Naproxen in the horse: Pharmacokinetics and side effects in the elderly

It is well-known that old animals show physiologic and/or pathologic variation that could modify the pharmacokinetics of drugs and the related pharmacodynamic response. In order to define the most appropriate therapeutic protocol in old horses, pharmacokinetic profile and safety of naproxen were investigated in horses aged over 18 years after oral administration for 5 days at the dose of 10 mg/kg b.w./day. After the first administration, the maximum concentration (Cmax 44.21 ± 9.21 μg/mL) was reached at 2.5 ± 0.58 h post-treatment, the harmonic mean terminal half-life was 6.96 ± 1.73 h, AUC0–24h was 459.71 ± 69.95 h μg/mL, MRT was 7.44 ± 0.74 h and protein binding was 98.47 ± 2.72%. No drug accumulation occurred with repeated administrations. No clinical and laboratory changes were detected after administration of naproxen. Gastric endoscopies performed after the treatment did not show pathological changes of the gastric mucosa.