In veterinary medicine, the treatment of wild birds is challenging because of the scarcity of scientific information with which to determine dosing regimens. Veterinarians involved in wildlife conservation sometimes use allometric scaling as a tool to make decisions on treatments for species in which few data are available. Frequently, allometric scaling is applied to estimate drug doses when pharmacokinetic data for the target species are not available. Special consideration must be given when a particular pharmacokinetic behavior is described for a specific species (eg, obligate scavengers). This could be important because slow drug elimination in species of the Gyps genus has been reported.1–3
The American black vulture (Coragyps atratus) is a New World vulture belonging to the order Cathartiforme. Its distribution includes the southern United States and South America, and it is classified as least concern in the red list of threatened species.4 Although this vulture species has genetic differences from members of the Gyps genus, the carrion-feeding lifestyle of vultures and the morphological convergence between Gyps spp and American black vultures,5,6 which also can involve shared physiologic characteristics, could be reflected in similar pharmacokinetic behavior of drugs.
Enrofloxacin is one of the most widely used fluoroquinolones in veterinary medicine. Because of its pharmacokinetic profile and spectrum of activity, enrofloxacin is an antimicrobial agent frequently administered to avian species in rehabilitation centers. It is commonly used to treat bacteremia or septicemia and infections of the soft tissues, skin, bones and joints, urinary tract, respiratory tract, and reproductive tract.7,8 Although enrofloxacin has been extensively evaluated in avian species, no pharmacokinetic studies have been performed with scavengers; therefore, the dose of enrofloxacin for black vultures is currently obtained by extrapolation from data for other bird species.9
The objectives of the study reported here were to evaluate the pharmacokinetic behavior of enrofloxacin after IV administration in black vultures, to compare (by means of the physiologic interpretation of Cl) the elimination of enrofloxacin with Cl for enrofloxacin in other avian species, and to evaluate whether allometric scaling is an appropriate tool for extrapolation of the dose of enrofloxacin in American black vultures. We hypothesized that allometric scaling would be a useful tool for extrapolation of drug doses in avian species.
Materials and Methods
Animals
Six healthy adult American black vultures housed in captivity at the Buenos Aires Zoological Garden were used in the study. Vultures were fed in a manner appropriate for the species. Body weight of the vultures ranged from 1.8 to 2.2 kg. Vultures were considered healthy on the basis of results of a complete physical examination, routine consumption of daily meals, maintenance of body weight, and results of hematologic tests. No drugs were administered to the vultures for at least 2 months before the start of the study. The study was performed at the Buenos Aires Zoological Garden, and it was approved by the Institutional Animal Care and Use Committee of the Veterinary Sciences School of the Universidad de Buenos Aires.
Experimental procedures
A 24-gauge catheter was placed in an ulnar vein of each vulture. A single dose (5 mg/kg) of a 5% injectable solution of enrofloxacina was administered IV. Blood samples (0.6 mL) were collected by use of a 27-gauge catheter placed in a medial tarsal vein. Blood samples were obtained immediately before (time 0) and 5, 15, and 35 minutes and 1, 2, 4, 6, 8, 11, 24, 29, 34, and 48 hours after enrofloxacin administration. Samples were immediately placed in tubes containing heparin.
Drug assay
Analytic standards of ofloxacin, enrofloxacin, and ciprofloxacin were purchased.b Sample processing and drug detection methods for enrofloxacin and ciprofloxacin were adapted from a method described elsewhere.10 Ofloxacin was used as an internal standard.
Enrofloxacin was quantified with a high-performance liquid chromatography system.c Extracted samples were injected into an ion-pairing C18 reverse-phase column (5 μm; 150 × 4.6 mm). Limit of quantification was 0.025 μg/mL for enrofloxacin and 0.050 μg/mL for ciprofloxacin; calibration curves were linear (R2 > 0.99) up to concentrations of 5 μg/mL. Intraday precision was < 8%, and interday precision was < 12%. Accuracy ranged from 82% to 120% and from 88% to 113% for enrofloxacin and ciprofloxacin, respectively.
Pharmacokinetic analysis
Plasma concentrations of enrofloxacin were evaluated by use of compartmental and noncompartmental analysis with a software package.d Data were used for additional computations.
Physiologic interpretation of Cl and dose calculation
Cardiac output of various species for which the pharmacokinetics of enrofloxacin was known was estimated by use of the allometric equation proposed in another study11 for the physiologic interpretation of body Cl in birds: CO = 290.7 × BW0.69, where CO is cardiac output in milliliters per minute and BW is body weight in kilograms.
The minimal physiologic model for total body Cl12 was applied. Considering the entire body as a system of drug reservoir, the extraction ratio was estimated as the percentage of drug cleared by the entire body during a single passage through the various organs that contributed to body Cl; the extraction ratio was calculated as Cl/CO, where CO is the previously determined cardiac output.
Because the MIC, MBC, and MPC for isolates cultured from black vultures were not known, MIC, MBC, and MPC values for Escherichia coli isolates cultured from chickens13 were used for extrapolation of the optimal dose of enrofloxacin for American black vultures. The optimal dose was calculated by use of the following equation: dose = ([AUC/MIC] × Cl × MIC)/(F × 24 hours), where F is bioavailability, which was assigned a value of 1. Values for MIC in the equation were replaced by values for MBC or MPC to calculate optimal doses for bactericidal action or resistance prevention, respectively. The optimal dose was calculated by use of the Cl determined for the pharmacokinetic analysis and also by use of the Cl predicted with allometric scaling.
Allometric computation
A simple allometric analysis of enrofloxacin pharmacokinetics was performed with pharmacokinetic data obtained for American black vultures in the present study and data obtained after IV administration in other avian species.10,14–30 Allometric analysis was performed for t½β, Vdss, Cl, MRT, and AUC weighted by dose (ie, AUC/dose). For allometric scaling, each parameter and the body weight were logarithmically transformed and fitted by use of least squares linear regression analysis to the following equation: log10 y = log10 a + (b × log10 BW), where y is a pharmacokinetic parameter (Cl, Vdss, t½β, MRT, and AUC/dose), a is the y-axis intercept, b is the slope, and BW is body weight.
Allometric scaling involved the use of data for up to 13 avian species. Allometric scaling was performed with the inclusion or exclusion of pharmacokinetic data obtained for American black vultures (13 or 12 avian species, respectively), with the exclusion of data obtained for ostriches and greater rheas (11 avian species), and with the exclusion of data obtained for American black vultures, ostriches, and greater rheas (10 avian species). Only when a good fit was found were the allometric equations obtained from those linear regressions used to predict pharmacokinetic parameters.
The predicted Vdss and Cl for American black vultures (value for BW in the equation was 2.0 kg) were obtained by use of allometric equations that included data for all avian species (n = 13), all avian species except for American black vultures (12), all avian species except for ostriches and greater rheas (11), and all avian species except for American black vultures, ostriches, and greater rheas (10). The 95% prediction interval and accuracy of the predicted values were calculated.
Statistical analysis
The analysis was performed with statistical software.e Normality of data was assessed with the Shapiro-Wilk test. Correlation analysis and least squares linear regression were performed. Goodness of fit was measured by use of R2 and the SD of residuals. An F test was used to evaluate the null hypothesis that the overall slope was 0, and the Wald-Wolfowitz runs test was used to determine whether fit of the data differed significantly from a straight line. Values were considered significant at P < 0.05.
Results
Mean plasma concentration-time curve and pharmacokinetic parameters obtained after IV administration were determined (Figure 1; Table 1). The Cl of enrofloxacin was 0.147 L/h·kg, Vdss was 3.47 L/kg, and t½β was 18.3 hours. Very low plasma enrofloxacin concentrations were detected, and only 1 sample from each of 2 vultures had a concentration greater than the limit of quantification. Because of the sparse number of points available for evaluation, pharmacokinetic analysis was not performed.
Mean ± SD plasma concentration of enrofloxacin over time in 6 American black vultures (Coragyps atratus) after IV administration of a single dose of enrofloxacin (5 mg/kg).
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.727
Mean ± SD plasma concentration of enrofloxacin over time in 6 American black vultures (Coragyps atratus) after IV administration of a single dose of enrofloxacin (5 mg/kg).
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.727
Mean ± SD plasma concentration of enrofloxacin over time in 6 American black vultures (Coragyps atratus) after IV administration of a single dose of enrofloxacin (5 mg/kg).
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.727
Pharmacokinetic parameters obtained for 6 American black vultures (Coragyps atratus) after IV administration of enrofloxacin (5 mg/kg).
| Analysis | Parameter | Arithmetic Mean | Median | Geometric mean | SD | Range* |
|---|---|---|---|---|---|---|
| Compartmenta | C0 (μg/mL) | 3.09 | 3.09 | 3.09 | 0.18 | 0.42 |
| α (h−1) | 0.552 | 0.598 | 0.548 | 0.074 | 0.167 | |
| β (h−1) | 0.038 | 0.040 | 0.038 | 0.004 | 0.010 | |
| AUC0-∞ (μg·h/mL) | 34.9 | 35.5 | 34.5 | 5.9 | 14.3 | |
| t½α (h)† | 1.3 | 1.2 | 1.3 | 0.2 | 0.4 | |
| t½β (h)† | 18.3 | 17.3 | 18.1 | 2.4 | 5.8 | |
| Vdc (L/kg) | 1.62 | 1.62 | 1.62 | 0.09 | 0.22 | |
| Cl (L/h·kg) | 0.147 | 0.141 | 0.145 | 0.026 | 0.060 | |
| Vdss (L/kg) | 3.47 | 3.44 | 3.45 | 0.45 | 1.09 | |
| Noncompartmental | AUC0-last (μg·h/mL) | 33.1 | 34.1 | 32.9 | 3.3 | 9.5 |
| AUC0-∞ (μg·h/mL) | 41.0 | 42.1 | 40.9 | 3.7 | 9.6 | |
| t½λ (h)† | 16.7 | 16.7 | 16.7 | 1.1 | 3.1 | |
| Vdz (L/kg) | 3.14 | 3.08 | 3.11 | 0.36 | 1.05 | |
| Cl (L/h·kg) | 0.13 | 0.12 | 0.13 | 0.01 | 0.03 | |
| MRT0-last (h)† | 16.4 | 16.2 | 16.3 | 0.8 | 2.1 | |
| MRT0-∞ (h)† | 24.6 | 24.2 | 24.5 | 1.8 | 4.8 | |
| tlast (h) | 48.0 | 48.0 | 48.0 | 0.0 | 0.0 | |
| Clast (μg/mL) | 0.33 | 0.33 | 0.33 | 0.04 | 0.09 |
The range was calculated as the maximum value minus the minimum value.
Value reported is the harmonic mean.
α = Distribution rate constant. AUC0-∞ = AUC from time zero to infinity. AUC0-last = AUC from time zero to the last quantifiable concentration. β = Elimination rate constant. C0 = Plasma concentration at time 0. Clast = Last quantifiable concentration. MRT0-∞ = MRT from time zero to infinity. MRT0_last = MRT from time zero to the last quantifiable concentration. tlast = Time of last quantifiable concentration. t½a = Distribution half-life. tl/2λ, = Terminal half-life. Vdc = Volume of distribution of the central compartment. Vdz = Volume of distribution based on the terminal phase.
The estimated cardiac output, extraction ratio, and extrapolated dose were calculated for the 6 American black vultures of the present study and avian species reported in other studies10,14–30 (Figure 2; Table 2). The extraction ratio for American black vultures was 1.04%. The allometric equation was used to fit data for all avian species (including vultures; 13 avian species), data that excluded vultures (12 avian species), data that included vultures but excluded ostriches and greater rheas (11 avian species), and data that excluded vultures, ostriches, and greater rheas (10 avian species). Allometric relationships between body weight and pharmacokinetic parameters for the American black vultures of the present study and avian species reported in other studies10,14–30 were evaluated (Figure 3; Tables 3 and 4). Only data for Vdss and, to a lesser extent, Cl had a good fit. When all 13 species were considered for the allometric calculation, high variability (94%) in the Vdss of enrofloxacin among species was explained by interspecies variation in body weight. Also, 69% of the variability in Cl could be attributed to differences in body weight. Slopes for both equations were significantly different from 0, but they did not differ significantly from a straight line.
Values for enrofloxacin Cl (gray bars) and the extraction ratio (black bars) for 6 American black vultures and avian species reported in other studies.10,14–30 The extraction ratio was obtained by use of an equation proposed in another study.12
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.727
Values for enrofloxacin Cl (gray bars) and the extraction ratio (black bars) for 6 American black vultures and avian species reported in other studies.10,14–30 The extraction ratio was obtained by use of an equation proposed in another study.12
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.727
Values for enrofloxacin Cl (gray bars) and the extraction ratio (black bars) for 6 American black vultures and avian species reported in other studies.10,14–30 The extraction ratio was obtained by use of an equation proposed in another study.12
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.727
Results for allometric scaling of Cl (A) and Vdss (B) for enrofloxacin in 6 American black vultures and avian species reported in other studies.10,14–30 BW = Body weight.
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.727
Results for allometric scaling of Cl (A) and Vdss (B) for enrofloxacin in 6 American black vultures and avian species reported in other studies.10,14–30 BW = Body weight.
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.727
Results for allometric scaling of Cl (A) and Vdss (B) for enrofloxacin in 6 American black vultures and avian species reported in other studies.10,14–30 BW = Body weight.
Citation: American Journal of Veterinary Research 80, 8; 10.2460/ajvr.80.8.727
Values for American black vultures and various avian species administered enrofloxacin.
| Extrapolated dose | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Avian species | Body weight (kg) | Dose (mg·kg) | CO (L/h·kg) | Cl (L/h·kg) | Extraction ratio (%)* | MIC | MBC | MPC | Reference |
| African penguin | 3.60 | 15 | 11.73 | 0.182 | 1.55 | 0.23 | 1.37 | 91 | 30 |
| Chicken | 0.65 | 10 | 19.93 | 0.618 | 3.10 | 0.77 | 4.64 | 309 | 26 |
| Chicken | 2.70 | 5 | 12.82 | 0.198 | 1.54 | 0.25 | 1.49 | 99 | 21 |
| Chicken | 2.70 | 5 | 12.82 | 0.179 | 1.40 | 0.22 | 1.34 | 89 | 21 |
| Chicken | 2.95 | 2.5 | 12.47 | 0.667 | 5.35 | 0.83 | 5.00 | 333 | 22 |
| Chicken | 2.98 | 5 | 12.43 | 0.250 | 2.01 | 0.31 | 1.88 | 125 | 17 |
| Chicken | 0.95 | 10 | 17.72 | 0.350 | 1.97 | 0.44 | 2.63 | 175 | 28 |
| Chicken | 1.68 | 10 | 14.85 | 0.130 | 0.88 | 0.16 | 0.98 | 65 | 14 |
| Chicken | 1.70 | 10 | 14.80 | 0.148 | 1.00 | 0.19 | 1.11 | 74 | 14 |
| Chicken | 1.82 | 10 | 14.49 | 0.132 | 0.91 | 0.17 | 0.99 | 66 | 14 |
| Chicken | 2.50 | 10 | 13.13 | 0.290 | 2.21 | 0.36 | 2.18 | 145 | 15 |
| Mean chicken | — | — | — | 2.04 | — | — | — | — | |
| Duck | 1.09 | 10 | 16.98 | 0.890 | 5.24 | 1.11 | 6.68 | 445 | 29 |
| Emu | 25.50 | 2.2 | 6.39 | 0.360 | 5.63 | 0.45 | 2.70 | 180 | 25 |
| Greater rhea | 2.97 | 15 | 12.45 | 3.950 | 31.74 | 4.94 | 29.63 | 1,975 | 19 |
| Houbara bustard | 1.29 | 10 | 16.12 | 0.343 | 2.13 | 0.43 | 2.57 | 172 | 16 |
| Ostrich | 43.50 | 5 | 5.42 | 4.560 | 84.20 | 5.70 | 34.20 | 2,280 | 18 |
| Pheasant | 0.95 | 10 | 17.72 | 0.600 | 3.39 | 0.75 | 4.50 | 300 | 27 |
| Quail | 0.20 | 10 | 28.64 | 1.520 | 5.31 | 1.90 | 11.40 | 760 | 23 |
| Quail | 0.20 | 10 | 28.64 | 1.400 | 4.89 | 1.75 | 10.50 | 700 | 27 |
| Mean quail | — | — | — | — | 5.10 | — | — | — | — |
| Red-tailed hawk | 1.25 | 15 | 16.28 | 0.240 | 1.47 | 0.30 | 1.80 | 120 | 24 |
| Southern crested caracara | 1.33 | 5 | 15.97 | 0.240 | 1.50 | 0.30 | 1.80 | 120 | 10 |
| Turkey | 6.95 | 2.5 | 9.56 | 0.502 | 5.25 | 0.63 | 3.77 | 251 | 22 |
| Turkey | 7.00 | 10 | 9.54 | 0.408 | 4.28 | 0.51 | 3.06 | 204 | 20 |
| Mean turkey | — | — | — | — | 4.76 | — | — | — | — |
| American black vulture | 2.00 | 10 | 14.07 | 0.147 | 1.04 | 0.18 | 1.10 | 73 | NA |
The extrapolated dose of enrofloxacin was determined for microorganisms with an MIC of 0.01 μg/mL, an MBC of 0.06 μg/mL, and an MPC of 4 μg/mL.
For the extraction ratio, values > 35% were considered high, values of approximately 15% were considered medium, and values < 5% were considered low.
— = Not determined. CO = Cardiac output. NA = Not applicable; represents results for the 6 American black vultures of the study reported here.
Values for pharmacokinetic parameters of enrofloxacin after IV administration to various avian species.
| Avian species | Body weight (kg)* | Cl (L/h·kg) | Vdss (L/kg) | t½β (h) | MRT (h) | AUC/dose (g·h/mL) | Reference |
|---|---|---|---|---|---|---|---|
| African penguin | 3.60 | 0.182 | 3.00 | 13.67 | 16.50 | 5.50 | 30 |
| Chicken | 0.65 | 0.618 | 3.90 | 5.56 | 6.38 | 1.62 | 26 |
| Chicken | 2.70 | 0.198 | 1.98 | 6.99 | 10.24 | 5.35 | 21 |
| Chicken | 2.70 | 0.179 | 1.77 | 7.52 | 9.96 | 5.91 | 21 |
| Chicken | 2.95 | 0.667 | 4.55 | 5.54 | 6.99 | 1.62 | 22 |
| Chicken | 2.98 | 0.250 | 2.92 | 10.96 | 12.50 | 4.34 | 17 |
| Chicken | 0.95 | 0.350 | 2.22 | 4.75 | 6.72 | 2.85 | 28 |
| Chicken | 1.68 | 0.130 | 2.40 | 4.16 | — | 2.76 | 14 |
| Chicken | 1.70 | 0.148 | 2.43 | 3.65 | — | 2.57 | 14 |
| Chicken | 1.82 | 0.132 | 2.48 | 4.34 | — | 2.81 | 14 |
| Chicken | 2.50 | 0.290 | 2.77 | 10.29 | 9.65 | 3.45 | 15 |
| Duck | 1.09 | 0.890 | 2.07 | 6.47 | 2.26 | 2.36 | 29 |
| Emu | 25.50 | 0.360 | 1.62 | 3.33 | 4.40 | 3.75 | 25 |
| Greater rhea | 2.97 | 3.950 | 5.01 | 2.66 | 1.23 | 0.24 | 19 |
| Houbara bustard | 1.29 | 0.343 | 2.98 | 5.63 | 8.13 | 2.98 | 16 |
| Ostrich | 43.50 | 4.560 | 3.40 | 0.78 | 0.70 | 0.19 | 18 |
| Pheasant | 0.95 | 0.600 | 3.70 | 4.60 | — | 1.81 | 27 |
| Quail | 0.20 | 1.520 | 5.36 | 2.45 | 3.53 | 0.66 | 23 |
| Quail | 0.20 | 1.400 | 4.60 | 2.30 | — | 0.71 | 27 |
| Red-tailed hawk | 1.30 | 0.240 | 2.30 | 19.40 | 9.58 | 4.15 | 24 |
| Southern crested caracara | 1.33 | 0.240 | 2.09 | 7.81 | 7.97 | 4.38 | 10 |
| Turkey | 6.95 | 0.502 | 4.06 | 6.05 | 8.32 | 2.09 | 22 |
| Turkey | 7.00 | 0.408 | 3.57 | 6.64 | 8.96 | 2.59 | 20 |
| American black vulture | 2.00 | 0.147 | 3.47 | 18.29 | 24.58 | 8.20 | NA |
Value reported is mean body weight. When the mean body weight was not included in the report, the value midway between the minimum and maximum value was used as an estimate.
See Table 2 for remainder of key.
Parameters determined for variables by use of allometric analysis of data for various avian species.
| No. of avian species* | Parameter | Cl | Vdss | t½β | MRT | AUC/dose |
|---|---|---|---|---|---|---|
| 13 | a | 0.46 | 3.17 | 6.77 | 6.72 | 2.35 |
| bt | 1.157 (0.232) | 0.939 (0.070) | −0.233 (0.174) | −0.227 (0.216) | −0.172 (0.241) | |
| R2 | 0.693 | 0.943 | 0.141 | 0.100 | 0.044 | |
| SDr | 0.496 | 0.149 | 0.372 | 0.451 | 0.515 | |
| P valued‡ | ||||||
| F test | < 0.001 | < 0.001 | 0.207 | 0.317 | 0.491 | |
| Wald-Wolfowitz runs test | 0.152 | 0.121 | 0.500 | 0.279 | 0.236 | |
| 12 | a | 0.52 | 3.13 | 6.08 | 5.77 | 2.07 |
| b† | 1.145 (0.229) | 0.940 (0.073) | −0.22 (0.165) | −0.207 (0.202) | −0.158 (0.236) | |
| R2 | 0.714 | 0.943 | 0.153 | 0.105 | 0.043 | |
| SDr | 0.488 | 0.155 | 0.353 | 0.421 | 0.503 | |
| P valued‡ | ||||||
| F test | < 0.001 | < 0.001 | 0.209 | 0.332 | 0.518 | |
| Wald-Wolfowitz runs test | 0.197 | 0.175 | 0.652 | 0.333 | 0.279 | |
| 11 | a | 0.43 | 3.09 | 6.74 | 7.07 | 2.53 |
| b† | 0.746 (0.168) | 0.860 (0.075) | 0.053 (0.179) | 0.106 (0.191) | 0.279 (0.150) | |
| R2 | 0.686 | 0.935 | 0.001 | 0.037 | 0.278 | |
| SDr | 0.284 | 0.128 | 0.303 | 0.317 | 0.253 | |
| P valued‡ | ||||||
| F test | 0.002 | < 0.001 | 0.775 | 0.593 | 0.096 | |
| Wald-Wolfowitz runs test | 0.024 | 0.110 | 0.833 | 0.048 | 0.024 | |
| 10 | a | 0.47 | 3.03 | 6.14 | 6.21 | 2.30 |
| b† | 0.750 (0.156) | 0.859 (0.077) | 0.049 (0.166) | 0.107 (0.164) | 0.275 (0.128) | |
| R2 | 0.743 | 0.939 | 0.011 | 0.058 | 0.364 | |
| SDr | 0.264 | 0.131 | 0.281 | 0.272 | 0.217 | |
| P valued‡ | ||||||
| F test | 0.001 | < 0.001 | 0.777 | 0.534 | 0.065 | |
| Wald-Wolfowitz runs test | 0.040 | 0.643 | 0.881 | 0.107 | 0.048 |
Allometric analysis was performed with data for all avian species (including vultures; 13 avian species), data that excluded vultures (12 avian species), data that included vultures but excluded ostriches and greater rheas (11 avian species), and data that excluded vultures, ostriches, and greater rheas (10 avian species).
Value reported is the mean (SE).
Values were considered significant at P < 0.05.
a = The y-axis intercept. b = Slope. SDr = SD of residuals.
Predicted Vdss and Cl for American black vultures (value for body weight = 2.0 kg) were obtained by use of allometric equations that accounted for data from all birds (13 avian species), excluded data for vultures (12 avian species), included data for vultures but excluded data for ostriches and greater rheas (11 avian species), and excluded data for vultures, ostriches, and greater rheas (10 avian species). The 95% confidence interval and accuracy of predicted values were determined (Table 5).
Predicted values for Vdss and Cl of enrofloxacin obtained by use of allometric equations with data for various avian species.
| No. of avian species* | ||||
|---|---|---|---|---|
| Variable | 13 | 12 | 11 | 10 |
| Cl (L/h·kg)† Predicted | ||||
| Mean (SD) | 0.521 (0.215) | 0.583 (0.245) | 0.352 (0.107) | 0.386 (0.111) |
| 95% Cl | 0.348 to 0.779 | 0.391 to 0.882 | 0.261 to 0.475 | 0.291 to 0.513 |
| Accuracy (%) 95% CI | 254 137 to 430 | 297 166 to 500 | 140 78 to 223 | 162 98 to 249 |
| Vdss (L/kg)‡ Predicted | ||||
| Mean (SD) 95% CI | 3.02 (0.37) 2.67 to 3.41 | 2.98 (0.38) 2.63 to 3.39 | 2.77 (0.38) 2.42 to 3.17 | 2.71 (0.39) 2.35 to 3.12 |
| Accuracy (%) 95% CI | −13 −2 to −23 | −14 −2 to −24 | −20 −9 to −30 | −22 −10 to −32 |
For the allometric analysis, body weight of the 6 American black vultures used in the equation was 2.0 kg. 95% CI = 95% confidence interval.
Value measured for the 6 American black vultures of the present study was 0.47 L/h·kg.
Value measured for the 6 American black vultures of the present study was 43.47 L/kg.
See Table 4 for remainder of key.
Discussion
In the study reported here, the pharmacokinetic behavior of enrofloxacin after IV administration to American black vultures was determined. Enrofloxacin had a high Vdss and low Cl after IV administration to black vultures, which resulted in a prolonged t½β. The value of t½β obtained in the present study is exceeded only by the value (19.4 hours) reported after administration (15 mg/kg, IV) to red-tailed hawks (Buteo jamaicensis).24
American black vultures had the lowest Cl for enrofloxacin, compared with Cl for other avian species (Table 2). However, it is extremely important to determine whether a given plasma Cl obtained in a study is really low. To perform a physiologic interpretation of plasma Cl, a minimal physiologic model for total body Cl12 was applied. Because liver and kidney blood flows represent approximately half of the cardiac output, the overall extraction ratio should be considered high for values > 35%, medium for values of approximately 15%, and low for values < 5%. By use of these breakpoints, the enrofloxacin extraction ratio calculated for American black vultures of the present study (1.04%) was in the low extraction group, and it was the lowest value of all birds that were evaluated. Ducks, quails, turkeys, and emus had values slightly > 5%. Emus have slow Cl, similar to that for houbara bustards; however, the percentage of extraction is higher in emus. The ratites (ie, ostriches and greater rheas) had the highest extraction and Cl values of all birds, although the predicted cardiac output was the lowest of the analyzed species. Ostriches had the highest Cl. This could be related to special pharmacokinetic behavior in ostriches, which has been reported for other drugs.31 It also could have been attributable to an allometrically underestimated cardiac output because it has been determined that the heart weight of ostriches is higher than the allometrically calculated heart weight.32 In that study,32 a heart weight-to-body weight ratio of 1.08 was reported for ostriches, which is as high as the ratio in many birds that weigh < 50 g. The value for ostriches is an outlier because according to the authors of the aforementioned study,32 it is related to the fact these birds are extraordinarily powerful runners. Despite the fact quail had the highest cardiac output (29 L/h·kg), this avian species had an extraction ratio of 5.1%.
In veterinary medicine, the treatment of wild birds is challenging because of the scarcity of available scientific information on which to base therapeutic dosing regimens. Frequently, allometric scaling is applied to estimate drug doses when pharmacokinetic data are not available. Dose extrapolation by use of basal metabolic rate ratios provides relatively lower doses for large birds and higher doses for small birds. The enrofloxacin dose predicted for American black vultures by use of the dose approved for chickens (10 mg/kg) and by applying an equation previously used to estimate a specific minimum energy cost33 (specific minimum energy cost = 78 × BW−0.25) or by use of the metabolic value for Coragyps spp (76.99 kcal/d·kg)34 would be 11 or 13 mg/kg, respectively.
The aforementioned metabolic scaling approach takes into consideration the fact that body metabolic rate can be used for almost any drug and any species; however, this is a simplistic view of the relationship between basal metabolic rate and drug pharmacokinetics. Another allometric approach would be to use scaling of pharmacokinetic parameters across animal species. Volume of distribution and Cl are pivotal parameters that are necessary to estimate loading and maintenance doses, respectively.
Although the use of allometric scaling to extrapolate doses would not be appropriate when a drug undergoes extensive metabolism, allometric analysis for enrofloxacin has been performed in various studies.35–38 However, to our knowledge, allometric analysis of enrofloxacin in avian species has been evaluated in only 1 study.38 The use of allometric scaling could be appropriate for extrapolating enrofloxacin pharmacokinetics in mammals, which would facilitate selection of an appropriate dose for avian species.35
In the allometric analysis for the present study, the best fit was obtained for Vdss and a good fit was achieved for Cl, with extremely high or high positive correlations.39 Investigators of another study38 performed allometric scaling with data for mammalian and avian species. Only 3 avian species (chickens, turkeys, and houbara bustards) were included in that study,38 and results similar to those described for the study reported here were found. However, higher R2 values were obtained in the present study for Vdss and Cl. When the values reported for mammalian species of that study38 were analyzed, slope was 0.75 and 0.78 for Cl and Vdss, respectively. In a study36 that involved the use of data for 16 vertebrate species (most of which were mammalian), lower values were obtained for goodness of fit; however, the allometric behavior of the pharmacokinetic parameters was similar among species. The pharmacokinetic parameters of mammals that reportedly correlate with body weight are Cl, t½β, and volume of distribution (exponents of 0.82, 0.06, and 0.90, respectively).35 In the study reported here, a negligible correlation was detected between body weight and t½β, MRT, and AUC/dose.
When Vdss was subjected to allometric analysis, goodness of fit and the predicted Vdss obtained with all 4 allometric analyses were similar. However, when allometric analysis of Cl was performed, goodness of fit was improved, and the predicted Cl for American black vultures was closer to the measured values when ratites were not included in the analysis. Similar to pharmacokinetic behavior of other drugs,31,40 enrofloxacin has a unexpected pharmacokinetic behavior in ostriches and greater rheas. A high Cl was found for these species, and the higher residual values for these species conformed to a straight line. In 1 study,31 authors reported that ostriches had more rapid elimination. Those authors believed that this contradicted the allometric relationship with body weight; therefore, a good correlation was not detected in that study. In the study reported here, use of allometric scaling that did not take into account data for ratites (ostriches and greater rheas) resulted in a predicted Cl for American black vultures of 0.352 L/h·kg; Table 5), which was closer to the measured value obtained for the present study but with a narrower 95% confidence interval (0.261 to 0.475 L/h·kg) and slightly higher accuracy. Although the inclusion of large animals in allometric scaling can substantially improve the prediction of enrofloxacin Cl in large mammalian species,37 the same is apparently not true for enrofloxacin Cl in birds. On the other hand, the exclusion of data from the ratites did not modify the goodness of fit for the Vdss estimation, which was considered adequate in all cases.
The maintenance dose calculated by use of an equation proposed in another study12 and with an MIC of 0.01 μg/mL would be 0.18 mg/kg/d if the Cl for the present study were used. If the predicted Cl (obtained by use of the equation that included data for 12 avian species [all avian species except American black vultures]) were to be used, the dose would be 0.73 mg/kg/d, which would be 300% as great as the dose obtained by use of the measured Cl for the present study. Analysis of results for the study reported here suggested that application of allometric principles for the prediction of Vdss could provide a suitable method for extrapolation of the enrofloxacin loading dose among avian species; however, the prediction of Cl would not be adequate, and therefore, the maintenance dose calculated with predicted Cl values would not be appropriate. Regardless, extreme caution should be taken when allometric approaches are used for extrapolation of drug doses for clinical cases. The mathematical assumptions as well as the limitations of this method should be considered.33
The slow elimination of certain drugs in Gyps vultures could be related to toxicosis issues.1–3,41 We believe slow elimination of drugs could be evident in other obligate scavengers belonging to the New World vultures (eg, Coragyps atratus) because extremely slow elimination was detected in the study reported here. Extreme caution should be used when the maintenance dose is extrapolated from data for other avian species (even those of the same family or extrapolated by use of allometric scaling) because of the important differences reported for the present study. Findings for the study reported here could be the starting point for more accurate extrapolation of drug doses for vultures, which reportedly have issues with drug elimination.1–3
If a maintenance dose were to be calculated with the equation proposed by other authors12 and an MIC value reported in another study,13 we suggest that for most of the species included in the comparisons in the present study, a recommended dose of 5 mg/kg could be adequate to achieve clinical success, except for avian species with a high extraction ratio. However, the dose obtained for the various species by use of MPC values could be an indication of the inability of these dosing regimens to completely eradicate pathogenic microorganisms and could result in the emergence of resistant bacteria.42 Therefore, analysis of results for the study reported here suggested that administration of enrofloxacin (5 mg/kg, IV) to American black vultures could have a therapeutic effect, although it would not prevent the emergence of resistant bacteria. Nevertheless, these results should be interpreted with extreme caution because the pharmacodynamic parameters were calculated for E coli isolates obtained from chickens,13 a species in which the selection pressure is probably high as a result of the intensive use of this class of antimicrobial. Further studies are necessary to evaluate the pharmacokinetic and pharmacodynamic behavior of enrofloxacin in American black vultures and the influence of infectious disease on the pharmacokinetics of this drug.
In the present study, American black vultures had the slowest elimination for enrofloxacin for all species evaluated. Analysis of the results suggested that the use of allometric principles can provide a suitable method for extrapolation of the enrofloxacin loading dose among avian species; however, allometric scaling could not be used to adequately predict the maintenance dose of enrofloxacin.
Acknowledgments
Supported as part of a UBACyT Project, Secretaría de Ciencia y Técnica, Universidad de Buenos Aires. Funding sources did not have any involvement in the study design, data analysis and interpretation, or writing and publication of the manuscript.
The authors declare that there were no conflicts of interest. None of the authors had any financial or personal relationships that could have inappropriately influenced or biased the content of the paper.
The authors thank Dr. M. A. Rivolta for providing permission to work at the Buenos Aires Zoological Garden, Mariano Diaz, Mar Sanz, and Carmen Muñoz Serrano for technical assistance, and Santiago Cano for assistance with the statistical analysis.
ABBREVIATIONS
| AUC | Area under the plasma concentration-time curve |
| Cl | Clearance |
| MBC | Minimum bactericidal concentration |
| MIC | Minimum inhibitory concentration |
| MPC | Mutant prevention concentration |
| MRT | Mean residence time |
| t½β | Elimination half-life |
| Vdss | Volume of distribution at steady state |
Footnotes
Baytril 5%, Bayer Hispania SL, Barcelona, Spain.
Sigma-Aldrich Corp, Madrid, Spain.
Spectra-Physics System, Madrid, Spain.
PCnonlin, version 4.0, SCI Software, Statistical Consultants Inc, Lexington, Ky.
Prism, version 4.00, GraphPad Software, La Jolla, Calif.
References
1. Naidoo V, Duncan N, Bekker L, et al. Validating the domestic fowl as a model to investigate the pathophysiology of diclofenac in Gyps vultures. Environ Toxicol Pharmacol 2007;24:260–266.
2. Naidoo V, Wolter K, Cuthbert R, et al. Veterinary diclofenac threatens Africa's endangered vulture species. Regul Toxicol Pharmacol 2009;53:205–208.
3. Garcia-Montijano M, Waxman S, de Lucas JJ, et al. Disposition of marbofloxacin in vulture (Gyps fulvus) after intravenous administration of a single dose. Res Vet Sci 2011;90:288–290.
4. International Union for Conservation of Nature. The IUCN red list of threatened species. Available at: www.iucnredlist.org. Accessed Feb 18, 2018.
5. Wink M. Phylogeny of Old and New World vultures (Aves: Accipitridae and Cathartidae) inferred from nucleotide sequences of the mitochondrial cytochrome b gene. Z Naturforsch C 1995;50:868–882.
6. Dermody BJ, Tanner CJ, Jackson AL. The evolutionary pathway to obligate scavenging in Gyps vultures. PLoS One 2011;6:e24635.
7. Carpenter JW. Pharmacotherapeutics in companion birds: an update and review, in Proceedings. 31st World Small Anim Vet Cong 2006;1–4.
8. López-Cadenas C, Sierra-Vega M, Garcia-Vieitez JJ, et al. Enrofloxacin: pharmacokinetics and metabolism in domestic animal species. Curr Drug Metab 2013;14:1042–1058.
9. Hawkins MG, Barron HW, Speer BL, et al. Antimicrobial agents used in birds. In: Carpenter JW, ed. Exotic animal formulary. 4th ed. St Louis: Elsevier Saunders, 2013;184–216.
10. Waxman S, Prados AP, de Lucas J, et al. Pharmacokinetic and pharmacodynamic properties of enrofloxacin in southern crested caracaras (Caracara plancus). J Avian Med Surg 2013;27:180–186.
11. Grubb BR. Allometric relations of cardiovascular function in birds. Am J Physiol 1983;245:H567–H572.
12. Toutain PL, Bousquet-Mélou A. Plasma clearance. J Vet Pharmacol Ther 2004;27:415–425.
13. Haritova A, Urumova V, Lutckanov M, et al. Pharmacokinetic-pharmacodynamic indices of enrofloxacin in Escherichia coli O78/H12 infected chickens. Food Chem Toxicol 2011;49:1530–1536.
14. Abd el-Aziz MI, Aziz MA, Soliman FA, et al. Pharmacokinetic evaluation of enrofloxacin in chickens. Br Poult Sci 1997;38:164–168.
15. Anadón A, Martínez-Larrañaga MR, Díaz MJ, et al. Pharmacokinetics and residues of enrofloxacin in chickens. Am J Vet Res 1995;56:501–506.
16. Bailey TA, Sheen RS, Silvanose C, et al. Pharmacokinetics of enrofloxacin after intravenous, intramuscular and oral administration in houbara bustard (Chlamydotis undulata macqueenii). J Vet Pharmacol Ther 1998;21:288–297.
17. Bugyei K, Black WD, McEwen S. Pharmacokinetics of enrofloxacin given by the oral, intravenous and intramuscular routes in broiler chickens. Can J Vet Res 1999;63:193–200.
18. de Lucas JJ, Rodriguez C, Waxman S, et al. Pharmacokinetics of enrofloxacin after single intravenous and intramuscular administration in young domestic ostrich (Struthio camelus). J Vet Pharmacol Ther 2004;27:119–122.
19. de Lucas JJ, Rodríguez C, Martella MB, et al. Pharmacokinetics of enrofloxacin following intravenous administration to greater rheas: a preliminary study. Res Vet Sci 2005;78:265–267.
20. Dimitrova DJ, Lashev LD, Yanev SG, et al. Pharmacokinetics of enrofloxacin in turkeys. Res Vet Sci 2007;82:392–397.
21. García Ovando H, Gorla N, Luders C, et al. Comparative pharmacokinetics of enrofloxacin and ciprofloxacin in chickens. J Vet Pharmacol Ther 1999;22:209–212.
22. Haritova A, Djeneva H, Lashev L, et al. Pharmacokinetics and PK/PD modelling of enrofloxacin in Meleagris gallopavo and Gallus domesticus. Bulg J Vet Med 2004;7:139–148.
23. Haritova A, Dimitrova D, Dinev T, et al. Comparative pharmacokinetics of enrofloxacin, danofloxacin, and marbofloxacin after intravenous and oral administration in Japanese quail (Coturnix japonica). J Avian Med Surg 2013;27:23–31.
24. Harrestien LA, Tell LA, Vulliet R, et al. Disposition of enrofloxacin in red-tailed hawks (Buteo jamaicensis) and great horned owls (Bubo virginianus) after a single oral, intramuscular or intravenous dose. J Avian Med Surg 2000;14:228–236.
25. Helmick KE, Boothe DM, Jensen JM. Disposition of single-dose intravenously administered enrofloxacin in emus (Dromaius novaehollandiae). J Zoo Wildl Med 1997;28:43–48.
26. Knoll U, Glünder G, Kietzmann K. Comparative study of the plasma pharmacokinetics and tissue concentrations of danofloxacin and enrofloxacin in broiler chickens. J Vet Pharmacol Ther 1999;22:239–246.
27. Lashev LD, Dimitrova DJ, Milanova A, et al. Pharmacokinetics of enrofloxacin and marbofloxacin in Japanese quails and common pheasants. Br Poult Sci 2015;56:255–261.
28. Soliman GA. Tissue distribution and disposition kinetics of enrofloxacin in healthy and E. coli infected broilers. Dtsch Tierarztl Wochenschr 2000;107:23–27.
29. Tansakul N, Poapolathep A, Klangkaew N, et al. Pharmacokinetics and withdrawal times of enrofloxacin in ducks. Kasetsart J Nat Sci 2005;39:235–239.
30. Wack AN, KuKanich B, Bronson E, et al. Pharmacokinetics of enrofloxacin after single dose oral and intravenous administration in the African penguin (Spheniscus demersus). J Zoo Wildl Med 2012;43:309–316.
31. Baert K, De Backer P. Comparative pharmacokinetics of three non-steroidal anti-inflammatory drugs in five bird species. Comp Biochem Physiol C Toxicol Pharmacol 2003;134:25–33.
32. Clark AJ. The influence of body weight on the size of the heart in warm-blooded animals. In: Clark AJ, ed. Comparative physiology of the heart. Cambridge, England: Cambridge University Press, 2015;71–86.
33. Hunter RP. Interspecies allometric scaling. In: Cunningham FE, Lees P, eds. Comparative and veterinary pharmacology. New York: Springer-Verlag Berlin Heidelberg, 2010;140–157.
34. Buckley NJ. Black vulture: Coragyps atratus. Birds of North America. Available at: birdsna.org/Species-Account/bna/species/blkvul/introduction. Accessed Feb 18, 2018.
35. Bregante MA, Saez P, Aramayona JJ, et al. Comparative pharmacokinetics of enrofloxacin in mice, rats, rabbits, sheep, and cows. Am J Vet Res 1999;60:1111–1116.
36. Cox SK, Cottrell MB, Smith L, et al. Allometric analysis of ciprofloxacin and enrofloxacin pharmacokinetics across species. J Vet Pharmacol Ther 2004;27:139–146.
37. Mahmood I. Application of allometric principles for the prediction of pharmacokinetics in human and veterinary drug development. Adv Drug Deliv Rev 2007;59:1177–1192.
38. Haritova AM, Lashev LD. Comparison of the pharmacokinetics of seven fluoroquinolones in mammalian and bird species using allometric analysis. Bulg J Vet Med 2009;12:3–24.
39. Mukaka MM. Statistics corner: a guide to appropriate use of correlation coefficient in medical research. Malawi Med J 2012;24:69–71.
40. de Lucas JJ, Rodríguez C, Waxman S, et al. Pharmacokinetics of marbofloxacin after intravenous and intramuscular administration to ostriches. Vet J 2005;170:364–368.
41. Naidoo V, Wolter K, Cuthbert R, et al. Veterinary diclofenac threatens Africa's endangered vulture species. Regul Toxicol Pharmacol 2009;53:205–208.
42. Olofsson SK, Marcusson LL, Komp Lindgren P. Selection of ciprofloxacin resistance in Escherichia coli in an in vitro kinetic model: relation between drug exposure and mutant prevention concentration. J Antimicrob Chemother 2006;57:1116–1121.