Mechanism of a Model Dipeptide Transport Across Blood Ocular Barriers Following Systemic Administration

Abstract

Abstract: : Purpose: The purpose of this study is to elucidate the mechanism of dipeptide transport across blood retinal and blood aqueous barriers following systemic administration and to provide functional evidence for the presence of a peptide transporter on these barriers. Methods: Ocular microdialysis technique and in vivo ocular uptake method was used to elucidate the mechanism of transport. New Zealand albino male rabbits were used. Glycylsarcosine (GlySar) was chosen as a model dipeptide. [³H] GlySar (20µCi/ml) was administered through the marginal ear vein with and without 500mM unlabelled GlySar. Concentration of [³H] GlySar in the aqueous and vitreous humor was determined by microdialysis for a period of 10 hrs. Blood samples were obtained from mid ear vein. In vivo ocular uptake was performed by administration of [³H] GlySar through the marginal ear vein. At the end of an experimental period, vitreous humor, retina, and aqueous humor were collected and analyzed for radioactivity. Time dependent ocular uptake, competitive inhibition studies and concentration dependent studies of [³H] GlySar were carried out. Results: Aqueous and vitreous to plasma ratio of [³H] GlySar at steady state were greater than one indicating active transport of GlySar across these barriers. The aqueous and vitreous penetration ratio given by AUC_aqueous/AUC _plasma and AUC_vitreous/AUC _plasma was 1.79±0.14 and 1.61±0.49 respectively. In the presence of unlabelled GlySar, the aqueous penetration ratio (1.30±0.18) of [³H] GlySar was observed to be significantly lower but vitreous penetration ratio (1.68±0.51) appears to be unchanged. Time dependent uptake of GlySar into vitreous humor, retina and aqueous humor for a period of 30 minutes following systemic administration was observed to be linear. Ocular uptake of GlySar was inhibited by peptide transporter substrates such as dipeptides (GlycylProline and carnosine) and captopril but not by non-substrates such as amino acids (glycine and sarcosine). Concentration dependent self-inhibition of GlySar ocular uptake was also observed. Conclusions: The results indicate that dipeptide is transported across blood retinal and blood aqueous barriers via a carrier-mediated process. Oligopeptide transport system is involved in the transport of GlySar across these barriers. This information may be utilized to design transporter/receptor targeted drug delivery systems for efficient ocular uptake from systemic administration.

Keywords: retina • pump/barrier function • anterior segment