Differential enhancement of antidepressant penetration into the brain in mice with abcb1ab (mdr1ab) P-Glycoprotein gene disruption

Differential Enhancement of Antidepressant Penetration into the Brain in Mice with abcb1ab (mdr1ab) P-Glycoprotein Gene Disruption Manfred Uhr, Markus T. Grauer, and Florian Holsboer Background: Mice with a genetic disruption (knockout) of the multiple drug resistance (abcb1ab) gene were used to examine the effect of the absence of the drug-transporting P-glycoprotein (P-gp) at the blood– brain barrier on the uptake of the antidepressants venlafaxine, paroxetine, mirtazapine, and doxepin and its metabolites into the brain. Methods: One hour after subcutaneous injection of venlafaxine, paroxetine, mirtazapine, or doxepin, knockout and wildtype mice were sacrificed, and the drug concentrations in brain, spleen, kidney, liver, and plasma were measured. Results: The cerebrum concentrations of doxepin, venlafaxine, and paroxetine were higher in knockout mice, demonstrating that these substances are substrates of P-gp and that abcb1ab activity at the level of the blood– brain barrier reduces the penetration of these substances into the brain. In contrast, brain distribution of mirtazapine was indistinguishable between the groups. Conclusions: The differences reported here in brain penetration of antidepressant drugs that depend on the presence of the abcb1ab gene may offer an explanation for poor or nonresponse to antidepressant treatment. Furthermore, they may be able to explain in part the discrepancies between plasma levels of an antidepressant and its clinical effects and side effects. Biol Psychiatry 2003;54: 840 – 846 © 2003 Society of Biological Psychiatry Key Words: Drug resistance multiple, P-glycoprotein, mice knockout, brain, antidepressive agents

Introduction

T

he 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 From the Max Planck Institute of Psychiatry, Munich, Germany. Address reprint requests to Dr. Manfred Uhr, Max Planck Institute of Psychiatry, Kraepelinstrasse 10, D-80804 Munich, Germany. Received August 22, 2002; revised December 13, 2002; accepted January 1, 2003.

© 2003 Society of Biological Psychiatry

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, 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 0006-3223/03/$30.00 doi:10.1016/S0006-3223(03)00074-X

Antidepressant Penetration into the Brain in abcb1ab(⫺/⫺) Mice

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, 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.

Methods and Materials Materials Venlafaxine and o-desmethylvenlafaxine were kindly provided by Wyeth-Pharma GMBH (Mu¨ nster, 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 individually and maintained on a 12-hour light– dark cycle (lights on at 7 AM), with food and water ad libitum. For age and body weight see Table 1. Abcb1ab double knockout mice, originally produced by A. Schinkel by sequential gene targeting in 129/Ola E14 embryonic stem cells (Schinkel et al 1997) and backcrossed seven times (N7) to FVB/N from the C57BL/6 ⫻ 129 chimera, and FVB/N wildtype mice were received from Taconic (Germantown, PA; FVB/Tac-[KO]Pgy2 N7). A homozygous colony is maintained at the Max Planck Institute of Psychiatry on the N7 FVB/N background through intercrossing of homozygous mice. Western blot analysis carried out by Schinkel (Schinkel et al 1997) and again by our laboratory (unpublished data) shows that P-gp is not expressed by the knockout mice.

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Figure 1. Structural formula of doxepin, venlafaxine, paroxetine, and mirtazapine.

Experimental Procedures The analytical method was validated, has been described elsewhere (Uhr et al 2000), and was modified for the drugs used in this investigation. Doxepin, venlafaxine, paroxetine, and mirtazapine dissolved in .9% sodium chloride and .5% ethanol were administered subcutaneous (s.c.) to the nape of the neck. The proportions of ethanol and sodium chloride used for s.c. administration were the final concentrations of the agent in the vehicle. The concentrations used are given in Table 1. The total volume injected was 10 ␮L/g body weight of mouse. One hour after injection, the mice were anesthetized with halothane and decapitated. Trunk blood was collected in ethylenediamine tetraacetate– coated tubes and centrifuged at 3000 g for 5 min for the determination of the plasma concentration of the substances. Cerebrum, liver, spleen, and kidney were dissected and weighed and then homogenized in the fivefold volume of Hank’s Balanced Salts Solution (HBSS), .02 mol/L hydroxyethlpiperazineethanesulfonic acid (HEPES) buffer, pH 7.2 with a polytron PT 1200 Kinematic AG (Lucern, Switzerland). The homogenates were frozen at –20°C until further use.

Extraction Procedure After thawing, the samples were homogenized in a Branson sonifier at 4°, and .05 mL of an internal standard solution (protriptyline 10 ␮g/mL) and .4 mL of 2 mol/L sodium-carbonate buffer (adjusted to pH ⫽ 10.5 with NaOH) were added to .4 mL of plasma or homogenate and vortexed. We added 5 mL n-hexan with various isoamyl alcohol concentrations (Table 1), and the samples were mixed for 30 min at room temperature. After centrifugation at room temperature for 15 min at 3000 g, the organic layer was transferred to a tube containing .3 mL of .18 mol/L phosphoric acid, mixed for 30 min, and centrifuged at 3000 g for 10 min. The organic layer was then discarded, and an aliquot of the aqueous phase was injected for chromatographic separation. The extraction recoveries were ⬎ 90% for doxepin,

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Table 1. Details of the Animals, Experimental and Extraction Procedure, and the High-Performance Liquid Chromatography

Animals Sex Group size (n) Age (weeks) Weight abcb1ab(⫺/⫺) Weight abcb1ab(⫹/⫹) Experimental Procedures Subcutaneous administered substance Extraction Procedure Isoamylalcohol (plasma extraction) Isoamylalcohol (organ extraction) High-Performance Liquid Chromatography Mobile phase gradient (% B) Detection ultraviolet (nm) Detection fluorescence ex/em [nm]

Doxepine

Venlafaxine

Paroxetine

Mirtazapine

Male 10 12–15 28.0 ⫾ .3 27.4 ⫾ .3

Male 10 12–16 28.9 ⫾ .4 30.0 ⫾ .4

Male 9 26 –32 39.0 ⫾ 1.3 38.1 ⫾ 1.6

Male 10 13–16 31.7 ⫾ .7 31.2 ⫾ .3

10 ␮g/g bw

5 ␮g/g bw

1 ␮g/g bw

1 ␮g/g bw

1.5% 0%

.5% .5%

.5% .5%

0% 0%

12.5–25 254 —

0 –30 — 225/305

15–30 — 295/365

0 –25 — 295/370

bw, body weight; B, acetonitrile; ex, extinction; em, emission.

d-doxepin, mirtazapine, paroxetine, venlafaxine, and 36% for d-venlafaxine.

High-Performance Liquid Chromatography A Beckman 166 variable-wavelength ultraviolet (UV) detector, a Merck L-7480 fluorescence detector, and a Beckman gradient pump 126 Solvent Module equipped with a Beckman autoinjector 508 autosampler were used for the high-performance liquid chromatography (HPLC) analysis. Separations were made on a Luna 5 ␮ C18(2) 250 ⫻ 4.6 mm column (Phenomenex, Torrance, CA). The mobile phases A (acetonitrile–water, 43.6 mmol/L orthophosphoric acid, 35.9 mmol/L triethylamine [15:85]), and B (acetonitrile), were degased for 15 min in an ultrasonic bath immediately before use. The column temperature was 60°C and the flow of the mobile phase was 1.0 mL/min. A mobile phase gradient was used for the chromatographic analysis of venlafaxine, paroxetine, mirtazapine, and doxepin and its metabolites (Table 1). The substances and its metabolites were determined by UV absorption or fluorescence at the described wavelength (Table 1). The coefficient of variance was less than 15% for the different methods used. To avoid differences due to day-to-day variability, experimental procedure, extraction procedure and HPLC were carried out in alternating order.

Quantification Plasma and organ samples were calibrated by using spiked samples at different concentrations. The concentrations were in the measurement range to the respective substances. Quantification was performed by calculating the analyte: internal-standard peak-area ratio, and a regression model was fitted to the peak-area ratio of each compound to internal standard versus concentration.

Statistical Analysis The statistical procedure was designed and carried out by the Department of Biostatistics of the Max Planck Institute. Differences in the plasma and organ concentrations of the substances under examination in the mutants and the wildtype mice were

investigated and, if found, were tested for significance by one-factorial multivariate analysis of variance (MANOVA). The plasma and organ concentrations were the dependent variables and group, a between-subjects factor with two levels (mutants vs. wildtype mice), was the independent variable. When a significant group effect was found for an organ sample or plasma, univariate F tests followed to identify the variables whose differences between the two groups contributed significantly to the global group effect. As a nominal level of significance ␣ ⫽ .05 was accepted and corrected (reduced according to the Bonferroni procedure) for all a posteriori tests (univariate F tests) to keep type I errors less than or equal to .05.

Results Doxepin One hour after s.c. injection of 10 ␮g 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 values ⬍ .05). For d-doxepin we calculated F(5,14) ⫽ 6.59 with the significance of F ⫽ .002, to which only the concentrations in the cerebrum and liver contributed. The cerebrum concentrations for doxepin and d-doxepin were 1.2and 1.5-fold higher, respectively, in the knockout mice than in the control animals (Table 2, Figure 2). There were no significant differences in the doxepin and d-doxepin concentrations in plasma, spleen, and kidney.

Venlafaxine One hour after s.c. injection of 5 ␮g venlafaxine/g body weight, concentrations of venlafaxine and d-venlafaxine

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Table 2. Organ Concentrations of Antidepressant Drugs and Metabolites 1 hour after Subcutaneous Injection

Substance Doxepin (F[5,14] ⫽ 3.10, significance of F ⫽ .043)

abcb1ab(⫺/⫺) (ng/mL)

abcb1ab(⫹/⫹) (ng/mL)

Organ

Mean ⫾ SEM

Mean ⫾ SEM

Ratio

p Value

pla cer spl kid liv

414.4 ⫾ 18.9 3139.6 ⫾ 97.4 3203.5 ⫾ 204.7 5595.0 ⫾ 303.8 535.0 ⫾ 38.7

365.4 ⫾ 15.7 2609.0 ⫾ 154.8 3362.0 ⫾ 190.0 5069.8 ⫾ 277.9 397.6 ⫾ 37.6

1.1 1.2 1.0 1.1 1.3

ns ⬍.05a ns ns ⬍.05a

pla cer spl kid liv

11.1 ⫾ .9 27.0 ⫾ 1.8 66.2 ⫾ 3.5 228.8 ⫾ 10.9 199.7 ⫾ 18.2

9.5 ⫾ 1.2 18.3 ⫾ 1.8 70.1 ⫾ 4.0 200.1 ⫾ 11.7 145.1 ⫾ 14.4

1.2 1.5 .9 1.1 1.4

ns ⬍.05a ns ns ⬍.05a

pla cer spl kid liv

227.7 ⫾ 17.1 2014.3 ⫾ 98.3 2169.4 ⫾ 189.4 2236.6 ⫾ 181.8 826.2 ⫾ 68.5

178.4 ⫾ 11.3 877.9 ⫾ 27.3 1971.9 ⫾ 115.2 1976.2 ⫾ 75.3 603.8 ⫾ 29.3

1.3 2.3 1.1 1.1 1.4

⬍.05a ⬍.05a ns ns ⬍.05a

pla cer spl kid liv

47.4 ⫾ 2.8 110.8 ⫾ 4.2 251.5 ⫾ 21.4 476.9 ⫾ 33.0 226.0 ⫾ 19.5

42.6 ⫾ 3.1 59.4 ⫾ 2.7 259.4 ⫾ 11.4 457.7 ⫾ 20.6 215.7 ⫾ 14.3

1.1 1.9 1.0 1.0 1.0

ns ⬍.05a ns ns ns

pla cer spl kid liv

150.4 ⫾ 31.5 455.0 ⫾ 33.8 848.6 ⫾ 62.0 1528.0 ⫾ 82.6 585.3 ⫾ 33.9

110.5 ⫾ 7.9 212.8 ⫾ 29.5 875.9 ⫾ 80.0 1740.6 ⫾ 114.9 648.7 ⫾ 71.7

1.4 2.1 1.0 .9 .9

ns ⬍.05a ns ns ns

pla cer spl kid liv

54.7 ⫾ 6.2 163.3 ⫾ 14.3 198.3 ⫾ 12.9 280.5 ⫾ 15.3 129.5 ⫾ 18.2

41.2 ⫾ 2.9 127.9 ⫾ 9.8 189.5 ⫾ 15.5 260.1 ⫾ 23.4 103.9 ⫾ 7.2

1.3 1.3 1.0 1.1 1.2

ns ns ns ns ns

D-Doxepin (F[5,14] ⫽ 6.59, significance of F ⫽ .002)

Venlafaxine (F[5,14] ⫽ 118.9, significance of F ⬍ .001)

D-Venlafaxine (F[5,14] ⫽ 24.5, significance of F ⬍ .001)

Paroxetine (F[5,14] ⫽ 5.72, significance of F ⫽ .006)

Mirtazapine (F[5,14] ⫽ 1.47, significance of F ⫽ .264)

pla, plasma; cer, cerebrum; spl, spleen; kid, kidney; liv, liver. a Significant difference.

were 2.3 times and 1.9 times higher, respectively, which was significant (Table 2, Figure 2).

Paroxetine One hour after s.c. injection of 1 ␮g paroxetine/g body weight, the cerebrum paroxetine concentration of the abcb1ab(⫹/⫹) control mice was less than 50% of the concentration in the cerebrum of the abcb1ab(⫺/⫺) mutant mice (Table 2).

Mirtazapine One hour after s.c. injection of 1 ␮g/g body weight, there were no significant differences in the mirtazapine organ concentrations between the groups examined.

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 cerebral disorders, because drugs that are systemically administered need to penetrate into the brain tissue from the circulation, a process that is affected, among other things, by P-gp (van Asperen et al 1997). Located at the apical membrane of the small capillary endothelial cells, P-glycoprotein is able to transport its substrates against the concentration gradient back into the blood, thereby inhibiting transit across the BBB for numerous compounds. Using abcb1a knockout mice in vivo, Schinkel et al (1994) demonstrated by that P-gp prevents penetration of a variety of structurally different

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Figure 2. Organ and plasma concentrations of doxepin, venlafaxine, paroxetine, and mirtazepine and its metabolites in extracts obtained one hour after subcutaneous injection from abcb1ab (⫺/⫺) knockout mice (solid bars) and abcb1ab (⫹/⫹) wildtype control mice (shaded bars). The concentrations are shown as percent of the control. An asterisk indicates a significant difference between the knockout mutants and the control mice (univariate F tests in multivariate analysis of variance, p values ⬍ .05). pla, plasma; cer, cerebrum; spl, spleen; kid, kidney; liv, liver. Values shown as means ⫾ SEM.

substances into the brain, and more groups of substances have been reported to be substrates of P-gp since that time (von Moltke and Greenblatt 2000). As this study and a previous report from our group (Uhr et al 2000) have shown, various antidepressants are also substrates of P-gp, raising important questions about therapy with CNS drugs. Our study showed that P-gp exports different antidepressants at different rates. When penetration into the brain is compared between both groups, the ratio of abcb1ab(⫺/⫺)/ control animals is quite low for doxepin (1.2) and d-doxepin (1.5); however, the ratio is much higher for venlafaxine, d-venlafaxine, and paroxetine. Penetration of these substances into the brain of the abcb1ab(⫺/⫺) exceeds that of the control group by 200%. It has to be taken into account that the doses used in this study are close to the upper limit of the human dose. This is important because P-gp is probably a saturable transporter, and thus lower doses of the antidepressants would likely have resulted in even greater differences between the two groups.

It was recently shown for the antidepressant amitriptyline and its metabolites that the ratio ranges from 1.8 – 4.5 in abcb1a(⫺/⫺) mice compared with the control group (Uhr et al 2000). Compared with strong P-gp substrates such as cytostatic agents, digoxine, or ivermectine (Mayer et al 1996, Schinkel et al 1995b), the ratio for the previously mentioned antidepressants is lower, but effects in the range of a two- to fourfold difference in cerebral concentrations may well be of clinical relevance. So far, it has not been possible to predict the affinity of a substrate to P-gp from the chemical structure or from substrate properties, such as hydrophobicity, lipophobicity, charge, or size. Currently, structural similarities or differences do not explain why venlafaxine and amitriptyline are substrates, but mirtazapine is not. For this reason, experiments regarding the different antidepressants have to be carried out individually. One limitation of our study is the fact that concentrations were measured after one hour; steady-state conditions might

Antidepressant Penetration into the Brain in abcb1ab(⫺/⫺) Mice

differ and are more closely related to human use, in which antidepressants are given over longer periods of time. The important role of P-gp at the BBB was confirmed by the fact that uptake of the antidepressants and their metabolites did not differ significantly for organs that either lack P-gp completely or whose P-gp is not located in the endothelial cells, such as the spleen, kidney, and liver (Table 2). Moderately increased doxepin and venlafaxine concentrations in the liver of the abcb1ab(⫺/⫺) group can be explained by delayed detoxification due to the absence of the P-gp in the bilary canalicular membrane of hepatocytes. Small differences in the plasma concentrations of venlafaxine also result from different pharmacokinetics of detoxification between the groups (Sparreboom et al 1997). The coefficient of variance for the drug concentrations in the cerebrum was about 20% in both the abcb1ab(⫺/⫺) group as well as in the control groups. This low value is due to the analytical methods used and the fact that in each group the mice have the same genetic background and live under standardized living conditions. These experimental conditions ensure that the expression and function of P-gp are the same in each group. In humans undergoing therapy, the situation is, of course, more complex. Clinicians have long observed high rates of poor or no response to antidepressant treatment, noting that plasma levels and clinical effects and side effects do not directly correlate. We speculate that this is at least in part due to P-gp. Recently, genetic polymorphisms of P-gp have been described in humans (Cascorbi et al 2001; Kerb et al 2001b). These polymorphims can affect phenytoin plasma levels in healthy volunteers (Kerb et al 2001a), are associated with side effects of nortriptyline treatment in patients with depression (Roberts et al 2002), and affect intestinal uptake of digoxin (Hoffmeyer et al 2000). P-glycoprotein activity in the endothelial cells of the BBB may therefore prevent drugs from reaching the central nervous system at therapeutic concentrations, raising the possibility that the BBB can be a major obstacle for the successful treatment of CNS diseases. Although measurements of plasma concentrations are important, plasma levels of an antidepressant do not allow an easy and definite statement about its concentration in the brain. A patient with plasma concentrations of venlafaxine or paroxetine within the normal range may in fact be receiving an insufficient dose because penetration into the brain is reduced by an active retransport of the antidepressant drug through the P-gp located at the BBB. Besides genetic polymorphisms, P-gp function can also be affected by other medications taken concurrently. In cell culture and animal experiments, as well as in cancer therapy, a number of agents have successfully been used to suppress the activity of P-gp (Ford and Hait 1990). On the

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other hand, certain substances increase the expression and function of the P-gp (Seelig 1998). The popular nonprescription antidepressant St. John’s wort, for example, increases expression of P-gp (Hennessy et al 2002). In addition, one can suspect that changes associated with depression itself may affect the activity of P-gp and permeability of the BBB. It is important to note that no structural similarity exists between the antidepressants tested so far (Figure 1) that would allow one to predict which substance may be a substrate of the P-gp and which not. Therefore, each antidepressant compound needs to be tested with regard to its interaction with P-gp. Such interactions may be of particular relevance in cases of treatment resistance, one of the major drawbacks of antidepressant treatments. In addition, concurrent use of other medications must be taken into account. In sum, our study shows that antidepressants are substrates of P-gp and are exported actively out of the brain. Together with recent data about genetic polymorphisms of P-gp and the effects of other medications used concurrently, it raises important questions about future therapeutic strategies and is an important step toward an individually tailored, pharmacogenetically oriented antidepressive therapy. Similar to the CYP gene, individual P-gp polymorphisms and pharmacokinetics may require greater consideration in the future.

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