Precorneal barriers

The precorneal barriers are the first barriers that slow down the penetration of an active ingredient into the eye. They are comprised of the tear film and the conjunctiva [4]. 

After topical administration of an ophthalmic drug solution, the drug has to cross a succession of anatomical barriers before reaching the systemic circulation. These barriers (as shown in Figure 25.1) can be commonly classified as precorneal and corneal barriers [3]. 

Barriers to ocular drug delivery. The presence of epithelial tight junctions and rapid drainage of the instilled drug solution by tears from the precorneal area result in the permeation of less than 5 percent of the applied dose of a drug.66,67 The cellular calcium levels and actin filaments of the cytoskeleton play a major role in the integrity of tight... 

In order to research and develop an effective ophthalmic delivery system, a good understanding of the anatomy and physiology of the eye (the globe) is necessary. Figure 1 shows a cross section through the human eye. This chapter will mainly focus on the precorneal area and the transport barriers present in the eye. 

The lubricating ointments are composed of a mixture of white petrolatum, mineral oil, liquid or alcohol lanolin, and sometimes a preservative. These highly viscous formulations cause considerable blurring of vision, and consequently they are used primarily at bedtime, in critically ill patients, or in very severe dry-eye conditions. Such aqueous and ointment formulations are only fair substitutes for the precorneal tear film, which is truly a poorly understood lipid, aqueous, and mucin trilaminar barrier. 

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