Effect of probenecid on fluorescein transport in the central nervous system using in vitro and in vivo models

Purpose. The purpose of this study was to characterize the function of multidrug resistance-associated proteins (MRPs) (or MRP-like organic anion transport systems) in the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) using both an in vitro BBB model and an in vivo microdialysis model. Methods. In vitro functional studies were performed using bovine brain microvessel endothelial cells (BBMEC). The accumulation of fluorescein, an anionic fluorescent dye, in BBMEC was determined with and without the presence of inhibitors of various efflux transport proteins. In vivo microdialysis simultaneously monitored fluorescein concentrations in cortical extracellular fluid and cerebrospinal fluid. The effect of probenecid on the in vivo distribution of fluorescein was studied using a balanced crossover design in the rat. Results. In vitro experiments showed that probenecid, indomethacin, LY-329146, and all MRP inhibitors significantly increased (two- to threefold) the accumulation of fluorescein in BBMEC, whereas LY-335979, a P-gp inhibitor, had no effect on the accumulation of fluorescein. Probenecid significantly increased fluorescein plasma concentration and the plasma free fraction in vivo. The distribution of fluorescein across the BBB and BCSFB was enhanced by 2.2- and 1.9-fold, respectively, when probenecid was coadministered, even after correction for increased fluorescein plasma concentrations and free fraction. Conclusions. These results demonstrate that MRPs or MRP-like transport system(s) may play an important role in fluorescein distribution across both BBB and BCSFB. This study showed that microdialysis proved to be a powerful in vivo technique for the study of transport systems in the central nervous system, and in vitro/in vivo correlations are possible using these model systems.