Pharmacokinetics of the Bovine Formulation of Enrofloxacin (Baytril 100) in Horses

C. Boeckh, C. Buchanan, A. Boeckh, S. Wilkie, C. Davis, T. Buchanan, and D. Boothe Pharmacokinetics of the Bovine Formulation of Enrofloxacin (Baytril 100) in Horses Christine Boeckh, DVM, MS a Charles Buchanan, DVM a Albert Boeckh, DVM b Scott Wilkie, BS b Cal Davis, DVM a Terrell Buchanan, DVM a Dawn Boothe, DVM, PhD b a Brazos Valley Equine Hospital 6999 Highway 6 Navasota, TX 77868 b Department of Veterinary Physiology and Pharmacology College of Veterinary Medicine Texas A&M University College Station, TX 77843-4466 ABSTRACT Following approval of a concentrated injectable formulation of enrofloxacin for cattle (Baytril 100 Injectable, Bayer Corp. Agricultural Division, Shawnee Mission, KS), equine practitioners have started administering this preparation both parenterally and orally to horses, despite the lack of pharmacokinetic data in this species. Six healthy horses received enrofloxacin at 7.5 mg/kg both orally and intravenously, with the sequence being randomly assigned and at least 1 week of washout allowed between administrations. Blood samples were collected from each horse at various intervals after drug administration to study the pharmacokinetic profile of this product. As concentration-dependent antimicrobials, fluorinated quinolones such as enrofloxacin are most efficacious when the inhibitory quotient is at least 8. In this study, based on inhibitory quotients between 8 and 10, the bovine injectable formulation of enrofloxacin administered to horses intravenously or orally once daily at 7.5 mg/kg may be effective against susceptible organisms whose minimum inhibitory concentration for 90% of the isolates is lower than 0.5 µg/ml. INTRODUCTION Enrofloxacin is a popular fluorinated quinolone antimicrobial recently approved in the United States for treatment of bovine respiratory disease. This formulation of enrofloxacin (Baytril 100 Injectable, Bayer Corp. Agricultural Division, Shawnee Mission, KS) is an arginine-based injectable product containing 100 mg enrofloxacin per ml. Other approved formulations include tablets (27, 68, and 136 mg) and injectable (2.27 mg/ml) for use in small animals. Although not approved for use in horses, equine practitioners have taken advantage of the potential therapeutic ben- 129

Veterinary Therapeutics Vol. 2, No. 2, Spring 2001 efits of enrofloxacin, using both the oral and injectable small animal formulations for treatment of bacterial infections in their patients. Previous reports support the efficacy and safety of enrofloxacin in horses. 1 3 One disadvantage of using small animal formulations of enrofloxacin, however, is the number of tablets or volume of the injectable product needed to administer an appropriate dose to an adult horse. Following approval of the concentrated injectable formulation of enrofloxacin for cattle, equine practitioners have started administering this preparation both parenterally and orally to horses, despite the lack of pharmacokinetic data in this species. The objective of this study was to establish the pharmacokinetic profile, including bioavailability, of the bovine arginine-based enrofloxacin formulation (Baytril 100 Injectable) administered either intravenously or orally in horses. MATERIALS AND METHODS Horses Six healthy adult horses (four geldings and two mares), weighing from 400 to 450 kg each and ranging in age from 3 to 8 years, were selected for the study. The study design was reviewed and approved by the Texas A&M University Veterinary Clinical Research Committee, which assures the humane and ethical use of client-owned animals in research. Animals were individually housed in stalls at a private equine referral hospital and fed commercial equine pellet food and coastal hay. Each horse received the drug both orally and intravenously, with the sequence being randomly assigned and at least 1 week of washout allowed between administrations. All horses received a thorough physical examination at the start of the trial and were clinically healthy throughout the study. Treatment Animals initially received enrofloxacin at 7.5 mg/kg on Day 0 either intravenously or orally. This dosage rate is within the range used in dogs and cats (5 20 mg/kg) as well as for treatment of bovine respiratory disease (7.5 12.5 mg/kg). On the day of treatment, animals treated intravenously were fitted with two jugular catheters, the right one for drug administration and the left one for blood sample collection. The right catheter was removed immediately after drug administration, and the left catheter remained for the duration of sample collection. When receiving the drug orally, animals were fitted with only one catheter; a nasogastric tube was used for drug administration, ensuring that no drug loss would occur during dosing. Evaluations A blood sample (7 ml) was collected from each horse through the left jugular catheter, transferred to glass tubes, and allowed to clot at room temperature. Blood samples were collected immediately before dosing and 2, 5, 10, 15, 20, 30, 45 minutes and 2, 3, 4, 6, 8, 10, and 12 hours after intravenous administration. Samples were collected immediately before dosing and 15, 30, 45, 60, 90 minutes and 2, 3, 4, 6, 8, and 12 hours after oral dosing. Clotted blood samples were centrifuged at 1250 g for harvesting of the serum. Serum was frozen at 20 C until analysis, which occurred within 1 month of sample collection. One standard of a known concentration of enrofloxacin prepared in equine serum was frozen and analyzed with each batch of samples for assurance of sample stability during storage. Samples were analyzed by high-performance liquid chromatography using a procedure previously described and validated for simultaneous measurement of both enrofloxacin and its active metabolite ciprofloxacin. 4 Briefly, samples were defrosted and centrifuged through a 10-kD membrane at 1750 g for 2 hours. After 130

C. Boeckh, C. Buchanan, A. Boeckh, S. Wilkie, C. Davis, T. Buchanan, and D. Boothe centrifugation, enrofloxacin and ciprofloxacin were separated from one another and from other serum constituents by a reverse phase C- 18 column, then detected and quantified by fluorescence with excitation at 280 nm and detection at 450 nm. The lower and upper limits of quantification for this study were 0.06 and 6.79 µg/ml, respectively, for enrofloxacin and 0.02 and 6.75 µg/ml for ciprofloxacin. Precision was 91.8% for enrofloxacin and 91.3% for ciprofloxacin; accuracy was 105.8% for enrofloxacin and 110.0% for ciprofloxacin. Analysis Serum concentrations versus time were subjected to linear regression using computer modeling software for determination of maximum concentration (C max ); time to maximum concentration (T max ); area under the concentration versus time curve (AUC) to infinity by the trapezoid method; mean residence time (MRT); appearance (k a ) and disappearance (k el ) constants; appearance (t1/2a) and disappearance (t1/2) half-lives; clearance (Cl); apparent volume of distribution (Vd); rate constant for the terminal portion of the curve (Beta); y intercept for the terminal portion of the curve (B); and bioavailability (F) (Table 1). The model selected for each animal was always that which minimized residuals, based on Akaike s method for modeling criteria. 5 Appearance and disappearance rate constants were determined, respectively, from the initial and terminal components of the plasma drug concentration versus time curves. 6 Each rate constant was used to determine respective t1/2a and t1/2. 7 Inhibitory quotients (IQs; peak serum concentrations: minimum inhibitory concentration [MIC] ratio of the infecting microbe) for selected pathogens were calculated using previously published MIC data from large animal field isolates. 8 Descriptive statistics were reported for enrofloxacin and ciprofloxacin as the mean ± standard deviation. Pharmacokinetic data were evaluated for statistical significance using Student s t-test. Differences were considered to be significant at P.05. RESULTS There was no indication of drug degradation during storage. After intravenous administration of enrofloxacin, mean serum concentrations, extrapolated from the terminal phase (B), were 4.31 ± 3.43 µg enrofloxacin/ml and 0.37 ± 0.11 µg ciprofloxacin/ml (Table 1, Figure 1). After oral administration, mean C max was 2.2 ± 0.71 µg/ml for enrofloxacin and 0.38 ± 0.12 µg/ml for ciprofloxacin. Times to peak concentrations were similar for the two compounds: 54.11 ± 18.80 minutes for enrofloxacin and 58.73 ± 13.45 minutes for ciprofloxacin (Table 1, Figure 1). The t1/2 of ciprofloxacin was longer than for the parent compound after intravenous administration but not after oral dosing (Table 1). Bioavailability of enrofloxacin in this study was 65.6% ± 14.9%. Significant differences between the routes of administration were not detected for any variable measured. Inhibitory quotients for enrofloxacin, based on B for intravenous administration and on C max for oral administration, divided by the MIC for 90% of selected isolates (MIC 90 ), 8,9 ranged from 3.6 to 150 for the isolates evaluated (Table 2). DISCUSSION Results of this study suggest that oral or intravenous administration of a commercially available bovine injectable formulation of enrofloxacin is potentially useful as an antimicrobial for horses. The calculated apparent Vd of enrofloxacin was 2.49 L/kg, consistent with results (2.9 L/kg) from a previous equine study using the same dosage rate 10 as well as a canine 131

Veterinary Therapeutics Vol. 2, No. 2, Spring 2001 TABLE 1. Selected Pharmacokinetic Parameters for Enrofloxacin and Ciprofloxacin After Intravenous or Oral Administration of Enrofloxacin at 7.5 mg/kg Intravenous Oral Enrofloxacin Ciprofloxacin Enrofloxacin Ciprofloxacin Mean SD Mean SD Mean SD Mean SD C max (µg/ml) NA NA 0.27 0.16 2.22 0.71 0.38 0.12 T max (hr) NA NA 1.39 0.85 0.90 0.31 0.98 0.22 AUC ([ug/ml] hr) 14.37 3.58 1.95 0.93 9.78 4.03 2.08 0.52 t1/2a (hr) NA NA 0.25 0.19 0.33 0.18 0.29 0.15 t1/2 (hr) 5.92 4.04 9.53 5.08 10.7 3.30 11.35 4.46 MRT (hr) 3.67 0.96 5.11 0.67 3.83 1.38 4.86 0.19 K a (hr 1 ) NA NA 4.65 3.38 3.53 3.57 3.22 2.16 K el (hr 1 ) 0.17 0.11 0.08 0.03 0.18 0.28 0.07 0.02 B (µg/ml) 4.31 3.43 0.37 0.11 0.95 0.59 0.24 0.07 Beta (hr 1 ) 0.18 0.12 0.26 0.03 0.07 0.02 0.06 0.03 Vd (B) (L/kg) 2.49 1.25 NA NA NA NA NA NA Vd (area) (L/kg) 4.20 2.17 NA NA NA NA NA NA Cl (ml/kg/h) 561.25 191.30 NA NA NA NA NA NA F (%) NA NA NA NA 65.6 14.9 NA NA AUC = area under the concentration versus time curve; B = Y intercept for the terminal portion of the curve; Beta = rate constant for the terminal portion of the curve; Cl = clearance; C max = maximum concentration; F = bioavailability; k a = appearance constant; k el = disappearance constant; MRT = mean residence time; NA = not applicable; t1 /2a = appearance half-life; t1 /2 = disappearance half-life; T max = time to maximum concentration; Vd (area) = apparent volume of distribution (area); Vd (B) = apparent volume of distribution (B). 100.00 Concentration (µg/ml) 10.00 1.00 0.10 Enrofloxacin (intravenous treatment) Enrofloxacin (oral treatment) Ciprofloxacin (intravenous treatment) Ciprofloxacin (oral treatment) 0.01 0 2 4 6 8 10 12 Time (hr) Figure 1. Mean (± SD) concentrations of enrofloxacin and its metabolite, ciprofloxacin, after intravenous or oral administration of enrofloxacin at 7.5 mg/kg. 132

C. Boeckh, C. Buchanan, A. Boeckh, S. Wilkie, C. Davis, T. Buchanan, and D. Boothe TABLE 2. MIC 90 and Inhibitory Quotients for Enrofloxacin, Ciprofloxacin, and Enrofloxacin + Ciprofloxacin against Selected Pathogenic Organisms Pasteurella Escherichia coli multocida Salmonella Mycoplasma (n = 109) (n = 63) (n = 234) (n = 18) MIC 90 (µg/ml) 0.05 0.06 0.06 0.25 INHIBITORY QUOTIENTS Intravenous enrofloxacin 86.0 71.7 71.7 17.2 Intravenous ciprofloxacin 3.8 3.2 3.2 0.8 Intravenous enrofloxacin + ciprofloxacin 90.0 75.0 75.0 18.0 Oral enrofloxacin 18.0 15.0 15.0 3.6 Oral ciprofloxacin 4.8 4.0 4.0 1.0 Oral enrofloxacin + ciprofloxacin 22.8 19.0 19.0 4.6 MIC 90 = minimum inhibitory concentration for 90% of the isolates. study in which Vd ranged from 1.2 to 3.4 L/kg. 11 The large Vd suggests lipid solubility and good distribution into tissues, an important attribute for efficacy of an antimicrobial. Bioavailability of enrofloxacin in this study (65.6%) was lower than previously reported (78.3%) in horses 10 or other species, such as dogs and cats. 12 Nevertheless, a dosage rate of 7.5 mg/kg should provide sufficient tissue drug concentrations to ensure efficacy, based on the lipid solubility and the high Vd of enrofloxacin. Elimination half-life (t1/2) in horses in this study was more than twice as long as previously reported in dogs 11 but similar to another study in horses in which the t1/2 was 6.1 hours after intravenous injection of enrofloxacin at 5 mg/kg. 13 This longer half-life may be an advantage in horses because the drug will have longer contact with the bacterial organism. However, because fluorinated quinolones as a class are described as concentration dependent, the clinical relevancy of a longer half-life is not clear. Indeed, these drugs can be administered once daily despite a relatively short half-life. As concentration-dependent antimicrobials, fluorinated quinolones such as enrofloxacin are most efficacious when the IQ is at least 8. 14 The MIC 90 for enrofloxacin and pathogenic organisms in horses apparently has not been published. Although IQ data in this study are based on MIC 90 data cultured from bovine rather than equine samples, they can provide a useful guide for pathogens infecting horses. The IQ does not take into consideration the active metabolite ciprofloxacin, which acts in an additive fashion with its parent compound 15 and has similar antimicrobial activity. The combined B of enrofloxacin and ciprofloxacin was 4.7 µg/ml after intravenous administration and 2.6 µg/ml after oral administration of enrofloxacin. This additive effect should lead to a higher IQ and thus greater efficacy. CONCLUSION Based on IQs of 8 and above, the bovine injectable formulation of enrofloxacin (Baytril 100) administered to horses intravenously or orally once daily at 7.5 mg/kg may be effective against susceptible organisms whose MIC 90 is lower than 0.5 µg/ml. REFERENCES 1. Bertone AL, Tremaine WH, Macoris DG, et al: Effect of long-term administration of an injectable enrofloxacin solution on physical and musculoskeleta 133

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