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 0.1 mg/kg butorphanol. Blood and urine samples were collected at time 0 and at various times for up to 120 h and analyzed using LC-MS. Mean±SD systemic clearance, steady-state volume of distribution, and terminal elimination half-life were 11.5±2.5 mL/min/kg, 1.4±0.3 L/kg, and 5.9±1.5 h, respectively. Butorphanol plasma concentrations were below the limit of detection (LOD) (0.01 ng/mL) by 48 h post administration. Urine butorphanol concentrations were below the LOD (0.05 ng/mL) of the assay in seven of 10 horses by 120 h 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 24 h.

This study extends previous studies describing the pharmacokinetics of TA following i.m. and intra-articular administration to the horse and suggests that plasma and urine concentrations are not a good indicator of synovial fluid concentrations. Furthermore, results of this study supports an extended withdrawal time for TA given i.m.

Methylprednisolone acetate (MPA) is commonly administered to performance horses, and therefore, establishing appropriate withdrawal times prior to performance is critical. The objectives of this study were to describe the plasma pharmacokinetics of MPA and time-related urine and synovial fluid concentrations following intra-articular administration to sixteen racing fit adult Thoroughbred horses. Horses received a single intra-articular administration of MPA (100 mg). Blood, urine, and synovial fluid samples were collected prior to and at various times up to 77 days postdrug administration and analyzed using tandem liquid chromatography-mass spectrometry (LC-MS/MS). Maximum measured plasma MPA concentrations were 6.06 ± 1.57 at 0.271 days (6.5 h; range: 5.0-7.92 h) and 6.27 ± 1.29 ng/mL at 0.276 days (6.6 h; range: 4.03-12.0 h) for horses that had synovial fluid collected (group 1) and those that did not (group 2), respectively. The plasma terminal half-life was 1.33 ± 0.80 and 0.843 ± 0.414 days for groups 1 and 2, respectively. MPA was undetectable by day 6.25 ± 2.12 (group 1) and 4.81 ± 2.56 (group 2) in plasma and day 17 (group 1) and 14 (group 2) in urine. MPA concentrations in synovial fluid remained above the limit of detection (LOD) for up to 77 days following intra-articular administration, suggesting that plasma and urine concentrations are not a good indicator of synovial fluid concentrations.

Fentanyl appears to be consistently well tolerated following i.v. administration of 4 μg/kg bwt to foals ranging in age from 1 to 6 weeks. The results of this study support further study of fentanyl for clinical use in foals.

Behavioural and physiological responses to i.v. fentanyl in young foals are dose related. As with mature horses, the window of fentanyl plasma concentrations related to possible clinically desirable actions appears relatively narrow.