Pridinol, chemically known as 1,1-diphenyl-3-piperidino-1-propanol, is a centrally acting muscle relaxant that alleviates muscle spasms and pain by antagonizing muscarinic acetylcholine receptors. In veterinary use, it is marketed as an injectable formulation under the name Loxeen and is used to reduce muscle tension and improve mobility in animals. However, its use in competitive environments such as horseracing and equestrian sports is strictly regulated by authorities including the Federation of Equestrian Institutions (FEI), the Horseracing Integrity and Safety Authority (HISA), and the Japan Racing Association (JRA) in order to uphold fair competition and animal welfare.
In Japanese horseracing, a withdrawal period is typically applied to allow sufficient drug clearance from the horse’s system prior to racing. Such intervals are generally determined through pharmacokinetic/pharmacodynamic (PK/PD) modeling. The Toutain approach, in particular, is employed to estimate the time required for plasma and urine concentrations to fall below a predefined irrelevance level, ensuring that no pharmacological effects remain that could influence performance or affect animal welfare.
In line with this framework, the present study investigated the intravenous pharmacokinetics of pridinol in Thoroughbreds, applying the Toutain model to derive a scientifically sound withdrawal period. Each horse received a single 20 mg dose of pridinol mesylate via the jugular vein, followed by plasma and urine sampling over 72 hours. Drug concentrations were quantified by liquid chromatography–tandem mass spectrometry (LC-MS/MS), and pharmacokinetic parameters were calculated using a three-compartment model. The estimated irrelevant concentrations were 0.00284 ng/mL for plasma and 0.000612 ng/mL for urine. Key pharmacokinetic parameters included a clearance rate of 1.27 L/hr/kg, a steady-state volume of distribution of 2.07 L/kg, and a urine-to-plasma concentration ratio at steady state of 0.211. These results may support the establishment of regulatory screening limits for pridinol in equine sports.