Pharmacokinetics and pharmacodynamics of butorphanol following intravenous administration to the horse

Butorphanol is a narcotic analgesic commonly used in horses. Currently, any detectable concentration of butorphanol in biological samples collected from performance horses is considered a violation. The primary goal of the study reported here was to update the pharmacokinetics of butorphanol following intravenous administration, utilizing a highly sensitive liquid chromatography-mass spectrometry (LC-MS) assay that is currently employed in many drug-testing laboratories. An additional objective was to characterize behavioral and cardiac effects following administration of butorphanol. Ten exercised adult horses received a single intravenous dose of 0.1mg/kg butorphanol. Blood and urine samples were collected at time 0 and at various times for up to 120h and analyzed using LC-MS. Mean±SD systemic clearance, steady-state volume of distribution, and terminal elimination half-life were 11.5±2.5mL/min/kg, 1.4±0.3L/kg, and 5.9±1.5h, respectively. Butorphanol plasma concentrations were below the limit of detection (LOD) (0.01ng/mL) by 48h post administration. Urine butorphanol concentrations were below the LOD (0.05ng/mL) of the assay in seven of 10 horses by 120h post drug administration. Following administration, horses appeared excited as noted by an increase in heart rate and locomotion. Gastrointestinal sounds were markedly decreased for up to 24h.