Apparent lack of Mrp1-mediated efflux at the luminal side of mouse blood-brain barrier endothelial cells

Purpose. The purpose of this work was to determine mrp1-mediated efflux across the luminal membrane of endothelial cells at the blood brain barrier (BBB) in mice. Methods. The transport of radiolabeled etoposide, 17beta-estradiol-D-17beta-glucuronide (E(2)17betaG), vincristine, and doxorubicin across the BBB of mrp1(-/-) and wild-type mice was evaluated by in situ brain perfusion. Etoposide transport was also determined in P-glycoprotein-deficient mdr1a(-/-) mice perfused with both etoposide and mrp1 inhibitors like probenecid or MK571. Cerebral vascular volume was determined by co-perfusion with labeled sucrose. Results. Sucrose perfusion indicated that the vascular space was close to normal in all the studies, indicating that the BBB remained intact. The transport of etoposide, E(2)17betaG, vincristine, and doxorubicin into the brain was not affected by the lack of mrp1. Trans-efflux studies in mrp1-deficient mice with etoposide and E(2)17betaG confirmed that mrp1 was not involved in the efflux of these substrates across the BBB. There was also a significant P-gp-mediated efflux of etoposide in studies with P-glycoprotein-deficient mdr1a(-/-) mice. Perfusion of mdr1a(-/-) mice etoposide plus probenecid or MK571 did not affect the brain transport of etoposide. Conclusion. Efflux mediated by mrp1 does not seem to occur across the luminal membrane of the endothelial cells forming the mouse BBB.