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ABSTRACT.Purpose: This study aimed to: (1) determine the relative efficiencies of topical and systemic absorption of drugs delivered by eyedrops to the anterior and posterior segments of the eye; (2) establish whether dexamethasone-cyclodextrin eyedrops deliver significant levels of drug to the retina and vitreous in the rabbit eye, and (3) compare systemic absorption following topical application to the eye versus intranasal or intravenous delivery. Methods: In order to distinguish between topical and systemic absorption in the eye, we applied 0.5% dexamethasone-cyclodextrin eyedrops to one (study) eye of rabbits and not to the contralateral (control) eye. Drug levels were measured in each eye. The study eye showed the result of the combination of topical and systemic absorption, whereas the control eye showed the result of systemic absorption only. Systemic absorption was also examined after intranasal and intravenous administration of the same dose of dexamethasone. Results: In the aqueous humour dexamethasone levels were 170 ± 76 ng ⁄ g (mean ± standard deviation) in the study eye and 6 ± 2 ng ⁄ g in the control eye. Similar ratios were seen in the iris and ciliary body. In the retina the dexamethasone level was 33 ± 7 ng ⁄ g in the study eye and 14 ± 3 ng ⁄ g in the control eye. Similar ratios were seen in the vitreous humour. Systemic absorption was similar from ocular, intranasal and intravenous administration. Conclusions: Absorption after topical application dominates in the anterior segment. Topical absorption also plays a significant role in delivering dexamethasone to the posterior segment of the rabbit eye. In medication administered to the retina, 40% of the drug reaches the retina via the systemic route and 60% via topical penetration. Dexamethasone-cyclodextrin eyedrops deliver a significant amount of drug to the rabbit retina.

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