Penetration of topically administered corticosteroids in normal equine ocular fluids

Background: Topical dexamethasone and prednisolone are currently the mainstay treatment for equine ophthalmic inflammatory diseases, such as equine recurrent uveitis. Comparative pharmacokinetic studies in horses are lacking and current guidelines are mainly based on empirical data and extrapolation from other species.
Objectives: To investigate the penetration and local concentrations of topically applied dexamethasone and prednisolone in normal equine ocular fluids and serum.
Study Design: Prospective randomized experimental pharmacokinetic study.
Methods: Twenty-one Shetland ponies without ophthalmic disease were treated bilaterally topically every 2 hours during 24 hours to obtain steady-state drug concentrations. One eye was treated with 0.15 mg dexamethasone disodium phosphate (0.1%) and the other eye was simultaneously treated with 1.5 mg prednisolone acetate (1%). Serum samples were taken prior to induction of general anaesthesia. Aqueous and vitreous humour samples were taken during euthanasia at time points after administration of the last dose (t=5min, t=15min, t=30min, t=60min, t=90 min, t=120min, t=180min). Each pony was randomly assigned to one time point and 3 ponies were sampled per time point. Dexamethasone and prednisolone concentrations were measured by liquid chromatography-mass spectrometry.