine (N4-AcSDM) were estimated in blood and various tissues of laying hens. The tissue contents (ppm) of SDM and N4-AcSDM after the withdrawal of SDM, which was fed to hens at 400 ppm diet for 5 successive days, were determined by HPLC. The elimination half-life (t1/2) of N4-AcSDM in the liver, ovary and muscle was estimated to be 4.3 h with a 95% confidence interval from 3.6 to 5.3 h. No significant difference between t1/2 of N4-AcSDM in the tissues and that of SDM (4.4 h) in the blood, kidney, muscle, ovary and adipose tissue was observed. On the other hand, the t1/2 of N4-AcSDM in the kidney (8.1 h) was significantly longer than that in the above 3 tissues.     

Sulfamethazine residues in uncooked edible tissues of port following recommended oral administration and withdrawal.

Commercial feed rations containing sulfamethazine at the level of 110 ppm were fed for a period of 65 days to market pigs in a study simulating normal farm practices. The levels of sulfamethazine at the end of medication were in excess of 10 ppm in liver and kidney and up to 2.6 ppm in muscle tissues. Concentrations of sulfamethazine in tissues from pigs after withdrawal of medicated feed depleted to 0.1 ppm within nine days. The rate of depletion was similar for all tissues. It was observed that storage of tissue samples at freezer temperature (-20 degrees C) for 30 days further reduces sulfamethazine levels by 3 to 20% of their original value.     

A PBPK model for midazolam in four avian species

A physiologically based pharmacokinetic (PBPK) model was developed for midazolam in the chicken and extended to three other species. Physiological parameters included organ weights obtained from 10 birds of each species and blood flows obtained from the literature. Partition coefficients for midazolam in tissues vs. plasma were estimated from drug residue data obtained at slaughter. The avian models include separate compartments for venous plasma, liver, kidney, muscle, fat and all other tissues. An estimate of total body clearance from an earlier in vitro study was used as a starting value in the model, assuming almost complete removal of the parent compound by liver metabolism. The model was optimized for the chicken with plasma and tissue data from a pharmacokinetic study after intravenous midazolam (5 mg/kg) dose. To determine which parameters had the most influence on the goodness of fit, a sensitivity analysis was performed. The optimized chicken model was then modified for the turkey, pheasant and quail. The models were validated with midazolam plasma and tissue residue data in the turkey, pheasant and quail. The PBPK models in the turkey, pheasant and quail provided good predictions of the observed tissue residues in each species, in particular for liver and kidney.     

Evaluation of a Bacillus stearothermophilus tube test as a screening tool for anticoccidial residues in poultry

A Bacillus stearothermophilus var. calidolactis C953 tube test was evaluated for its ability in detecting the residue of selected anticoccidial drugs in poultry, specically sulfamethazine, furazolidone, and amprolium. Various concentrations of each drug were injected into chicken liver and kidney tissues and these tissues were tested to determine the drug detection limits for each drug. The detection limit was defined as the drug concentration at which 95% of the test results were interpreted as positive. The limits of detection in liver tissue were 0.35 µg/ml for furazolidone, 0.70 µg/ml for sulfamethazine and 7.80 µg/ml for amprolium. In kidney tissues, they were 0.30 µg/ml for furazolidone, 0.54 µg/ml for sulfamethazine, and 7.6 µg/ml for amprolium. It was concluded that this tube test could be used to screen for the residue of these three drugs in poultry.     

A physiologically based pharmacokinetic model for the prediction of the depletion of methyl‐3‐quinoxaline‐2‐carboxylic acid,the marker residue of olaquindox,in the edible tissues of pigs

To estimate the consumer exposure to olaquindox (OLA) residues in porcine edible tissues, a physiologically based pharmacokinetic (PBPK) model for methyl‐3‐quinoxaline‐2‐carboxylic acid (MQCA), the marker residue of OLA, was developed in pigs based on the assumptions of the flow‐limited distribution, hepatic metabolism, and renal excretion. The model included separate compartments corresponding to blood, muscle, liver, kidney, adipose, and an extra compartment representing the remaining carcass. Physiological parameters were determined from literatures. Plasma protein binding, partition coefficients, and renal clearance for MQCA were determined in in vitro and in vivo studies. The metabolic conversion of OLA to MQCA was assumed as a simple, one‐step process, and an apparent first‐order rate constant (k) was employed to describe this metabolic process. The PBPK model was optimized and validated with plasma and tissue data from literatures and our study. Sensitivity analysis and Monte Carlo simulation were also implemented to estimate the influence of model parameters on the goodness of fit. When compared with the observed data, the PBPK model underestimated the MQCA level in all compartments at the early time points, whereas gave excellent predictions of MQCA concentration in porcine edible tissues at later time points. The correlation coefficients between the predicted and observed values were over 0.88. The consistency between the model predictions and the real residues of OLA in pigs proved the good applicability of our model in food safety risk assessment.     

A physiologically based pharmacokinetic model for oxytetracycline residues in sheep

A physiologically based pharmacokinetic model (PBPK) for oxytetracycline (OTC) residues in sheep was developed using previously published data from a combined serum pharmacokinetic and tissue residue study [Craigmill et al. (2000) J. Vet. Pharmacol. Ther.23, 345]. Physiological parameters for organ weights and tissue blood flows were obtained from the literature. The tissue/serum partition coefficients for OTC were estimated from the serum and tissue residue data obtained at slaughter. The model was developed to include all of the tissues for which residue data were available (serum, kidney, liver, fat, muscle and injection site), and all of the remaining tissues were combined into a slowly perfused compartment with low permeability. Total body clearance of OTC calculated in the previous study was used as the starting value for clearance in the PBPK model, with the kidney being the only eliminating organ. The model was built using ACSL (Advanced Continuous Simulation Language) Graphic Modeler, and the model was fit to the serum and tissue data using the ACSL Math/Optimizer software (AEgis Technologies Group, Inc., Huntsville, AL, USA). A sensitivity analysis was also performed to determine which parameters had the greatest effect on the goodness of fit. Numerous strategies were tested to model the injection site, and a model providing a biexponential absorption of the drug from the injection bolus gave the best fit to the experimental data. The model was validated using the clearance parameters calculated from the traditional pharmacokinetic model for each individual animal in the PBPK model. This simple PBPK model well predicted OTC residues in sheep tissues after intramuscular dosing with a long-acting preparation and may find use for other species and other veterinary drugs.     

A physiologically based pharmacokinetic model for valnemulin in rats and extrapolation to pigs

A flow-limited, physiologically based pharmacokinetic (PBPK) model for predicting the plasma and tissue concentrations of valnemulin after a single oral administration to rats was developed, and then the data were extrapolated to pigs so as to predict withdrawal interval in edible tissues. Blood/tissue pharmacokinetic data and blood/tissue partition coefficients for valnemulin in rats and pigs were collected experimentally. Absorption, distribution and elimination of the drug were characterized by a set of mass-balance equations. Model simulations were achieved using a commercially available software program. The rat PBPK model better predicted plasma and tissue concentrations. The correlation coefficients of the predicted and experimentally determined values for plasma, liver, kidney, lung and muscle were 0.96, 0.94, 0.96, 0.91 and 0.91, respectively. The rat model parameters were extrapolated to pigs to estimate valnemulin residue withdrawal interval in edible tissues. Correlation (R(2) ) between predicted and observed liver, kidney and muscle were 0.95, 0.97 and 0.99, respectively. Based on liver tissue residue profiles, the pig model estimated a withdrawal interval of 10 h under a multiple oral dosing schedule (5.0 mg/kg, twice daily for 7.5 days). PBPK models, such as this one, provide evidence of the usefulness in interspecies PK data extrapolation over a range of dosing scenarios and can be used to predict withdrawal interval in pigs.     

Sulfamethazine blood/tissue correlation study in swine

Seventy market-weight hogs (90 to 113 kg) were used in a feeding study to determine the correlation of serum sulfamethazine concentrations with sulfamethazine concentrations in liver and muscle at time of slaughter. Test groups were fed medicated feeds prepared from commercial medicated premixes containing 110 g of sulfamethazine/metric ton for 30 days. Fifteen days before hogs were slaughtered, test groups were given maintenance feeds containing 1.1 to 13.9 g of sulfamethazine/metric ton and were fed these diets until slaughtered. Comparison of data from positive- and negative-control groups indicated that total withdrawal of sulfamethazine in the feed was not necessary for the liver to contain less than the allowed tolerance of 0.1 mg of sulfamethazine/kg of liver at slaughter. Feed concentrations of up to 2 g of sulfamethazine/metric ton could be tolerated in withdrawal feeds before liver sulfamethazine values exceeded 0.1 mg/kg of liver. Serum/tissue sulfamethazine ratios were erratic in hogs given 1.1 to 2.7 g of sulfamethazine/metric ton, but became less variable in hogs given greater than 5.7 g/metric ton. Feed concentrations greater than 8 g of sulfamethazine/metric ton produced values greater than 0.1 mg/kg of muscle and values of about 0.4 mg/kg of liver. When serum sulfamethazine concentrations alone were used as a predictor for tissue sulfamethazine values, 100% of the liver values exceeded 0.10 mg/kg of liver when sulfamethazine in serum was greater than 0.45 mg/L. However, 57.4% of samples having serum concentrations between 0.10 and 0.45 mg/L had associated sulfamethazine values greater than 0.1 mg/kg of liver. All hogs having serum sulfamethazine concentrations less than 0.1 mg/L had sulfamethazine concentrations less than 0.1 mg/kg of liver.     

Total radioactive residues and clenbuterol residues in swine after dietary administration of [14C]clenbuterol for seven days and preslaughter withdrawal periods of zero, three, or seven days

Nine barrows (23.8 +/- 0.9 kg) and 9 gilts (23.1 +/- 0.9 kg) were used to determine the disposition of radiocarbon after oral [14C]clenbuterol (4-amino-alpha-[t-butylaminomethyl]-3,5-dichlorobenzyl [7-(14)C]alcohol hydrochloride) administration and to determine total and parent residues in edible tissues. Three barrows and three gilts, housed in metabolism crates, were fed 1 ppm [14C]clenbuterol HCl for seven consecutive days in three separate trials; a single barrow and gilt from each trial was slaughtered after 0-, 3-, or 7-d preslaughter withdrawal periods. Urine and feces were collected during the dosing and the withdrawal period; edible and inedible tissues were collected at slaughter. Total recovery of radiocarbon was 94.2 +/- 6.5%. Total clenbuterol absorption was greater than 75% for barrows and 60% for gilts. Total radioactive residues in tissues were not different (P > 0.05) between barrows and gilts. Concentrations of parent clenbuterol in liver, kidney, skeletal muscle, adipose tissue, and lung did not differ between barrows and gilts (P > 0.05). Total radioactive and parent residues declined in tissues as withdrawal period increased. After the 0-d withdrawal period, total liver residues (286 ppb) were approximately equal to lung residues, twice those of the kidney, and about 15 times those of adipose tissue and skeletal muscle. After a 7-d withdrawal period, total radioactive residues in liver (15 ppb) were roughly three times greater than lung, kidney, and adipose tissue total residues and about 13 times those of skeletal muscle total residues. Parent clenbuterol represented 79, 63, 42, 67, and 100% of the total radioactive residue in adipose tissue, kidney, liver, lung, and skeletal muscle, respectively, in hogs slaughtered with a 0-d withdrawal period. With increasing withdrawal period, the percentage of total radioactive residue present as parent clenbuterol within edible tissues (including lung) decreased, so that after a 7-d withdrawal period, 7, 16, and 29% of the total residue was composed of parent clenbuterol in kidney, liver, and lung, respectively. After a 7-d withdrawal period, parent clenbuterol exceeded the European maximum residue limit (0.5 ppb) 4.6-fold in liver and 2.4-fold in lung. In muscle, clenbuterol was approximately 40 times the limit after a 0-d withdrawal period but had dropped below 0.5 ppb after a 3-d withdrawal period. Results from this study indicate that clenbuterol HCl is well absorbed in swine and that the use of clenbuterol in this species in an off-label manner is inconsistent with human food safety standards used in developed countries.     

Sulfamethazine residues in swine: comparison of on-farm monitoring methods

Three sulfamethazine-residue detection methods were used to evaluate samples collected from five swine farms over a 12-month period. All cooperating farms included sulfamethazine in swine diets at various stages of production, for growth promotion or disease control, and followed recommended drug withdrawal periods. Swine finishing ration, swine urine, and swine serum from market-weight animals were tested monthly for the presence of sulfamethazine. Thin-layer chromatograph (TLC) analysis of swine urine was the gold standard by which three other test method-sample combinations were compared. Samples were analyzed for sulfamethazine using TLC (feed), competitive enzyme immunoassay (serum), and agar-diffusion swab test (urine). The relative sensitivities and specificities of sulfamethazine-residue detection for the three combinations were: (1) TLC analysis (27%, 94%); (2) competitive enzyme immunoassay analysis (58%, 59%); (3) agar-diffusion swab test (78%, 12%). None of the three methods tested was individually adequate for on-farm monitoring of sulfonamide residues. Sulfamethazine residues in swine urine were found on 43.3% of the monthly farm visits and in 19.7% of all swine tested.     

磺胺二甲嘧啶在麻黄鸡组织中残留消除规律研究

用添加临床剂量100 mg/kg磺胺二甲嘧啶的饲料喂养40日龄麻黄鸡,连续用药3 d,分别在停药0、12、24、48、72、96、120、144、192、240、360、480、600 h时间点对肌肉、肝和肾组织采样,检测磺胺二甲嘧啶残留浓度。停药25 d后,肌肉组织残留量为0.054 mg/kg,肝和肾组织未检出磺胺二甲嘧啶,结果表明磺胺二甲嘧啶在麻黄鸡组织中停药期为25 d,《中国兽药典》2005年版中规定磺胺二甲嘧啶10 d的停药期是不尽合理的。     

Stimulation of lipogenesis by insulin in swine adipose tissue: antagonism by porcine growth hormone

The effects of physiological (1, 10 ng/ml) and pharmacological (1,000 ng/ml) concentrations of insulin (INS) and porcine growth hormone (pGH) on lipid metabolism were determined in short-term (2 h) and long-term (26, 50 h) incubations of swine adipose tissue. The short-term effects of three different commercial sources of bovine serum albumin (BSA) on adipose tissue metabolism were also evaluated. Two of the three BSA preparations were found to be unsuitable for inclusion in the short-term incubation buffer because they caused a stimulation of lipid synthesis in adipose tissue and masked the stimulatory effects of insulin. Physiological concentrations of insulin stimulated glucose metabolism in 2-h incubations by 100% in adipose tissue from 80-kg swine. After a 26-h incubation period, INS maintained rates of glucose metabolism at levels comparable to maximally stimulated rates in fresh tissue. Insulin also enhanced glucose metabolism following 50-h incubations; however, rates were less than for 2- or 26-h incubations. Glucose metabolism was also stimulated in adipose tissue from 127-kg swine when incubated for 2 h with INS; however, INS responsiveness declined with increasing body weight. Lipogenesis and glucose oxidation were partially maintained by INS using tissue from the heavier swine. A pharmacological but not physiological concentration of pGH stimulated glucose metabolism in short-term incubations by 50% in adipose tissue from 80-kg swine, and by 10% in adipose tissue from 127-kg swine. Long-term culture of adipose tissue in the presence of pGH had no effect on glucose metabolism. Physiological levels of pGH directly antagonized the stimulation of glucose metabolism by INS in short- and long-term incubations. In summary, these results are the first to establish that swine adipose tissue is quite sensitive to insulin and that pGH directly antagonizes insulin action.     

Enzymes of glucose and fatty acid metabolism of liver, kidney, skeletal muscle, adipose tissue and mammary gland of lactating and non-lactating sheep

The effect of peak lactation on the activities of a number of enzymes of glucose and lipid metabolism of perirenal and subcutaneous adipose tissue, skeletal muscle, liver, kidney cortex and mammary parenchyma of sheep are described. Enzymes studied included hexokinase (glucose utilization), pyruvate carboxylase (gluconeogenesis), pyruvate dehydrogenase (glucose oxidation and production of acetyl CoA for fatty acid synthesis), acetyl CoA carboxylase (fatty acid synthesis) and glycerol-3-phosphate acyltransferase (fatty acid esterification). Major changes that were found include a decrease in activities of enzymes of fatty acid synthesis and esterification in adipose tissues, decreased activity of pyruvate dehydrogenase in muscle and adipose tissues and increased pyruvate carboxylase; there was no change in activities of enzyme of fatty acid esterification in liver. Activities of hexokinase, acetyl CoA carboxylase and glycerol-3-phosphate acyltransferase have been estimated per tissue; this shows the quantitative importance of limiting glucose utilization by muscle and of suppression of fatty acid synthesis in adipose tissue for efficient partitioning of nutrients for milk production.     

Pharmacokinetics of sulfamethazine in male, female and castrated male swine

The concentration of sulfamethazine in plasma and sulfamethazine and its metabolites in urine were compared in male, female and castrated male swine. A surgical technique for placement of catheters in the urinary bladder was used to facilitate the collection of urine in males and castrated males. The elimination rate of sulfamethazine from plasma and the excretion of parent drug and metabolites into urine did not differ significantly among females, males and castrated male swine.