Cetirizine in horses: Pharmacokinetics and pharmacodynamics following repeated oral administration

The pharmacokinetics of the histamine H1-antagonist cetirizine and its effect on histamine-induced cutaneous wheal formation were studied in six healthy horses following repeated oral administration. After three consecutive administrations of cetirizine (0.2 mg/kg body weight, bw) every 12 h, the trough plasma concentration of cetirizine was 16 ± 4 ng/mL (mean ± SD) and the wheal formation was inhibited by 45 ± 23%. After four additional administrations of cetirizine (0.4 mg/kg bw) every 12 h, the trough plasma concentration was 48 ± 15 ng/mL and the wheal formation was inhibited by 68 ± 11%. The terminal half-life was about 5.8 h. A pharmacokinetic/pharmacodynamic link model showed that the maximal inhibition of wheal formation was about 95% and the EC50 about 18 ng/mL. It is concluded that cetirizine in doses of 0.2–0.4 mg/kg bw administered at 12 h intervals exhibits favourable pharmacokinetic and pharmacodynamic properties without causing visible side effects, and the drug may therefore be a useful antihistamine in equine medicine.