Principles of pharmacology in the eye.

TLDR: The least invasive routes for ocular drug administration are topical (e.g. eyedrops) and systemic formulations as discussed by the authors, however, topical drugs often display limited bioavailability due to many physical and biochemical barriers including the pre-corneal tear film, the structure and biophysiologic properties of the cornea, the limited volume that can be accommodated by the cul-desac, the lacrimal drainage system and reflex tearing.

Abstract: The eye is a highly specialised organ that is subject to a huge range of pathology. Both local and systemic disease may affect different anatomical regions of the eye. The least invasive routes for ocular drug administration are topical (e.g. eyedrops) and systemic (e.g. tablets) formulations. Barriers that subserve as protection against pathogen entry also restrict drug permeation. Topically administered drugs often display limited bioavailability due to many physical and biochemical barriers including the pre-corneal tear film, the structure and biophysiologic properties of the cornea, the limited volume that can be accommodated by the cul-de-sac, the lacrimal drainage system and reflex tearing. The tissue layers of cornea and conjunctiva are further key restrictors to drug delivery. Using carriers that enhance viscosity or bind to the ocular surface increase bioavailability. Matching the pH and polarity of drug molecules to the tissue layers allows greater penetration. Drug delivery to the posterior segment is a greater challenge and currently the standard route is via intravitreal (IVT) injection, notwithstanding the risks of endophthalmitis and retinal detachment with frequent injections. Intraocular implants that allow sustained drug release are at different stages of development. Novel exciting therapeutic approaches include methods for promoting transscleral delivery, sustained release devices, nanotechnology and gene therapy.