Designing lipid nanoparticles for topical ocular drug delivery.

Topically-applied dosage forms, such eye drops, are the most used formulations in the treatment of ocular diseases. Nonetheless, because of the special protection of the eye associated with the ocular surface, drug bioavailability and subsequent therapeutic efficiency obtained with these conventional dosage forms are very low. Recently, novel drug delivery systems have been proposed to solve the main drawbacks of conventional formulations. Nanotechnology, and more specifically lipid nanoparticles, have emerged as promising "modified eyedrops" with the purpose of improving therapeutic efficiency without compromising drug safety and patient compliance. The purpose of this review is to offer an overview of lipid nanoparticles as feasible alternatives to the conventional topically-applied dosage forms. We discuss the main limitations of topical ocular drug delivery and describe how correctly designed lipid nanoparticles can be highly valuable tools to overcome the constraints imposed by the ocular surface. Special emphasis is placed on the description of production methods and bulk materials used in the development of lipid nanoparticles for ophthalmic use and how both issues will determine the physicochemical and biopharmaceutical properties of the developed nanosystems.