Comparative Pharmacokinetics of Gentamicin in Healthy Young-Adult and Geriatric Horses

Aging may modify the pharmacokinetic disposition and excretion of gentamicin, although drug dose adjustments in aged horses are uncommon in clinical practice. Since high-dose, once daily dosing of gentamicin is considered therapeutically most effective, a comparative single-dose study was conducted to evaluate the differences in pharmacokinetics between healthy young-adult (5-10 years) and geriatric (≥ 25 years) horses receiving 6.6 mg/kg gentamicin intravenously. Blood samples were collected at designated time-points following drug administration and frozen at -80°C until assayed by a validated immunoassay. Gentamicin plasma concentrations versus time plots were analyzed by noncompartmental analysis using commercial software (WinNonlin-v8.4). Baseline physical examination and hematological parameters did not differ between age groups, except for a lower mean bodyweight in the geriatric group (477 ± 4 kg vs. 402 ± 6 kg). None of the pharmacokinetic parameters were statistically different between age groups. The oldest geriatric horse (41 years) had a longer half-life and lower clearance of 5.3 h and 22.79 L/h, respectively, compared to a range of 1.39-2.56 h and 26.39-40.59 L/h for the remainder of the geriatric group (25-29 years). Further studies may be indicated in horses > 30 years old to determine if dose reduction is necessary in this population.