Evaluation of the pharmacokinetics and bioavailability of intravenously and orally administered amiodarone in horses.
OBJECTIVE
To determine the clinical effects and pharmacokinetics of amiodarone after single doses of 5 mg/kg administered orally or intravenously.


METHODS
6 healthy adult horses.


METHODS
In a cross over study, clinical signs and electrocardiographic variables were monitored and plasma and urine samples were collected. A liquid chromatography-mass spectrometry method was used to determine the percentage of protein binding and to measure plasma and urine concentrations of amiodarone and the active metabolite desethylamiodarone.


RESULTS
No adverse clinical signs were observed. After IV administration, median terminal elimination half-lives of amiodarone and desethylamiodarone were 51.1 and 75.3 hours, respectively. Clearance was 0.35 L/kg x h, and the apparent volume of distribution for amiodarone was 31.1 L/kg. The peak plasma desethylamiodarone concentration of 0.08 microg/mL was attained 2.7 hours after IV administration. Neither parent drug nor metabolite was detected in urine, and protein binding of amiodarone was 96%. After oral administration of amiodarone, absorption of amiodarone was slow and variable; bioavailability ranged from 6.0% to 33.7%. The peak plasma amiodarone concentration of 0.14 microg/mL was attained 7.0 hours after oral administration and the peak plasma desethylamiodarone concentration of 0.03 microg/mL was attained 8.0 hours after administration. Median elimination half-lives of amiodarone and desethylamiodarone were 24.1 and 58.6 hours, respectively.


CONCLUSIONS
Results indicate that the pharmacokinetic distribution of amiodarone is multicompartmental. This information is useful for determining treatment regimens for horses with arrythmias. Amiodarone has low bioavailability after oral administration, does not undergo renal excretion, and is highly protein-bound in horses.
De Clercq
Dominique
D
Department of Large Animal Internal Medicine, Ghent University, Salisburylaan 133, B-9820, Merelbeke, Belgium.
Baert
Kris
K
Croubels
Siska
S
van Loon
Gunther
G
Maes
An
A
Tavernier
Rene
R
Deprez
Piet
P
De Backer
Patrick
P
D000284:Administration, Oral; D000638:Amiodarone / Q000008:administration & dosage* / Q000097:blood / Q000493:pharmacokinetics*; D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000008:administration & dosage* / Q000097:blood / Q000493:pharmacokinetics*; D019540:Area Under Curve; D001682:Biological Availability; D018592:Cross-Over Studies; D005260:Female; D017619:Hemibody Irradiation; D006736:Horses; D007275:Injections, Intravenous
D000889:Anti-Arrhythmia Agents; D000638:Amiodarone