review of intravitreal implants for extended drug delivery to the posterior segment of the eye

intravitreal injection is the most common approach for drug delivery to the posterior segment in humans. however, following the injection, drugs are rapidly eliminated from the vitreous, with half-lives up to a few days. depending on the rate of clearance from the vitreous, large boluses and frequent administrations may be required to ensure therapeutic levels over an extended period of time. multiple intraocular injections can lead to an increased likelihood of complications, such as vitreous hemorrhage, retinal detachment, endophthalmitis and cataract. in order to decrease the occurrence of complications associated with intravitreal injections, sustained-release drug delivery systems are desired to provide drug release into the posterior segment for a longer period of time (months or even years) compared to direct injection. this article reviews the current status of various intravitreal implants, both approved and in clinical development. these implants are: vitraset, retisert, ozurdex (posurdex), i-vation, iluvien (medidur) and nt-501. a search of the english language pubmed database was conducted with keywords of “intraocular implant”, “intravitreal implant”, “sustained-release” and “intravitreal drug delivery”.