Pharmacokinetics of cannabidiol and its metabolites for the definition of ineffective plasma and urinary concentrations in biological liquids of sport horses

Cannabidiol (CBD) is increasingly used in veterinary medicine, although its pharmacokinetics in horses remain variable due to the heterogeneity of the available oral CBD formulations. As a substance controlled by regulatory bodies, CBD demands clear guidelines to distinguish pharmacologically active concentrations from irrelevant residues. This study evaluated the pharmacokinetics of a CBD oral formulation and its major metabolites (7-hydroxy-cannabidiol-7OH-CBD and 7-carboxy-cannabidiol-7COOH-CBD) in horses, defining also its pharmacologically irrelevant plasma (IPC) and urinary (IUC) concentrations. Six standardbred mares received a single 200mg CBD oral dose. Plasma and urine were collected before administration and up to 120 hours and analyzed using LC/HRMS. Pharmacokinetic parameters were determined applying non-compartmental analysis and IPC-IUC were calculated with Toutain and Lassourd’s pharmacokinetic/pharmacodynamic approach. CBD exhibited a short half-life (2.52±1.54h) and low Cmax (1.98±1.29ng/mL), reflecting extensive first-pass metabolism and high interindividual variability. Conversely, 7COOH-CBD displayed a prolonged half-life (32.47±9.51h) and higher systemic exposure, whereas 7OH-CBD was not quantified in plasma. The calculated IPC and IUC for CBD were 0.37pg/mL and 31.36pg/mL, respectively, with a withdrawal time (WT) of 1.2 days. For 7COOH-CBD, the IPC was 158.29pg/mL, IUC 55.40pg/mL, and WT 10.5 days. No THC was detected in any samples. These findings provide thresholds for CBD and 7COOH-CBD following administration of this oral formulation and support rational WT for equine athletes. The prolonged detectability of 7COOH-CBD highlights the need for regulations distinguishing active from irrelevant residual concentrations to ensure fair competition.