Systemic Dexamethasone Concentration in Horses After Continued Topical Treatment With an Ophthalmic Preparation of Dexamethasone

Abstract

Objective - To determine concentrations of dexamethasone in serum and urine of horses treated repeatedly with a topically administered ophthalmic dexamethasone preparation. Animals - 4 clinically normal horses (2 mares, 2 geldings). Procedure - 0.1% dexamethasone ophthalmic ointment was administered to the left eye of each horse every 5 to 9 hours for 8 consecutive days, yielding an estimated cumulative dexamethasone dose of 6.4 μg/kg of body weight. Serum and urine samples were obtained before the first dexamethasone treatment, on days 4 and 8 of treatment, and 24, 48, and 96 hours after cessation of treatment. To detect small concentrations of dexamethasone, serum and urine samples were analyzed by use of a competitive enzyme immunoassay. Results - During the period of continued topical treatment, serum dexamethasone concentrations increased to between 0.10 and 0.49 ng/ml, then decreased below the limit of detection (0.06 ng/ml) within 24 hours after cessation of treatment. Dexamethasone also was detected in urine samples at concentrations of up to 0.98 ng/ml. Conclusions - Repeated topical administration of dexamethasone ophthalmic ointment generated low, but detectable glucocrticoid concentrations in serum and urine. Clinical Relevance - Because treatment of performance horses with dexamethasone is prohibited for most types of competitions and because enhanced glucocorticoid detection methods may result in positive test results, owners and trainers may wish to reconsider entering horses in competitions during periods of treatment with ophthalmic dexamethasone preparations.