Contribution of P‐glycoprotein to bunitrolol efflux across blood–brain barrier

Abstract In this study, we investigated the mechanism of the blood–brain barrier (BBB) transport of bunitrolol (BTL), as a model of β‐blocker, in vivo and in vitro . In order to define the contribution of P‐glycoprotein (P‐gp) to the active efflux of BTL from brain to blood, we examined the in vivo brain distribution of BTL in mdr1a (−/−) mice with a disrupted mdr1a gene. After intravenous administration of BTL to mdr1a (−/−) mice, the brain concentration and K p value of BTL were significantly increased as compared with those in mdr1a (+/+) mice. Next, the contribution of the mdr1a P‐gp to in vitro uptake of BTL was compared in LV500 cells and L cells (mouse mdr1a ‐expressing cells and host cells, respectively). The intracellular accumulations of [ 3 H]vinblastine and BTL by LV500 cells were lower than those by L cells, but were significantly increased by verapamil, a P‐gp inhibitor. Furthermore, the BTL uptake by KB‐VJ300 cells, which express human P‐gp, was also significantly lower than that by KB host cells, and was increased by verapamil. The steady‐state uptake of BTL by LLC‐GA5‐COL300 cells, expressing human P‐gp, was significantly increased in the presence of 20 μM cyclosporin A (another P‐gp inhibitor), which had no effect in the LLC‐PK1 host cells. On the other hand, the steady‐state intracellular accumulation of BTL by MBEC4 cells, which express mdr1b P‐gp instead of mdr1a P‐gp, was not significantly changed in the presence of verapamil. This finding suggested that BTL is not a good substrate for mdr1b P‐gp. In conclusion, our results suggest that BTL is transported from brain to blood by mdr1a P‐gp in mice and by MDR1 in humans, and this presumably accounts for the low brain distribution of BTL. Copyright © 1999 John Wiley & Sons, Ltd.