The majority of topical formulations used in veterinary ophthalmology are approved for human use but not for animals. Knowledge about absorption and distribution of these drugs in ocular structures in dogs and cats is limited. In general, the pharmacokinetics of ophthalmic preparations depends on the time the drug remains in the precorneal tear film, how it is metabolized, and how it diffuses across the cornea and conjunctiva. The formulation (ointment vs suspension) also affects contact time. Dexamethasone is commonly used in treating inflammatory eye diseases, but little is known about its absorption and distribution in the feline eye. In this study, 19 cats without ocular disease were treated with either dexamethasone ointment (1 mg/g) or suspension (1 mg/mL) in both eyes. Cats were euthanized at 3 hours or 6 hours after treatment and the eyes were harvested immediately. Dexamethasone concentration was measured in various structures of the eye by radioimmunoassay. The study showed that concentrations of the drug in the third eyelid, cornea, and aqueous humor were similar for both formulations 3 hours after administration. However, at 6 hours these concentrations were significantly higher in cats that received ointment. Dexamethasone reached therapeutic levels only in the third eyelid, cornea, and iris. The results suggest that dexamethasone is most effectively delivered in ointment form but reaches sufficient concentrations only in the anterior structures of the eye.

Commentary: This study was of relatively simple design but showed interesting data to suggest that dexamethasone ointment may be superior to drops in treating inflammatory eye disease and may be used every 6 hours for optimal dosing. More studies are needed to determine the absorption and distribution of dexamethasone preparations in the diseased feline eye.—Heather Troyer, DVM, Diplomate ABVP

Tissue distribution of dexamethasone in feline ocular structures following single topical application of dexamethasone as an ointment or suspension. Bessonova J, Meyer-Lindenberg A, Bäumer W, Kietzmann M. VET OPHTHALMOL 14:109-113, 2011.