Systemic detectability of dexamethasone and prednisolone after eye drop application in horses

Background Equine sport agencies list steroids as prohibited substances for competing horses. Objectives The objective of this study was to investigate if the controlled substances dexamethasone and prednisolone are detectable in equine serum and urine samples during and after treatment with eye drops and if this can generate a positive doping test. Study design Prospective cohort study. Methods The study cohort included 11 horses. One eye of the horses was treated with either dexamethasone (Maxitrol® 0.1%, n = 5 eyes) or prednisolone (Pred forte® 1%, n = 6 eyes) eye drops 3 times daily for 14 days. Dexamethasone and prednisolone concentrations were determined in serum and urine at day 0 (negative control), 1, 7, 14, 15, 17 and 21 using liquid chromatography‐tandem mass spectrometry. Blood samples were collected within 2 hours post application. Urine samples were collected during spontaneous urination. Results All serum samples (range: 0.7‐43 ng/mL, mean 2.1 ng/mL) and urine samples (range 1.2‐5 ng/mL, mean 0.8 ng/mL) showed measurable amounts of dexamethasone during the course of treatment. Concentrations in both serum and urine samples were below limit of detection (LOD) 24 hours after the last dexamethasone treatment (day 15). All serum samples (range 1.1‐32.5 ng/mL, mean 6.4 ng/mL) and urine samples (range 3.7‐19 ng/mL, mean 4.6 ng/mL) were positive for prednisolone during treatment. Urine samples were below LOD on day 15; serum samples on day 21. Conclusions Dexamethasone and prednisolone eye drops can induce detectable drug levels in serum and urine samples of horses after a 14‐day treatment plan. This can lead to a positive doping result. All samples tested negative (below LOD of the analytical method) for dexamethasone one day and for prednisolone one week after treatment cessation.