Plasma concentrations of flunixin in the horse: its relationship to thromboxane B2 production


Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, PA 19348, USA.


The effects of the intravenous (i.v.) administration of 1.1 mg/kg of flunixin meglumine on thromboxane B2 (TxB2) concentrations were studied in sedentary and 2-year-old horses in training. The baseline TxB2 serum concentrations generated during clotting were 2.89 ± 81, 2.19 ± 0.25 and 0.88 ± 0.12 ng/ml for the 2-year-old Thoroughbreds in training, sedentary horses under 10 and over 10 years old, respectively. There was a significant difference in baseline TxB2 concentrations between older and younger horses (P < 0.005). Significant reduction in TxB2 production from baseline were noted at 1 (P < 0.01) and 4 h (P < 0.01) but not at 8 h after flunixin administration. The percent reduction in serum TxB2 concentration at 1 h after the administration of flunixin was 68.6 ± 7.8 and 45.2 ± 6.8 for the training and sedentary horses, respectively; the differences were significant (P < 0.04). Serum concentrations of TxB2 returned to baseline values by 12–16 h after flunixin administration. The results of this study indicate a difference in the TxB2 concentrations of older vs. younger horses and a difference in the suppression of TxB2 after the administration of flunixin in 2-year-old Thoroughbreds in training compared to sedentary horses. The results of this study suggest that the detection of low concentrations of flunixin in urine 24 h post-administration may not represent pharmacologic effective concentrations of flunixin in plasma.