Serum concentrations and pharmacokinetics of enrofloxacin after intravenous and intragastric administration to mares

Serum concentrations and pharmacokinetics of enrofloxacin were studied in 6 mares after intravenous (TV) and intragastric (IG) administration at a sin gle dose rate of 7.5 mg/kg body weight. In experiment 1, an injectable form ulation of enrofloxacin (100 mg/mL) was given TV. At 5 min after injection, mean serum concentration was 9.04 mu g/mL and decreased to 0.09 mu g/mL by 24 h. Elimination half-life was 5.33 +/- 1.05 h and the area under the ser um concentration vs time curve (AUC) was 21.03 +/- 5.19 mg h/L. In experime nt 2, the same injectable formulation was given IG. The mean peak serum con centration was 0.94 +/- 0.97 mu g/mL at 4 h after administration and declin ed to 0.29 +/- 0.12 mu g/mL by 24 h. Absorption of this enrofloxacin prepar ation after IG administration was highly variable, and for this reason, pha rmacokinetic values for each mare could not be determined; In experiment 3, a poultry formulation (32.3 mg/mL) was given IG. The mean peak serum conce ntration was 1.85 +/- 1.47 mu g/mL at 45 min after administration and decli ned to 0.19 +/- 0.06 mu g/mL by 24 h. Elimination half-life was 10.62 +/- 5 .33 h and AUC was 16.30 +/- 4.69 mg.h/L. Bioavailability was calculated at 78.29 +/- 16.55%. Minimum inhibitory concentrations of enrofloxacin were de termined for equine bacterial culture specimens submitted to the microbiolo gy laboratory over an Ii-month period. The minimum inhibitory concentration of enrofloxacin required to inhibit 90% of isolates (MIC90) was 0.25 mu g/ mL for Staphylococcus aureus, Escherichia coil, Salmonella spp., Klebsiella spp., and Pasteurella spp. The poultry formulation was well tolerated and could be potentially useful in the treatment of susceptible bacterial infec tions in adult horses. The injectable enrofloxacin solution should not be u sed orally.