Pharmacokinetics of a single dose of flunixin transdermal formulation in American bullfrogs (Lithobates catesbeianus).

Objective: To determine the pharmacokinetics of a single dose of flunixin transdermal formulation in American bullfrogs (Lithobates catesbeianus).

Methods: Clinically healthy, purpose-bred adult bullfrogs housed at the North Carolina State University College of Veterinary Medicine were enrolled in a sparse-sampling population study. Frogs were administered 3.3 mg/kg transdermal flunixin meglumine (Banamine Transdermal; Merck Animal Health) on the dorsum via micropipette under manual restraint in July of 2022. Frogs were maintained in individual containers out of water for 4 hours and randomly assigned to 2 of the following venipuncture time points: 1, 2, 4, 8, 12, or 24 hours, with 7 frogs sampled per time point. Blood was collected from the popliteal sinus. Ultra performance liquid chromatography-tandem mass spectrometry was used to determine plasma flunixin concentrations. Data were analyzed using noncompartmental analysis.

Results: Flunixin was detected in all samples collected from 21 bullfrogs (9 males and 12 females). A mean peak plasma concentration of 2.39 µg/mL was reached between 1 and 2 hours. The elimination half-life was 15.0 hours. Plasma concentrations were similar across individuals at 1, 2, and 4 hours (range at 1 and 2 hours, 2.32 to 2.55 µg/mL) but were variable at 8, 12, and 24 hours (range at 24 hours, 0.16 to 1.79 µg/mL). Mucus and/or epithelial loss was noted at the drug application site in 18 of 21 frogs. No additional clinical signs or mortality occurred.

Conclusions: Transdermal flunixin was systemically absorbed, and plasma concentrations exceeded established therapeutic ranges in other species. Most frogs developed mild cutaneous lesions.

Clinical relevance: Transdermal flunixin was detected in plasma for 24 hours; however, variability in plasma concentrations over time and topical side effects may limit its use.