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A. Sinha, L.W. Maines, D.A. Antonetti, C.D. Smith; Expression and Activity of P–Glycoprotein in the Retina . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3956.
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Purpose:The Blood Retinal Barrier (BRB), like the Blood Brain Barrier (BBB), poses a challenge for drug delivery into the eye. P–Glycoprotein (Pgp), an eflux transporter contributing to the BBB, has been extensively studied in the brain microvasculature and certain tumours. However, its expression and activity in the retina is not well established. Optimization of drug delivery into the eye can be achieved by evaluating the presence and function of Pgp in the retina. The objective of this study was to establish the expression and activity of Pgp in an in vitro model of the BRB and in vivo. Methods: Expression of Pgp in bovine retinal endothelial cells (BREC) was established by western blotting and immunocytochemistry. Immunohistochemistry was performed to investigate Pgp expression in the rat retina. Pgp activity was evaluated in BRECs by intracellualr accumulation assay (IAA) of 3[H] taxol, a radiolabeled anti–tumor drug normally effluxed by Pgp. Verapamil and Pgp–4008, specific inhibitors of Pgp, were used to measure the efflux activity of Pgp. Effect of hydrocortisone on Pgp expression and function was also studied in BRECs by western blotting and IAA of 3[H] taxol. Results: Western blotting indicated expression of Pgp in BRECs and immunocytochemistry established distribution pattern of Pgp. Immunohistochemistry revealed Pgp to be located in the microvessels of the rat retina. Functional IAA demonstrated relatively high levels of taxol efflux activity. Pre–treatment with inhibitors resulted in marked increase in intracellular accumulation of 3[H] taxol, demonstrating Pgp specific activity. Treating BRECs with a physiological concentration of glucocorticoid increased expression of Pgp. Conclusions: Data obtained from this study suggests that Pgp is expressed in the retina and is functionally active in primary BREC culture. Thus, BREC culture is an appropriate model to study drug efflux and delivery across the BRB. Further studies to elucidate Pgp function in both in vitro and in vivo models will yield novel, non–invasive drug delivery methods across the BRB.
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