Table 1.
Summary of the different ocular barriers and their main functions.
| Segment of the Eye | Ocular Drug Barriers | Main Functions | |
|---|---|---|---|
| Anterior | Static | Cornea | Allows passive transport of hydrophobic drugs and blocks transport of hydrophilic drugs. |
| Conjunctiva | Formation of tear film, lipophilic affinity to drugs due to its paracellular spaces. | ||
| Blood Aqueous Barrier | Limitation of ion and small solute diffusion and hydrophilic drugs. Regulation of aqueous humor secretion through ion transporters and gap junctions, connexin 43. | ||
| Efflux pumps (ABC proteins) | Enhance or restrict transportation of substrates and drug bioavailability, including both hydrophilic and hydrophobic. | ||
| Dynamic | Tear film, turnover, and drainage | Mucous layer creates a hydrophilic gel layer clearing pathogens and restricting drug delivery, especially hydrophobic drugs. Production and flow of lacrimal fluid avoids the accumulation of drugs. | |
| Conjunctival lymph and blood flow | Maintain metabolic function and immune protection. | ||
| Aqueous humor | Protects avascular structures of the anterior chamber and provides nutrition to maintain ocular homeostasis. Easy drug distribution pathway for the anterior segment. | ||
| Choroidal lymph and blood circulation | Responsible for 85% of the eye’s perfusion and helps in drug clearance by decreasing the concentration of hydrophilic drugs. | ||
| Posterior | Static | Sclera | Permeation dependent on chemical or structural properties. Allows a more rapidly hydrophilic permeation than hydrophobic. |
| Bruch’s membrane | Helps in the prevention of hydrophilic compounds permeation. | ||
| Blood Retinal Barrier | Regulates the passage of solutes to the subretinal space. Presents with hydrophilic permeation. | ||
| Efflux pumps | Enhance or restrict transportation of substrates and drug bioavailability, including both hydrophilic and hydrophobic. | ||
| Dynamic | Choroidal lymph and blood circulation | Responsible for 85% of the eye’s perfusion and helps in drug clearance by decreasing the concentration of hydrophilic drugs. | |
| Anterior and Posterior | Metabolic | Cytochrome P450 | Metabolize substances by conjugation or oxidation to avoid drug accumulation. Or convert prodrugs into active drugs modifying the solubility, bioavailability, and concentration. |
| Enzymes: peptidases, esterases | |||