Tissue distribution of dexamethasone in canine ocular compartments following topical application of dexamethasone-21-isonicotinate and oxytetracycline HCl.

OBJECTIVE: To study the dexamethasone (DXM) concentration at different time points in various compartments of the canine eye following topical application of DXM-21-isonicotinate and oxytetracycline hydrochloride ANIMALS STUDIED: Thirty dogs to be euthanized for reasons not related to this study were selected and their ocular health status evaluated. Selected animals were treated with DXM-oxytetracycline ointment and euthanized after 6, 11 or 16 h. PROCEDURE: The concentration of DXM was determined in the following compartments of the eye: third eyelid, cornea, aqueous humor, iris, lens, vitreous body and choroid/retina. The DXM concentration in the eye was measured by radioimmunoassay. The applied amount of DXM was 0.04 mg in 0.2 mL ointment. Dogs were treated once with Corti Biciron eye ointment (DXM-21-isonicotinate and oxytetracycline hydrochloride, S & K Pharma, Perl, Germany) and were euthanized 6, 11 and 16 h after treatment. RESULTS: At 6 h following topical application the mean DXM concentration was highest in the anterior structures of the eye (third eyelid: 18 ng/g, cornea: 36 ng/g). The concentration in the posterior structures was below detection level. A decreased DXM concentration in the anterior structures was measured 11 and 16 h after treatment. CONCLUSION: It could be demonstrated that therapeutically relevant concentrations of DXM after a single topical administration are only achieved in anterior structures of the eye. A dosing interval of 6-11 h is recommended to achieve therapeutic drug concentration in those structures. The posterior structures of the eye are not reached by topical administration.