Species dependent gentamicin pharmacokinetics and nephrotoxicity in the young horse

Gentamicin pharmacokinetics and nephrotoxic potential were evaluated in twelve 2 to 3 month-old horses. Whereas recent evidence in our clinic indicated that young horses may be especially susceptible to gentamicin nephrotoxicity, young rabbits and rats are usually resistant. Gentamicin (4.5 mg/kg) was given by rapid intravenous injection. Serum gentamicin concentrations over a 13-hour period were fitted to an open, two-compartment, pharmacokinetic model. Subsequently, the same horses were divided into groups of 3 horses each. Each group received 0, 2.2, 4.4 or 8.8 mg gentamicin/kg, intramuscularly, every 12 hours for 15 days. Renal function was monitored. Peak and trough gentamicin concentrations were monitored daily. Renal sections were collected for histopathologic and electron microscopic examination. The (mean +/- SD) serum halflife was 194 +/- 37 minutes, total body clearance (ClB) was 1.65 +/- 0.79 mL/min/kg and volume of distribution at steady state (Vd(ss)) was 30.6 +/- 9.4 L/100 kg. Decreased renal function, as detected by elevated serum urea nitrogen or creatinine concentrations, was detected only in the two youngest foals (including animals in both the 4.4 and 8.8 mg/kg dose groups). The trough serum gentamicin concentrations of these 2 horses increased over time. These horses had the lowest ClB and Vd(ss) in the intravenous study. Morphologic changes were seen in kidneys of all treated horses and were similar to those occurring with gentamicin toxicity in other species. Our results support the clinical impression that very young horses may be more susceptible than adult horses, and adults of other species, to gentamicin nephrotoxicity.