ABSTRACT
Controlled and sustained delivery of ophthalmic drugs continues to remain a major focus area in the field of pharmaceutical drug delivery with the emergence of new, more potent drugs and biological response modifiers that may also have very short biological half-lives. The major objective of clinical therapeutics is to provide and maintain adequate concentration of drugs at the site of action. In ocular drug delivery, the physiological constraints imposed by the protective mechanisms of the eye lead to poor absorption of drugs with very small fractions of the instilled dose penetrating the cornea and reaching the intraocular tissues. The reasons for inefficient drug delivery include rapid turnover, lacrimal drainage, reflex blinking, and drug dilution by tears (1,2). The limited permeability of cornea also contributes to the low absorption of ocular drugs. As shown in Figure 1, tear drainage causes a major portion of the administered dose to be transported via the nasolacrimal duct to the gastrointestinal (GI) tract, where it may be absorbed, leading to unwanted systemic side effects and occasional toxicity due to the drug (3). The rapid elimination of administered eye drops often results in a short duration of the ocular therapeutic effect, making a frequent dosing regimen necessary.
