Mechanism of a model dipeptide transport across blood-ocular barriers following systemic administration

The purposes of this study were to provide functional evidence for the presence of a peptide transporter on blood-ocular barriers and to elucidate the mechanism of a dipeptide transport across these barriers following systemic administration. Glycylsarcosine was chosen as a model dipeptide and [(3)H] glycylsarcosine was administered through the marginal ear vein of New Zealand white rabbits. At the end of an experimental period, vitreous humor, retina and aqueous humor were collected. Time dependent uptake of glycylsarcosine into ocular tissues was studied at 5, 10, 15 and 30 min. Competitive inhibition studies were performed by intravenous administration of [(3)H] glycylsarcosine with and without various inhibitors. Concentration-dependent ocular uptake of glycylsarcosine was carried out by administration of various concentrations of unlabelled glycylsarcosine spiked with a fixed amount of [(3)H] glycylsarcosine. Time-dependent uptake of glycylsarcosine into vitreous humor, retina and aqueous humor for a period of 30 min following systemic administration was linear. Ocular uptake of glycylsarcosine was inhibited by peptide transporter substrates such as dipeptides (glycylproline and carnosine) and captopril but not by non-substrates such as amino acids. Concentration-dependent self-inhibition of glycylsarcosine ocular uptake was also observed. The results indicate that model dipeptide is transported across blood-ocular barriers via a carrier-mediated process. In conclusion, an oligopeptide transport system is involved in the transport of glycylsarcosine across blood-ocular barriers. This information may be utilized to design transporter/receptor targeted drug delivery systems for efficient ocular uptake from systemic administration.