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.