2. Transscleral drug delivery to the posterior segment of the eye: Particulate and colloidal formulations and biopharmaceutical considerations

Diseases or disorders affecting the posterior segment of the eye, such as diabetic retinopathy, age related macular degeneration and retinitis pigmentosa, are the leading causes of decreased visual acuity and blindness. Successful treatment modalities require generation and maintenance of therapeutically effective drug concentrations at the target sites: vitreous, retina and/or choroid. Drug delivery to the posterior ocular segment is, however, limited by several physiological processes. Topical instillation, the most favored local mode of application, is challenged by tear flow, structural complexity of the cornea, diffusion distance, counter-directional intraocular convection and conjunctival, scleral and choroidal vasculature and lymphatics. Although systemic administration offers an alternative route, the blood-ocular barriers restrict the diffusion of a wide variety of Correspondence/Reprint request: Dr. Soumyajit Majumdar, Associate Professor, Department of Pharmaceutics, Research Associate Professor, Research Institute of Pharmaceutical Sciences, The University of Mississippi University, MS-38677, USA. E-mail: [email protected] Ramesh Srirangam & Soumyajit Majumdar 34 therapeutic agents from the systemic circulation into the ocular chambers. In recent years, transscleral delivery approach has gained in prominence rapidly. A relatively large scleral surface area with favorable permeability characteristics presents opportunities for efficient and effective delivery to the retina and vitreous. This chapter will explore the recent literature on transscleral drug delivery and disposition, with a particular emphasis on novel drug delivery systems like particulate and colloidal formulations.