Single and multiple-dose pharmacokinetics of tepoxalin and its active metabolite after oral administration to rabbits (Oryctolagus cuniculus) C. G. POLLOCK* J. W. CARPENTER* D. E. KOCH   & R. P. HUNTER  ,1 Departments of *Clinical Sciences; and   Zoological Pharmacology Laboratory, Anatomy and Physiology, College of Veterinary Medicine, Kansas State University, Manhattan, KS, USA Pollock, C. G., Carpenter, J. W., Koch, D. E., Hunter, R. P. Single and multiple- dose pharmacokinetics of tepoxalin and its active metabolite after oral admin- istration to rabbits (Oryctolagus cuniculus). J. vet. Pharmacol. Therap. 31, 171– 174. The anti-inflammatory agent, tepoxalin, was administered to eight healthy 6-month-old female New Zealand white rabbits once daily at an oral dose of 10 mg ⁄ kg. Blood samples were obtained immediately before and at 0.25, 0.5, 1, 2, 3, 4, 6, 8, 12, and 24 h postadministration on days 1 and 10. Tepoxalin and its active metabolite, RWJ 20142, concentrations were determined in plasma by use of high-performance liquid chromatography with mass spectrometry. C max of the parent compound was reached between 3 and 8h of drug administration, with a harmonic mean t 1 ⁄ 2 of 3.6 h. Peak tepoxalin plasma concentrations were 207 ± 49 ng ⁄ mL. After oral administration, the metab- olite RWJ 20142 achieved C max in plasma 2–8 h after administration, with a t 1 ⁄ 2 of 1.9–4.8 h (harmonic mean 2.8 h). Peak plasma concentrations of RWJ 20142 on day 1 were 2551 ± 1034 ng ⁄ mL. (Paper received 12 June 2007; accepted for publication 11 December 2007) C. G. Pollock, 14265 Superior Road, Cleveland Heights, OH 44118, USA. E-mail: christal7@mac.com 1 Present address: Elanco Animal Health, 2001 West Main Street, Greenfield, IN 46140, USA Tepoxalin is a nonsteroidal anti-inflammatory drug (NSAID) with multiple targets. As a dual inhibitor of cyclooxygenase (COX) and 5-lipoxygenase (LOX), tepoxalin inhibits the synthesis of inflammatory prostaglandins and leukotrienes and other inflammatory mediators, such as tumor necrosis factor, inter- leukin-2, and interleukin-6 (Argentieri et al., 1994; Ritchie et al., 1995; Goossens & Berthomme, 2001; Papich, 2004). Eight 6-month-old female New Zealand white rabbits (Oryc- tolagus cuniculus) were obtained from a private breeder. Rabbits were deemed healthy based on physical examination, hemato- crit, and total protein. During a 2-week acclimation period and subsequent treatment periods, rabbits were fed grass hay and water ad libitum as well as alfalfa-based pellets (Bunny Basics 15 ⁄ 23; Oxbow Hay Company, Murdock, NE, USA). The Kansas State University Institutional Animal Care and Use Committee approved the study protocol. A 50 mg, rapidly disintegrating tepoxalin tablet (Zubrin, Schering-Plough, Union, NJ, USA) was dissolved in 1 mL of tap water within a 3-mL syringe. Tepoxalin was administered to rabbits at a dose of 10 mg ⁄ kg once every 24 h for 10 days at 7 AM. Heparinized blood samples (2 mL) were obtained from the central ear artery or the lateral ⁄ medial saphenous vein imme- diately before and at 0.25, 0.5, 1, 2, 3, 4, 6, 8, 12, and 24 h after administration of tepoxalin on days 1 and 10. Blood samples were then obtained immediately before drug adminis- tration and at 3 h postadministration on days 3, 5, and 7. Plasma was obtained after centrifugation for 10 min at 1000 g and stored at approximately at )70 °C until analyzed. Plasma concentrations of tepoxalin and its active metabolite, RWJ 20142, were determined by use of validated high- performance liquid chromatography ⁄ mass spectroscopy (HPLC ⁄ MS) assay (Burinsky et al., 1996; Waldman et al., 1996). The extraction of tepoxalin and its metabolite from rabbit plasma used a 200 lL aliquot. To this aliquot was added 20 lL of 20 lg ⁄ mL celecoxib, and then 1 mL of methyl-t-butyl ether (MTBE) was added. The sample was vortexed for 1 min and centrifuged for 10 min at 1000 g. The supernatant was transferred to a fresh 1.5 mL microcentrifuge tube, dried under nitrogen in a 30 °C water bath, and reconstituted with 100 lL of mobile phase. Samples were assayed within 30 days of collection. At least 30 days of stability had been demonstrated for both compounds. The limit of quantification for this method was 1 ng ⁄ mL for both tepoxalin and its metabolite RWJ 20142, and a linear standard curve, weighted 1 ⁄ x, from 1 to 5000 ng ⁄ mL was used for quantification of both compounds. Using quality control samples of 10, 100, and 1000 ng ⁄ mL the intraday variation obtained for tepoxalin was 7.0%, 9.0%, and 7.2%, intraday J. vet. Pharmacol. Therap. 31, 171–174, doi: 10.1111/j.1365-2885.2007.00937.x. SHORT COMMUNICATION Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Publishing Ltd 171