Systemic dexamethasone concentration in horses after continued topical treatment with an ophthalmic preparation of dexamethasone

Objective-To determine concentrations of dexamethasone in serum and urine o f horses treated repeatedly with a topically administered ophthalmic dexame thasone preparation. Animals-4 clinically normal horses (2 mares, 2 geldings), Procedure-0.1% dexamethasone ophthalmic ointment was administered to the le ft eye of each horse every 5 to 9 hours for 8 consecutive days, yielding an estimated cumulative dexamethasone dose of 6.4 mu g/kg of body weight. Ser um 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 urin e samples were analyzed by use of a competitive enzyme immunoassay. Results-During the period of continued topical treatment, serum dexamethaso ne 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 o f treatment. Dexamethasone also was detected in urine samples at concentrat ions of up to 0.98 ng/ml. Conclusions-Repeated topical administration of dexamethasone ophthalmic oin tment generated low, but detectable glucocorticoid concentrations in serum and urine, Clinical Relevance-Because treatment of performance horses with dexamethaso ne is prohibited for most types of competitions and because enhanced glucoc orticoid detection methods may result in positive test results, owners and trainers may wish to reconsider entering horses in competitions during peri ods of treatment with ophthalmic dexamethasone preparations.