Pharmacokinetics of an injectable long‐acting parenteral formulation of doxycycline hyclate in pigs

Based on its ideal PK/PD ratios, doxycycline hyclate (DOX‐h), a time‐dependant antibacterial, is ideally expected to achieve sustained plasma drug concentrations at or slightly above the MIC level for as long as possible between dosing intervals. Pursuing this end, a poloxamer‐based matrix was used to produce a 10% long‐acting injectable preparation (DOX‐h‐LA) and its serum concentrations vs. time profile investigated after its injection to pigs in the pericaudal s.c. by parallel design. Results were compared with the forced oral bolus dose and i.v. pharmacokinetics of DOX‐h. For this study, 12 recently weaned pigs per group were included in this trial, and a dose of 20 mg/kg was injected in all cases. DOX‐h‐LA showed the greatest values for bioavailability (115.38%); maximum serum concentration (Cmax) value was 1.5 ± 0.2 with a time to reach Cmax of 3.41 ± 0.04 h and an elimination rate constant of 70.93 ± 0.87 h. Considering minimum effective serum concentration of 0.5 μg/mL, a dose interval of at least 5 days can be achieved for DOX‐h‐LA, whereas p.o. and i.v. dosing of DOX‐h may only last 11 and 15 h, respectively. Pigs were slaughtered on day 30 after this trial, and no visible remnants of the preparation were detected neither fibrosis was observed after a thorough macroscopic and histopathological analysis.     

Doxorubicin area under the curve is an important predictor of neutropenia in dogs with naturally occurring cancers

Doxorubicin (DOX) area‐under‐the‐curve (AUC) was calculated for 40 dogs with spontaneously occurring cancers using a previously validated limited‐sampling approach. All dogs were administered a dose of 30 mg/m² by intravenous infusion and serum samples were collected at 5, 45 and 60 minutes post‐infusion. DOX and its major metabolite, doxorubicinol (doxol), were quantified in serum samples using high‐performance liquid chromatography tandem‐mass spectrometry. Wide interpatient variability was observed in the predicted DOX AUC with a coefficient of variation of 34%. A significant relationship was found between DOX AUC and absolute white blood cell count (P = 0.003), absolute neutrophil count (ANC; P = 0.002) and surviving fraction of neutrophils (P = 0.03) approximately 1 week after dosing (nadir). No changes in other hematologic parameters (red blood cells, platelets, lymphocytes, haemoglobin) were found to correlate with DOX AUC. The absolute dose (mg) and the dose per unit body weight (mg/kg) were not significantly correlated with nadir ANC. No relationships were found between maximum serum doxol concentration and myelosuppression. Baseline ANC was also significantly correlated to nadir ANC and a model was constructed using baseline ANC and DOX AUC that significantly described the nadir ANC. These findings demonstrate the important relationship between systemic DOX exposure and degree of neutropenia in dogs, and suggest a potential for individualized, pharmacokinetically‐guided DOX dosing in dogs.     

Interindividual variability in plasma concentrations after systemic exposure of swine to dietary doxycycline supplied with and without paracetamol: a population pharmacokinetic approach

Anorexigenic substances released during infection may hinder the therapeutic efficacy of in-feed antibiotics. Paracetamol (acetaminophen; PARA) inhibits the anorexia of infection and seems to improve the clinical efficacy of doxycycline (DOX) against bacterial respiratory disease in swine herds. In order to verify whether PARA selectively stimulates intake of DOX-medicated feed in diseased pigs, we documented the pharmacokinetics (PK) of DOX when coadministered with PARA and examined the effect of in-feed PARA on the interindividual variability in plasma concentrations after systemic exposure to in-feed DOX in swine herds with respiratory disease. Systemic exposure to DOX was measured with the area under the curve (AUC) of its plasma concentrations over time. First, a rich-sampling PK study of in-feed and i.v. DOX (10 mg/kg of BW) and PARA (30 and 10 mg/kg of BW, respectively) was performed on 5 pigs. The PK profiles of in-feed DOX were used in mathematical simulations to determine 5 optimal sampling times for the farm-based population PK study. A randomized, blind, parallel PK study was performed in 2 herds with bacterial respiratory disease, where liquid feed was fortified with DOX alone (5 mg x kg of BW(-1) x meal(-1)) or combined with PARA (15 mg x kg of BW(-1) x meal(-1)). Medicated meals were given twice, 12 h apart, to group-housed growing pigs (n > 50 pigs x treatment(-1) x herd(-1), totaling 215 pigs). Plasma concentrations of DOX and PARA were measured with HPLC. At variance with our expectations, PARA decreased (P = 0.069) mean AUC of in-feed DOX and did not decrease its variability (P > 0.34). Mean AUC of DOX increased with feed intake and with initial exposure to DOX, and was greater in sick animals. Therefore, symptomatic PARA-induced improvement in bacterial respiratory disease control with DOX is more likely caused by its analgesic/antipyretic effects than by its orexigenic effect. Interindividual variation in the AUC of DOX was large in pigs given group medication, even when sufficient feeding space was allowed and the amount of feed offered was greater than their requirements. Therefore, future studies to improve the efficacy of group antibiotic therapy should focus on feeding behavior characteristics as well as biopharmaceutical properties of medicated feeds.     

Integration and modelling of pharmacokinetic and pharmacodynamic data to optimize dosage regimens in veterinary medicine

In veterinary drug development procedures, pharmacokinetic (PK) and pharmacodynamic (PD) data have generally been established in separate, parallel studies to assist in the design of dosage schedules for subsequent evaluation in clinical trials. This review introduces the concept of PK/PD modelling, an approach in which PK and PD data are generated in the same study, and used to derive numerical values for PD parameters based on drug plasma concentrations. The PD parameters define the efficacy, potency and slope (sensitivity) of the concentration-effect relationship. It is proposed that the parameters derived from PK/PD modelling may be used as an alternative and preferred approach to dose titration studies for selecting rational dosage regimens (both dose and dosing interval) for further evaluation in clinical trials. In PK/PD modelling, the explicative variable for effect is the plasma concentration profile. The PK/PD approach provides several advantages over dose-titration studies, including determination of a projected dosage regimen by investigation of a single dose, in contrast to dose-ranging studies which by definition require testing of multiple dosage. Implementation of PK/PD modelling in the veterinary drug development process is currently constrained by the limited number of veterinary studies performed to date, and the consequently limited understanding of PK/PD concepts and their absence from regulatory authority guidelines. Nevertheless, PK/PD modelling has major potential for rational dosage regimen determination, as it considers and quantifies the two main sources of interspecies variability (PK and PD). It is therefore applicable to interspecies extrapolation and to multiple species drug development. As well as the currently limited appreciation of PK/PD principles in the veterinary scientific community, a further constraint in implementing PK/PD modelling is the need to validate PK/PD approaches and thereby gain confidence in its value by pharmaceutical companies and regulatory authorities.     

Population pharmacokinetics of isoniazid in the treatment of Mycobacterium tuberculosis among Asian and African elephants (Elephas maximus and Loxodonta africana)

We recently described the clinical presentation and treatment of 18 elephants from six herds infected with TB. Treatment protocols and methods varied between herds to include both oral and rectal dosing using multiple drug doses and formulations. In this paper we present information regarding the pharmacokinetics (PK) of isoniazid (INH) in elephants and provide suggestions regarding initial treatment regimens. Forty-one elephants received INH daily by either oral or rectal administration with different formulations. Population PK analysis was performed using Non-linear Mixed Effect Modeling (NONMEM). Results of oral administration indicated that compared with premixed INH solution, the drug exposure was highest with a suspension prepared freshly with INH powder. When INH was concomitantly given as an admixture over food, Tmax was delayed and variability in drug absorption was significantly increased. Compared with oral administration, similar drug exposures were found when INH was dosed rectally. The data generated suggest that a starting dose of 7.5 mg/kg of INH is appropriate for initial TB treatment in elephants when premixed solution is administered directly into the oropharynx or rectal vault and 4 mg/kg are when INH is administered following immediate suspension from powdered form.     

Pharmacokinetic Modeling of Doxorubicin Pharmacokinetics in Dogs Deficient in ABCB1 Drug Transporters

Background: The identification of dogs defective in ATP‐binding cassette transporter B1 (ABCB1, MDR1) activity has prompted questions regarding pharmacokinetics (PK), efficacy and toxicity of ABCB1 substrates in these dogs. Hypothesis/Objectives: Dogs defective in ABCB1 activity (ABCB1_null) have doxorubicin (DOX) PK different from that of normal dogs (ABCB1_wt). Utilization of a physiologically based pharmacokinetic (PBPK) model allows computer simulation to study this polymorphism's impact on DOX PK. Animals: None. Methods: A virtual ABCB1_wt dog population was generated and DOX distribution, elimination, and metabolism simulated by PBPK modeling. An in silico population of virtual dogs was generated by Monte Carlo simulation, with variability in physiologic and biochemical parameters consistent with the dog population. This population was used in the PBPK model. The ABCB1 components of the model were inactivated to generate an ABCB1_null population and simulations repeated at multiple doses. Resulting DOX levels were used to generate PK parameters. Results: DOX exposures in the ABCB1_null population were increased in all simulated tissues including serum (24%) and gut (174%). Estimated dosages in the ABCB1_null population to approximate exposure in the ABCB1_wt population at a dose of 30 mg/m² were 24.8 ± 3.5 mg/m² for serum and 10.7 ± 5.9 mg/m² for gut. Conclusions and Clinical Importance: These results suggest that serum DOX concentrations are not indicative of tissue exposure, especially those with appreciable ABCB1 activity, and that gastrointestinal (GI) toxicosis would be dose limiting in ABCB1_null populations. Dosage reductions necessary to prevent GI toxicosis likely result in subtherapeutic concentrations, thereby reducing DOXs efficacy in ABCB1_null dogs.     

Activity of cefquinome against extended‐spectrum β‐lactamase‐producing Klebsiella pneumoniae in neutropenic mouse thigh model

Increasing prevalence of extended‐spectrum β‐lactamase (ESBL)‐producing Klebsiella pneumoniae (K. pneumoniae) is of clinical concern. The objective of our study was to examine the in vivo activity of cefquinome against ESBL‐producing K. pneumoniae strain using a neutropenic mouse thigh infection model. Cefquinome kinetics and protein binding in infected neutropenic mice were measured by liquid chromatography–tandem mass spectrometry (LC‐MS/MS). Dose‐fractionation studies over a 24‐h dose range of 2.5–320 mg/kg were administered every 3, 6, 12, or 24 h. The percentage of the dosing interval that the free‐drug serum levels exceed the MIC (%fT > MIC) was the PK–PD index that best correlated with cefquinome efficacy (R² = 86%). Using a sigmoid E_max model, the magnitudes of %fT > MIC producing net bacterial stasis, a 1‐log₁₀ kill and a 2‐log₁₀ kill over 24 h, were estimated to be 20.07%, 29.57%, and 55.12%, respectively. These studies suggest that optimal cefquinome PK/PD targets are not achieved in pigs, sheep, and cattle at current recommended doses (1?2 mg/kg). Further studies with higher doses in the target species are needed to ensure therapeutic concentration, if cefquinome is used for treatment of K. pneumoniae infection.     

蒙特卡罗模拟法在抗微生物药物药动学和药效学研究中的应用

常见病原微生物对抗微生物药物的耐药性正逐渐增加,为了达到最佳治疗效果,临床用药必须根据药动学与药效学数据调整给药方案。药动学能够提供药物浓度在组织、体液和感染部位的经时过程,而药效学则反映药物对致病菌的杀灭或抑制能力。蒙特卡罗模拟法则是利用统计学抽样来获得数学方程的近似解的一种方法,目前采用蒙特卡罗模拟法进行实时模拟正成为国际上研究抗微生物药物的药动学和药效学的热点。论文就蒙特卡罗模拟法的原理、拟合过程及其在估算细菌对药物的敏感性折点、比较药动-药效参数以选择最优药物等方面做一综述。     

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.     

Temozolomide alone or in combination with doxorubicin as a rescue agent in 37 cases of canine multicentric lymphoma

Temozolomide (TMZ) is an alkylating agent previously used in conjunction with doxorubicin (DOX) to treat dogs with relapsed lymphoma. However, there are very limited data for this drug when used as single agent. The aim of this retrospective study was to evaluate the efficacy and toxicity of TMZ in dogs with relapsed multicentric lymphoma that failed multi‐agent chemotherapy protocols, and compare the outcome to a group of dogs receiving the same drug in combination with DOX. Twenty‐six patients were included in the TMZ group and 11 in the TMZ/DOX group. Responses were evaluated via retrospective review of the medical records. The overall median survival time (MST) for both groups was 40 days (range 1‐527 days). For the TMZ group, median time to progression (TTP) was 15 days (range 1‐202 days) and MST 40 days (range 1‐527 days), with an overall response rate (ORR) of 32% and 46% recorded toxicities. For the TMZ/DOX group, median TTP was 19 days (range 2‐87 days) and MST 24 days (range 3‐91 days), with an ORR of 60% and 63% recorded toxicities. However, a proportion of haematological toxicoses may have gone undetected due to the absence of associated clinical signs. The difference in MST and TTP between the 2 groups was not statistically significant. Similarly, no negative prognostic factors were identified. Although responses were generally short lived, this study suggests that TMZ may achieve similar efficacy to TMZ/DOX whilst being associated with a lower frequency of recorded toxicities.     

Pharmacokinetics and pharmacokinetic/pharmacodynamic relationships for angiotensin-converting enzyme inhibitors

The pharmacokinetic (PK) properties and the pharmacokinetic/pharmacodynamic (PK/PD) relationships for the angiotensin-converting enzyme (ACE) inhibitors (ACEIs), such as enalaprilat, benazeprilat, imidaprilat and ramiprilat, differ from those of conventional drugs. This is because of their immediate and saturable binding to an ACE pool which is partly circulating (and contributing to the measured plasma concentration), and partly noncirculating (tissular), being anchored to the endothelium of vessels and not measurable by the analytical technique. A physiologically based model is required to allow appropriate interpretation of the different phases of the disposition curve of ACEI. The protracted terminal phase observed for all ACEIs is not a conventional elimination phase but a phase dependent on ACEI dissociation from ACE. In contrast, the phase which reflects ACEI elimination (and which is interpreted as a distribution phase for a conventional drug) has a short half-life, thus explaining the absence of drug accumulation during repeated dosing and mild kidney failure. ACE inhibition is the surrogate endpoint generally selected for establishing a PK/PD relationship and for simulating dosage regimen scenarios in order to decide on the appropriate dosage regimen for ACEIs.     

RT09Bayesian approaches in dosage selection

There are several means whereby dosage schedules for clinical use may be set, some more appropriate and scientific than others! The challenge of the 21st century must be for colleagues in the pharmaceutical industry, those serving registration bodies and academic colleagues to pool their expertise with the objective of designing dosage schedules for clinical use, which are based on the application of sound scientific principles appropriate for each drug class. In this Roundtable Session colleagues of international standing will review (a) pharmacological and other sources of variability in the responses to drugs; (b) the advantages and limitations of pre‐clinical studies for dose selection; (c) the roles of population PK and population PK/PD together with Monte Carlo simulations in dosage regimen selection; (d) Bayesian approaches to dosage selection and (e) regulatory guidelines on the type and extent of studies required for selecting dosages. There is no unanimity amongst stakeholders on either the principles or the methods underlying dosage schedule design. Dose titration studies have long been the principal means of fixing doses but PK‐PD and population PK‐PD studies are now challenging more traditional approaches. The papers and discussion in this Roundtable Session will provide a critical basis for future advances in this crucial area of therapeutic drug usage. Getting the doses right means that the patient will receive maximum benefit, in terms of optimal efficacy with minimal toxicity, and hence correct dosing will contribute enormously to animal welfare.     

Interpreting culture and susceptibility data in critical care: perks and pitfalls

Problem – The need for immediate, effective antimicrobial therapy in the critical care patient must be tempered by approaches which simultaneously minimize emergence of antimicrobial resistance. Ideally, therapy will successfully resolve clinical signs of infection, while eradicating infecting pathogens such that the risk of resistance is avoided. Increasing limitations associated with empirical antimicrobial choices direct the need for culture and susceptibility data as a basis of therapy. Even so, such in vitro data should be utilized within its limitations. Objectives – To demonstrate the attributes and limitations of patient and population culture and susceptibility (pharmacodynamic) data in the selection of antimicrobial drugs and to demonstrate the design of individualized dosing regimens based on integration of pharmacodynamic (PD) and pharmacokinetic (PK) data. Diagnosis – Limitations in culture and susceptibility testing begin with sample collection and continue through drug selection and dose design. Among the challenges in interpretation is discrimination between pathogens and commensals. Properly collected samples are critical for generation of data relevant to the patient's infection. Data are presented as minimum inhibitory concentrations (MICs). The MIC facilitate selection of the most appropriate drug, particularly when considered in the context of antimicrobial concentrations achieved in the patient at a chosen dose. Integration of MIC data with key PK data yields the C_max:MIC important to efficacy of concentration‐dependent drugs and T>MIC, which guides use of time‐dependent drugs. These indices are then used to design dosing regimens that are more likely to kill all infecting pathogens. In the absence of patient MIC data, population data (eg, MIC₉₀) may serve as a reasonable surrogate. Conclusions – Properly collected, performed, and interpreted culture and susceptibility data are increasingly important in the selection of and design of dosing regimens for antimicrobial drugs. Integration of PK and PD data as modified by host and microbial factors supports a hit hard, exit fast approach to therapy that will facilitate efficacy while minimizing resistance.     

Pharmacokinetic/pharmacodynamic relationship of cefquinome against Pasteurella multocida in a tissue‐cage model in yellow cattle

The cephalosporin antimicrobial drug cefquinome was administered to yellow cattle intravenously (i.v.) and intramuscularly (i.m.) at a dose of 1 mg/kg of body weight in a two‐period crossover study. The pharmacokinetic (PK) properties of cefquinome in serum, inflamed tissue‐cage fluid (exudate), and noninflamed tissue‐cage fluid (transudate) were studied using a tissue‐cage model. The in vitro and ex vivo activities of cefquinome in serum, exudate, and transudate against a pathogenic strain of Pasteurella multocida (P. multocida) were determined. A concentration‐independent antimicrobial activity of cefquinome was confirmed for levels lower than 4 × MIC. Integration of in vivo pharmacokinetic data with the in vitro MIC provided mean values for the time that drug levels remain above the MIC (T > MIC) in serum was 14.10 h after intravenous and 14.46 h after intramuscular dosing, indicating a likely high level of effectiveness in clinical infections caused by P. multocida of MIC 0.04 μg/mL or less. These data may be used as a rational basis for setting dosing schedules, which optimize clinical efficacy and minimize the opportunities for emergence of resistant organisms.     

Pharmacokinetics and bioavailability of oral firocoxib in adult,mixed‐breed goats

Pharmacokinetic (PK) studies of oral firocoxib in large animal species have been limited to horses, preruminating calves, and adult camels. The aim of this study was to describe pharmacokinetics and bioavailability of firocoxib in adult goats. Ten healthy adult goats were administered 0.5 mg/kg firocoxib intravenously (i.v.) and per os (p.o.) in a randomized, crossover study. Plasma firocoxib concentrations were measured over a 96‐hr period for each treatment using HPLC and mass spectrometry, and PK analysis was performed. The p.o. formulation reached mean peak plasma concentration of 139 ng/ml (range: 87–196 ng/ml) in 0.77 hr (0.25–2.00 hr), and half‐life was 21.51 hr (10.21–48.32 hr). Mean bioavailability was 71% (51%–82%), indicative of adequate gastrointestinal absorption of firocoxib. There were no negative effects observed in any animal, and all blood work values remained within or very near reference range at the study's conclusion. Results indicate that oral firocoxib is well‐absorbed and rapidly reaches peak plasma concentrations, although the concentration also decreased quickly prior to the terminal phase. The prolonged half‐life may suggest tissue accumulation and higher plasma concentrations over time, depending on dosing schedule. Further studies to determine tissue residue depletion, pharmacodynamics, and therapeutic concentrations of firocoxib in goats are necessary.     

Phase I lead‐in and subsequent randomized trial assessing safety and modulation of regulatory T cell numbers following a maximally tolerated dose doxorubicin and metronomic dose cyclophosphamide combination chemotherapy protocol in tumour‐bearing dogs

Maximally tolerated dose (MTD) and metronomic dose chemotherapeutic approaches alter the immune system and the angiogenic process in different yet potentially complementary ways. A combination of MTD doxorubicin (MTD‐DOX) and metronomic cyclophosphamide (mCTX) protocol was evaluated for safety and effect on circulating regulatory T (Treg) cells. We found that mCTX can be safely administered with MTD‐DOX in tumour‐bearing dogs. Both combination DOX/mCTX and single‐agent DOX resulted in significant depletions of circulating lymphocytes throughout the chemotherapy cycle without apparent selectivity for Tregs. The indiscriminant lymphocyte depletions were similar between dogs randomized to receive DOX and dogs randomized to receive DOX/mCTX, suggesting this effect is because of DOX alone. These findings may have implications as to the therapeutic benefit (or lack thereof) of concurrent combination MTD and metronomic protocols. Future investigations are required to determine the effects and indeed the efficacy of concurrent versus sequential applications of MTD and metronomic chemotherapy protocols.     

Use of Body Surface Area (BSA)-Based Dosages to Calculate Chemotherapeutic Drug Dose in Dogs: I. Potential Problems with Current BSA Formulae

The dose of most cancer chemotherapeutic drugs administered to dogs is calculated on the basis of estimated body surface area (BSA); however, results of some chemotherapy trials have revealed that this dosing method increases toxicosis in small dogs. The current formula used to estimate BSA in dogs may be inaccurate or the assumption that BSA correlates with chemotherapeutic drug exposure may be unfounded. Results presented in this review suggest that canine BSA estimates may be inaccurate because the values for the constant (K) and exponent (a) in the formulae (BSA = K.W^a) are incorrect or because a linear parameter such as body length is lacking from the formulae. Results that suggest the relationship between BSA and the physiologic/pharmacologic factors that influence drug exposure may not be closely correlated are also presented. Studies are warranted to determine whether there are dosing methods that normalize chemotherapeutic drug toxicity in dogs.     

Pharmacokinetics and pharmacodynamics of glycopyrrolate following a continuous‐rate infusion in the horse

Glycopyrrolate (GLY) is an antimuscarinic agent that is used in humans and domestic animals primarily to reduce respiratory tract secretions during anesthesia and to reverse intra‐operative bradycardia. Although GLY is used routinely in veterinary patients, there is limited information regarding its pharmacokinetic (PK) and pharmacodynamic (PD) properties in domestic animals, and an improved understanding of the plasma concentration–effect relationship in racehorses is warranted. To accomplish this, we characterize the pharmacokinetic–pharmacodynamic (PK‐PD) actions of GLY during and after a 2‐h constant‐rate intravenous infusion (4 μg/kg/h) and evaluate potential PK‐PD models for cardiac stimulation in adult horses. Measurements of plasma GLY concentrations, heart and respiration rates, and frequency of bowel movements were performed in six Thoroughbred horses. The time course for GLY disposition in plasma followed a tri‐exponential equation characterized by rapid disappearance of GLY from blood followed by a prolonged terminal phase. Physiological monitoring revealed significant (P < 0.01) increases in heart (>70 bpm) and respiratory rates accompanied by a marked and sustained delay in the frequency of bowel movements (1.1 ± 0.2 h [saline group] vs. 6.0 ± 2.0 h [GLY group]). Two of six horses showed signs of colic during the 8‐h observation period after the end of the GLY infusion, but were treated and recovered without further complications. The relationship between plasma GLY concentration and heart rate exhibited counterclockwise hysteresis that was adequately described using an effect compartment.