Blood–Retinal Barrier Drug Transport: Aiming to Develop Oral Administration for Retinal Diseases

The blood–retinal barrier (BRB) consists of the inner and outer blood–retinal barrier (abbreviated as inner BRB and outer BRB). The inner and outer BRB’s together regulate the transport of substances between the blood and neural retina via various membrane transporters. To utilize the oral administration for the pharmacotherapy of retinal diseases, it is necessary and important to understand many drug transport mechanisms in the BRB’s. This review discusses the BRB transport research reported over more than a quarter of a century, particularly the development of in vitro and in vivo transport evaluation systems as well as newly discovered BRB-specific cationic drug transport mechanisms.