Fexofenadine in horses: pharmacokinetics, pharmacodynamics and effect of ivermectin pretreatment

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<jats:p>The pharmacokinetics and the effects on inhibition of histamine‐induced cutaneous wheal formation of the histamine H<jats:sub>1</jats:sub>‐antagonist fexofenadine were studied in horse. The effect of ivermectin pretreatment on the pharmacokinetics of fexofenadine was also examined. After intravenous infusion of fexofenadine at 0.7 mg/kg bw the mean terminal half‐life was 2.4 h (range: 2.0–2.7 h), the apparent volume of distribution 0.8 L/kg (0.5–0.9 L/kg), and the total body clearance 0.8 L/h/kg (0.6–1.2 L/h/kg). After oral administration of fexofenadine at 10 mg/kg bw bioavailability was 2.6% (1.9–2.9%). Ivermectin pretreatment (0.2 mg/kg, p.o.) 12 h before oral fexofenadine decreased the bioavailability to 1.5% (1.4–2.1%). In addition, the area under the plasma concentration–time curve decreased 27%. Ivermectin did not affect the pharmacokinetics of i.v. administered fexofenadine. Ivermectin may influence fexofenadine absorption by interfering in intestinal efflux and influx pumps, such as P‐glycoprotein and the organic anion transport polypeptide family. Oral and i.v. fexofenadine significantly decreased histamine‐induced wheal formation, with a maximal duration of 6 h. A pharmacokinetic/pharmacodynamic link model indicated that fexofenadine in horse has antihistaminic effects at low plasma concentrations (EC<jats:sub>50</jats:sub> = 16 ng/mL). However, oral treatments of horses with fexofenadine may not be suitable due to the low bioavailability.</jats:p>

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