The blood-brain barrier (BBB) segregates the circulating blood from interstitial fluid in the brain and restricts drug permeability into the brain. Recent studies have revealed that the BBB exhibits not only blood-to-brain influx transport for the supply of nutrients, but also brain-to-blood efflux transport to excrete drugs and endogenous compounds. The influx transport system allows drugs to enter the brain. L-DOPA is transported into the brain by the large neutral amino acid transport system, system L. A cationic μ-opioid peptide analogue enters the brain by adsorptive-mediated endocytosis. In contrast, efflux transport limits the distribution of drugs in the brain. The ATP binding cassette transporter B1 (ABCB1) mediates the efflux transport of lipophilic drugs at the BBB by using ATP energy. Furthermore, organic anion transporter 3 (OAT3) is expressed at the BBB and mediates the efflux transport of homovanillic acid, a dopamine metabolite. This efflux transport is also likely to be involved in the transport of anionic drugs such as 6-mercaptopurine and acyclovir. Clarifying the BBB transport could give us important information allowing the development of better CNS drugs and improving our understanding of the relationship between CNS diseases and BBB functions.
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