P-Glycoprotein, a gatekeeper in the blood–brain barrier
Introduction
Work of the last 9–10 years has unequivocally demonstrated that the drug-transporting (or mdr1-type) P-glycoproteins form an important part of the blood–brain barrier. Immunohistochemistry and analysis of isolated brain capillaries, primary cultures of brain capillary endothelial cells and immortalized cell lines derived from these cells have established that P-glycoprotein is present in the endothelial cells that form the blood–brain barrier, and functionally active in transporting drugs from the brain (or basolateral) side to the blood (apical or luminal) side of these cells. Subsequent analysis of knockout mice lacking P-glycoprotein in the blood–brain barrier and other animal models treated with P-glycoprotein blocking (and other) agents demonstrated that in vivo, blood–brain barrier P-glycoprotein can prevent the accumulation of many compounds, including a variety of drugs, in the brain. Many of the original findings in this field were recently reviewed by Naito and Tsuruo [1]and Tsuji and Tamai [2]. This review, therefore, aims to first discuss some general features of P-glycoprotein relevant to the understanding of its functioning in the blood–brain barrier, and the possible consequences of interfering with its activity in vivo. I will further focus on the recent advances that have been made in this area and on some remaining controversies, while referring to the earlier reviews for more detailed information.
Section snippets
P-Glycoprotein and multidrug resistance
The drug-transporting P-glycoproteins were originally identified by their capacity to confer multidrug resistance to tumor cells against a range of anticancer drugs. The P-glycoprotein is localized in the plasma membrane of the cell, where it can actively extrude a variety of drugs from the cell, thus making it resistant to the cytotoxic activity of these drugs. The drug-transporting P-glycoproteins are N-glycosylated membrane proteins of about 1280 amino acids, the polypeptide chain consisting
Structure of the blood–brain barrier
The blood–brain barrier is physically formed by the blood capillary endothelial cells in the brain. In contrast to endothelial cells in capillary blood vessels in most other tissues, those in brain are closely joined to each other by tight junctions, and they cover the walls of the vessels as a continuous sheath, leaving no space between cells. Moreover, these endothelial cells demonstrate very little fenestration and pinocytosis (Fig. 2). As a result of this configuration, only very small
Ivermectin hypersensitivity of mice lacking mdr1a P-glycoprotein in the blood–brain barrier
The real impact of P-glycoprotein in the blood–brain barrier became only evident with the generation of knockout mice lacking mdr1a P-glycoprotein (mdr1a (−/−) mice). As a result of this knockout, these mice lack detectable P-glycoprotein in the brain capillary endothelial cells [23]. The consequences are dramatic. Whereas the mice behave perfectly normal under average laboratory conditions, they turned out to be almost 100-fold more sensitive to the neurotoxic pesticide ivermectin. This was
Experiments in normal mice and rats
As soon as it was recognized that P-glycoprotein in the blood–brain barrier might be an important determinant of the brain penetration of many drugs, attempts were initiated to enhance the brain penetration of drugs by administration of P-glycoprotein blockers. Some initial negative results in these attempts (see e.g., [67]) could be explained by the use of relatively inefficient P-glycoprotein blockers, the use of suboptimal administration protocols of the blocker, or a combination of these
Conclusions and caveats
In principle, almost any of the experimental approaches used so far in the analysis of blood–brain barrier P-glycoprotein may have its complications. Immunohistochemistry can be prone to false-positive and false-negative results, and cultured brain capillary endothelial cells may lose or alter part of their characteristic differentiation properties. P-Glycoprotein knockout mice may have undergone additional changes secondary to the loss of P-glycoprotein expression, and inhibitors of
Acknowledgements
I thank Mr J.W. Jonker and Dr J. Allen for critical reading of the manuscript. Most of the work at The Netherlands Cancer Institute described in this review was supported by grants NKI 92-41 and NKI 97-1434 of the Dutch Cancer Society.
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