P-glycoprotein-mediated efflux of phenobarbital at the blood-brain barrier evidence from transport experiments in vitro.

The purpose of the present studies was to investigate characteristics of phenobarbital (PB) transport across the blood-brain barrier (BBB). Cultured rat brain microvascular endothelial cells (rBMECs) were used as an in vitro BBB model. Experiments were conducted to examine time-, concentration- and temperature-dependent elements of PB uptake, and the effect of tested agents on the steady-state uptake of PB. PB efflux from rBMECs and the polarised transport of PB were examined to evaluate whether P-glycoprotein (P-gp) was involved in the PB efflux transport across BBB. The results demonstrated that the uptake of PB by rBMECs was in a time-, concentration- and temperature-dependent manner. P-gp modulators, cyclosporin A (CsA), ketoconazole and metabolic inhibitor dinitrophenol, increased the PB steady-state uptake by more than 50% (p < 0.01), and decreased the PB efflux by more than 50% (p < 0.01). Similar results were observed in the uptake and efflux studies of rhodamine-123 by co-administration of CsA. Further results were obtained in the polarised transendothelial transport of PB in the apical to basolateral (A-B) and basolateral to apical (B-A) directions either with or without CsA. The result showed that transport of PB in the B-A direction was significantly greater than the transport in the A-B direction (p < 0.05), and the co-administration of 50microM CsA almost inhibited P-gp-involved efflux in the rBMECs. Overall, these findings suggest that P-gp may contribute to the efflux transport of PB across BBB.