Ophthalmic drug delivery systems

In recent years, increased attention has been given to the development of new systems for the delivery of ophthalmic medication. These systems are of interest for several reasons: they generally require less frequent administration than eyedrops; some of them provide therapy with fewer drug side effects than eyedrops; and they can offer practical advantages in situations where repeated, self-medication by patients is not feasible. Further, by providing continuous delivery, they allow the use of some newly emerging drugs that have very short biological half-lives. Some ocular delivery systems extend the duration of drug action by enhancement of corneal absorption; these include soluble gels and emulsions, hydrophilic ocular inserts, ion-pair associations, pro-drugs, and liposomes. Other delivery systems provide for a controlled release of drugs, thus avoiding the pulse-entry with which side-effects are associated. These systems can be based on any of several different mechanisms, and include both erodible and nonerodible matrices. The various new systems that have been developed, or are known to be under development, are described in this review, along with their mechanisms and limitations, and with the therapeutic rationale for their use.