The protective filter around the retina, similar to the blood-brain barrier, that prevents most drugs from reaching the back of the eye via the bloodstream.This is why intravitreal injection — directly into the vitreous of the eye — is the standard delivery route for retinal therapeutics. Systemic dosing of anti-VEGF biologics or somatostatin analogs for retinal disease would require concentrations that could produce unacceptable systemic side effects. Local delivery solves the access problem but requires a clinical procedure. See also: intravitreal injection.