The Conjunctival Barrier in Ocular Drug Delivery

Abstract

Within the context of topical and local drug delivery to the eye, the mammalian conjunctiva functions as a unique biological barrier. Various model systems as in vitro tools have been refined and validated over the years to assess drug absorption across the conjunctiva. Passive and active drug transport as well as endocytic routes of transconjunctival drug permeation have been extensively characterized. The subconjunctival space offers the possibility of delivering multiple types of sustained release formulations to the eye. Additionally, the concept of transscleral/conjunctival iontophoresis has been proven as a viable option for topical, relatively noninvasive delivery of select categories of molecules. Furthermore, a number of ocular surface disorders have been associated with conjunctival phenotypes, making this tissue a potential drug target. The aim of this chapter is to facilitate the understanding of conjunctival (transport) physiology and establishment of in vitro models of conjunctiva validated for predicting drug delivery to the eye in health and disease. The models are evaluated in terms of assessing their electrical, morphological, and permeability properties as well as expression profiles of endogenous active transporter(s) and marker enzyme(s). Finally, we provide protocols and methods, as an appendix to this chapter, to be used for in vitro studies of the conjunctiva for testing preclinical biopharmaceutics.