Abstract
Lipophilicity and molecular weight have long been believed to be determinants of the transfer of drugs into the central nervous system(CNS) across the blood-brain barrier(BBB), which is formed by the brain capillary endothelial cells (BCECs). However, recent advances in studies on the BBB transport of xenobiotics, as well as nutrients and neuroactive agents, have led to a change in our concept of the BBB. It is no longer regarded as a static lipoidal membrane harrier of endothelial cells which have tight junctions without fenestrations, but rather is censidered to be a dynamic interface which has physiological functions for the specific and selective membrane transport of many compounds, as well as degradative enzyme activities. It has carrier-mediated transport systems for relatively small molecules, an absorptive-mediated endocytosis system for positively charged peptides and a receptor-mediated endocytosis system specific to certain other peptides. The recent discovery of P-glycoprotein as a specific efflux pump in addition to the above influx systems confirmed the idea that the BCECs form a well organized barrier that by actively and selectively controls the influx and efflux of compounds. From the viewpoint of drug delivery, although an increase in the lipophilicity of compounds is useful to enhance the permeation across the BBB, such a strategy would also increase the permeation into tissues other than brain. Accordingly, for the development of brain-specific drug delivery systems for neuroactive compounds, it is important to understand and utilize the specific transport mechanisms of the BCECs. Here I describe the physiological and molecular characterization of the transport mechanisms of the BBB for natural compounds, as well as bioactive xenobiotics.
