**2.3 Metabolic barriers**

Transporters are membrane-bound proteins that play an important role in the active transport of nutrients or xenobiotics. These transporters play a significant role in humans in the processes of drug absorption, distribution, metabolism, and elimination. The presence of both efflux and influx transporters has been reported in various ocular tissues. The presence of efflux pumps in various ocular tissues has shown to regulate the intracellular drug concentration needed to achieve therapeutic activity. Drug-metabolizing enzymes are present in many ocular tissues such as the ciliary body and RPE [57, 58]. As a defense mechanism, the eye possesses several sophisticated metabolizing enzyme systems that degrade drug molecules, including those of the cytochrome P-450 (CYP) family, which are the most important drugmetabolizing enzymes. CYP3A isozyme, for example, metabolizes over 50% of commercial drugs [59].
