P-Glycoprotein, a gatekeeper in the blood–brain barrier

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Abstract

The blood–brain barrier is a major impediment to the entry of many therapeutic drugs into the brain. P-Glycoprotein is an ATP-dependent drug transport protein that is predominantly found in the apical membranes of a number of epithelial cell types in the body, including the blood luminal membrane of the brain capillary endothelial cells that make up the blood–brain barrier. Since P-glycoprotein can actively transport a huge variety of hydrophobic amphipathic drugs out of the cell, it was hypothesized that it might be responsible for the very poor penetration of many relatively large (>400 Da) hydrophobic drugs in the brain, by performing active back-transport of these drugs to the blood. Extensive experiments with in vitro models and with knockout mice lacking blood–brain barrier P-glycoprotein or other animal models treated with blockers of P-glycoprotein have fully confirmed this hypothesis. Absence of functional P-glycoprotein in the blood–brain barrier leads to highly increased brain penetration of a number of important drugs. Depending on the pharmacological target of these drugs in the central nervous system (CNS), this can result in dramatically increased neurotoxicity, or fundamentally altered pharmacological effects of the drug. Given the variety of drugs affected by P-glycoprotein transport, it may be of tremendous therapeutic value to apply these insights to the development of drugs that should have either very poor or very good brain penetration, whichever is preferred for pharmacotherapeutic purposes. The clinical application of P-glycoprotein blockers should also be considered in order to improve the blood–brain barrier permeability of certain drugs that currently display insufficient brain penetration for effective therapy.

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.

References (79)

  • A Tsuji et al.

    P-glycoprotein as the drug efflux pump in primary cultured bovine brain capillary endothelial cells

    Life Sci.

    (1992)
  • L Jetté et al.

    Interaction of drugs with P-glycoprotein in brain capillaries

    Biochem. Pharmacol.

    (1995)
  • T Tatsuta et al.

    Functional involvement of P-glycoprotein in blood–brain barrier

    J. Biol. Chem.

    (1992)
  • A Tsuji et al.

    Restricted transport of cyclosporin A across the blood–brain by a multidrug transporter. P-glycoprotein

    Biochem. Pharmacol.

    (1993)
  • M.A Barrand et al.

    Comparisons of P-glycoprotein expression in isolated rat brain microvessels and in primary cultures of endothelial cells derived from microvasculature of rat brain, epididymal fat pad and from aorta

    FEBS Lett.

    (1995)
  • D Biegel et al.

    Isolation and culture of human brain microvessel endothelial cells for the study of blood–brain barrier properties in vitro

    Brain Res.

    (1995)
  • A Shirai et al.

    Transport of cyclosporin A across the brain capillary endothelial cell monolayer by P-glycoprotein

    Biochim. Biophys. Acta

    (1994)
  • G.R Lankas et al.

    P-glycoprotein deficiency in a subpopulation of CF-1 mice enhances avermectin-induced neurotoxicity

    Toxicol. Appl. Pharmacol.

    (1997)
  • D.R Umbenhauer et al.

    Identification of a P-glycoprotein-deficient subpopulation in the CF-1 mouse strain using a restriction fragment length polymorphism

    Toxicol. Appl. Pharmacol.

    (1997)
  • A Sakata et al.

    In vivo evidence for ATP-dependent and P-glycoprotein-mediated transport of cyclosporin A at the blood–brain barrier

    Biochem. Pharmacol.

    (1994)
  • Q Wang et al.

    Effect of the P-glycoprotein inhibitor, cyclosporin A, on the distribution of rhodamine-123 to the brain: an in vivo microdialysis study in freely moving rats

    Biochem. Biophys. Res. Comm.

    (1995)
  • S Desrayaud et al.

    Effect of the P-glycoprotein inhibitor, SDZ PSC 833, on the blood and brain pharmacokinetics of colchicine

    Life Sci.

    (1997)
  • J Van Asperen et al.

    The functional role of P-glycoprotein in the blood–brain barrier

    J. Pharm. Sci.

    (1997)
  • E.G Schuetz et al.

    Human MDR1 and mouse mdr1a P-glycoprotein alter the cellular retention and disposition of erythromycin, but not retinoic acid or benzo(a)pyrene

    Arch. Biochem. Biophys.

    (1998)
  • M. Naito, T. Tsuruo, Role of P-glycoprotein in the blood–brain barrier. In: S. Gupta, T. Tsuruo (Eds.), Multidrug...
  • J.A Endicott et al.

    The biochemistry of P-glycoprotein-mediated multidrug resistance

    Annu. Rev. Biochem.

    (1989)
  • M.M Gottesman et al.

    Biochemistry of multidrug resistance mediated by the multidrug transporter

    Annu. Rev. Biochem.

    (1993)
  • B.-F Pan et al.

    Enhanced transepithelial flux of cimetidine by Madin–Darby canine kidney cells overexpressing human P-glycoprotein

    J. Pharmacol. Exp. Ther.

    (1994)
  • D De Graaf et al.

    P-glycoprotein confers methotrexate resistance in 3T6 cells with deficient carrier-mediated methotrexate uptake

    Proc. Natl. Acad. Sci. USA

    (1996)
  • T Tsuruo et al.

    Overcoming of vincristine resistance in P388 leukemia in vivo and in vitro through enhanced cytotoxicity of vincristine and vinblastine by verapamil

    Cancer Res.

    (1981)
  • L Goldstein

    Clinical reversal of drug resistance

    Curr. Probl. Cancer

    (1995)
  • F Hyafil et al.

    In vitro and in vivo reversal of multidrug resistance by GF120918, an acridonecarboxamide derivative

    Cancer Res.

    (1993)
  • S.M Witherspoon et al.

    Flow cytometric assay of modulation of P-glycoprotein function in whole blood by the multidrug resistance inhibitor GG918

    Clin. Cancer Res.

    (1996)
  • A.H Dantzig et al.

    Reversal of P-glycoprotein-mediated resistance by a potent cyclopropyldibenzosuberane modulator, LY335979

    Cancer Res.

    (1996)
  • F Thiebaut et al.

    Cellular localization of the multidrug resistance gene product in normal human tissues

    Proc. Natl. Acad. Sci. USA

    (1987)
  • J.M Croop et al.

    The three mouse multidrug resistance (mdr) genes are expressed in a tissue-specific manner in normal mouse tissues

    Mol. Cell. Biol.

    (1989)
  • R.J Arceci et al.

    The gene encoding multidrug resistance is induced and expressed at high levels during pregnancy in the secretory epithelium of the uterus

    Proc. Natl. Acad. Sci. USA

    (1988)
  • A Devault et al.

    Two members of the mouse mdr gene family confer multidrug resistance with overlapping but distinct drug specificities

    Mol. Cell. Biol.

    (1990)
  • A.H Schinkel et al.

    Normal viability and altered pharmacokinetics in mice lacking mdr1-type (drug-transporting) P-glycoproteins

    Proc. Natl. Acad. Sci. USA

    (1997)
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