Tulathromycin assay validation and tissue residues after single and multiple subcutaneous injections in domestic goats (Capra aegagrus hircus)

Clothier, K. A., Leavens, T., Griffith, R. W., Wetzlich, S. E., Baynes, R. E., Riviere, J. E., Tell, L. A. Tulathromycin assay validation and tissue residues after single and multiple subcutaneous injections in domestic goats (Capra aegagrus hircus). J. vet. Pharmacol. Therap.  35 , 113–120. Tulathromycin is a macrolide antimicrobial labeled for treatment of bacterial pneumonia in cattle and swine. The purpose of the present research was to evaluate tissue concentrations of tulathromycin in the caprine species. A tandem mass spectrometry regulatory analytical method that detects the common fragment of tulathromycin in cattle and swine was validated with goat tissues. The method was used to study tulathromycin depletion in goat tissues (liver, kidney, muscle, fat, injection site, and lung) over time. In two different studies, six juvenile and 25 market‐age goats received a single injection of 2.5 mg/kg of tulathromycin subcutaneously; in a third study, 18 juvenile goats were treated with 2.5, 7.5, or 12.5 mg/kg tulathromycin weekly with three subcutaneous injections. Mean tulathromycin tissue concentrations were highest at injection site samples in all studies and all doses. Lung tissue concentrations were greatest at day 5 in market‐age goats while in the multi‐dose animals concentrations demonstrated dose‐dependent increases. Concentrations were below limit of quantification in injection site and lung by day 18 and in liver, kidney, muscle, and fat at all time points. This study demonstrated that tissue levels in goats are very similar to those seen in swine and cattle.     

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.     

Pharmacokinetics of tulathromycin following subcutaneous administration in meat goats

Tulathromycin is a triamilide antibiotic that maintains therapeutic concentrations for an extended period of time. The drug is approved for the treatment of respiratory disease in cattle and swine and is occasionally used in goats. To investigate the pharmacokinetics of tulathromycin in meat goats, 10 healthy Boer goats were administered a single 2.5 mg/kg subcutaneous dose of tulathromycin. Plasma concentrations were measured by ultra-high pressure liquid chromatography tandem mass spectrometry (UPLC–MS/MS) detection. Plasma maximal drug concentration (C_max) was 633 ± 300 ng/ml (0.40 ± 0.26 h post-subcutaneous injection). The half-life of tulathromycin in goats was 110 ± 19.9 h. Tulathromycin was rapidly absorbed and distributed widely after subcutaneous injection 33 ± 6 L/kg. The mean AUC of the group was 12,500 ± 2020 h ng/mL for plasma. In this study, it was determined that the pharmacokinetics of tulathromycin after a single 2.5 mg/kg SC injection in goats were very similar to what has been previously reported in cattle.     

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.     

Pharmacokinetics of tulathromycin in nonpregnant adult ewes

The objectives of this study were to determine plasma concentrations and pharmacokinetic parameters of tulathromycin after a single subcutaneous administration in the cervical region in sheep using the cattle labeled dose of 2.5 mg/kg. Six adult healthy ewes were administered tulathromycin on day 0. Blood samples were collected just prior to dosing and at selected time points for 360 h. Plasma samples were analyzed to determine tulathromycin concentrations, and noncompartmental analysis was performed for pharmacokinetic parameters. The mean maximum plasma concentration was 3598 ng/mL, the mean time to maximum concentration was 1.6 h, and the apparent elimination half‐life ranged from 68.1 to 233.1 h (mean 118 h). When comparing our results to goats and cattle, it appears sheep are more similar to cattle in regard to the concentrations observed and pharmacokinetic parameters. In summary, the pharmacokinetics of tulathromycin in sheep appear to be similar enough to those in goats and cattle to recommend similar dosing (2.5 mg/kg SC), assuming that the target pathogens have similar inhibitory concentrations.     

Effects of experimentally induced respiratory disease on the pharmacokinetics and tissue residues of tulathromycin in meat goats

Tulathromycin is a macrolide antibiotic commonly used for the treatment of respiratory disease in food animal species including goats. Recent research in pigs has suggested that the presence of disease could alter the pharmacokinetics of tulathromycin in animals with respiratory disease. The objectives of this study were (a) compare the plasma pharmacokinetics of tulathromycin in healthy goats as well as goats with an induced respiratory disease; and (b) to compare the tissue residue concentrations of tulathromycin marker in both groups. For this trial, disease was induced with Pasteurella multocida. Following disease induction, tulathromycin was administered. Samples of plasma were collected at various time points up to 312 hr posttreatment, when study animals were euthanized and tissue samples were collected. For PK parameters in plasma, V_z (control: 28.7 ± 11.9 ml/kg; experimental: 57.8 ± 26.6 ml/kg) was significantly higher (p = 0.0454) in the experimental group than the control group, and nonsignificant differences were noted in other parameters. Among time points significantly lower plasma concentrations were noted in the experimental group at 168 hr (p = 0.023), 216 hr (p = 0.036), 264 hr (p = 0.0017), 288 hr (p = 0.0433), and 312 hr (p = 0.0486). None of the goats had tissue residues above the US bovine limit of 5 µg/g at the end of the study. No differences were observed between muscle, liver, or fat concentrations. A significantly lower concentration (p = 0.0095) was noted in the kidneys of experimental goats when compared to the control group. These results suggest that the effect of respiratory disease on the pharmacokinetics and tissue residues appear minimal after experimental P. multocida infection, however as evidenced by the disparity in C_max, significant differences in plasma concentrations at terminal time points, as well as the differences in kidney concentrations, there is the potential for alterations in diseased versus clinical animals.     

Physiological parameter values for physiologically based pharmacokinetic models in food-producing animals. Part I: Cattle and swine

Physiologically based pharmacokinetic (PBPK) models for chemicals in food animals are a useful tool in estimating chemical tissue residues and withdrawal intervals. Physiological parameters such as organ weights and blood flows are an important component of a PBPK model. The objective of this study was to compile PBPK-related physiological parameter data in food animals, including cattle and swine. Comprehensive literature searches were performed in PubMed, Google Scholar, ScienceDirect, and ProQuest. Relevant literature was reviewed and tables of relevant parameters such as relative organ weights (% of body weight) and relative blood flows (% of cardiac output) were compiled for different production classes of cattle and swine. The mean and standard deviation of each parameter were calculated to characterize their variability and uncertainty and to allow investigators to conduct population PBPK analysis via Monte Carlo simulations. Regression equations using weight or age were created for parameters having sufficient data. These compiled data provide a comprehensive physiological parameter database for developing PBPK models of chemicals in cattle and swine to support animal-derived food safety assessment. This work also provides a basis to compile data in other food animal species, including goats, sheep, chickens, and turkeys.     

Concentrations of trimethoprim and sulphadoxine in tissues from goats and a cow.

The concentration of trimethoprim and sulphadoxine in plasma and tissue from goats and a cow have been determined after a single intravenous injection. Furthermore, the concentration of the two drugs and their metabolites in plasma and tissues have been determined after continuous intravenous infusion for 2½–3 hrs. Trimethoprim was present in all tissues but brain at higher concentrations than in plasma while the concentration of sulphadoxine in the different tissues were lower than in plasma. The highest concentration of the 2 drugs and their metabolites was found in the kidney. The distribution pattern of trimethoprim and sulphadoxine was similar in cow and goats.     

Pharmacokinetics of tulathromycin in fetal sheep and pregnant ewes

Macrolides are important antimicrobials frequently used in human and veterinary medicine in the treatment of pregnant women and pregnant livestock. They may be useful for the control of infectious ovine abortion, which has economic, animal health, and human health impacts. In this study, catheters were surgically placed in the fetal vasculature and amnion of pregnant ewes at 115 (±2) days of gestation. Ewes were given a single dose of 2.5 mg/kg tulathromycin subcutaneously, and drug concentrations were determined in fetal plasma, maternal plasma, and amniotic fluid at 4, 8, 12, 24, 36, 48, 72, 144, and 288 hr after drug administration. Pharmacokinetic parameters in maternal plasma were estimated using noncompartmental analysis and were similar to those previously reported in nonpregnant ewes. Tulathromycin was present in fetal plasma and amniotic fluid, indicating therapeutic potential for use against organisms in these compartments, though concentrations were lower than those in maternal plasma. Time‐course of drug concentrations in the fetus was quite different than that in the ewe, with plasma concentrations reaching a plateau at 4 hr and remaining at this concentration for the remainder of the sampling period (288 hr), raising questions about how tulathromycin may be transported into or metabolized and eliminated by the fetus.     

Pharmacokinetic profile of cefotaxime in goats

Cefotaxime was administered to goats intravenously, intramuscularly and subcutaneously to determine blood and urine concentration, kinetic behaviour and bioavailability. Following a single intravenous injection, the blood concentration-time curve indicated a two compartment open model, with an elimination half-life value (t1/2 beta) of 22.38 +/- 0.41 minutes. Both intramuscular and subcutaneous routes showed slower values, that is, 38.64 and 69.58 minutes. The apparent volume of distribution of cefotaxime in goats was less than 1 litre kg-1 and suggested a lower distribution in tissues than in blood. After intramuscular and subcutaneous injections peak plasma cefotaxime concentrations were 77.8 +/- 1.7 and 44.0 +/- 0.8 micrograms ml-1 at 29.6 and 40.4 minutes, respectively. The average bioavailability of cefotaxime given by intramuscular and subcutaneous injection was 1.08 and 1.25 times the intravenous availability, respectively. The cefotaxime concentration remained in urine 24 hours longer after subcutaneous injection than after intramuscular administration.     

Development of a multiroute physiologically based pharmacokinetic model for orbifloxacin in rabbits

To predict the orbifloxacin concentrations in rabbits after multiple routes of administration, a flow‐limited multiroute physiologically based pharmacokinetic (PBPK) model was developed. Three routes of administration (IV, IM, and PO) were incorporated into this model. Physiological parameters including tissue weights and blood flows through different tissues were obtained from the literature. The tissue/plasma partition coefficients (P_Xs) for noneliminating tissues were calculated according to the area method, while the P_Xs for kidney and the rest of the body compartment, together with other parameters for absorption and elimination, were optimized based on the published concentrations. The comparisons between predicted and observed orbifloxacin concentrations proved its validity, and the present model predicted available concentration data well, including those in liver, kidney, muscle, lung, heart, and plasma after oral, intravenous, or intramuscular administration. A local sensitivity analysis was also performed, which showed that the parameters for oral absorption were most influential on the orbifloxacin concentrations. This model was used to predict plasma and tissue concentrations after multiple oral or intramuscular administration. This study demonstrated the feasibility of predicting drug residues in minor species after multiple routes of administration in the extra‐label manner using the PBPK modeling.     

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.     

Standard PK/PD concepts can be applied to determine a dosage regimen for a macrolide: the case of tulathromycin in the calf

The pharmacokinetic (PK) profile of tulathromycin, administered to calves subcutaneously at the dosage of 2.5 mg/kg, was established in serum, inflamed (exudate), and noninflamed (transudate) fluids in a tissue cage model. The PK profile of tulathromycin was also established in pneumonic calves. For Mannheimia haemolytica and Pasteurella multocida, tulathromycin minimum inhibitory concentrations (MIC) were approximately 50 times lower in calf serum than in Mueller–Hinton broth. The breakpoint value of the PK/pharmacodynamic (PD) index (AUC_(0–24 h)/MIC) to achieve a bactericidal effect was estimated from in vitro time‐kill studies to be approximately 24 h for M. haemolytica and P. multocida. A population model was developed from healthy and pneumonic calves and, using Monte Carlo simulations, PK/PD cutoffs required for the development of antimicrobial susceptibility testing (AST) were determined. The population distributions of tulathromycin doses were established by Monte Carlo computation (MCC). The computation predicted a target attainment rate (TAR) for a tulathromycin dosage of 2.5 mg/kg of 66% for M. haemolytica and 87% for P. multocida. The findings indicate that free tulathromycin concentrations in serum suffice to explain the efficacy of single‐dose tulathromycin in clinical use, and that a dosage regimen can be computed for tulathromycin using classical PK/PD concepts.     

Use of population pharmacokinetic modeling and Monte Carlo simulation to capture individual animal variability in the prediction of flunixin withdrawal times in cattle

The objective of this study was to develop a population pharmacokinetic (PK) model and predict tissue residues and the withdrawal interval (WDI) of flunixin in cattle. Data were pooled from published PK studies in which flunixin was administered through various dosage regimens to diverse populations of cattle. A set of liver data used to establish the regulatory label withdrawal time (WDT) also were used in this study. Compartmental models with first‐order absorption and elimination were fitted to plasma and liver concentrations by a population PK modeling approach. Monte Carlo simulations were performed with the population mean and variabilities of PK parameters to predict liver concentrations of flunixin. The PK of flunixin was described best by a 3‐compartment model with an extra liver compartment. The WDI estimated in this study with liver data only was the same as the label WDT. However, a longer WDI was estimated when both plasma and liver data were included in the population PK model. This study questions the use of small groups of healthy animals to determine WDTs for drugs intended for administration to large diverse populations. This may warrant a reevaluation of the current procedure for establishing WDT to prevent violative residues of flunixin.     

Mathematical modeling and simulation in animal health – Part II: principles,methods, applications,and value of physiologically based pharmacokinetic modeling in veterinary medicine and food safety assessment

This review provides a tutorial for individuals interested in quantitative veterinary pharmacology and toxicology and offers a basis for establishing guidelines for physiologically based pharmacokinetic (PBPK) model development and application in veterinary medicine. This is important as the application of PBPK modeling in veterinary medicine has evolved over the past two decades. PBPK models can be used to predict drug tissue residues and withdrawal times in food‐producing animals, to estimate chemical concentrations at the site of action and target organ toxicity to aid risk assessment of environmental contaminants and/or drugs in both domestic animals and wildlife, as well as to help design therapeutic regimens for veterinary drugs. This review provides a comprehensive summary of PBPK modeling principles, model development methodology, and the current applications in veterinary medicine, with a focus on predictions of drug tissue residues and withdrawal times in food‐producing animals. The advantages and disadvantages of PBPK modeling compared to other pharmacokinetic modeling approaches (i.e., classical compartmental/noncompartmental modeling, nonlinear mixed‐effects modeling, and interspecies allometric scaling) are further presented. The review finally discusses contemporary challenges and our perspectives on model documentation, evaluation criteria, quality improvement, and offers solutions to increase model acceptance and applications in veterinary pharmacology and toxicology.     

磺胺二甲嘧啶生理药动学模型种间类推的应用

本研究应用磺胺二甲嘧啶在猪的生理药动学模型来预测其在绵羊体内的动力学过程。利用文献中已经建立的猪的生理模型,采用动物种间类推原理,将模型外推至绵羊,模拟磺胺二甲嘧啶在绵羊体内的血药和组织浓度,并估算可食性组织残留休药期。结果模拟休药期和文献休药期基本趋于一致,表明生理药动学模型种间类推是可行的。     

Influence of the route of administration on efficacy and tissue distribution of ivermectin in goat

The tissue concentration and efficacy of ivermectin after per os and subcutaneous administration were compared in goats experimentally infected with Trichostrongylus colubriformis (ivermectin-susceptible strain, INRA). Infected goats (n = 24) were treated per os (n = 9) or subcutaneously (n = 9) with ivermectin, 0.2 mg/kg, or kept as not treated controls. The faecal egg counts and small intestine worm counts were determined. Ivermectin concentration was measured in the plasma, gastrointestinal tract, lung, skin or hair, liver and adipose tissues at 0, 2, 7 and 17 days post-treatment. The efficacy of ivermectin against T. colubriformis infection in goat was 98.7 and 99.9% for subcutaneous and oral administration, respectively. Ivermectin concentration declined with time and only residual concentration was measured at 17 days post-treatment in plasma and gastrointestinal tract. Ivermectin concentration was higher after subcutaneous compared to per os injection in most of the tissue examined. In skin, hair and subcutaneous adipose tissue ivermectin persisted at significant concentrations 17 days post-treatment for both routes of administration. In our experimental conditions, ivermectin provides similar efficacy against T. colubriformis after subcutaneous or per os administration in goat. However, the lower ivermectin levels in tissues after per os administration suggest that the lasting of efficacy may be shortened after per os compared to subcutaneous administration especially in animals with poor body condition in pasture where re-infection occurs quickly after anthelmintic treatment.     

Kinetics and intrapulmonary disposition of tilmicosin after single and repeated oral bolus administrations to rabbits

Tilmicosin (TIM, Pulmotil®) was administered to eight rabbits by oral gavage at a dose of 12.5 mg/kg body weight for 2, 5, and 7 days, and its plasma kinetics and intrapulmonary disposition were investigated. TIM concentrations in plasma samples collected after days 1 and 6 of treatment were measured by high-performance liquid chromatography with ultraviolet detection. The pharmacokinetic parameters, obtained by non-compartmental analysis of TIM plasma concentrations, did not show any significant variations between days 1 and 6. From the second day of treatment, TIM concentrations attained in lung tissue and pulmonary alveolar macrophages (PAM) exceeded those in plasma by 7- and 400-fold, respectively, and high levels were maintained in lung tissues during the entire treatment duration. After the first day of withdrawal, a fast decline in TIM levels in both plasma and lung tissue was observed, but in PAM, much higher concentrations were maintained after 3 days of TIM withdrawal.     

Development of a physiologic-based pharmacokinetic model for estimating sulfamethazine concentrations in swine and application to prediction of violative residues in edible tissues

OBJECTIVE: To develop a flow-limited, physiologic-based pharmacokinetic model for use in estimating concentrations of sulfamethazine after IV administration to swine. SAMPLE POPULATION: 4 published studies provided physiologic values for organ weights, blood flows, clearance, and tissue-to-blood partition coefficients, and 3 published studies provided data on plasma and other tissue compartments for model validation. PROCEDURE: For the parent compound, the model included compartments for blood, adipose, muscle, liver, and kidney tissue with an extra compartment representing the remaining carcass. Compartments for the N-acetyl metabolite included the liver and the remaining body. The model was created and optimized by use of computer software. Sensitivity analysis was completed to evaluate the importance of each constant on the whole model. The model was validated and used to estimate a withhold interval after an IV injection at a dose of 50 mg/kg. The withhold interval was compared to the interval estimated by the Food Animal Residue Avoidance Databank (FARAD). RESULTS: Specific tissue correlations for plasma, adipose, muscle, kidney, and liver tissue compartments were 0.93, 0.86, 0.99, 0.94, and 0.98, respectively. The model typically overpredicted concentrations at early time points but had excellent accuracy at later time points. The withhold interval estimated by use of the model was 120 hours, compared with 100 hours estimated by FARAD. CONCLUSIONS AND CLINICAL RELEVANCE: Use of this model enabled accurate prediction of sulfamethazine pharmacokinetics in swine and has applications for food safety and prediction of drug residues in edible tissues.