Pharmacokinetics of gentamicin C₁, C_1a and C ₂ in horses after single intravenous dose

Gentamicin pharmacokinetics has not been studied in horses. Pharmacokinetics of gentamicin C¹, C_1a and C₂ components following i.v. administration of total gentamicin at 6.6 mg/kg bwt to 6 healthy mature horses was determined. Significant differences in clearance, half-life (t_1/2), and mean residence time (MRT) between the gentamicin C_1a and the 2 other components were found. The total body clearance (CL) of gentamicin C_1a was 1.62 ± 0.50 ml/min x kg and similar to the glomerular filtration rate (GFR) reported for horses. The CL of gentamicin C₁ and C₂ were 1.03 ± 0.08 ml/min x kg and 1.10 ± 0.15 ml/min x kg, respectively, and significantly slower than that of gentamicin C_1a. The values of apparent volume of distribution at steady state were 0.22 ± 0.05, 0.26 ± 0.12 and 0.23 ± 0.05 l/kg for gentamicin C₁, C_1a and C ₂, respectively. The MRT values were mean ± s.d. 3.6 ± 0.5, 2.7 ± 0.3 and 3.5 ± 0.4 h and the t_1/2 values were 3.1 (2.5-4.0), 2.4 (2.0-3.2) and 3.3 (2.4-4.3) h (harmonic mean and range) for gentamicin C₁, C_1a and C₂, respectively. The MRT and t_1/2 values for gentamicin C_1a were significantly shorter than those of gentamicin C₁ and C₂. It was concluded that the difference in pharmacokinetics between the gentamicin components has potential pharmacological and toxicological implications.