2014 by Taylor & Francis Group, LLC. The development of drug delivery for the transportation of drug in a bioavailable and safe manner to the target site is now becoming an exceedingly important area of pharmaceutics research. A large number of novel drug delivery technologies surface every year and targeting every body part as potential target sites. As a result, various smart drug delivery technologies with signicant outcomes have been reported for biopharmaceutical classication system (BCS) class-II and class-IV drugs, peptides, proteins, and so on. Further, among novel drug delivery technologies, advancement of nanotechnology in formulation development for biodegradable nanoparticles, nanoemulsions (NEs), vesicular systems, implants, bioadhesive systems, and so on are currently under intensive exploratory studies (Akhter et al., 2011, 2012). Apart from that, ocular drug delivery has remained one of the most challenging topic of research as it requires a series of specied characteristics for effective drug delivery (Tiffany, 1991; Van Ooteghem, 1987). Eye is a unique and challenging organ for therapeutic delivery on to the surface as well as in its interior structure (Sieg and Robinson, 1997; Shedden et al., 2001; Rupenthal et al., 2011). Most of its anatomical and physiological architecture interferes with the fate of the administered drug (Diepold et al., 1989; Hanrahan et al., 2012). Tears permanently wash the surface of the eye and enhance anti-infectious activity by the lysozyme and immunoglobulins (Tian et al., 2012). In addition, the drug may bind to tear proteins and conjunctival mucin (Davies, 2000). Moreover, corneal and conjunctival epithelia, along with the tear lm, build a complex barrier that prevents the intraocular absorption of topically applied drugs and results in low ocular bioavailability and unwanted systemic side effects (Lang, 1995; Bourlais et al., 1998; Jain et al., 2011a, b). Topical eye drops are the most desirable dosage form for ophthalmic disease and account for nearly 90% of the currently available ophthalmic formulations owing to their simplicity and preference in use by patients. However, conventional eye drops, most of which are present in solution form, usually have quite a limited therapeutic efcacy due to the low bioavailability (Davies et al., 1997; Jain et al., 2011a, b). In addition, topical ocular drug delivery is associated with rapid and extensive precorneal loss as a major drawback caused by drainage in the extraocular area and high tear uid turnover (Chrai and Robinson, 1974; Zignani et al., 1995; Zimmer et al., 1994; Bourlais et al., 1998; Wei et al., 2002). Typically, less than 5% of the drug applied penetrates the cornea/sclera and reaches the inner tissues, with the foremost part of the installed dose often absorbed systemically through the conjunctiva and nasolacrimal duct (Kupferman et al., 1974; MacKeen, 1980). For easy understanding, the ocular barriers and drug disposition in the eyes are illustrated in Figures 17.1 and 17.2, respectively.
