Pharmacokinetics of triamcinolone acetonide following intramuscular and intra-articular administration to exercised Thoroughbred horses.
Authors: Knych H K, Vidal M A, Casbeer H C, McKemie D S
Journal: Equine veterinary journal
Summary
# Editorial Summary: Triamcinolone Acetonide Pharmacokinetics in Exercised Thoroughbreds Triamcinolone acetonide (TA) remains a commonly used anti-inflammatory in performance horses, yet establishing evidence-based withdrawal times has proven challenging—a knowledge gap that motivated Knych and colleagues to characterise drug clearance in racing-fit animals following both intramuscular and intra-articular routes. Twelve Thoroughbreds received a single i.m. dose (0.1 mg/kg bodyweight) and, after washout, intra-articular TA (9 mg into the antebrachiocarpal joint), with plasma, urine and synovial fluid sampled over 60 days and analysed by liquid chromatography-mass spectrometry. Peak plasma concentrations differed substantially between routes—0.996 ng/ml at 13.2 hours (i.m.) versus 1.27 ng/ml at 6.5 hours (intra-articular)—yet the elimination profiles diverged markedly: i.m. administration demonstrated a terminal half-life of 11.4 days, remaining detectable in urine until Day 60, whilst intra-articular TA cleared from plasma by Day 7 and urine by Day 8 despite persisting above quantifiable levels in the injected joint for 21 days. This work challenges the assumption that plasma and urine concentrations reliably predict synovial fluid drug persistence, and supports substantially extended withdrawal periods for i.m. TA in competition horses—particularly important given the extended systemic circulation and urinary excretion profile observed in exercised animals.
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Practical Takeaways
- •Extended withdrawal times are required for IM triamcinolone acetonide in performance horses due to prolonged plasma detectability (52+ days), significantly longer than intra-articular administration (7 days)
- •IM and intra-articular routes have markedly different pharmacokinetic profiles; IM injection shows much longer systemic half-life (11.4 vs 0.78 days), requiring separate withdrawal time recommendations
- •When evaluating drug testing results, synovial fluid concentrations cannot be predicted from plasma or urine levels, particularly important for intra-articular joint injections that persist longer at the injection site
Key Findings
- •Intramuscular TA reached peak plasma concentration of 0.996 ng/ml at 13.2 hours with terminal half-life of 11.4 days; intra-articular administration peaked at 1.27 ng/ml at 6.5 hours with half-life of 0.78 days
- •Following IM administration, TA was undetectable in plasma by day 52 and urine by day 60; following intra-articular administration, undetectable by days 7 and 8 respectively
- •TA was undetectable in plasma and synovial fluid following IM administration, but remained above limit of quantitation in synovial fluid for 21 days after intra-articular administration
- •Plasma and urine concentrations are not reliable indicators of synovial fluid concentrations of TA