Cornea topical ocular drug delivery

The synchronized movement of the eyelids spreads the precorneal tearfilm across the cornea and pushes it toward the nasolacrimal duct. Precorneal drainage is quite efficient. An aqueous instilled dose leaves the precorneal area within 5 min of instillation in humans. Most of the drug absorbed by transcorneal penetration, without retention modification, is spread across the cornea by the eyelids in the first minutes postdosing. In the precorneal space transcorneal penetration is limited by solution drainage, lacrimation and tear dilution, tear turnover, conjunctival absorption, and the corneal epithelium. Slowing down tear film turnover has well-established benefits to topical ocular drug delivery. 

Consequently, frequent instillation of eye drops is required, resulting in pulsed administration and patient noncompliance. Clearly, the main prerequisite for absorption of drugs into the eye is good corneal penetration and prolonged contact time with the corneal epithelium. Many intrinsic barriers, such as the cornea barrier, blood-aqueous barrier (BAB), and blood-retinal barrier (BRB), restrict ocular drug delivery (Figure 51.2). The barriers related to topical drug administration route are as follows. 

Typically topical ocular drug administration is accomplished by eye drops. - These are liquid preparations that contain drug substances that are used in ocular drug delivery. The drug substance must be active on the surface of the eye or the internal region of the eye after passage through the cornea or conjunctiva. ... 

Topically applied ophthalmic drugs are used for their local effects (see Chapter 63) requiring absorption of the drug through the cornea corneal infection or trauma thus may result in more rapid absorption. Ophthalmic delivery systems that provide prolonged duration of action (e.g., suspensions and ointments) are useful, as are ocular inserts providing continuous delivery of drug. 

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