Kinetic and biochemical analysis of carrier-mediated efflux of drugs through the blood-brain and blood-cerebrospinal fluid barriers: importance in the drug delivery to the brain

In this manuscript, our recent studies on the transporters on the blood-bra in barrier and blood-cerebrospinal fluid (CSF) barrier responsible for the excretion of ligands from the central nervous system (CNS) to the blood are summarized. By comparing the brain entry of quinidine in normal and mdr 1a knock out mice, the predominant role of P-glycoprotein in the brain distri bution of this compound was demonstrated. In addition to P-glycoprotein, th e presence of transporters responsible for the efflux of organic anions fro m the brain has been suggested by a pharmacokinetic analysis of the CNS dis tribution of cefodizime, a third generation cephalosporin antibiotic. This suggestion was confirmed by demonstrating the presence of a specific mechan ism for the elimination of p-aminohippuric acid from the brain after microi njection into the cerebral hemisphere. In vitro, the energy-dependent lumin al preferential efflux of glutathione-bimane was demonstrated in a monolaye r of MBEC4 cells which were derived from mouse brain endothelial cells. Stu dies with isolated membrane vesicles from MBEC4 cells suggested the presenc e of a primary active transporter(s) for organic anions, and Western blot a nalysis indicated the presence of multidrug resistance associated protein ( MRP1) and/or its related transporters on MBEC4 cells and freshly isolated r at cerebral endothelial cells. The transcellular transport of 17 beta estra diol 17 beta-D-glucuronide (E(2)17 beta G) across the choroid plexus was al so demonstrated by examining the efflux of this compound from CSF after int racerebroventricular administration. The functional significance of organic anion transporting polypeptide (oatp-1) on the brush border membrane of th e choroid plexus was demonstrated by comparing the uptake of E(2)17 beta G into the isolated choroid plexus and oatp-1 transfected COS-7 cells; in add ition, reverse transcription-polymerase chain reaction and Western blot ana lysis indicated the presence of MRP in the choroid plexus. Together with th e direction of transcellular transport, the basolateral localization of MRP on the choroid plexus was suggested. By regulating the activity of these e fflux transporters, it is possible to improve the brain entry of certain su bstrates. (C) 1999 Elsevier Science B.V. All rights reserved.