Population pharmacokinetics in veterinary medicine: potential use for therapeutic drug monitoring and prediction of tissue residues

Phenylbutazone pharmacokinetics in southern white rhinoceros (Ceratotherium simum simum) after oral administration

Southern white rhinoceros (Ceratotherium simum simum) frequently develop painful conditions, such as traumatic injuries or osteoarthritis, necessitating the administration of pain-relieving medications. One of the preferred treatments is the nonsteroidal anti-inflammatory drug phenylbutazone because of the availability of oral formulations and the familiarity of its use in horses. For the main study, a single oral dose of phenylbutazone at 2 mg/kg was administered to healthy adult rhinoceros (n = 33) housed at six North American zoological institutions. Each rhinoceros had up to four blood samples collected under voluntary behavioural restraint at up to four predetermined time points (0, 1, 1.5, 2, 3, 4, 6, 8, 10, 24, 30 and 48 h). Drug analysis was performed by high-performance liquid chromatography. The population pharmacokinetic parameters were calculated with nonlinear mixed-effects modelling, and analysis showed a peak concentration (C_MAX ) of 3.8 µg/ml at 1.8 h and an elimination half-life of 9 h. The concentrations achieved were similar to what has been reported for horses and were within the half maximal effective concentration for horses for at least 10 h. A multi-dose trial in five rhinoceros receiving 2 mg/kg orally once daily for five days found mild accumulation at a predicted factor of 1.2. This study represents the first pharmacokinetic data of phenylbutazone in any rhinoceros species.

A history of antimicrobial drugs in animals: Evolution and revolution

The evolutionary process of antimicrobial drug (AMD) uses in animals over a mere eight decades (1940-2020) has led to a revolutionary outcome, and both evolution and revolution are ongoing, with reports on a range of uses, misuses and abuses escalating logarithmically. As well as veterinary therapeutic perspectives (efficacy, safety, host toxicity, residues, selection of drug, determination of dose and measurement of outcome in treating animal diseases), there are also broader, nontherapeutic uses, some of which have been abandoned, whilst others hopefully will soon be discontinued, at least in more developed countries. Although AMD uses for treatment of animal diseases will continue, it must: (a) be sustainable within the One Health paradigm; and (b) devolve into more prudent, rationally based therapeutic uses. As this review on AMDs is published in a Journal of Pharmacology and Therapeutics, its scope has been made broader than most recent reviews in this field. Many reviews have focused on negative aspects of AMD actions and uses, especially on the question of antimicrobial resistance. This review recognizes these concerns but also emphasizes the many positive aspects deriving from the use of AMDs, including the major research-based advances underlying both the prudent and rational use of AMDs. It is structured in seven sections: (1) Introduction; (2) Sulfonamide history; (3) Nontherapeutic and empirical uses of AMDs (roles of agronomists and veterinarians); (4) Rational uses of AMDs (roles of pharmacologists, clinicians, industry and regulatory controls); (5) Prudent use (residue monitoring, antimicrobial resistance); (6) International and inter-disciplinary actions; and (7) Conclusions.

Integration of Food Animal Residue Avoidance Databank (FARAD) empirical methods for drug withdrawal interval determination with a mechanistic population-based interactive physiologically based pharmacokinetic (iPBPK) modeling platform: example for flunixin meglumine administration

Violative chemical residues in animal-derived food products affect food safety globally and have impact on the trade of international agricultural products. The Food Animal Residue Avoidance Databank program has been developing scientific tools to provide appropriate withdrawal interval (WDI) estimations after extralabel drug use in food animals for the past three decades. One of the tools is physiologically based pharmacokinetic (PBPK) modeling, which is a mechanistic-based approach that can be used to predict tissue residues and WDIs. However, PBPK models are complicated and difficult to use by non-modelers. Therefore, a user-friendly PBPK modeling framework is needed to move this field forward. Flunixin was one of the top five violative drug residues identified in the United States from 2010 to 2016. The objective of this study was to establish a web-based user-friendly framework for the development of new PBPK models for drugs administered to food animals. Specifically, a new PBPK model for both cattle and swine after administration of flunixin meglumine was developed. Population analysis using Monte Carlo simulations was incorporated into the model to predict WDIs following extralabel administration of flunixin meglumine. The population PBPK model was converted to a web-based interactive PBPK (iPBPK) framework to facilitate its application. This iPBPK framework serves as a proof-of-concept for further improvements in the future and it can be applied to develop new models for other drugs in other food animal species, thereby facilitating the application of PBPK modeling in WDI estimation and food safety assessment.

Mathematical modeling and simulation in animal health. Part III: Using nonlinear mixed-effects to characterize and quantify variability in drug pharmacokinetics

A common feature of human and veterinary pharmacokinetics is the importance of identifying and quantifying the key determinants of between-patient variability in drug disposition and effects. Some of these attributes are already well known to the field of human pharmacology such as bodyweight, age, or sex, while others are more specific to veterinary medicine, such as species, breed, and social behavior. Identification of these attributes has the potential to allow a better and more tailored use of therapeutic drugs both in companion and food-producing animals. Nonlinear mixed effects (NLME) have been purposely designed to characterize the sources of variability in drug disposition and response. The NLME approach can be used to explore the impact of population-associated variables on the relationship between drug administration, systemic exposure, and the levels of drug residues in tissues. The latter, while different from the method used by the US Food and Drug Administration for setting official withdrawal times (WT) can also be beneficial for estimating WT of approved animal drug products when used in an extralabel manner. Finally, NLME can also prove useful to optimize dosing schedules, or to analyze sparse data collected in situations where intensive blood collection is technically challenging, as in small animal species presenting limited blood volume such as poultry and fish.

Models of antimicrobial pressure on intestinal bacteria of the treated host populations

Antimicrobial drugs are used to treat pathogenic bacterial infections in animals and humans. The by-stander enteric bacteria of the treated host's intestine can become exposed to the drug or its metabolites reaching the intestine in antimicrobially active form. We consider which processes and variables need to be accounted for to project the antimicrobial concentrations in the host's intestine. Those include: the drug's fraction (inclusive of any active metabolites) excreted in bile; the drug's fractions and intestinal segments of excretion via other mechanisms; the rates and intestinal segments of the drug's absorption and re-absorption; the rates and intestinal segments of the drug's abiotic and biotic degradation in the intestine; the digesta passage time through the intestinal segments; the rates, mechanisms, and reversibility of the drug's sorption to the digesta and enteric microbiome; and the volume of luminal contents in the intestinal segments. For certain antimicrobials, the antimicrobial activity can further depend on the aeration and chemical conditions in the intestine. Model forms that incorporate the inter-individual variation in those relevant variables can support projections of the intestinal antimicrobial concentrations in populations of treated host, such as food animals. To illustrate the proposed modeling framework, we develop two examples of treatments of bovine respiratory disease in beef steers by oral chlortetracycline and injectable third-generation cephalosporin ceftiofur. The host's diet influences the digesta passage time, volume, and digesta and microbiome composition, and may influence the antimicrobial loss due to degradation and sorption in the intestine. We consider two diet compositions in the illustrative simulations. The examples highlight the extent of current ignorance and need for empirical data on the variables influencing the selective pressures imposed by antimicrobial treatments on the host's intestinal bacteria.

The present and future of withdrawal period calculations for milk in the European Union: focus on heterogeneous, nonmonotonic data

Harmonization of the method for calculating the withdrawal period for milk dates from the 1990s. European harmonization has led to guidance with three accepted methods for determining the withdrawal period for milk that are currently applicable. These three methods can be used by marketing authorization holders, but, in some cases, their diversity can lead to very different withdrawal periods. This is particularly the case when concentrations in milk are nonmonotonic and heterogeneous, meaning that concentrations strictly increase and then strictly decrease with significant interindividual variability in the time to reach the maximal concentration. Here, we first describe the concepts associated with the different methods used in the harmonized approach currently applicable for the determination of milk withdrawal periods, and then, we propose the application of a modern pharmacometric tool. Finally, with a nonmonotonic heterogeneous dataset, we illustrate the usefulness of this tool in comparison with the three currently applicable methods and discuss the limitations and advantages of each method.

Mathematical modeling and simulation in animal health. Part I: Moving beyond pharmacokinetics

The application of mathematical modeling to problems in animal health has a rich history in the form of pharmacokinetic modeling applied to problems in veterinary medicine. Advances in modeling and simulation beyond pharmacokinetics have the potential to streamline and speed-up drug research and development programs. To foster these goals, a series of manuscripts will be published with the following goals: (i) expand the application of modeling and simulation to issues in veterinary pharmacology; (ii) bridge the gap between the level of modeling and simulation practiced in human and veterinary pharmacology; (iii) explore how modeling and simulation concepts can be used to improve our understanding of common issues not readily addressed in human pharmacology (e.g. breed differences, tissue residue depletion, vast weight ranges among adults within a single species, interspecies differences, small animal species research where data collection is limited to sparse sampling, availability of different sampling matrices); and (iv) describe how quantitative pharmacology approaches could help understanding key pharmacokinetic and pharmacodynamic characteristics of a drug candidate, with the goal of providing explicit, reproducible, and predictive evidence for optimizing drug development plans, enabling critical decision making, and eventually bringing safe and effective medicines to patients. This study introduces these concepts and introduces new approaches to modeling and simulation as well as clearly articulate basic assumptions and good practices. The driving force behind these activities is to create predictive models that are based on solid physiological and pharmacological principles as well as adhering to the limitations that are fundamental to applying mathematical and statistical models to biological systems.

A population pharmacokinetic approach to describe cephalexin disposition in adult and aged dogs

This study was conducted in order to characterize the pharmacokinetics of orally administered cephalexin to healthy adult and aged dogs, using a population pharmacokinetic approach. Two hundred and eighty-six cephalexin plasma concentrations obtained from previous pharmacokinetic studies were used. Sex, age, pharmaceutical formulation, and breed were evaluated as covariates. A one-compartment model with an absorption lag-time (Tlag) best described the data. The final model included age (adult; aged) on apparent volume of distribution (Vd/F), apparent elimination rate (ke/F), and Tlag; sex (female; male) on ke/F, and breed (Beagle; mixed-breed) on Vd/F. Addition of the covariates to the model explained 78% of the interindividal variability (IIV) in Vd/F, 36% in ke/F, and 24% in Tlag, respectively. Formulation did not affect the variability of any of the pharmacokinetic parameters. Tlag was longer, whereas Vd/F and ke/F were lower in aged compared to adult animals; in female aged dogs ke/F was lower than in male aged dogs; however, the differences were of low magnitude. Different disposition of cephalexin may be expected in aged dogs.

Interspecies mixed-effect pharmacokinetic modeling of penicillin G in cattle and swine

Extralabel drug use of penicillin G in food-producing animals may cause an excess of residues in tissue which will have the potential to damage human health. Of all the antibiotics, penicillin G may have the greatest potential for producing allergic responses to the consumer of food animal products. There are, however, no population pharmacokinetic studies of penicillin G for food animals. The objective of this study was to develop a population pharmacokinetic model to describe the time-concentration data profile of penicillin G across two species. Data were collected from previously published pharmacokinetic studies in which several formulations of penicillin G were administered to diverse populations of cattle and swine. Liver, kidney, and muscle residue data were also used in this study. Compartmental models with first-order absorption and elimination were fit to plasma and tissue concentrations using a nonlinear mixed-effect modeling approach. A 3-compartment model with extra tissue compartments was selected to describe the pharmacokinetics of penicillin G. Typical population parameter estimates (interindividual variability) were central volumes of distribution of 3.45 liters (12%) and 3.05 liters (8.8%) and central clearance of 105 liters/h (32%) and 16.9 liters/h (14%) for cattle and swine, respectively, with peripheral clearance of 24.8 liters/h (13%) and 9.65 liters/h (23%) for cattle and 13.7 liters/h (85%) and 0.52 liters/h (40%) for swine. Body weight and age were the covariates in the final pharmacokinetic models. This study established a robust model of penicillin for a large and diverse population of food-producing animals which could be applied to other antibiotics and species in future analyses.

Tetracycline residues in porcine stomach after administration via drinking water on a swine farm

Tetracycline is a broad-spectrum antibiotic used to treat infections in swine. The maximum residue levels of tetracycline in pork stomach tissue in Russia, Europe, and the United States are 10, 200, and 2,000 ppb, respectively. This difference in accepted safety levels may be the reason why stomach tissues that the United States exports continue to be residue violators in overseas markets. In this study, 30 pigs at two different stages of production (weanling and finisher) were treated with tetracycline at 22 mg/kg of body weight per day for a total of 5 days via a water medicator. Blood samples were collected at 0, 72, 78, 96, and 102 h after the start of medication. The medication was stopped at 120 h, and blood samples were again collected at 126, 144, 168, 192, and 216 h after exposure. Five animals were slaughtered for stomach tissue 0, 24, 48, 96, and 192 h after the drug was flushed from the water line. All blood and tissue samples were analyzed by high-performance liquid chromatography-UV methods. The tetracycline levels in plasma were below the level of detection after the U.S.-labeled withdrawal time of 4 days. The stomach tissue residues averaged 671.72, 330.31, 297.77, 136.36, and 268.08 ppb on withdrawal days 0, 1, 2, 4, and 8, respectively. Using the U.S. Food and Drug Administration tolerance limit method and a population-based pharmacokinetic model with Monte Carlo simulation, a withdrawal interval was estimated. This study demonstrated that tetracycline residues are still detectable in the stomach tissues after the established United States withdrawal time of 4 days. These residue levels may explain why stomach tissues tested in Russia and Europe show positive residues for tetracycline, even though the meat may pass inspection here in the United States prior to export.

Veterinary pharmacology: history, current status and future prospects

Veterinary therapeutics, based on the art of Materia Medica, has been practised for countless centuries, but the science of veterinary pharmacology is of very recent origin. This review traces the contribution of Materia Medica to veterinary therapeutics from the Egyptian period through to the Age of Enlightenment. The first tentative steps in the development of the science of veterinary pharmacology were taken in the 18th century, but it was not until the mid 20th century that the science replaced the art of Materia Medica. This review traces the 20th century developments in veterinary pharmacology, with emphasis on the explosion of knowledge in the 35 year period to 2010. The range of factors which have influenced the current status of the discipline are reviewed. Future developments are considered from the perspectives of what might be regarded as desirable and those innovations that might be anticipated. We end with words of encouragement for young colleagues intent upon pursuing a career in veterinary pharmacology.

Population pharmacokinetics of cefquinome in pigs

This study was performed in 145 pigs to develop a population pharmacokinetics (PPK) model by i.m. administration of cefquinome (CEQ) at the dose of 2 mg/kg in the neck muscle. Serum physiological and biochemical parameters for each pig were determined before administration. After administration, 2-4 samples were collected at random, with the sampling point evenly distributed in the three periods (<1 h, 1-4 h and >4 h). The plasma concentration of CEQ was determined by high performance liquid chromatography with UV detector. The pharmacostatistical analyses of concentration-time data, weight, age, gender, serum physiological and biochemical parameters were performed with nonlinear mixed effect modeling (NONMEM). A one-compartmental model with first-order absorption and elimination adequately described the data from the study group. The optimal random effect model of pharmacokinetics parameters was of log-normal distribution and the residual errors assumed a mixed-type model (proportional and additive) to best explain intra-individual variability. Covariate analysis showed that body weight is positively correlated with apparent volume of distribution (V/F) and body clearance (CL/F). The typical PPK parameters of Ka , CL, and V were 0.564/h, 5.15 L/h, and 1.36 L, respectively.

Evaluation of factors important in modeling plasma concentrations of tetracycline hydrochloride administered in water in swine

OBJECTIVE

To model the plasma tetracycline concentrations in swine (Sus scrofa domestica) treated with medication administered in water and determine the factors that contribute to the most accurate predictions of measured plasma drug concentrations.

SAMPLE

Plasma tetracycline concentrations measured in blood samples from 3 populations of swine.

PROCEDURES

Data from previous studies provided plasma tetracycline concentrations that were measured in blood samples collected from 1 swine population at 0, 4, 8, 12, 24, 32, 48, 56, 72, 80, 96, and 104 hours and from 2 swine populations at 0, 12, 24, 48, and 72 hours hours during administration of tetracycline hydrochloride dissolved in water. A 1-compartment pharmacostatistical model was used to analyze 5 potential covariate schemes and determine factors most important in predicting the plasma concentrations of tetracycline in swine.

RESULTS

2 models most accurately predicted the tetracycline plasma concentrations in the 3 populations of swine. Factors of importance were body weight or age of pig, ambient temperature, concentration of tetracycline in water, and water use per unit of time.

CONCLUSIONS AND CLINICAL RELEVANCE

The factors found to be of importance, combined with knowledge of the individual pharmacokinetic and chemical properties of medications currently approved for administration in water, may be useful in more prudent administration of approved medications administered to swine. Factors found to be important in pharmacostatistical models may allow prediction of plasma concentrations of tetracycline or other commonly used medications administered in water. The ability to predict in vivo concentrations of medication in a population of food animals can be combined with bacterial minimum inhibitory concentrations to decrease the risk of developing antimicrobial resistance.

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.

Clinical interpretation of pharmacokinetic and pharmacodynamic data in zoologic companion animal species

The treatment and prevention of pain in zoologic companion animals is difficult because of the lack of data available on the safety and efficacy of analgesics. Pharmacokinetic (PK)-pharmacodynamic (PD) studies integrate changes in drug concentrations and changes in the drug's effect. All experimental studies assessing the PDs of analgesics have limitations in animals, but the data provided by experimental studies are valuable in designing dosages. Placebo-controlled, randomized, and blinded clinical trials provide the best PK and PD data, but are rarely performed in major veterinary species because of the number of animals required for the study, lack of preliminary PK and PD data in a given species, species-specific differences in PK and PD, and ethical and toxicologic concerns. The usefulness and limitations as well as considerations for interpreting PK, PD, and controlled clinical studies are discussed. An example of allometric analysis of buprenorphine in mammals is also included.

New technologies for application to veterinary therapeutics

The purpose of this contribution is to review new technologies and make an educated prediction as to how they will impact veterinary pharmacology over the coming decades. By examining past developments, it becomes evident that change is incremental and predictable unless either a transforming discovery or a change in societal behaviour occurs. In the last century, both discoveries and behaviours have dramatically changed medicine, pharmacology and therapeutics. In this chapter, the potential effects of six transforming technologies on veterinary therapeutics are examined: continued advances in computer technology, microfluidics, nanotechnology, high-throughput screening, control and targeted drug delivery and pharmacogenomics. These should lead to the more efficacious and safer use of existing medicants, and the development of novel drugs across most therapeutic classes through increases in our knowledge base, as well as more efficient drug development. Although this growth in technology portends major advances over the next few decades, economic and regulatory constraints must still be overcome for these new drugs or therapeutic approaches to become common practise.

The future of veterinary therapeutics: A glimpse towards 2030

The purpose of this article is to make an educated guess as to what veterinary pharmacology will look like in two decades. By examining the past, it is evident that change is incremental unless a transforming discovery occurs. In the last few decades, such events have dramatically changed medicine and pharmacology, however they have not percolated through the system to the effect that novel drugs have replaced our traditional armamentarium. The effect of six transforming technologies (continued advances in computer technology, microfluidics, nanotechnology, high-throughput screening, control and targeted drug delivery, pharmacogenomics) on veterinary therapeutics is examined. These should lead toward more efficacious and safer drugs across most therapeutic classes due to both increases in our knowledge base as well as more efficient drug development. Shorter term improvements in drug delivery should be seen. Although this growth in technology would portend major advances over the next few decades, economic and regulatory constraints must still be overcome for these new drugs or therapeutic approaches to become common practice.

Effects of physiological covariables on pharmacokinetic parameters of clomipramine in a large population of cats after a single oral administration

This study was conducted to confirm an interindividual variability in pharmacokinetic parameters of clomipramine in a large population of cats and to identify potential covariables that would explain the presence of such pharmacokinetic variability after a single dose of Clomicalm. Clomipramine hydrochloride was administered orally according to a weight-dose chart from 0.32 to 0.61 mg/kg, to 76 cats and five blood samples were then taken by direct venipuncture at 1, 3, 6, 12, and 24 h. Plasma concentrations of clomipramine and desmethylclomipramine (DCMP) were measured by LC-MS/MS. The Standard Two-Stage technique was used to assess differences and detect correlations between pharmacokinetic parameter estimates and individual covariables. A large interindividual variability in all pharmacokinetic parameters (CV% 64-124) was detected. Statistically significant gender-related differences were detected in MR and Cl/F, where female cats had a higher mean MR (0.53) and faster Cl/F (0.36 L/h.kg) than males (0.36 and 0.21 L/h.kg, respectively). No correlation could be found between clomipramine AUC0-24 h or DCMP AUC0-24 h and sedation scores. Further feline studies are required to assess these findings after multiple dosing of clomipramine and DCMP to allow clinical extrapolation.

Application of risk assessment and management principles to the extralabel use of drugs in food-producing animals

A risk assessment of the food safety implications of drugs used in food-producing animals is an essential component of the regulatory approval process for products containing these drugs. This ensures that there is negligible risk to human health if these drugs are used according to the instructions that appear on the approved label. A relative paucity of approved products for veterinary species; however, forces veterinarians worldwide to use drugs in an extralabel manner to treat disease and alleviate suffering in animals. In food-producing animals, this may result in residues that are potentially harmful to the human consumer. This review describes how risk assessment principles can be extended to evaluate the risks posed by different classes of extralabel drug use. Risk management practices in the United States and Europe are summarized and contrasted to illustrate the application of these principles.

Population pharmacokinetics of marbofloxacin in horses: preliminary analysis

Population pharmacokinetic of marbofloxacin was investigated on 21 healthy and 16 diseased horses to assess interindividual variability of drug exposure. Demographic, physiologic and disease covariables were tested using mixed effects models. As a preliminary analysis, this study has demonstrated that none of the tested covariables were significant in regression models for compartmental volumes or clearance of distribution, but the clinical status of the horse (healthy/diseased) was a significant covariable (P < 0.01) for systemic clearance. Clearance had a lower mean and a higher variance for diseased horses than healthy horses, with respectively a mean of 0.209 and 0.284 L/h/kg and a coefficient of variation of 52 and 15%. Consequently, variability of AUC was greater in diseased horses. Considering an AUC/MIC ratio below 60 h as a prediction of poor efficacy, a dosage regimen of 2 mg/kg intravenous was deemed to be inadequate for 19% of diseased horses if the MIC of the bacteria was 0.1 microg/mL. However 93% of diseased horses could achieve a ratio above 125 h, predicting a very good efficacy, for the MIC(90) of Enterobacteriacae (0.027 microg/mL).

Pharmacokinetic/pharmacodynamic relationships of antimicrobial drugs used in veterinary medicine

The rise in incidence of antimicrobial resistance, consumer demands and improved understanding of antimicrobial action has encouraged international agencies to review the use of antimicrobial drugs. More detailed understanding of relationships between the pharmacokinetics (PK) of antimicrobial drugs in target animal species and their action on target pathogens [pharmacodynamics (PD)] has led to greater sophistication in design of dosage schedules which improve the activity and reduce the selection pressure for resistance in antimicrobial therapy. This, in turn, may be informative in the pharmaceutical development of antimicrobial drugs and in their selection and clinical utility. PK/PD relationships between area under the concentration time curve from zero to 24 h (AUC(0-24)) and minimum inhibitory concentration (MIC), maximum plasma concentration (C(max)) and MIC and time during which plasma concentrations exceed the MIC have been particularly useful in optimizing efficacy and minimizing resistance. Antimicrobial drugs have been classified as concentration-dependent where increasing concentrations at the locus of infection improve bacterial kill, or time-dependent where exceeding the MIC for a prolonged percentage of the inter-dosing interval correlates with improved efficacy. For the latter group increasing the absolute concentration obtained above a threshold does not improve efficacy. The PK/PD relationship for each group of antimicrobial drugs is 'bug and drug' specific, although ratios of 125 for AUC(0-24):MIC and 10 for C(max):MIC have been recommended to achieve high efficacy for concentration-dependent antimicrobial drugs, and exceeding MIC by 1-5 multiples for between 40 and 100% of the inter-dosing interval is appropriate for most time-dependent agents. Fluoroquinolones, aminoglycosides and metronidazole are concentration-dependent and beta-lactams, macrolides, lincosamides and glycopeptides are time-dependent. For drugs of other classes there is limited and conflicting information on their classification. Resistance selection may be reduced for concentration-dependent antimicrobials by achieving an AUC(0-24):MIC ratio of greater than 100 or a C(max):MIC ratio of greater than 8. The relationships between time greater than MIC and resistance selection for time-dependent antimicrobials have not been well characterized.

Extrapolated withdrawal-interval estimator (EWE) algorithm: a quantitative approach to establishing extralabel withdrawal times

The extralabel use of drugs can be defined as the use of drugs in a manner inconsistent with their FDA-approved labeling. The passage of the Animal Medicinal Drug Use Clarification Act (AMDUCA) in 1994 and its implementation by the FDA-Center for Veterinary Medicine in 1996 has allowed food animal veterinarians to use drugs legally in an extralabel manner, as long as an appropriate withdrawal period is established. The present study introduces and validates with simulated and experimental data the Extrapolated Withdrawal-Period Estimator (EWE) Algorithm, a procedure aimed at predicting extralabel withdrawal intervals (WDIs) based on the label and pharmacokinetic literature data contained in the Food Animal Residue Avoidance Databank (FARAD). This is the initial and first attempt at consistently obtaining WDI estimates that encompass a reasonable degree of statistical soundness. Data on the determination of withdrawal times after the extralabel use of the antibiotic oxytetracycline were obtained both with simulated disposition data and from the literature. A withdrawal interval was computed using the EWE Algorithm for an extralabel dose of 25 mg/kg (simulation study) and for a dose of 40 mg/kg (literature data). These estimates were compared with the withdrawal times computed with the simulated data and with the literature data, respectively. The EWE estimates of WDP for a simulated extralabel dose of 25 mg/kg was 39 days. The withdrawal time (WDT) obtained for this dose on a tissue depletion study was 39 days. The EWE estimate of WDP for an extralabel intramuscular dose of 40 mg/kg in cattle, based on the kinetic data contained in the FARAD database, was 48 days. The withdrawal time experimentally obtained for similar use of this drug was 49 days. The EWE Algorithm can obtain WDI estimates that encompass the same degree of statistical soundness as the WDT estimates, provided that the assumptions of the approved dosage regimen hold for the extralabel dosage regimen. Population models could be fitted to fragmentary data to predict residue concentrations in tissues, validate the EWE estimates, and obtain WDI estimates.

Rational dosing of antimicrobial drugs: animals versus humans

The rational dosing of antimicrobial drugs depends on knowledge of physiology, anatomy and pathology, including disease condition, and in major respects these differ between animals and humans and between species of animal. These differences lead to species variation in drug pharmacokinetics, which can be profound. This review highlights selected aspects of species differences in pharmacokinetics and considers underlying mechanisms by reference to ruminant and non-ruminant mammals, birds, fish and bees. For all species it is desirable and should be possible to design dosage schedules based on knowledge of drug pharmacokinetics and pharmacodynamics. There have been many attempts to integrate pharmacokinetic and pharmacodynamic data to provide dosage schedules which optimize efficacy and minimize opportunities for the development of antimicrobial resistance in both laboratory animal studies and human clinical trials. However, there have been relatively few studies in animal species of major veterinary interest. This review summarizes recent studies in four ruminant species (calf, sheep, goat and camel) which have used PK-PD integration to determine for the fluoroquinolone, danofloxacin, AUC/MIC ratios producing (a) bacteriostasis (b) bactericidal activity and (c) elimination of bacteria. Future possible developments in dosage schedule design are considered.

Pharmacokinetic/pharmacodynamic integration in drug development and dosage-regimen optimization for veterinary medicine

Pharmacokinetic (PK)/pharmacodynamic (PD) modeling is a scientific tool to help developers select a rational dosage regimen for confirmatory clinical testing. This article describes some of the limitations associated with traditional dose-titration designs (parallel and crossover designs) for determining an appropriate dosage regimen. It also explains how a PK/PD model integrates the PK model (describing the relationship between dose, systemic drug concentrations, and time) with the PD model (describing the relationship between systemic drug concentration and the effect vs time profile) and a statistical model (particularly, the intra- and interindividual variability of PK and/or PD origin). Of equal importance is the utility of these models for promoting rational drug selection on the basis of effectiveness and selectivity. PK/PD modeling can be executed using various approaches, such as direct versus indirect response models and parametric versus nonparametric models. PK/PD concepts can be applied to individual dose optimization. Examples of the application of PK/PD approaches in veterinary drug development are provided, with particular emphasis given to nonsteroidal anti-inflammatory drugs. The limits of PK/PD approaches include the development of appropriate models, the validity of surrogate endpoints, and the acceptance of these models in a regulatory environment.

Mixed effects modeling of the disposition of gentamicin across domestic animal species

An interspecies pharmacokinetic model for gentamicin was developed using the mixed effects modeling approach and serum disposition data obtained from the Food Animal Residue Avoidance Databank (FARAD). Data that met a priori quality criteria was obtained from the database and analysed using the traditional double logarithmic analysis and the mixed effects modeling approach. Body weight, brain weight and fever were the covariates of interest in our study. Population pharmacokinetic models across species were developed and validated with swine data. The parameter volume of distribution was modeled as a function of body weight. The total clearance was initially modeled as a function of body weight. The predictability performance of the model improved dramatically when the parameter brain weight was included in the covariate model for clearance. This was a surprising finding worthy of further study. The covariate fever seemed to influence the magnitude of the volume of distribution, although the scarcity of data pertaining to diseased animals makes this finding uncertain. We conclude that the pharmacokinetic characteristics of drugs such as gentamicin, can be predicted across species using a population pharmacokinetics modeling approach, and that clinical features that affect species in a similar manner can be also explored in this fashion.

Pharmacokinetics and milk discard times of pirlimycin after intramammary infusion: a population approach

A population pharmacokinetic approach was used to analyse milk concentration data to determine whether milk discard times and the clearance of intramammary infusions of pirlimycin could be adequately predicted by readily available demographic variables. Milk samples were collected at 12 hourly milking intervals after dosing with pirlimycin during product development from both normal cows (primary data) and cows with naturally occurring mastitis (validation data) and pirlimycin concentration was determined by microbial inhibition assay. The data were analysed by the conditional estimation/ maximum likelihood population approach within the computer program PPharm and fitted a two compartment open model. Bayesian estimates of individual parameters allowed solutions for each cow, predicting the time after last dosing by which milk concentration reached the target safe concentration. From this population of times, the 95% confidence interval of the 99th percentile was defined as the milk discard time. After elimination of one very low producing outlier, the calculated discard time agreed with the label recommendation of 36 h (3 milkings, USA) after the last dose. Milk pirlimycin clearance was strongly and positively correlated to the logarithm of the kilograms of milk produced in 24 h at time of dosing (r2=0.939). Agreement was strong at most time points between predicted and measured pirlimycin concentrations in milk from cows with mastitis. This alternative method for determining milk discard times was compared to existing recommendations.