Differential enhancement of antidepressant penetration into the brain in mice with abcb1ab (mdr1ab) P-Glycoprotein gene disruption
Introduction
The blood–brain barrier (BBB) protects the brain against potentially toxic substances and helps to maintain a constant internal environment. Endothelial cells of the brain capillaries form the BBB, and tight junctions between them as well as their low endocytotic activity restrict any transport through the intercellular spaces or through transcytosis into the brain. The ability of a drug to cross the BBB has been related to its molecular weight, hydrophobicity, degree of ionization, protein, and tissue binding (Saunders et al 1999). Strong hydrophobicity, as defined by a high octanol–water partition coefficient, has been regarded as a prerequisite for the ability of a drug to cross the BBB. Nevertheless, access to the brain has been shown to be limited for several hydrophobic substances. The reason for this is that many of these substances are substrates of P-glycoprotein (P-gp), expressed in the BBB cells by the abcb gene. P-glycoprotein acts as an extrusion pump for many xenobiotic compounds Schinkel et al 1995a, Schinkel et al 1996, von Moltke and Greenblatt 2000.
P-glycoprotein, a 170-kDa glycoprotein, is a member of a phylogenetically highly conserved superfamily of adenosin-triphosphate (ATP)-binding cassette (ABC) transport proteins and shares many features with numerous bacterial and eucaryotic ABC transport proteins Doige and Ames 1993, Gottesman et al 1995, Higgins 1992. P-gp is encoded by the ABCB1 gene in humans and the abcb1a (also called mdr1a or mdr3) and abcb1b (also called mdr1b or mdr1) gene in mice (Devault and Gros 1990). Although abcb1a and abcb1b are not always expressed in the same organs, the overall distribution of these genes in mice tissue coincides roughly with that of the single ABCB1 gene in humans, suggesting that abcb1a and abcb1b together function in the same manner as human ABCB1 Meijer et al 1998, van de Vrie et al 1998. The ABCB1 P-glycoprotein is a 1280 amino acid, glycosylated plasma membrane protein. It can actively transport its substrates against a concentration gradient, using ATP hydrolysis as an energy source. The transported substrates for P-gp include anticancer drugs such as anthracyclines, alkaloids, and the immunosuppressive agent cyclosporin A; the glycoside digoxin; the synthetic glucocorticoid dexamethasone; physiologic steroids such as cortisol, corticosterone, aldosterone, progesterone (Uhr et al 2002), and 17-beta estradiol Bello-Reuss et al 2000, Gosland et al 1993; the antidepressant drug amitriptyline (Uhr et al 2000); and local anesthetics and the anthelmintic drug ivermectin (Seelig 1998). Regarding central nervous system (CNS) drugs, little in vitro data are available regarding whether certain substances are substrates of P-gp. One in vitro study showed that the antidepressant citalopram is a P-gp substrate (Rochat et al 1999).
ABCB-type P-gp exists in the apical membrane of intestinal epithelial cells (Mukhopadhyay et al 1988), in the biliary canalicular membrane of hepatocytes (Thiebaut et al 1987), and in the lumenal membrane of proximal tubular epithelial cells in the kidney. High levels of ABCB1 P-gp have been found in the lumenal membrane of the endothelial cells that line small blood capillaries and form the blood–brain and blood–testis barriers Cordon-Cardo et al 1989, Cordon-Cardo et al 1990, Thiebaut et al 1989, Tsuji et al 1992
Because the activity of the P-gp–encoding ABCB gene differs among individuals, drug concentrations in brain tissue may vary, depending on P-gp levels. A high gene activity could explain why a patient does not reach sufficient drug concentrations in the brain and therefore fails to benefit from therapy. In such cases, administration of drugs that are not affected by the P-gp elimination system is warranted. For this reason, it is of clinical importance to know whether a given drug for the treatment of CNS diseases is a P-gp substrate. This is particularly important for antidepressant treatment, which is burdened with a nonresponse rate between 15%–30%.
We hypothesized that various antidepressant compounds are differentially recognized as substrates for P-gp and used abcb1ab P-gp knockout mouse (Schinkel et al 1994) to test the hypothesis. With these mouse mutants, we examined whether the brain uptake of four structurally different antidepressants, venlafaxine, paroxetine, mirtazapine, and doxepin (Figure 1) and their metabolites, is affected by P-gp.
Section snippets
Materials
Venlafaxine and o-desmethylvenlafaxine were kindly provided by Wyeth-Pharma GMBH (Münster, Germany). Mirtazapine was obtained from Thiemann Arzneimittel GMBH (Waltrop, Germany), Paroxetin from SmithKline Beecham (Betchworth, UK), and doxepin from Mack (Illertissen, Germany). Protriptyline was purchased from RBI (Natwick, Massachusetts). All other chemicals were obtained in the purest grade available from Merck (Darmstadt, Germany).
Animals
Male abcb1ab(−/−) mice and FVB/N wildtype mice were housed
Doxepin
One hour after s.c. injection of 10 μg doxepin/g body weight the doxepin and d-doxepin concentrations in the cerebrum and liver were found to be different in the abcb1ab(−/−) mutant and the wildtype control animals. Analysis of variance revealed a significant group effect on the doxepin concentrations (Wilks multivariate test of significance; effect of group: F[5,14] = 3.10; significance of F = .043), to which only the concentrations in the cerebrum and liver contributed (univariate F test; p
Discussion
The major finding of this investigation is that doxepin, venlafaxine, and paroxetine as well as the metabolites d-doxepin and d-venlafaxine turned were substrates of P-glycoprotein in our mouse P-gp mutant model. In contrast, penetration of the antidepressant mirtazapine did not differ between abcb1ab(−/−) mice and the control group in brain or in any other organs studied.
Successful drug treatment requires sufficient bioavailability of the drug in the affected tissue. This can be critical for
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