Eye: anatomy, physiology and barriers to drug delivery

Abstract: This chapter describes current knowledge on the ocular structure, function and barrier properties to drug delivery. A broad description of different ocular parts and their histology is provided. Different routes of drug administration such as topical, oral, intravenous, subconjunctival, periocular and intravitreal routes are employed to treat ocular ailments. Critical anatomical and physiological factors that regulate drug absorption are discussed. Of the drug administration routes mentioned, topical for anterior ocular tissues, and periocular and intravitreal for back of the eye tissues are mostly preferred and recommended. Ocular drug absorption is impeded by static, dynamic and metabolic barriers. Static ocular barriers include corneal epithelium, blood aqueous barrier, sclera, retinal pigment epithelium and blood capillary endothelial cells, whereas the dynamic ocular barriers include tear drainage, conjunctival blood and lymph clearance, and choroidal blood and lymphatic circulations. These barriers are segregated and described as anterior segment and posterior segment barriers. As a barrier, the role of ocular efflux transporters, which reduces intracellular xenobiotic concentration, is briefly described.