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

This research explores how a topical dexamethasone treatment in horses impacts systemic concentrations of the drug, revealing that the drug can be detected in serum and urine after continued use, even when used topically for eye treatment. It concludes that this finding implicates potential complications for performance horses enrolled in competitions, due to restrictions on dexamethasone use.

Research Methodology

• Four clinically normal horses (two mares, two geldings) were included in the study.
• These horses were treated with a 0.1% dexamethasone ophthalmic ointment in their left eye every 5-9 hours for eight consecutive days. This resulted in an estimated cumulative dose of 6.4 micrograms of dexamethasone per kilogram of body weight.
• Samples of the horses’ serum and urine were collected at the start of the study, on days 4 and 8 during treatment, and then 24, 48, and 96 hours after the end of treatment.
• To detect the small quantities of dexamethasone, the researchers utilized a competitive enzyme immunoassay to analyze the serum and urine samples.

Findings

• Dexamethasone concentrations in the horses’ serum ranged between 0.10 and 0.49 ng/ml during the period of topical treatment.
• Within 24 hours after ending the treatment, dexamethasone levels decreased below the limit of detection (0.06 ng/ml).
• Dexamethasone was also detected in the urine samples, with concentrations reaching up to 0.98 ng/ml.

Conclusions

• The results of the study indicate that topically applied ophthalmic dexamethasone leads to low, yet detectable, glucocorticoid concentrations in both serum and urine of horses.
• The presence of dexamethasone in systemic concentrations may have implications for performance horses. As dexamethasone treatment is prohibited in most competitions, the persistent detectable levels of the drug could result in positive drug test results for these horses.
• Consequently, the researchers suggest that owners and trainers may need to rethink the participation of horses in competitions during periods when they are undergoing treatment with topical ophthalmic dexamethasone preparations.