192 ICRAV 2026 1. Equine Research Institute, Japan Racing Association, Shimotsuke, Tochigi, Japan 2. Graduate School of Agriculture, Tokyo University of Agriculture and Technology, Fuchu, Tokyo, Japan 3. K.L. Maddy Equine Analytical Chemistry Laboratory (Pharmacology Section), School of Veterinary Medicine, University of California, Davis, California, USA 4. School of Agricultural, Environmental and Veterinary Sciences, Charles Sturt University, Wagga, New South Wales, Australia 5. Laboratory of Racing Chemistry, Utsunomiya, Tochigi, Japan 6. Equine Department Main Office, Japan Racing Association, Tokyo, Japan 7. London Representative Office, Japan Racing Association, London, United Kingdom 8. Comparative Biomedical Sciences, The Royal Veterinary College, London, United Kingdom ABSTRACT A meta-analysis of flunixin plasma and urine concentrations in 65 horses across Japan, the USA, the UK, and Australia was conducted using a nonlinear mixed-effects population model (POP model). The objective of this study was to provide regulatory authorities responsible for international harmonization with a balanced synthesis of available data to facilitate decisions on appropriate screening limits for medication control. The dataset included 55 horses administered with a single dose of flunixin meglumine (1.1 mg/kg or 500 mg/horse), and 10 horses administered multiple doses (1.1 mg/kg every 24 h for 5 days). Irrelevant plasma concentration (IPC) and irrelevant urine concentration (IUC) were estimated to be 1.9 ng/mL and 70.2 ng/mL, respectively, with a typical urine-to-plasma ratio (Rss) of 35.9. Under the current International Federation of Horseracing Authorities (IFHA) screening limits (SL) (1 ng/mL for plasma; 100 ng/mL for urine), plasma exhibited longer detection times (DTs) than urine—up to 41.4 h after a single dose and 125.6 h after multiple doses. This mismatch arises because the ISL ratio (100) exceeds the observed Rss (35.9) and becomes more pronounced after multiple dosing due to the plasma ISL intersecting a slow terminal elimination phase. Increasing the current plasma ISL from 1 to 3 ng/mL—while keeping the current urine ISL at 100 ng/mL—could better align the plasma and urine DTs. Taisuke Kuroda1, 2, Heather K Knych3, Glenys K Noble4, Yohei Minamijima5, Gary Ngai-Wa Leung5, Motoi Nomura1, Fumiaki Mizobe6, Yuhiro Ishikawa6, Kanichi kusano7, Pierre-Louis Toutain8 Meta-analysis of international pharmacokinetic data of flunixin in horses: supporting regulatory harmonization and enabling prescribers to individualize detection times through a Bayesian paradigm
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