Cortical bone concentrations of enrofloxacin in dogs

Cortical bone concentrations of enrofloxacin were determined over time in dogs after SC administration of the drug. Nineteen healthy adult dogs were anesthetized and were given 2.5 or 5.0 mg of enrofloxacin/kg of body weight, SC. Serial serum and bone samples were obtained for determination of enrofloxacin concentrations at intervals until 8 hours after drug administration. Cortical bone samples were procured by surgical disarticulation of successive second phalanges. Additional cortical bone samples were taken from long bones in 4 dogs. Mean +/- SD peak serum enrofloxacin concentration was 0.54 +/- 0.10 micrograms/ml for the 2.5-mg/kg dosage and 0.97 +/- 0.34 micrograms/ml for the 5.0-mg/kg dosage. Serum concentration was significantly higher than bone concentration for each dosage. Mean peak bone concentrations reached 29% of peak serum values: 0.15 +/- 0.09 micrograms/g and 0.29 +/- 0.09 micrograms/g for 2.5-mg/kg and 5.0-mg/kg dosages, respectively. Serum concentration for the 5.0-mg/kg dosage was significantly greater than that for the 2.5-mg/kg dosage for all times, whereas bone concentrations for the 5.0-mg/kg dosage were significantly higher at all times after 180 minutes. For the duration of the study, cortical bone concentrations of enrofloxacin at either dosage exceeded the minimum inhibitory concentration (MIC) for the Enterobacteriaceae, but reliably exceeded the MIC for Staphylococcus sp only at the 5.0-mg/kg dosage. At no time did cortical bone concentrations of enrofloxacin exceed the MIC for Pseudomonas aeruginosa at either dosage. To validate extrapolation of data from the second phalanx to long bones and from anesthetized to awake dogs, 16 healthy dogs being euthanatized in unrelated studies were given 2.5 or 5.0 mg of enrofloxacin/kg, sc. These dogs were not anesthetized but were euthanatized at 60, 120, or 240 minutes after drug administration, and multiple cortical bone samples were taken. Antibiotic concentrations in the second phalanx were not significantly different from those in long bones. Comparison of enrofloxacin concentrations in cortical bone of awake and anesthetized dogs suggested no differences between groups. We concluded that general anesthesia and use of the antibiotic concentrations in the second phalanx as representative of those in long bones did not affect results of this study.