Tracking Pharmacokinetics of a Single Dose of Flunixin Transdermal Formulation in American Bullfrogs Using UHPLC–MS/MS

A joint study between North Carolina State University (Raleigh, NC) and the North Carolina Zoo (Asheboro, NC) aimed to assess the pharmacokinetics of a single dose of topical transdermal flunixin meglumine in American bullfrogs (Lithobates catesbeianus). The team used ultrahigh-pressure liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) to determine plasma flunixin concentrations. A paper based on this research was published in the American Journal of Veterinary Research (1).

Nonsteroidal anti-inflammatory drugs (NSAIDs) in mammals are a mainstay of analgesic therapy, as they decrease prostaglandin synthesis via competitive inhibition of cyclooxygenase enzymes and, as a result, provide anti-inflammatory, antipyretic, and analgesic effects (2). The current science presumes that this mechanism is conserved in frogs and toads; a prior study in American bullfrogs (Lithobates catesbeianus) demonstrated decreased serum prostaglandin levels in response to meloxicam administration postinjury (3). The unique semipermeable skin of amphibians allows for easy absorption of gas, water, and electrolyte exchange, which researchers have taken advantage of for the administration of topical drugs (4–8).

Recently formulated and made commercially available in the US specifically for transdermal use in cattle, the NSAID flunixin meglumine is FDA-approved for fever associated with bovine respiratory disease or mastitis and for control of pain from foot rot in cattle. A single study investigating the pharmacokinetics of this product in a toad species demonstrated favorable results (9).

The researchers detected flunixin in all samples collected from 21 bullfrogs (nine 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 or epithelial loss was noted at the drug application site in 18 of 21 frogs, but no additional clinical signs or mortality occurred (1).

The authors acknowledge several limitations in their study, such as a lack of pharmacodynamic assessment, a small sample size, the need for a sparse sampling protocol and population-based pharmacokinetic approach, and the lack of bioavailability data. Further assessment of safety using additional diagnostics (such as blood biochemistry, euthanasia, and histopathology) was beyond the study’s scope. Finally, while repeated doses of NSAIDs are commonly clinically indicated in clinical settings, the pharmacokinetics and provisional safety of multiple doses of transdermal flunixin meglumine were not evaluated in the study discussed (1).

Pair of bullfrogs. © Mary Durden - stock.adobe.com

References

1. Heniff, A. C.; Mzyk, D. A.; Giles, C. B.; et al. Pharmacokinetics of a Single dose of Flunixin Transdermal Formulation in American Bullfrogs (Lithobates catesbeianus). Am. J. Vet. Res. 2025, 1–8. DOI: 10.2460/ajvr.24.12.0414

2. Papich, M. G. Papich Handbook of Veterinary Drugs-E-Book: Papich Handbook of Veterinary Drugs-E-Book; Elsevier Health Sciences, 2020.

3. Minter, L. J.; Clarke, E. O.; Gjeltema, J. L.; et al. Effects of Intramuscular Meloxicam Administration on Prostaglandin E2 Synthesis in the North American Bullfrog (Rana catesbeiana). J. Zoo Wildl. Med. 2011, 42 (4), 680–685. DOI: 10.1638/2011-0126.1

4. Ardente, A. J.; Barlow, B. M.; Burns, P.; Goldman, R.; Baynes, R. E. Vehicle Effects on in vitro Transdermal Absorption of Sevoflurane in the Bullfrog, Rana catesbeiana. Environ. Toxicol. Pharmacol. 2008, 25 (3), 373–379. DOI:10.1016/j.etap.2007.12.001

5. Chinnadurai, S. K.; Kane, L. P. Advances in Amphibian Clinical Therapeutics. J. Exot. Pet. Med. 2014, 23 (1), 50–55. DOI:10.1053/j.jepm.2013.11.008

6. D’Agostino, J. J.; West, G.; Boothe, et al. Plasma Pharmacokinetics of Selamectin After a Single Topical Administration in the American Bullfrog (Rana catesbeiana). J. Zoo Wildl. Med. 2007, 38 (1), 51–54. DOI:10.1638/06-054.1

7. Llewelyn, V. K.; Berger, L.; Glass, B. D. Percutaneous Absorption of Cemicals: Developing an Understanding for the Treatment of Disease in Frogs. J. Vet. Pharmacol. Ther. 2016, 39 (2),109–121. DOI:10.1111/jvp.12264

8. Rifkin, A.; Visser, M.; Barrett, K.; Boothe, D.; Bronson, E. The Pharmacokinetics of Topical Itraconazole in Panamanian Golden Frogs (Atelopus zeteki). J. Zoo Wildl. Med. 2017, 48 (2), 344–351. DOI:10.1638/2015-0218R2.1

9. Scott, G.; Louis, M. M.; Balko, J. A.; et al. Pharmacokinetics of Transdermal Flunixin Meglumine Following a Single Dose in Marine Toads (Rhinella marina). Vet. Med. Int. 2020, 2020 (1), 8863537. DOI: 10.1155/2020/8863537