Pharmacokinetics and safety of repeated oral dosing of acetaminophen in adult horses.

Abstract

BACKGROUND: There are no published studies on the pharmacokinetics of acetaminophen at the dosage used clinically (20 mg/kg), nor has the safety of multiple doses in horses been investigated. OBJECTIVE: Define the pharmacokinetic parameters of oral acetaminophen at 20 mg/kg in adult horses as a single dose, and twice daily for 14 days to assess the safety of multiple dosing. STUDY DESIGN: Pharmacokinetic study, multiple dose safety study. METHODS: Eight healthy Thoroughbred geldings were given acetaminophen (20 mg/kg; 500 mg tablets) orally as a single dose followed by doses every 12 h for 14 days. Serial blood samples were collected for determination of plasma acetaminophen concentrations using high performance liquid chromatography with ultraviolet detection. Serum biochemical analysis, gastroscopy and liver biopsy were examined during the safety study. RESULTS: Following a single dose, mean maximum concentration (C) was 16.61 μg/mL at 1.35 h (T), and drug concentration was below the lower limit of detection in most horses by 24 h. Elimination half-life (T) was 2.78 h. No significant accumulation was noted following multiple doses. Average Cof acetaminophen following multiple oral dosing was 15.85 μg/mL, with a Tof 0.99 h and Tof 4 h. Serum activities of sorbitol dehydrogenase were significantly decreased and total bilirubin concentrations were significantly increased following the last dose. No statistically significant changes were noted in gastroscopy scores. MAIN LIMITATIONS: Only one dose level (20 mg/kg) was studied, sample size was small and only a single breed and sex was used, with no pretreatment liver biopsies. CONCLUSION: This study described the pharmacokinetics of acetaminophen following single and multiple 20 mg/kg oral doses in adult horses and demonstrated the safety of acetaminophen with multiple oral dosing over 14 days. The summary is available in Portuguese - see Supporting information.