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

The pharmacokinetics and the effects on inhibition of histamine-induced cutaneous wheal formation of the histamine H1-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 (EC50 = 16 ng/mL). However, oral treatments of horses with fexofenadine may not be suitable due to the low bioavailability.