Gary N. Y. Chan

The complexities of antiretroviral drug–drug interactions: role of ABC and SLC transporters

Treatment of human immunodeficiency virus (HIV) infection involves a combination of several antiviral agents belonging to different pharmacological classes. This combination is referred to as highly active antiretroviral therapy (HAART). This treatment has proved to be very effective in suppressing HIV replication, but antiretroviral drugs have complex pharmacokinetic properties involving extensive drug metabolism and transport by membrane-associated drug carriers. Combination drug therapy often introduces complex drug-drug interactions that can result in toxic or sub-therapeutic drug concentrations, compromising treatment. This review focuses on the role of ATP-binding cassette (ABC) membrane-associated efflux transporters and solute carrier (SLC) uptake transporters in antiretroviral drug disposition, and identifies clinically important antiretroviral drug-drug interactions associated with changes in drug transport.

Up-regulation of P-glycoprotein by HIV protease inhibitors in a human brain microvessel endothelial cell line.

A major concern regarding the chronic administration of antiretroviral drugs is the potential for induction of drug efflux transporter expression (i.e., P‐glycoprotein, P‐gp) at tissue sites that can significantly affect drug distribution and treatment efficacy. Previous data have shown that the inductive effect of human immunodeficiency virus protease inhibitors (PIs) is mediated through the human orphan nuclear receptor, steroid xenobiotic receptor (SXR or hPXR). The objectives of this study were to investigate transport and inductive properties on efflux drug transporters of two PIs, atazanavir and ritonavir, at the blood–brain barrier by using a human brain microvessel endothelial cell line, hCMEC/D3. Transport properties of PIs by the drug efflux transporters P‐gp and multidrug resistance protein 1 (MRP1) were assessed by measuring the cellular uptake of 3H‐atazanavir or 3H‐ritonavir in P‐gp and MRP1 overexpressing cells as well as hCMEC/D3. Whereas the P‐gp inhibitor, PSC833, increased atazanavir and ritonavir accumulation in hCMEC/D3 cells by 2‐fold, the MRP inhibitor MK571 had no effect. P‐gp, MRP1, and hPXR expression and localization were examined by Western blot analysis and immunogold cytochemistry at the electron microscope level. Treatment of hCMEC/D3 cells for 72 hr with rifampin or SR12813 (two well‐established hPXR ligands) or PIs (atazanavir or ritonavir) resulted in an increase in P‐gp expression by 1.8‐, 6‐, and 2‐fold, respectively, with no effect observed for MRP1 expression. In hCMEC/D3 cells, cellular accumulation of these PIs appears to be primarily limited by P‐gp efflux activity. Long‐term exposure of atazanavir or ritonavir to brain microvessel endothelium may result in further limitations in brain drug permeability as a result of the up‐regulation of P‐gp expression and function.

Regulation of P‐glycoprotein by orphan nuclear receptors in human brain microvessel endothelial cells

J. Neurochem. (2011) 118, 163–175.

1α,25-Dihydroxyvitamin D3-liganded vitamin D receptor increases expression and transport activity of P-glycoprotein in isolated rat brain capillaries and human and rat brain microvessel endothelial cells.

Induction of the multidrug resistance protein 1 (MDR1)/P‐glycoprotein (P‐gp) by the vitamin D receptor (VDR) was investigated in isolated rat brain capillaries and rat (RBE4) and human (hCMEC/D3) brain microvessel endothelial cell lines. Incubation of isolated rat brain capillaries with 10 nM of the VDR ligand, 1α,25‐dihydroxyvitamin D3 [1,25(OH)2D3] for 4 h increased P‐gp protein expression fourfold. Incubation with 1,25(OH)2D3 for 4 or 24 h increased P‐gp transport activity (specific luminal accumulation of NBD‐CSA, the fluorescent P‐gp substrate) by 25–30%. In RBE4 cells, Mdr1b mRNA was induced in a concentration‐dependent manner by exposure to 1,25(OH)2D3. Concomitantly, P‐gp protein expression increased 2.5‐fold and was accompanied by a 20–35% reduction in cellular accumulation of the P‐gp substrates, rhodamine 6G (R6G), and HiLyte Fluor 488‐labeled human amyloid beta 1‐42 (hAβ42). In hCMEC/D3 cells, a 3 day exposure to 100 nM 1,25(OH)2D3 increased MDR1 mRNA expression (40%) and P‐gp protein (threefold); cellular accumulation of R6G and hAβ42 was reduced by 30%. Thus, VDR activation up‐regulates Mdr1/MDR1 and P‐gp protein in isolated rat brain capillaries and rodent and human brain microvascular endothelia, implicating a role for VDR in increasing the brain clearance of P‐gp substrates, including hAβ42, a plaque‐forming precursor in Alzheimers disease.

Role of P-Glycoprotein in the Distribution of the HIV Protease Inhibitor Atazanavir in the Brain and Male Genital Tract

ABSTRACT The blood-testis barrier and blood-brain barrier are responsible for protecting the male genital tract and central nervous system from xenobiotic exposure. In HIV-infected patients, low concentrations of antiretroviral drugs in cerebrospinal fluid and seminal fluid have been reported. One mechanism that may contribute to reduced concentrations is the expression of ATP-binding cassette drug efflux transporters, such as P-glycoprotein (P-gp). The objective of this study was to investigate in vivo the tissue distribution of the HIV protease inhibitor atazanavir in wild-type (WT) mice, P-gp/breast cancer resistance protein (Bcrp)-knockout (Mdr1a−/−, Mdr1b−/−, and Abcg2−/− triple-knockout [TKO]) mice, and Cyp3a−/− (Cyp) mice. WT mice and Cyp mice were pretreated with a P-gp/Bcrp inhibitor, elacridar (5 mg/kg of body weight), and the HIV protease inhibitor and boosting agent ritonavir (2 mg/kg intravenously [i.v.]), respectively. Atazanavir (10 mg/kg) was administered i.v. Atazanavir concentrations in plasma (Cplasma), brain (Cbrain), and testes (Ctestes) were quantified at various times by liquid chromatography-tandem mass spectrometry. In TKO mice, we demonstrated a significant increase in atazanavir Cbrain/Cplasma (5.4-fold) and Ctestes/Cplasma (4.6-fold) ratios compared to those in WT mice (P < 0.05). Elacridar-treated WT mice showed a significant increase in atazanavir Cbrain/Cplasma (12.3-fold) and Ctestes/Cplasma (13.5-fold) ratios compared to those in vehicle-treated WT mice. In Cyp mice pretreated with ritonavir, significant (P < 0.05) increases in atazanavir Cbrain/Cplasma (1.8-fold) and Ctestes/Cplasma (9.5-fold) ratios compared to those in vehicle-treated WT mice were observed. These data suggest that drug efflux transporters, i.e., P-gp, are involved in limiting the ability of atazanavir to permeate the rodent brain and genital tract. Since these transporters are known to be expressed in humans, they could contribute to the low cerebrospinal and seminal fluid antiretroviral concentrations reported in the clinic.

Induction of P-Glycoprotein by Antiretroviral Drugs in Human Brain Microvessel Endothelial Cells

ABSTRACT The membrane-associated drug transporter P-glycoprotein (P-gp) plays an essential role in drug efflux from the brain. Induction of this protein at the blood-brain barrier (BBB) could further affect the ability of a drug to enter the brain. At present, P-gp induction mediated by antiretroviral drugs at the BBB has not been fully investigated. Since P-gp expression is regulated by ligand-activated nuclear receptors, i.e., human pregnane X receptor (hPXR) and human constitutive androstane receptor (hCAR), these receptors could represent potential pathways involved in P-gp induction by antiretroviral drugs. The aims of this study were (i) to determine whether antiretroviral drugs currently used in HIV pharmacotherapy are ligands for hPXR or hCAR and (ii) to examine P-gp function and expression in human brain microvessel endothelial cells treated with antiretroviral drugs identified as ligands of hPXR and/or hCAR. Luciferase reporter gene assays were performed to examine the activation of hPXR and hCAR by antiretroviral drugs. The hCMEC/D3 cell line, which is known to display several morphological and biochemical properties of the BBB in humans, was used to examine P-gp induction following 72 h of exposure to these agents. Amprenavir, atazanavir, darunavir, efavirenz, ritonavir, and lopinavir were found to activate hPXR, whereas abacavir, efavirenz, and nevirapine were found to activate hCAR. P-gp expression and function were significantly induced in hCMEC/D3 cells treated with these drugs at clinical concentrations in plasma. Together, our data suggest that P-gp induction could occur at the BBB during chronic treatment with antiretroviral drugs identified as ligands of hPXR and/or hCAR.

In vivo induction of P-glycoprotein expression at the mouse blood-brain barrier: an intracerebral microdialysis study.

Intracerebral microdialysis was utilized to investigate the effect of P‐glycoprotein (a drug efflux transporter) induction at the mouse blood–brain barrier (BBB) on brain extracellular fluid concentrations of quinidine, an established substrate of P‐glycoprotein. Induction was achieved by treating male CD‐1 mice for 3 days with 5 mg/kg/day dexamethasone (DEX), a ligand of the nuclear receptor, pregnane X receptor, and a P‐glycoprotein inducer. Tandem liquid chromatography mass spectrometric method was used to quantify analytes in dialysate, blood and plasma. P‐glycoprotein, pregnane X receptor and Cyp3a11 (metabolizing enzyme for quinidine) protein expression in capillaries and brain homogenates was measured by immunoblot analysis. Following quinidine i.v. administration, the average ratio of unbound quinidine concentrations in brain extracellular fluid (determined from dialysate samples) to plasma at steady state (375–495 min) or Kp, uu, ECF/Plasma in the DEX‐treated animals was 2.5‐fold lower compared with vehicle‐treated animals. In DEX‐treated animals, P‐glycoprotein expression in brain capillaries was 1.5‐fold higher compared with vehicle‐treated animals while Cyp3a11 expression in brain capillaries was not significantly different between the two groups. These data demonstrate that P‐gp induction mediated by DEX at the BBB can significantly reduce quinidine brain extracellular fluid concentrations by decreasing its brain permeability and further suggest that drug–drug interactions as a result of P‐gp induction at the BBB are possible.

Role of nuclear receptors in the regulation of drug transporters in the brain

ATP-binding cassette membrane-associated drug efflux transporters and solute carrier influx transporters, expressed at the blood-brain barrier, blood-cerebrospinal fluid barrier, and in brain parenchyma, are important determinants of drug disposition in the central nervous system. Targeting the regulatory pathways that govern the expression of these transporters could provide novel approaches to selectively alter drug permeability into the brain. Nuclear receptors are ligand-activated transcription factors which regulate the gene expression of several metabolic enzymes and drug efflux/influx transporters. Although efforts have primarily been focused on investigating these regulatory pathways in peripheral organs (i.e., liver and intestine), recent findings demonstrate their significance in the brain. This review addresses the role of nuclear receptors in the regulation of drug transporter functional expression in the brain. An in-depth understanding of these pathways could guide the development of novel pharmacotherapy with either enhanced efficacy in the central nervous system or minimal associated neurotoxicity.

Molecular and Functional Characterization of P-Glycoprotein In Vitro

The blood-brain barrier (BBB) physically and metabolically functions as a neurovascular interface between the brain parenchyma and the systemic circulation, and regulates the permeability of several endogenous substrates and xenobiotics in and out of the central nervous system. Several membrane-associated transport proteins, such as P-glycoprotein (P-gp), multidrug resistance-associated proteins, breast cancer resistance protein, and organic anion transporting polypeptides, have been characterized at the BBB and identified to play a major role in regulating the brain bioavailability of several pharmacological agents. This chapter reviews several well-established techniques for the study of the molecular expression, cellular localization, and functional activity of transport proteins in primary and immortalized cell culture systems of the BBB. In particular, we describe the molecular characterization of P-gp/MDR1 at the transcript level using semiquantitative polymerase chain reaction (PCR), at the protein level using immunoblotting, and at the cellular level using immunofluorescence. In addition, the uptake/efflux and transepithelial flux studies, which characterize P-gp transport activity, are described.

Role of CNS transporters in the pharmacotherapy of HIV-1 associated neurological disorders.

Membrane-associated drug transporters are important determinants of antiretroviral drug disposition in the central nervous system during HIV-1 infection. A number of influx and efflux transport proteins expressed at the blood-brain barrier, blood-cerebrospinal fluid barrier and in brain parenchyma cellular compartments (i.e., astrocytes, microglia) have been implicated in the traffic of many antiretroviral drugs into and out of the brain. In particular, members of the ATP-binding cassette membrane associated transporter superfamily and Solute Carrier family are known to be involved in the efflux and/or influx of drugs, respectively. As a result, changes in the functional expression of these transporters can alter the disposition and distribution of drugs in the brain. Moreover, antiretroviral therapy itself and/or pathological events (i.e., inflammation, oxidative stress) associated with viral infection may affect the functional expression of these transporters. This review summarizes recent knowledge on the role of drug transporters in regulating brain antiretroviral drug transport in the context of HIV-1 infection.