Pharmacokinetics and bioavailability of tolfenamic acid in sheep

The pharmacokinetics, bioavailability, and tolerability of tolfenamic acid (TA) were determined after treating sheep with TA via different routes and doses. This crossover study was carried out with a washout period of 15 days. In the study, 16 clinically healthy sheep were randomly assigned to two equal groups. In the first group (n = 8), animals received TA by intravenous (IV), intramuscular (IM), subcutaneous (SC), or oral (OR) routes at 2 mg/kg. In the second group (n = 8), TA was administered intravenously to each sheep at 2, 4, 8, and 16 mg/kg. Plasma samples were analyzed with a high-performance liquid chromatography assay. Noncompartmental pharmacokinetic analyses were used to evaluate the data. The area under the concentration-time curves (AUC_0-∞ ), elimination half-life (t_1/2ʎz ), and the mean residence time (MRT) significantly differed among the administration routes at 2 mg/kg of TA. Following IM, SC, and OR administrations, TA demonstrated different peak concentrations (C_max ) and time to reach C_max (T_max ), with a bioavailability of 163%, 127%, and 107%, respectively. The dose-normalized AUC_0-∞ revealed a significant difference among the dose groups; however, the relationship between dose and AUC_0-∞ was linear. Both t_1/2ʎz and MRT increased depending on the dose. Although the total clearance (Cl_T ) decreased depending on dose, the volume of distribution at steady-state (V_ss ) increased. Tolfenamic acid indicated a long half-life and high bioavailability following IM, SC, and OR administrations at 2 mg/kg. TA exhibited linear kinetics and was well tolerated by the animals, except at 16 mg/kg. Thus, TA may be used in different routes and doses (≤8 mg/kg) in sheep; however, further studies are needed to determine the clinical efficacy of TA during the inflammatory and painful conditions and the pharmacokinetics and safety of repeated administration in sheep.