Treatment of uveitis often relies on topical corticosteroid medication to control inflammation. A study examined the pharmacokinetic potential of using dexamethasone or prednisolone topically in the eye to achieve this objective. [Hermans H, van den Berg EMH, Slenter IJM, et al. Penetration of topically administered dexamethasone disodium phosphate and prednisolone acetate into the normal equine ocular fluids. Equine Veterinary Journal Oct 2021, Vol. 54, Issue 5, pp. 865-872; https://doi.org/10.1111/evj.13526.]
Study on Topical Ophthalmic Corticosteroid Medication
The study looked at penetration of two topical ophthalmic corticosteroids in the aqueous humor, vitreous humor, and uptake into the systemic circulation. Sixteen ponies in the study did not have active anterior uveitis. Some of the others experienced eyelid laceration (n=1), incipient cataract (n=3), or posterior cortical cataract (n=1). Researchers medicated all 21 Shetland ponies bilaterally every two hours over the course of 24 hours with either 0.15 mg dexamethasone disodium phosphate (0.1%) or 1.5 mg prednisolone acetate (1%). They administered dexamethasone in one eye and prednisolone in the other. They did not measure a quantifiable amount of the alternative drug in the contralateral eye. This indicates that systemic circulation did not bring medication to the other eye.
Concentrations of the drugs plateaued up to three hours following administration. Researchers collected serum prior to collection of intraocular samples under anesthesia and then euthanasia based on time since last dose – 5, 15, 30, 60, 90, 120, or 180 minutes.
More than 95% (20/21) of the ponies experienced hyperemia of the conjunctiva (14% severe and 81% mild). Those receiving prednisolone experienced minor ocular discomfort. This was likely due to its suspension formulation that contains particles. The study results demonstrate that effective dexamethasone and prednisolone concentrations were measured in the anterior chamber. However, drug concentrations in the vitreous of the posterior chamber were “negligible,” while systemic uptake was below the limit of detection. Other methods of administration are necessary to improve corticosteroid concentrations in the posterior segment of the globe–subconjunctival, peribulbar injection, or systemic (oral) medication.
None of these ponies had ocular disease with an inflammatory condition. Those situations might alter uptake through the blood-aqueous barrier. Further studies need to address this difference.