A review on recent drug delivery systems for posterior segment of eye

Eye is the unique sense organ with complex and sophisticated anatomy and physiology. Being most instrumental for vision, it is secured by varied protective barriers; ranging from static (membranous) to dynamic (vascular) barrier. Although these barriers are very efficient to protect eye from exogenous substances and external stress, it is caught by various irreversible vision impairing ailments like cataract, conjunctivitis, glaucoma, uveitis, diabetic retinopathy (DR), diabetic macular edema (DME), age related macular degeneration (AMD), cytomegalovirus (CMV) retinitis, retinitis pigmentosa (RP), retinal vein occlusion (RVO), endophthalmitis affecting both anterior and posterior segment of eye. The treatment needed to reach the site of action is restricted by its characteristic barriers. The protective mechanism turns into hurdles when it comes to drug delivery especially in case of posterior segment of eye. Most common and preferable routes for ocular drug delivery are topical and systemic routes owing to their compliance and non-invasive nature, however they turned inefficient in delivering drugs to posterior segment. Currently, other local routes like intraocular and periocular (subconjunctival, sub-tenon, posterior juxtascleral, retrobulbar, peribulbar) are being explored and are showing positive outcomes in terms of symptomatic relief for a certain time period. But as these are invasive techniques, they also have some hidden long-term drawbacks on other side. Various advancements have been achieved till date in delivery of drug to posterior segment of eye, however despite these advancements; there is need of non-invasive or preferably less invasive technique considering prolonged treatments for such ailments. At times, dependency on invasive techniques may cause problems like patient incompliance, inflammation, contact cataract, retinal detachment, endophthalmitis etc. Here, in this review, barriers in ocular delivery, routes and recent advances in drug delivery to eye including patented commercial formulations with emphasis on posterior segment will be discussed.