Implants for drug delivery to the posterior segment of the eye: a focus on stimuli-responsive and tunable release systems.

Efficient drug delivery to the posterior segment of the eye is a challenging task for the formulation scientist. Current treatment of chronic back-of-the-eye conditions requires frequent intravitreal injections of drug containing solutions due to the short half-life and limited tissue permeation of the administered molecules. Sustained release ocular delivery systems offering reduced administration frequencies have therefore gained popularity over recent years with a few implants already on the market and many more in the pipeline. However, current implants generally release drug at a predetermined rate without the ability to alter release rates. As required drug concentrations may change over the course of treatment due to the individual patient's clinical response, implants from which release rates can be tuned could optimize treatment efficacy. This article provides an overview of diseases of the posterior segment of the eye, describes currently available implants to treat such conditions and discusses advantages and disadvantages of various implant locations. Finally, stimuli-responsive drug delivery technologies that have been investigated for, or have the potential to be applied to, drug delivery to the back of the eye will be discussed. Emphasis is hereby placed on polymeric implants responsive to an electric current, light or a magnetic field to achieve tunable drug release.