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FOUNDATION OVERVIEW

The human eye is a complex sensory organ responsible for sight. Disease in the ocular system can result in vision impairment or blindness. Systemic absorption of ocular agents is affected due to the relative seclusion of the eye from systemic access by the blood-retinal, blood-aqueous, and blood-vitreous barriers.

Drug-Delivery Strategies

Ocular drug delivery poses a unique challenge due to the anatomy and structure of the eye. Ocular barriers do not only discourage the entry of foreign matter, but also exclude the active absorption of ophthalmic agents. An optimal, ocular drug delivery system will improve drug bioavailability and provide a controlled release of the drug at the target site. Multiple delivery systems have been developed for ocular disease management. A majority of ocular agents are available in solutions, but for compounds with poor stability in solution, suspension forms are used to enhance delivery. Characteristics of various routes of ocular drug administration are summarized in Table 68-1. Certain formulations increase the contact time between a drug and the eye surface. Examples include gels, ointments, ocular inserts, collagen shields, and drug-eluting contact lenses. Gels release drugs by diffusion following erosion of soluble polymers. Ointments are utilized in delivery of antibacterial agents, cycloplegic drugs, or mitotic agents. Ophthalmic ointments are primarily mixtures of a petrolatum base and mineral oil with or without lanolin. The petrolatum base acts as a lubricant, mineral oil helps the ointment melt at body temperature, and lanolin aids in water absorption. Ocular inserts function by zero-order kinetics, whereby drug release is constant over time. Drugs follow zero-order kinetics when there is a rate-limiting barrier such as a carrier system saturated by drug excess. The ganciclovir intravitreal implant used for treating cytomegalovirus (CMV), is an example of drug dosing by zero-order kinetics.

TABLE 68-1Characteristics of Ocular Routes of Drug Administration

Pharmacokinetics

The pharmacokinetic principles that apply to systemically administered drugs do not fully apply to ophthalmic drugs. This is due to the eye’s unique structure; several of its fluids and tissues are almost completely transparent and each ...

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