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S. Tamilvanan, M. Abdulrazik, S. Benita; Ocular and Regional Tissue Distribution of Topically Applied Ocular Therapeutics Part A: How Does a Topically Applied Drug Penetrate to the Back of the Eye? . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4270.
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Purpose: To evaluate the efficiency and the route of topical brimonidine penetration to the back of the eye Methods: After single instillation of 50µl of 3H-radiolabeled Alphagan® solution (0.2%) in the cul-de-sac of the right eye, three male albino rabbits (2-2.5 kg) were sacrificed at each time point (5, 15, 30 and 60 min). Both eyes were dissected. Both Eyes specimens of aqueous humor, cornea, iris, lens, vitreous, conjunctiva, sclera, ciliary body, choroid, and retina were weighted, and blood samples were measured. Samples were stored at -80 0C until combustion in tissue oxidizer and radioactive liquid scintillation counting. Results: The highest brimonidine levels in the treated eye were detected in the ocular surface tissues (cornea and conjunctiva) and the lowest in lens and vitreous. Vitreal brimonidine AUC was only 8.85% of aqueous humor AUC (P<0.01)and 15.7% of retinal AUC (p<0.001). Choroidal brimonidine AUC was 196% higher than retinal AUC (p<0.001), which in turn was 636% higher than vitreous AUC (p<0.001). Conclusions: Good retinal brimonidine availability was achieved following topical application. Our results suggest that the route of brimonidine delivery to the retina following topical application is mainly through the choroid rather than the vitreous. The penetration of brimonidine to the vitreous from both the aqueous humor and the retina is poor, and the vitreal drug levels should not be used as an indicator of retinal drug availability. Our data and the published evidence suggest a "roundabout" pattern of ocular drug delivery, with inefficient axial drug delivery from the surface to the core of the eye (lens and vitreous) compared to far more efficient lateral delivery, mainly through the uveal tract tissues.
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