Pharmacokinetic drug-drug interactions often occur at the level of P-glycoprotein (Pgp). To study possible interactions caused by the newer antidepressants we investigated citalopram, fluoxetine, fluvoxamine, paroxetine, reboxetine, sertraline, and venlafaxine and their major metabolites desmethylcitalopram, norfluoxetine, paroxetine-metabolite (paroxetine-M), desmethylsertraline, N-desmethylvenlafaxine, and O-desmethylvenlafaxine for their ability to inhibit Pgp. Pgp inhibition was studied by a fluorometric assay using calcein-acetoxymethylester as Pgp substrate and two different cell systems: L-MDR1 cells (model for human Pgp) and primary porcine brain capillary endothelial cells (pBCECs, model for the blood-brain barrier). Both cell systems proved to be suitable for the evaluation of Pgp inhibitory potency of drugs. All antidepressants tested except O-desmethylvenlafaxine showed Pgp inhibitory activity with sertraline, desmethylsertraline, and paroxetine being the most potent, comparable with the well known Pgp inhibitor quinidine. In L-MDR1 cells fluoxetine, norfluoxetine, fluvoxamine, reboxetine, and paroxetine-M revealed intermediate Pgp inhibition and citalopram, desmethylcitalopram, venlafaxine, and N-desmethylvenlafaxine were only weak inhibitors. The ranking order was similar in pBCECs. The fact that some of the compounds tested exert Pgp inhibitor effects at similar concentrations as quinidine suggests that pharmacokinetic drugdrug interactions between the newer antidepressants and Pgp substrates should now be thoroughly studied in vivo.P-glycoprotein (Pgp) is a member of the ATP-binding cassette superfamily of membrane transport proteins, responsible for the efflux of many drugs. It represents a major component of the blood-brain barrier (Schinkel et al., 1994) and the intestinal barrier (van Asperen et al., 1998), and it contributes to renal and biliary elimination of drugs (Kusuhara et al., 1998;Chiou et al., 2000). At the blood-brain barrier Pgp is localized in the apical membrane of brain capillary endothelial cells and transports substrates toward the blood compartment (Cordon-Cardo et al., 1989;van Asperen et al., 1997). Therefore, Pgp can limit the penetration into and retention within the brain and thus modulate effectiveness and central nervous system toxicity of numerous compounds. In contrast, the absence of active Pgp as observed in mdr-1 knockout mice lacking Pgp and thus exhibiting unrestricted access of Pgp substrates to the brain yields significantly increased central nervous system concentrations often exceeding those observed in wild-type mice by orders of magnitude (Schinkel et al., 1994(Schinkel et al., , 1996. Pgp is also highly expressed in the apical membrane of epithelial cells in the small and large intestine, where it transports drugs out of the cells into the intestinal lumen (Cordon-Cardo et al., 1989;van Asperen et al., 1998), thus limiting bioavailability of compounds such as paclitaxel and human immunodeficiency virus protease inhibitors (Sparreboom et al., 1997...

Taken together, our study demonstrates significant inhibition of BCRP by many anti-HIV drugs. These results suggest that inhibition of BCRP might contribute to drug-drug interactions observed during HAART in vivo and possibly also the superior effectiveness of combination antiretroviral therapy.

ABSTRACT:Many drug interactions with drugs used for the therapy of human immunodeficiency virus (HIV) occur at the level of different cytochrome P450 isozymes. Increasing evidence suggests that antiretrovirals may also modify activity and expression of active drug transport systems. Such interactions may alter drug absorption, elimination, and also drug distribution and reach clinical importance if thereby access to the target site is affected. Beyond P-glycoprotein, the family of multidrug resistance-related proteins (MRP/ABCC) substantially contributes to the elimination of numerous drugs and their metabolites. Because the interaction of MRPs with non-HIV protease inhibitor antiretrovirals has not been studied thoroughly, we investigated whether important non-nucleoside reverse transcriptase inhibitors (NNRTI) (delavirdine, efavirenz, and nevirapine), nucleoside reverse transcriptase inhibitors (NRTI) (abacavir, emtricitabine, and lamivudine), and tenofovir as a nonnucleotide reverse transcriptase inhibitor can interact with MRP1, MRP2, and MRP3 in vitro. Inhibition of these ABC transporters was quantified by confocal laser-scanning microscopy using the 5-chloromethylfluorescein diacetate assay. With the exception of abacavir, which had no effect on MRP3, all the test compounds increased intracellular 5-chloromethylfluorescein fluorescence in a concentration-dependent manner, and this effect was observed in all the overexpressing cell lines but not in the parental cell line, indicating inhibition of MRP1, MRP2, and MRP3. In conclusion, the present study provides the first evidence for a significant and concentration-dependent inhibition of MRPs by NNRTI, NRTI, and tenofovir, which was most pronounced for delavirdine, efavirenz, and emtricitabine, suggesting that this might contribute to some of the known drug interactions impairing HIV therapy and also to the superior effectiveness of combination pharmacotherapy.Infections with the human immunodeficiency virus (HIV) are typically treated with drug combinations consisting of at least three different antiretroviral drugs. Essential components of this highly active antiretroviral therapy (HAART) are the HIV protease inhibitors (HPI), the non-nucleoside and nucleoside reverse transcriptase inhibitors (NNRTI and NRTI), and the nucleotide reverse transcriptase inhibitor tenofovir. Whereas this combination therapy substantially improves the clinical prognosis for patients infected with HIV, it concurrently increases the risk for drug-drug interactions (Piscitelli and Gallicano, 2001;de Maat et al., 2003).Many drug interactions with antiretrovirals, but by far not all and particularly not those with NRTI, occur at the level of different cytochrome P450 isozymes (Dasgupta and Okhuysen, 2001;Piscitelli and Gallicano, 2001;de Maat et al., 2003). Indeed, increasing evidence suggests that antiretrovirals may also modify activity and expression of active drug transport systems. Such interactions may determine drug absorption, elimination, and also drug distribution and reac...

Abstract. Efavirenz, an important component of human immunodeficiency virus 1 (HIV-1) therapy, causes substantial drug interactions as an inducer of cytochromes and the transporter ABCB1. So far its effect on the expression of other transporters is unknown. We therefore investigated the effect of long-term exposure of cells to efavirenz on expression of a large number of important drug transporters and on cell proliferation as a surrogate of intracellular availability. LS180 cells were used as a surrogate for the major site of drug interactions and Jurkat cells were used as a surrogate for the main target cells of HIV therapy. Cells were treated with efavirenz over 4 weeks and mRNA expression of drug transporters was repeatedly quantified. After 4 weeks, efavirenz significantly up-regulated the mRNA of ABCB1, ABCG2, ABCC2, ABCC3, ABCC5, and SLCO3A1 in LS180 cells and ABCG2, ABCC1, ABCC4, ABCC5, and SLCO2B1 in Jurkat cells. However these changes in transporter expression did not influence cell proliferation indicating that intracellular efavirenz concentrations were likely not altered. Efavirenz induces mRNA expression of several drug transporters critically modulating the kinetics of other drugs. While these expressional changes will most likely not influence the efficiency of efavirenz itself, they might change the effect of other co-administered drugs.

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