Gentamicin kinetics after intramuscular administration in horses

Abstract

Gentamicin blood levels and pharmacokinetic parameters were studied in 14 horses after intramuscular administration (musculus serratus ventralis) of two doses, 3 and 6 mg/kg of body weight. Microbiological cylinder plate assay was used to determine gentamicin serum concentrations, using Staphylococcus epidermidis (ATCC 12228) as a test organism. Samples were collected 5, 10, 15, 20, 30, 45, 60, 120, 240, 360 and 720 minutes after antibiotic administration. The results were best fitted to a one compartment model with a constant of absorption (ka) of 5.14 ± 1.56 h^-1 and 5.48 ± 0.75 ^h-1 elimination half-life (t) of 4.58 ± 0.31 h and 4.60 ± 0.92 h, volume of distribution (Vd) 0.26 ± 0.13 1* kg^-1 and 0.42 ± 0.16 1* kg^-1 and total body clearance (Cl_total) of 0.64 ± 0.35 ml*min^-1 kg and 1.06 ± 0.34 ml*min^-1 *kg^-1 for doses of 3 and 6 mg/kg, respectively. Absolute bioavailability was 101.48 ± 10.49% and 93.98 ± 19.48%, C_max was 10.96 ± 6.20 µg/ml and 12.10 ± 8.78 µg/ml and t_max was 42.36 ± 13.55 min and 40.44 ± 12.63 min for the low and high dose, respectively. Results showed that serum concentrations over 1 µg/ml persisted up to twelve hours in animals treated intramuscularly. It was concluded that intramuscular administration of doses between 2.5 and 3.0 mg/kg every 12 hours are adequate to maintain therapeutic serum concentrations of gentamicin in the horse.