Figure 1.

Schematic illustration of ocular structures and barriers. The primary physiologic obstacle against topically instilled drugs is the tear film. The cornea is the main route for drug transport into the anterior chamber (I). The retinal pigment epithelium and the retinal capillary endothelium are main barriers against systemically administered drugs (II). Intravitreal injection is an invasive strategy to reach the vitreous (III). Administered drugs can be carried out of the anterior chamber by venous blood flow after diffusion across the iris surface (1) or by aqueous humor outflow (2). Drugs may be removed from the vitreous cavity through diffusion into the anterior chamber (3) or by the blood-retinal barrier (4). Figure 1 is taken from Barar et al., 2009 [6].