Ocular drug delivery corneal barriers

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. 

For in vitro toxicity studies and assessment of the barrier function, drug transport, cell physiology, and metabolism as well as the development of delivery systems, cell culture models provide powerful systems for scientific research. As the corneal epithelium is the main barrier for ocular penetration, various corneal epithelial cell cultures were established besides the corneal constructs that mimic the whole cornea and serve as reductionist models for the ocular barrier. In general, two types of cell culture models are available primary cell cultures and immortalized, continuous cell lines. 

As previously mentioned, systemic delivery is limited because of the isolation of ocular tissues from the systemic circulation thus, topical delivery is often the preferred administration route owing to ease of access and patient compliance, particularly when treating infections of the anterior segment such as keratitis sicca, conjunctivitis, or blepharitis and diseases such as glaucoma or uveitis that require the drug to be diffused across the corneal barrier [19,20]. However, drainage, lacrimation and tear dilution, tear turnover, conjunctival absorption, and the corneal epithelium all limit corneal drug penetration [21,22]. 

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