Pharmacokinetics of phenylbutazone in neonatal foals

Jeff R. Wilcke From the Departments of Biomedical Sciences (Wilcke) and Large Animal Clinical Sciences (Crisman), Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg, VA 24061, and Departments of Veterinary Medicine Administration and Veterinary Clinical Sciences (Sams), and Veterinary Physiology and Pharmacology (Gerken), College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.

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Mark V. Crisman From the Departments of Biomedical Sciences (Wilcke) and Large Animal Clinical Sciences (Crisman), Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg, VA 24061, and Departments of Veterinary Medicine Administration and Veterinary Clinical Sciences (Sams), and Veterinary Physiology and Pharmacology (Gerken), College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.

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Richard A. Sams From the Departments of Biomedical Sciences (Wilcke) and Large Animal Clinical Sciences (Crisman), Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg, VA 24061, and Departments of Veterinary Medicine Administration and Veterinary Clinical Sciences (Sams), and Veterinary Physiology and Pharmacology (Gerken), College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.

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Diane F. Gerken From the Departments of Biomedical Sciences (Wilcke) and Large Animal Clinical Sciences (Crisman), Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg, VA 24061, and Departments of Veterinary Medicine Administration and Veterinary Clinical Sciences (Sams), and Veterinary Physiology and Pharmacology (Gerken), College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.

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Summary

Single doses (2.2 mg/kg of body weight) of phenylbutazone (pbz) were administered iv to 6 neonatal horses (5 to 17 hours old at time of dosing). Plasma concentrations of pbz and its metabolite oxyphenbutazone were monitored serially for 120 hours after drug administration. Pharmacokinetic variables were calculated, using 1- and 2-compartment open models. Descriptive equations from the best model for each foal were then used to derive model-independent variables describing pbz disposition. Median volume of distribution at steady-state was 0.274 L/ kg (range, 0.190 to 0.401 L/kg). Median terminal half-life was 7.4 (6.4 to 22.1) hours, and median total plasma clearance of pbz for foals in this study was 0.018 L/kg/h (range, 0.013 to 0.038 L/kg/h). Volume of distribution was larger, half-life was longer, and total clearance was lower, compared with similar values reported for administration of pbz to adult horses.

Summary

Single doses (2.2 mg/kg of body weight) of phenylbutazone (pbz) were administered iv to 6 neonatal horses (5 to 17 hours old at time of dosing). Plasma concentrations of pbz and its metabolite oxyphenbutazone were monitored serially for 120 hours after drug administration. Pharmacokinetic variables were calculated, using 1- and 2-compartment open models. Descriptive equations from the best model for each foal were then used to derive model-independent variables describing pbz disposition. Median volume of distribution at steady-state was 0.274 L/ kg (range, 0.190 to 0.401 L/kg). Median terminal half-life was 7.4 (6.4 to 22.1) hours, and median total plasma clearance of pbz for foals in this study was 0.018 L/kg/h (range, 0.013 to 0.038 L/kg/h). Volume of distribution was larger, half-life was longer, and total clearance was lower, compared with similar values reported for administration of pbz to adult horses.

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