Role of membrane transporters and metabolizing enzymes in ocular drug delivery.

Ocular disorders can significantly lower a patient's quality of life. Centers for Disease Control and Prevention's Vision Health Initiative have estimated that the number of people affected by age-related ocular diseases may be doubled in the United States by 2030. Although availability of newer therapeutics has improved the prognosis of ocular diseases, poor ocular bioavailability still remains a major concern. Combinations of pharmacodynamic and pharmacokinetic barriers have been known to determine the amount of drug delivered to the target tissue. However, presence of membrane transporters and metabolizing enzymes pose a significant challenge to ocular drug disposition. Scientific literature confirms the expression of efflux/ATP-binding cassette transporters, influx/solute carrier transporters and several metabolic enzymes including oxidoreductases, hydrolases and transferases in different ocular tissues. Therefore, this review article describes the anatomical features of the eye and various barriers regulating ocular drug disposition. Differential expression of membrane transporters and metabolizing enzymes in normal and diseased states are briefly discussed. Further, the significance of transporter- metabolism interplay in ophthalmic drug design and various ocular drug delivery strategies are also outlined.