- Email: [email protected]
Population Pharmacokinetics of Tobramycin in Horses
Journal of Equine Veterinary Science 32 (2012) 531-535
Journal of Equine Veterinary Science journal homepage: www.j-evs.com
Original Research
Population Pharmacokinetics of Tobramycin in Horses Aneliya Haritova PhD a , Dinyo Bakalov DVM b, Huben Hubenov PhD b, Lubomir Lashev DSci a a b
Department of Pharmacology, Physiology of Animals and Physiological Chemistry, Faculty of Veterinary Medicine, Trakia University, Stara Zagora, Bulgaria Department of Veterinary Surgery, Faculty of Veterinary Medicine, Trakia University, Stara Zagora, Bulgaria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 December 2010 Received in revised form 3 July 2011 Accepted 23 December 2011 Available online 4 February 2012
Population pharmacokinetics of tobramycin was investigated in 28 healthy horses, with an aim to assess interindividual variability in the disposition of the antibiotic. Additionally, a covariate model for improved prediction of the concentrations in a particular animal was developed. A two-compartmental model best described the data. The final population covariate regression model was based on relationships between body weight and central and peripheral volumes of distribution, and between creatinine clearance and systemic tobramycin clearance. The value of population systemic tobramycin clearance and its interindividual variation (CV) were 0.087 L.hr1.kg1 and 6.0%, respectively. The respective values for central and peripheral volumes of distribution were 0.652 L.kg1 with CV of 17.7% and 1.56 L.kg1 with CV of 4.5%. In horses with decreased glomerular filtration rate, lower tobramycin clearance is predicted with the population model that requires administration of lower dose than that accepted for treatment of horses with normal kidney function. Population pharmacokinetic analysis allows study of basic disposition of tobramycin in horses with sparse data. The prediction power of the regression model was improved by inclusion of covariables such as body weight and creatinine clearance. This model can be used in direct patient care for the construction of dosing strategy in individual clinical cases. Ó 2012 Elsevier Inc. All rights reserved.
Keywords: Horses Population pharmacokinetics Tobramycin
1. Introduction Aminoglycoside antibiotics are extensively used in the treatment of infections in horses, mainly those caused by Pseudomonas spp. and Enterobacteriaceae strains [1]. Tobramycin is an antibiotic of the aminoglycoside group active against gram-negative and some gram-positive bacteria, with higher activity against several gramnegative microorganisms than gentamicin [2,3]. As an aminoglycoside drug, it exhibits concentration-dependent bactericidal activity. Tobramycin pharmacokinetics was earlier investigated in healthy horses after a single intravenous administration, and it does not differ from the other
Corresponding author at: Aneliya Haritova, PhD, Department of Pharmacology, Physiology of Animals and Physiological Chemistry, Faculty of Veterinary Medicine, Trakia University, Students’ Campus, 6000 Stara Zagora, Bulgaria. E-mail address: [email protected] (A. Haritova). 0737-0806/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jevs.2011.12.010
aminoglycoside antibiotics [4]. Tobramycin shows rapid distribution to most fluids in the body, including peritoneal, pleural, pericardial, and synovial fluids [5]. High concentrations of tobramycin are observed in the kidneys, which is typical for aminoglycosides [6]. It is eliminated through the kidney by glomerular filtration. Accumulation cannot be expected for this compound because it shows high capacity of elimination in horses, comparable with that of amikacin. It was higher than those of gentamicin, streptomycin, and neomycin. Knowledge for pharmacokinetics of aminoglycosides is often obtained from designed trials with standardized groups of healthy horses. However, large variability of pharmacokinetic parameters of these antibiotics has been observed among horses [1]. Pharmacokinetic parameters vary between individual animals, and they may even differ within an animal during treatment or progression of disease. Difficulty to determine the doses of aminoglycosides is linked to the variation of systemic clearance and
532
A. Haritova et al. / Journal of Equine Veterinary Science 32 (2012) 531-535
volume of distribution. Therefore, there exists a concern regarding the appropriate dose selection for treatment of individual animals. Traditional pharmacokinetic studies cannot recognize sources of variability. In contrast, population pharmacokinetic methods consider these variations, so-called random effects. Random effects are associated with the variability in pharmacokinetic parameters between animals and variability in measured drug concentration of an animal. According to previous studies, variability of systemic clearance has been related to body weight, age, serum creatinine concentration, and creatinine clearance (Clcr). Variability in volume of distribution has been related to body weight, age, gender, and serum creatinine. Thus, population approach has a significant power for the individualization and optimization of dosage regimen in clinical cases. It allows refining the dosing regimen by considering link between the individual pharmacokinetic parameters and factors such as age, gender, body weight, and physiological and health status [1,7]. Current investigation was planned as a second stage of a pilot kinetic study of tobramycin in horses. The aim of the present study was to estimate the population pharmacokinetic parameters of tobramycin in horses. A population pharmacokinetic model, which accounts for effect of Clcr and body weight on tobramycin disposition, was developed. 2. Material and Methods 2.1. Drug Tobramycin sulfate (activity: 690 IU, provided by Actavis Ltd, Sofia, Bulgaria) was administered as a 20% w/v aqueous solution, prepared ex tempore. 2.2. Animals and Experimental Design Data from 28 horses (12 males and 16 females) of various breeds weighing 255-710 kg were analyzed. Age of the studied population was between 1 and 16 years. The animals were provided by an experimental farm of Trakia University, Stara Zagora, Bulgaria. They were fed with commercial food 4-5 hours after injection of the antibiotic and had free access to water. All horses underwent a complete physical examination. Hematological, biochemical plasma, and urine parameters were assessed. Creatinine values in plasma (Crplasma) and urine (Crurine) were also measured. Clcr was estimated according to the following equation:
Clcr ¼ ðCrurine :x Flowurine
Crplasma
Tobramycin was administered intravenously in vena jugularis at a dose of 4 mg/kg body weight between 08:00 and 09:00 hours in the morning. Blood samples of 5 mL each were collected by venipuncture from the opposite vena jugularis in heparinized tubes before treatment and 0.083, 0.25, 0.5, 2, 4, 6, 8, 10, 12, and 24 hours after treatment from seven horses (five females and two males) and 1, 3, and 7 hours after treatment from 21 animals. The samples were centrifuged within 1 hour (1,400 g, 10 minutes), and plasma was stored at 20 C until the time of analysis.
2.3. Tobramycin Assay Antibiotic concentrations were determined within 2 days after sampling by microbiological assay, as described in the pilot pharmacokinetic study [4]. In comparison with high-performance liquid chromatography analysis, this assay is more sensitive in the detection of low plasma and urine values. Shortly, Bacillus subtilis ATCC 6633 reference tester strain for quantitative determination of tobramycin was used. The limit of detection and limit of quantification values were 0.024 and 0.048 mg/mL, respectively. 2.4. Pharmacokinetic Analysis Plasma concentration curves versus time were interpreted by a two-compartmental model on the basis of knowledge about tobramycin disposition in horses obtained in a previously conducted study [4]. All concentrationetime data were analyzed simultaneously by nonlinear mixed effects modeling using the Monolix V2.2 (INRIA, Monolix group, France, http://group.monolix. org). Population parameters were estimated using the stochastic approximation expectation maximization algorithm. Variances of pharmacokinetic parameters were also computed. The model was parameterized in terms of systemic clearance (Cl), intercompartmental distribution clearance (Q), volume of distribution in the central compartment (V1), and volume of distribution in the peripheral compartment (V2) (model number 20 from the program library). 2.5. Regression Model Regression models were developed to examine potential relationship between subject clinical characteristics such as body weight, age, gender, creatinine in plasma, and Clcr (fixed effects) and exposure to tobramycin. Unknown variabilities in measured drug concentrations and pharmacokinetic parameters were defined as random effects. Regression models were built using a stepwise procedure, starting with a model without covariables and consequently adding covariates one by one and testing its significance. The likelihood ratio test was used to compare models, and a P value of .05 was considered significant. The forms of the general covariate models were as follows:
logCli ¼ qCl;1 þ qCl;2: cov1 þ qCl;3: cov2 þ .qCl;n: covn1 þhCli logQ i ¼ qQ ;1 þ qQ ;2: cov1 þ qQ ;3: cov2 þ .qQ ;n: covn1 þ hQi logV1i ¼ qV1;1 þ qV1;2: cov1 þ qV1;3: cov2 þ .qV1;n: covn1 þ hV1i logV2i ¼ qV2;1 þ qV2;2: cov1 þ qV2;3: cov2 þ .qV2;n: covn1 þ hV2i
A. Haritova et al. / Journal of Equine Veterinary Science 32 (2012) 531-535
where Cli, Qi, V1i, and V2i are systemic clearance, Q, volume of the central compartment (V1), and volume of the peripheral compartment, respectively, of the ith horse. Cov are fixed effects variables, and q are proportionality constants that quantify the influence of the variable on the pharmacokinetic parameters when related to a variable. Independent random variables are represented by hCli, hQi, hV1i, and hV2i. Respective interindividual variances u2 were also calculated. The concentrationetime profile was described by the following equation:
yij ¼ f Xij
;
4i þ g Xij
;
4i εij; 1 i N; 1 j
ni where yij is the jth observation of subject i, N is the number of subjects, and ni is the number of observations of subject i; xij are the regression variables; εij is within-group errors. 2.6. Dose Estimation Maintenance dose (Dm) was estimated using the following equation:
Dm ¼
Cp :ClB :s F
where Cp represents the desired average plasma concentration of antibiotic, ClB is the total body clearance, s is the dosage interval, and F is bioavailability. F equals 1, as tobramycin was given by intravenous injection. 3. Results Results from the different regression models, without (model 1) and with covariables (models 2, 3, and 4), are shown in Table 1. Interindividual variability of systemic clearance, Q, V1, and V2 estimated for the model with no covariable is expressed in percentage and was 5.5%, 66.5%, 123%, and 41.7%, respectively. After analysis with all four
533
tested covariables taken together, evidence of a correlation between body weight, gender, creatinine or Clcr, and age and the calculated pharmacokinetic parameters Cl, Q, V1, and V2 were not revealed. Systemic clearance was related to body weight, Clcr, creatinine levels in blood, and gender, whereas the other three parameters Q, V1, and V2 were related to body weight and gender. A significant difference (P < .009) was found between model 1 and models with covariables, but no significant difference was demonstrated between models with covariables. The model with covariables body weight and Clcr (model 2, Fig. 1) was chosen as the final model because of the best fit between predicted and observed concentrations. Additional reason was the impact of Clcr on prediction of clearance of aminoglycosides such as tobramycin. Body weight, as a covariable for volumes of distribution, and Clcr, as a covariable for systemic clearance, decreased interindividual variability of estimated parameters (Table 1). Values of total body clearance, calculated with model 2, were used for estimation of dosage regimens (Table 2).
4. Discussion Population pharmacokinetic analysis is found to be useful in veterinary medicine because it allows defining factors that could influence pharmacokinetics of antibiotics and could decrease the risk of usage of inaccurate dosages in subpopulations of animals. Application of a suitable dosage regimen can decrease the risk of emergence of resistant strains of pathogenic bacteria and at the same time can avoid development of toxic concentrations. This approach was widely used in human medicine, especially for aminoglycosides, which are drugs with concentrationdependent efficacy [8,9]. Additionally, population pharmacokinetics has high utility in direct patient care in clinical cases by taking into account physiologic and pathophysiologic factors such as renal impairment. The experience with this class of drugs showed that dosing of aminoglycosides could be precise during the course of
Table 1 Population pharmacokinetic parameters of tobramycin administered intravenously at a dose rate of 4 mg/kg to healthy horses (n ¼ 28), and population pharmacokinetic models Pharmacokinetic Variable
Mean SEM
Population Parameter (u2, CV%)
Model 1, no covariates 0.11 0.02 0.11 (5.5) ClB (l.h1.kg1) 0.06 0.05 0.036 (123) V1 (l.kg1) 1 0.13 0.07 0.6 (41.7) V2 (l.kg ) Q (l.h1.kg1) 0.22 0.15 0.155 (66.5) Model 2, covariates body weight and creatinine clearance 0.11 0.01 0.087 (6.0) ClB (l.h1.kg1) V1 (l.kg1) 0.089 0.009 0.652 (17.7) V2 (l.kg1) 0.22 0.05 1.56 (4.5) Q (l.h1.kg1) 0.19 0.04 0.164 (77.5) Model 3, covariates body weight and plasma concentrations of creatinine ClB (l.h1.kg1) 0.11 0.01 0.093 (3.4) V1 (l.kg1) 0.06 0.02 0.357 (31.3) V2 ( l.kg1) 0.17 0.06 0.495 (30.2) Q (l.h1.kg1) 0.24 0.12 0.544 (71.6)
Covariate Model
e e e e Cl ¼ 0.087.(Clcri/MClcr)0.058.e(hCli) V1 ¼ 0.652.(bwi/Mbw)0.005.e(hV1i) V2 ¼ 1.56.(bwi/Mbw)0.0055.e(hV2i) e Cl ¼ 0.093.( bwi/Mbw)0.0004.e(hCli) þ 0.093.(Clcri/MClcr)0.0037.e(hCli) V1 ¼ 0.357.(bwi/Mbw)0.0045.e(hV1i) V2 ¼ 0.495.(bwi/Mbw)0.0026.e(hV2i) Q ¼ 0.544.(Qi/MQ)0.058.e(hQ)
ClB, total body clearance of tobramycin; V1 and V2, volumes of distribution in central and peripheral compartment, respectively; Q, intercompartmental clearance; Clcri, creatinine clearance of an individual animal; MClcr, geometric mean of creatinine clearance; e, exponent; h, random variable; u2, interindividual variance; bwi, body weight of an individual animal; Mbw, geometric mean of body weight; Qi, intercompartmental clearance of an individual animal; MQ, geometric mean intercompartmental clearance.
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A. Haritova et al. / Journal of Equine Veterinary Science 32 (2012) 531-535
Fig. 1. Fit to the data of 28 horses receiving tobramycin intravenously at a dose of 4 mg/kg. The dots represent all observed plasma concentrations of tobramycin. The solid line is the mean pharmacokinetic profile of the population.
treatment, taking into account wide interpatient and, from time to time, plasma concentration variability during the treatment of an individual animal [10]. Population pharmacokinetic models have been used for estimation of the individual pharmacokinetic parameters such as total body clearance and volume of distribution needed for individualization of dosage [11]. Individual dosage regimen during the treatment can be revised by using patient profile (age, gender, body weight) and records from its biochemical parameters. Plasma concentrations of aminoglycoside per patient can be monitored during treatment at only a few time intervals [12,13]. Once created, model and a database with patient information can be used in clinics for dosage adjustment in critically ill patients. Availability of userfriendly software can help in introduction of this procedure as a routine practice in clinical care for horses. In our study, using a population approach, experimental data were fitted to a two-compartmental model, in accordance with earlier reports on tobramycin disposition [4,14,15]. Estimated mean value of population for systemic clearance of tobramycin is very close to previously published data in horses [4]. Similar range of values have been reported for humansdsystemic clearance between 0.041 and 0.071 L/hr/kg; Vd ranged from 0.49 to 0.94 L/kg, and V1 ranged from 0.294 to 0.335 L/kg [16,17]. In human studies, pharmacokinetic parameters of aminoglycosides have been associated in population models with gender, age, body weight, Clcr, and clinical status [17-19]. In an earlier investigation of population pharmacokinetics with gentamicin in horses, age, body Table 2 Doses of tobramycin, calculated on the basis of total body clearance, predicted with a population model related to creatinine clearance (model 2) Creatinine Clearance
Predicted ClB(L.hr1.kg1)
Model 2, Cl ¼ 0.087.(Clcri/MClcr)0.058.e(hCli) 2.5 mL.min1.kg1 0.104 1.5 mL.min1.kg1 1.101 0.098 1 mL.min1.kg1 0.5 mL.min1.kg1 0.094
Dose (mg.kg1/24 hr)
mg/mL
MIC 1 mg/mL
mg/mL
1.25 1.20 1.18 1.13
2.50 2.40 2.35 2.25
5.0 4.8 4.7 4.5
MIC 0.5
MIC 2
ClB, total body clearance of tobramycin; MIC, minimal inhibitory concentration of tobramycin.
weight, and serum creatinine concentrations were considered in the model [1]. In our study, the inclusion of body weight and Clcr in the model considerably reduced unexplained interindividual variability, especially for volumes of distribution, V1 and V2, and total body clearance of tobramycin (model 2 vs. model 1). This suggests that only a simple measurement such as body weight can significantly improve the population model to allow determination of pharmacokinetics in patient groups during routine usage of the drug. V1 can be viewed as the apparent volume from which tobramycin elimination occurs through kidney. Prediction of maximum desirable and minimum effective steady-state plasma concentrations is based on values of volumes of distribution. Clcr was considered as a covariable because it was expected to have a positive contribution to the regression equation for tobramycin clearance. This assumption was based on the properties of tobramycinda polar polycationic molecule with low protein binding that is eliminated by glomerular filtration [20]. Incorporation of Clcr in population model was preferred because a single measurement of plasma concentration of creatinine might not properly reflect actual kidney function. As in previously published studies, correlation between creatinine values in plasma and volumes of distribution was not found in the present study [18]. Addition of gender as a covariable did not significantly improve the prediction power of population model (covariate model is not shown). Therefore, model 2 was chosen as a final model for estimation of pharmacokinetic parameters and dose calculations. The population model was used to calculate maintenance dosage regimen for a patient according to its clinical condition. Minimum inhibitory concentration, required to inhibit the growth of 90% of microorganisms (MIC90) for susceptible pathogenic strains such as Pseudomonas aeruginosa ranged between 0.5 and 8 mg/mL. For example, for a horse with Clcr of 2.5 mL.min1.kg1, the population model 2 predicts a clearance of 0.104 l.h1.kg1. For a target concentration equal to 2 mg/mL, a dose of 5 mg.kg1/24 hr is required. Estimated tobramycin clearance was 0.094 l.h1.kg1, if one assumes that a horse was with severely impaired kidney function and Clcr of 0.5 mL.min1.kg1. In this case, the animal has to be treated with a dose of 4.5 mg.kg1/24 hr. The availability of pharmacokinetic model and covariate regression model for patient subpopulations is important for dosage refinement and better prediction of drug concentrations among individual animals. Such a population model gives an opportunity to keep the concentrations within safe and therapeutic windows. Although the present study was conducted with a relatively small group of animals, it demonstrates that tobramycin pharmacokinetics in horses is closely related to factors such as body weight and Clcr (model 2). The question about tobramycin disposition in diseased horses remains unanswered. According to the published data, pharmacokinetics of aminoglycosides is highly dependent on the clinical status of the patients, and changes in systemic clearance have been found in patients with impaired renal function [21,22]. Alterations in glomerular filtration and volume of extracellular fluid can be expected in diseased animals [23,24]. These changes are important for the pharmacokinetics of tobramycin and other aminoglycosides. Therefore, application of population
A. Haritova et al. / Journal of Equine Veterinary Science 32 (2012) 531-535
pharmacokinetics in diseased horses could be helpful in design of dosage regimens in a particular animal. References [1] Martin-Jimenez T, Papich MG, Riviere JE. Population pharmacokinetics of gentamicin in horses. Am J Vet Res 1998;59:1589-98. [2] Okubo T, Iyobe S. Antibacterial activities and PK/PD parameters of aminoglycosides against recent clinical isolates of gram-negative rods. Jpn J Antibiot 2002;55:514-23. [3] Wilson DW. Rational selection of antimicrobials for use in horses. AAEP 2001;47:75-93. [4] Hubenov H, Bakalov D, Krastev S, Yanev S, Haritova A, Lashev L. Pharmacokinetic studies on tobramycin in horses. J Vet Pharmacol Ther 2007;30:353-7. [5] Inclan G, Suarez E, Calvo R, Aguirre C, Macheras P, Gazouli M, et al. Bicompartmental kinetics of tobramycin analysed with a wide range of covariates. Int J Antimicrob Agents 2005;26:304-11. [6] Duffull SB, Kirkpatrick CM, Begg EJ. Comparison of two Bayesian approaches to dose-individualization for once-daily aminoglycoside regimens. Br J Clin Pharmacol 1997;43:125-35. [7] Martín-Jiménez T, Riviere JE. Mixed effects modeling of the disposition of gentamicin across domestic animal species. J Vet Pharmacol Ther 2001;24:321-32. [8] Kirkpatrick CM, Begg EJ, Barclay ML, Duffull SB. Aminoglycoside dosage regimens after therapeutic drug monitoring. Clin Pharmacokinet 2002;41:791-2. [9] McKellar QA, Sanchezbruni SF, Jones DG. Pharmacokinetic/ pharmacodynamic relationships of antimicrobial drugs used in veterinary medicine. J Vet Pharmacol Ther 2004;27:503-14. [10] David SB, Jelliffe RW. A Bayesian approach to tracking patients having changing pharmacokinetic parameters. J Pharmacokinet Pharmacodyn 2004;31:75-107. [11] Avent M, Rogers B, Cheng A, Paterson D. Current use of aminoglycosides; indications, pharmacokinetics and monitoring for toxicity. Intern Med J 2011;41:441-9. [12] Conil JM, Georges B, Ruiz S, Rival T, Seguin T, Cougot P, et al. Tobramycin disposition in ICU patients receiving a once daily regimen: population approach and dosage simulations. Br J Clin Pharmacol 2011;71:61-71.
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[13] Roberts JA, Field J, Visser A, Whitbread R, Tallot M, Lipman J, et al. Using population pharmacokinetics to determine gentamicin dosing during extended daily diafiltration in critically ill patients with acute kidney injury. Antimicrob Agents Chemother 2010;54: 3635-40. [14] Dinev TG. Comparison of the pharmacokinetics of five aminoglycoside and aminocyclitol antibiotics using allometric analysis in mammal and bird species. Res Vet Sci 2008;84:107-18. [15] Inclan G, Suarez E, Calvo R, Aguirre C, Macheras P, Gazouli M, et al. Bicompartmental kinetics of tobramycin analysed with a wide range of covariates. Int J Antimicrob Agents 2005;26:304-11. [16] Pacifici GM. Clinical pharmacokinetics of aminoglycosides in the neonate: a review. Eur J Clin Pharmacol 2009;65:419-27. [17] Touw DJ, Knox AJ, Smyth A. Population pharmacokinetics of tobramycin administered thrice daily and once daily in children and adults with cystic fibrosis. J Cyst Fibros 2007;6:327-33. [18] Jauregizar N, Wald JA, Astobieta A, Rodriguez Sasiain JM, Lukas JC, Calvo R. Population pharmacokinetics of netilmicin in short-term prophylactic treatment. Br J Clin Pharmacol 2003;55: 552-9. [19] de Hoog M, Schoemaker RC, van den Anker JN, Vinks AA. NONMEM and NPEM2 population modeling: a comparison using tobramycin data in neonates. Ther Drug Monit 2002;24:359-65. [20] Matthews I, Kirkpatrick C, Holford N. Quantitative justification for target concentration interventiondparameter variability and predictive performance using population pharmacokinetic models for aminoglycosides. Br J Clin Pharmacol 2004;58:8-19. [21] Xuan D, Lu JF, Nicolau DP, Nightingale CH. Population pharmacokinetics of tobramycin in hospitalized patients receiving once-daily dosing regimen. Int J Antimicrob Agents 2000;15:185-91. [22] Bracco D, Landry C, Dubois MJ, Eggimann P. Pharmacokinetic variability of extended interval tobramycin in burn patients. Burns 2008;34:791-6. [23] Frazier DL, Dix LP, Bowman KF, Thompson C, Riviere JE. Increased gentamicin nephrotoxicity in normal and diseased dogs administered identical serum drug concentration profiles: increased sensitivity in subclinical renal dysfunction. J Pharmacol Exp Ther 1986;239:946-51. [24] Hunter RP, Brown SA, Rollins JK, Nelligan DF. The effects of experimentally induced bronchopneumonia on the pharmacokinetics and tissue depletion of gentamicin in healthy and pneumonic calves. J Vet Pharmacol Ther 1991;14:276-92.
Journal of Equine Veterinary Science journal homepage: www.j-evs.com
Original Research
Population Pharmacokinetics of Tobramycin in Horses Aneliya Haritova PhD a , Dinyo Bakalov DVM b, Huben Hubenov PhD b, Lubomir Lashev DSci a a b
Department of Pharmacology, Physiology of Animals and Physiological Chemistry, Faculty of Veterinary Medicine, Trakia University, Stara Zagora, Bulgaria Department of Veterinary Surgery, Faculty of Veterinary Medicine, Trakia University, Stara Zagora, Bulgaria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 December 2010 Received in revised form 3 July 2011 Accepted 23 December 2011 Available online 4 February 2012
Population pharmacokinetics of tobramycin was investigated in 28 healthy horses, with an aim to assess interindividual variability in the disposition of the antibiotic. Additionally, a covariate model for improved prediction of the concentrations in a particular animal was developed. A two-compartmental model best described the data. The final population covariate regression model was based on relationships between body weight and central and peripheral volumes of distribution, and between creatinine clearance and systemic tobramycin clearance. The value of population systemic tobramycin clearance and its interindividual variation (CV) were 0.087 L.hr1.kg1 and 6.0%, respectively. The respective values for central and peripheral volumes of distribution were 0.652 L.kg1 with CV of 17.7% and 1.56 L.kg1 with CV of 4.5%. In horses with decreased glomerular filtration rate, lower tobramycin clearance is predicted with the population model that requires administration of lower dose than that accepted for treatment of horses with normal kidney function. Population pharmacokinetic analysis allows study of basic disposition of tobramycin in horses with sparse data. The prediction power of the regression model was improved by inclusion of covariables such as body weight and creatinine clearance. This model can be used in direct patient care for the construction of dosing strategy in individual clinical cases. Ó 2012 Elsevier Inc. All rights reserved.
Keywords: Horses Population pharmacokinetics Tobramycin
1. Introduction Aminoglycoside antibiotics are extensively used in the treatment of infections in horses, mainly those caused by Pseudomonas spp. and Enterobacteriaceae strains [1]. Tobramycin is an antibiotic of the aminoglycoside group active against gram-negative and some gram-positive bacteria, with higher activity against several gramnegative microorganisms than gentamicin [2,3]. As an aminoglycoside drug, it exhibits concentration-dependent bactericidal activity. Tobramycin pharmacokinetics was earlier investigated in healthy horses after a single intravenous administration, and it does not differ from the other
Corresponding author at: Aneliya Haritova, PhD, Department of Pharmacology, Physiology of Animals and Physiological Chemistry, Faculty of Veterinary Medicine, Trakia University, Students’ Campus, 6000 Stara Zagora, Bulgaria. E-mail address: [email protected] (A. Haritova). 0737-0806/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jevs.2011.12.010
aminoglycoside antibiotics [4]. Tobramycin shows rapid distribution to most fluids in the body, including peritoneal, pleural, pericardial, and synovial fluids [5]. High concentrations of tobramycin are observed in the kidneys, which is typical for aminoglycosides [6]. It is eliminated through the kidney by glomerular filtration. Accumulation cannot be expected for this compound because it shows high capacity of elimination in horses, comparable with that of amikacin. It was higher than those of gentamicin, streptomycin, and neomycin. Knowledge for pharmacokinetics of aminoglycosides is often obtained from designed trials with standardized groups of healthy horses. However, large variability of pharmacokinetic parameters of these antibiotics has been observed among horses [1]. Pharmacokinetic parameters vary between individual animals, and they may even differ within an animal during treatment or progression of disease. Difficulty to determine the doses of aminoglycosides is linked to the variation of systemic clearance and
532
A. Haritova et al. / Journal of Equine Veterinary Science 32 (2012) 531-535
volume of distribution. Therefore, there exists a concern regarding the appropriate dose selection for treatment of individual animals. Traditional pharmacokinetic studies cannot recognize sources of variability. In contrast, population pharmacokinetic methods consider these variations, so-called random effects. Random effects are associated with the variability in pharmacokinetic parameters between animals and variability in measured drug concentration of an animal. According to previous studies, variability of systemic clearance has been related to body weight, age, serum creatinine concentration, and creatinine clearance (Clcr). Variability in volume of distribution has been related to body weight, age, gender, and serum creatinine. Thus, population approach has a significant power for the individualization and optimization of dosage regimen in clinical cases. It allows refining the dosing regimen by considering link between the individual pharmacokinetic parameters and factors such as age, gender, body weight, and physiological and health status [1,7]. Current investigation was planned as a second stage of a pilot kinetic study of tobramycin in horses. The aim of the present study was to estimate the population pharmacokinetic parameters of tobramycin in horses. A population pharmacokinetic model, which accounts for effect of Clcr and body weight on tobramycin disposition, was developed. 2. Material and Methods 2.1. Drug Tobramycin sulfate (activity: 690 IU, provided by Actavis Ltd, Sofia, Bulgaria) was administered as a 20% w/v aqueous solution, prepared ex tempore. 2.2. Animals and Experimental Design Data from 28 horses (12 males and 16 females) of various breeds weighing 255-710 kg were analyzed. Age of the studied population was between 1 and 16 years. The animals were provided by an experimental farm of Trakia University, Stara Zagora, Bulgaria. They were fed with commercial food 4-5 hours after injection of the antibiotic and had free access to water. All horses underwent a complete physical examination. Hematological, biochemical plasma, and urine parameters were assessed. Creatinine values in plasma (Crplasma) and urine (Crurine) were also measured. Clcr was estimated according to the following equation:
Clcr ¼ ðCrurine :x Flowurine
Crplasma
Tobramycin was administered intravenously in vena jugularis at a dose of 4 mg/kg body weight between 08:00 and 09:00 hours in the morning. Blood samples of 5 mL each were collected by venipuncture from the opposite vena jugularis in heparinized tubes before treatment and 0.083, 0.25, 0.5, 2, 4, 6, 8, 10, 12, and 24 hours after treatment from seven horses (five females and two males) and 1, 3, and 7 hours after treatment from 21 animals. The samples were centrifuged within 1 hour (1,400 g, 10 minutes), and plasma was stored at 20 C until the time of analysis.
2.3. Tobramycin Assay Antibiotic concentrations were determined within 2 days after sampling by microbiological assay, as described in the pilot pharmacokinetic study [4]. In comparison with high-performance liquid chromatography analysis, this assay is more sensitive in the detection of low plasma and urine values. Shortly, Bacillus subtilis ATCC 6633 reference tester strain for quantitative determination of tobramycin was used. The limit of detection and limit of quantification values were 0.024 and 0.048 mg/mL, respectively. 2.4. Pharmacokinetic Analysis Plasma concentration curves versus time were interpreted by a two-compartmental model on the basis of knowledge about tobramycin disposition in horses obtained in a previously conducted study [4]. All concentrationetime data were analyzed simultaneously by nonlinear mixed effects modeling using the Monolix V2.2 (INRIA, Monolix group, France, http://group.monolix. org). Population parameters were estimated using the stochastic approximation expectation maximization algorithm. Variances of pharmacokinetic parameters were also computed. The model was parameterized in terms of systemic clearance (Cl), intercompartmental distribution clearance (Q), volume of distribution in the central compartment (V1), and volume of distribution in the peripheral compartment (V2) (model number 20 from the program library). 2.5. Regression Model Regression models were developed to examine potential relationship between subject clinical characteristics such as body weight, age, gender, creatinine in plasma, and Clcr (fixed effects) and exposure to tobramycin. Unknown variabilities in measured drug concentrations and pharmacokinetic parameters were defined as random effects. Regression models were built using a stepwise procedure, starting with a model without covariables and consequently adding covariates one by one and testing its significance. The likelihood ratio test was used to compare models, and a P value of .05 was considered significant. The forms of the general covariate models were as follows:
logCli ¼ qCl;1 þ qCl;2: cov1 þ qCl;3: cov2 þ .qCl;n: covn1 þhCli logQ i ¼ qQ ;1 þ qQ ;2: cov1 þ qQ ;3: cov2 þ .qQ ;n: covn1 þ hQi logV1i ¼ qV1;1 þ qV1;2: cov1 þ qV1;3: cov2 þ .qV1;n: covn1 þ hV1i logV2i ¼ qV2;1 þ qV2;2: cov1 þ qV2;3: cov2 þ .qV2;n: covn1 þ hV2i
A. Haritova et al. / Journal of Equine Veterinary Science 32 (2012) 531-535
where Cli, Qi, V1i, and V2i are systemic clearance, Q, volume of the central compartment (V1), and volume of the peripheral compartment, respectively, of the ith horse. Cov are fixed effects variables, and q are proportionality constants that quantify the influence of the variable on the pharmacokinetic parameters when related to a variable. Independent random variables are represented by hCli, hQi, hV1i, and hV2i. Respective interindividual variances u2 were also calculated. The concentrationetime profile was described by the following equation:
yij ¼ f Xij
;
4i þ g Xij
;
4i εij; 1 i N; 1 j
ni where yij is the jth observation of subject i, N is the number of subjects, and ni is the number of observations of subject i; xij are the regression variables; εij is within-group errors. 2.6. Dose Estimation Maintenance dose (Dm) was estimated using the following equation:
Dm ¼
Cp :ClB :s F
where Cp represents the desired average plasma concentration of antibiotic, ClB is the total body clearance, s is the dosage interval, and F is bioavailability. F equals 1, as tobramycin was given by intravenous injection. 3. Results Results from the different regression models, without (model 1) and with covariables (models 2, 3, and 4), are shown in Table 1. Interindividual variability of systemic clearance, Q, V1, and V2 estimated for the model with no covariable is expressed in percentage and was 5.5%, 66.5%, 123%, and 41.7%, respectively. After analysis with all four
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tested covariables taken together, evidence of a correlation between body weight, gender, creatinine or Clcr, and age and the calculated pharmacokinetic parameters Cl, Q, V1, and V2 were not revealed. Systemic clearance was related to body weight, Clcr, creatinine levels in blood, and gender, whereas the other three parameters Q, V1, and V2 were related to body weight and gender. A significant difference (P < .009) was found between model 1 and models with covariables, but no significant difference was demonstrated between models with covariables. The model with covariables body weight and Clcr (model 2, Fig. 1) was chosen as the final model because of the best fit between predicted and observed concentrations. Additional reason was the impact of Clcr on prediction of clearance of aminoglycosides such as tobramycin. Body weight, as a covariable for volumes of distribution, and Clcr, as a covariable for systemic clearance, decreased interindividual variability of estimated parameters (Table 1). Values of total body clearance, calculated with model 2, were used for estimation of dosage regimens (Table 2).
4. Discussion Population pharmacokinetic analysis is found to be useful in veterinary medicine because it allows defining factors that could influence pharmacokinetics of antibiotics and could decrease the risk of usage of inaccurate dosages in subpopulations of animals. Application of a suitable dosage regimen can decrease the risk of emergence of resistant strains of pathogenic bacteria and at the same time can avoid development of toxic concentrations. This approach was widely used in human medicine, especially for aminoglycosides, which are drugs with concentrationdependent efficacy [8,9]. Additionally, population pharmacokinetics has high utility in direct patient care in clinical cases by taking into account physiologic and pathophysiologic factors such as renal impairment. The experience with this class of drugs showed that dosing of aminoglycosides could be precise during the course of
Table 1 Population pharmacokinetic parameters of tobramycin administered intravenously at a dose rate of 4 mg/kg to healthy horses (n ¼ 28), and population pharmacokinetic models Pharmacokinetic Variable
Mean SEM
Population Parameter (u2, CV%)
Model 1, no covariates 0.11 0.02 0.11 (5.5) ClB (l.h1.kg1) 0.06 0.05 0.036 (123) V1 (l.kg1) 1 0.13 0.07 0.6 (41.7) V2 (l.kg ) Q (l.h1.kg1) 0.22 0.15 0.155 (66.5) Model 2, covariates body weight and creatinine clearance 0.11 0.01 0.087 (6.0) ClB (l.h1.kg1) V1 (l.kg1) 0.089 0.009 0.652 (17.7) V2 (l.kg1) 0.22 0.05 1.56 (4.5) Q (l.h1.kg1) 0.19 0.04 0.164 (77.5) Model 3, covariates body weight and plasma concentrations of creatinine ClB (l.h1.kg1) 0.11 0.01 0.093 (3.4) V1 (l.kg1) 0.06 0.02 0.357 (31.3) V2 ( l.kg1) 0.17 0.06 0.495 (30.2) Q (l.h1.kg1) 0.24 0.12 0.544 (71.6)
Covariate Model
e e e e Cl ¼ 0.087.(Clcri/MClcr)0.058.e(hCli) V1 ¼ 0.652.(bwi/Mbw)0.005.e(hV1i) V2 ¼ 1.56.(bwi/Mbw)0.0055.e(hV2i) e Cl ¼ 0.093.( bwi/Mbw)0.0004.e(hCli) þ 0.093.(Clcri/MClcr)0.0037.e(hCli) V1 ¼ 0.357.(bwi/Mbw)0.0045.e(hV1i) V2 ¼ 0.495.(bwi/Mbw)0.0026.e(hV2i) Q ¼ 0.544.(Qi/MQ)0.058.e(hQ)
ClB, total body clearance of tobramycin; V1 and V2, volumes of distribution in central and peripheral compartment, respectively; Q, intercompartmental clearance; Clcri, creatinine clearance of an individual animal; MClcr, geometric mean of creatinine clearance; e, exponent; h, random variable; u2, interindividual variance; bwi, body weight of an individual animal; Mbw, geometric mean of body weight; Qi, intercompartmental clearance of an individual animal; MQ, geometric mean intercompartmental clearance.
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A. Haritova et al. / Journal of Equine Veterinary Science 32 (2012) 531-535
Fig. 1. Fit to the data of 28 horses receiving tobramycin intravenously at a dose of 4 mg/kg. The dots represent all observed plasma concentrations of tobramycin. The solid line is the mean pharmacokinetic profile of the population.
treatment, taking into account wide interpatient and, from time to time, plasma concentration variability during the treatment of an individual animal [10]. Population pharmacokinetic models have been used for estimation of the individual pharmacokinetic parameters such as total body clearance and volume of distribution needed for individualization of dosage [11]. Individual dosage regimen during the treatment can be revised by using patient profile (age, gender, body weight) and records from its biochemical parameters. Plasma concentrations of aminoglycoside per patient can be monitored during treatment at only a few time intervals [12,13]. Once created, model and a database with patient information can be used in clinics for dosage adjustment in critically ill patients. Availability of userfriendly software can help in introduction of this procedure as a routine practice in clinical care for horses. In our study, using a population approach, experimental data were fitted to a two-compartmental model, in accordance with earlier reports on tobramycin disposition [4,14,15]. Estimated mean value of population for systemic clearance of tobramycin is very close to previously published data in horses [4]. Similar range of values have been reported for humansdsystemic clearance between 0.041 and 0.071 L/hr/kg; Vd ranged from 0.49 to 0.94 L/kg, and V1 ranged from 0.294 to 0.335 L/kg [16,17]. In human studies, pharmacokinetic parameters of aminoglycosides have been associated in population models with gender, age, body weight, Clcr, and clinical status [17-19]. In an earlier investigation of population pharmacokinetics with gentamicin in horses, age, body Table 2 Doses of tobramycin, calculated on the basis of total body clearance, predicted with a population model related to creatinine clearance (model 2) Creatinine Clearance
Predicted ClB(L.hr1.kg1)
Model 2, Cl ¼ 0.087.(Clcri/MClcr)0.058.e(hCli) 2.5 mL.min1.kg1 0.104 1.5 mL.min1.kg1 1.101 0.098 1 mL.min1.kg1 0.5 mL.min1.kg1 0.094
Dose (mg.kg1/24 hr)
mg/mL
MIC 1 mg/mL
mg/mL
1.25 1.20 1.18 1.13
2.50 2.40 2.35 2.25
5.0 4.8 4.7 4.5
MIC 0.5
MIC 2
ClB, total body clearance of tobramycin; MIC, minimal inhibitory concentration of tobramycin.
weight, and serum creatinine concentrations were considered in the model [1]. In our study, the inclusion of body weight and Clcr in the model considerably reduced unexplained interindividual variability, especially for volumes of distribution, V1 and V2, and total body clearance of tobramycin (model 2 vs. model 1). This suggests that only a simple measurement such as body weight can significantly improve the population model to allow determination of pharmacokinetics in patient groups during routine usage of the drug. V1 can be viewed as the apparent volume from which tobramycin elimination occurs through kidney. Prediction of maximum desirable and minimum effective steady-state plasma concentrations is based on values of volumes of distribution. Clcr was considered as a covariable because it was expected to have a positive contribution to the regression equation for tobramycin clearance. This assumption was based on the properties of tobramycinda polar polycationic molecule with low protein binding that is eliminated by glomerular filtration [20]. Incorporation of Clcr in population model was preferred because a single measurement of plasma concentration of creatinine might not properly reflect actual kidney function. As in previously published studies, correlation between creatinine values in plasma and volumes of distribution was not found in the present study [18]. Addition of gender as a covariable did not significantly improve the prediction power of population model (covariate model is not shown). Therefore, model 2 was chosen as a final model for estimation of pharmacokinetic parameters and dose calculations. The population model was used to calculate maintenance dosage regimen for a patient according to its clinical condition. Minimum inhibitory concentration, required to inhibit the growth of 90% of microorganisms (MIC90) for susceptible pathogenic strains such as Pseudomonas aeruginosa ranged between 0.5 and 8 mg/mL. For example, for a horse with Clcr of 2.5 mL.min1.kg1, the population model 2 predicts a clearance of 0.104 l.h1.kg1. For a target concentration equal to 2 mg/mL, a dose of 5 mg.kg1/24 hr is required. Estimated tobramycin clearance was 0.094 l.h1.kg1, if one assumes that a horse was with severely impaired kidney function and Clcr of 0.5 mL.min1.kg1. In this case, the animal has to be treated with a dose of 4.5 mg.kg1/24 hr. The availability of pharmacokinetic model and covariate regression model for patient subpopulations is important for dosage refinement and better prediction of drug concentrations among individual animals. Such a population model gives an opportunity to keep the concentrations within safe and therapeutic windows. Although the present study was conducted with a relatively small group of animals, it demonstrates that tobramycin pharmacokinetics in horses is closely related to factors such as body weight and Clcr (model 2). The question about tobramycin disposition in diseased horses remains unanswered. According to the published data, pharmacokinetics of aminoglycosides is highly dependent on the clinical status of the patients, and changes in systemic clearance have been found in patients with impaired renal function [21,22]. Alterations in glomerular filtration and volume of extracellular fluid can be expected in diseased animals [23,24]. These changes are important for the pharmacokinetics of tobramycin and other aminoglycosides. Therefore, application of population
A. Haritova et al. / Journal of Equine Veterinary Science 32 (2012) 531-535
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