Epithelial and/or endothelial cells sealed by tight junctions serve as the major membrane barriers for the transport of nutrients, ions, and drugs into intraocular tissues. The principal membrane barriers of the eye are located in the cornea, conjunctiva, iris-ciliary body, lens epithelium, and retina (Fig. 1). Various specialized transport processes in these barriers control the movement of solutes into and out of intraocular chambers. These processes maintain the visual function, control intraocular pressure, provide nutrients to avascular cornea and lens, and protect ocular tissues from xenobiotics. An alteration in the function of these transporters is often the underlying cause of various ocular diseases. In addition, these transport processes can play a role in drug transport.
Drug therapy is useful in the treatment of corneal epitheliopathy, keratitis, conjunctivitis, extracellular infections, glaucoma, iritis, and cataract—diseases that afflict the anterior segment of the eye—and vitreoproli-ferative disorders, endophthalmitis of bacterial and fungal origins, uveitis, viral retinitis, diabetic retinopathy, and macular degeneration—diseases that afflict the posterior segment of the eye. Ocular drug therapy often involves topical or systemic administration of drugs. Therefore, for effective delivery to intraocular tissues, drugs must penetrate across cornea, conjunctiva, and/ or sclera following topical administration or across endothelial barriers
Current affiliation: Novartis Pharmaceuticals, East Hanover, New Jersey, U.S.A.
along with the epithelial barriers of the iris-ciliary body or retina following systemic administration. However, as these barriers restrict the transport of therapeutic agents, the ocular bioavailability of drugs following topical or systemic administration is low. These barriers also play a role in the drug clearance following periocular or intraocular administration.
This chapter describes the various ocular epithelial and endothelial barriers and the associated ion and solute transport processes in the eye. The discussion includes ocular drug transport processes as well as recently identified drug efflux pumps.
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