Drug delivery to the retina: challenges and opportunities

Retinal drug delivery is a challenging area in the field of ophthalmic drug delivery. An ideal drug delivery system for the retina and vitreous humor has not yet been found, despite extensive research. Drug delivery to retinal tissue and vitreous via systemic administration is constrained due to the presence of a blood-retinal barrier (BRB) which regulates permeation of substances from blood to the retina. Although intravitreal administration overcomes this barrier, it is associated with several other problems. In recent years, transporter targeted drug delivery has become a clinically significant drug delivery approach for enhancing the bioavailabilities of drug molecules with poor membrane permeability characteristics. Various nutrient transporters, which include peptide, amino acid, folate, monocarboxylic acid transporters and so on, have been reported to be expressed on the retina and BRB. Prodrug derivatisation of drug molecules which target these transporters could result in enhanced ocular bioavailability. Highlighted in this review are various strategies currently employed for drug delivery to the posterior chamber, and novel opportunities that can be exploited to enhance ocular bioavailability of drugs.