Induction of P-glycoprotein expression by HIV protease inhibitors in cell culture

Physiologically Based Pharmacokinetic Modeling to Assess Ritonavir-Digoxin Interactions and Recommendations for Co-Administration Regimens

BACKGROUND

Digoxin is a commonly used cardiac glycoside drug in clinical practice, primarily transported by P-glycoprotein (P-gp) and susceptible to the influence of P-gp inhibitors/inducers. Concurrent administration of ritonavir and digoxin may significantly increase the plasma concentration of digoxin. Due to the narrow therapeutic window of digoxin, combined use may lead to severe toxic effects.

PURPOSE

Utilize a Physiology-Based Pharmacokinetic (PBPK) model to simulate and predict the impact of the interaction between ritonavir and digoxin on the pharmacokinetics (PK) of digoxin, and provide recommendations for the combined medication regimen.

METHODS

Using PK-Sim®, develop individual PBPK models for ritonavir and digoxin. Simulate the exposure in a drug-drug interaction (DDI) scenario by implementing ritonavir's inhibition of P-glycoprotein (P-gp) on digoxin. Evaluate the performance of the models by comparing the predicted and observed plasma concentration-time curves and predicted versus observed PK parameter values. Finally, adjust the dosing regimen for the combined therapy based on the changes in exposure.

RESULTS

According to the model simulations, the steady-state exposure of digoxin increased by 86.5% and 90.2% for oral administration, and 80.2% and 90.2% for intravenous administration, respectively, when 0.25 mg or 0.5 mg of digoxin was administered concurrently with ritonavir. By reducing the dose of digoxin by 45% or doubling the oral administration interval, similar steady-state concentrations can be achieved compared to when the drugs are not co-administered.

CONCLUSIONS

In clinical practice, the influence of drug interactions on the plasma concentration changes of digoxin within the body should be considered to ensure the safety and effectiveness of treatment.

The Anti-Cancer Properties of the HIV Protease Inhibitor Nelfinavir

Simple Summary

To this day, cancer remains a medical challenge despite the development of cutting-edge diagnostic methods and therapeutics. Thus, there is a continual demand for improved therapeutic options for managing cancer patients. However, novel drug development requires decade-long time commitment and financial investments. Repurposing approved and market-available drugs for cancer therapy is a way to reduce cost and the timeframe for developing new therapies. Nelfinavir is an anti-infective agent that has extensively been used to treat acquired immunodeficiency syndrome (AIDS) in adult and pediatric patients. In addition to its anti-infective properties, nelfinavir has demonstrated potent off-target anti-cancer effects, suggesting that it could be a suitable candidate for drug repurposing for cancer. In this review, we systematically compiled the therapeutic benefits of nelfinavir against cancer as a single drug or in combination with chemoradiotherapy, and outlined the possible underlying mechanistic pathways contributing to the anti-cancer effects.

Abstract

Traditional cancer treatments may lose efficacy following the emergence of novel mutations or the development of chemoradiotherapy resistance. Late diagnosis, high-cost of treatment, and the requirement of highly efficient infrastructure to dispense cancer therapies hinder the availability of adequate treatment in low-income and resource-limited settings. Repositioning approved drugs as cancer therapeutics may reduce the cost and timeline for novel drug development and expedite the availability of newer, efficacious options for patients in need. Nelfinavir is a human immunodeficiency virus (HIV) protease inhibitor that has been approved and is extensively used as an anti-infective agent to treat acquired immunodeficiency syndrome (AIDS). Yet nelfinavir has also shown anti-cancer effects in in vitro and in vivo studies. The anti-cancer mechanism of nelfinavir includes modulation of different cellular conditions, such as unfolded protein response, cell cycle, apoptosis, autophagy, the proteasome pathway, oxidative stress, the tumor microenvironment, and multidrug efflux pumps. Multiple clinical trials indicated tolerable and reversible toxicities during nelfinavir treatment in cancer patients, either as a monotherapy or in combination with chemo- or radiotherapy. Since orally available nelfinavir has been a safe drug of choice for both adult and pediatric HIV-infected patients for over two decades, exploiting its anti-cancer off-target effects will enable fast-tracking this newer option into the existing repertoire of cancer chemotherapeutics.

ABC Transporters in Extrahepatic Tissues: Pharmacological Regulation in Heart and Intestine

ATP binding cassette (ABC) transporters are transmembrane proteins expressed in secretory epithelia like the liver, kidneys and intestine, in the epithelia exhibiting barrier function such as the blood-brain barrier and placenta, and to a much lesser extent, in tissues like reproductive organs, lungs, heart and pancreas, among others. They regulate internal distribution of endogenous metabolites and xenobiotics including drugs of therapeutic use and also participate in their elimination from the body. We here describe the function and regulation of ABC transporters in the heart and small intestine, as examples of extrahepatic tissues, in which ABC proteins play clearly different roles. In the heart, they are involved in tissue pathogenesis as well as in protecting this organ against toxic compounds and druginduced oxidative stress. The small intestine is highly exposed to therapeutic drugs taken orally and, consequently, ABC transporters localized on its surface strongly influence drug absorption and pharmacokinetics. Examples of the ABC proteins currently described are Multidrug Resistance-associated Proteins 1 and 2 (MRP1 and 2) for heart and small intestine, respectively, and P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP) for both organs.

Emerging Role of Proteases in the Pathogenesis of Chronic Rhinosinusitis with Nasal Polyps

Chronic rhinosinusitis with nasal polyps (CRSwNP) is a heterogeneous upper airway disease with multiple etiologies. Clinically, CRSwNP can be classified into either eosinophilic or non-eosinophilic subtypes. The eosinophilic phenotype of CRSwNP is widely thought to be highly associated with recurrence of nasal polyps or surgical failure. Epithelial cells have a crucial role in the development of Th2-biased airway diseases. Recent studies have shown that a wide range of external stimuli such as allergens and microorganisms can elicit the release of epithelial-derived Th2-driving cytokines and chemokines. Protease activity is a feature common to these multiple environmental insults and there is growing evidence for the concept that an imbalance of proteases and protease inhibitors in the epithelial barrier leads to both the initiation and maintenance of chronic eosinophilic airway inflammation. In this review, we analyze recent work on the role of proteases in the development of the sinonasal mucosal type 2 immune response with an emphasis on the molecular pathways promoting adaptive Th2 cell immunity.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Role and modulation of drug transporters in HIV-1 therapy

Current treatment of human immunodeficiency virus type-1 (HIV-1) infection involves a combination of antiretroviral drugs (ARVs) that target different stages of the HIV-1 life cycle. This strategy is commonly referred to as highly active antiretroviral therapy (HAART) or combined antiretroviral therapy (cART). Membrane-associated drug transporters expressed ubiquitously in mammalian systems play a crucial role in modulating ARV disposition during HIV-1 infection. Members of the ATP-binding cassette (ABC) and solute carrier (SLC) transporter superfamilies have been shown to interact with ARVs, including those that are used as part of first-line treatment regimens. As a result, the functional expression of drug transporters can influence the distribution of ARVs at specific sites of infection. In addition, pathological factors related to HIV-1 infection and/or ARV therapy itself can alter transporter expression and activity, thus further contributing to changes in ARV disposition and the effectiveness of HAART. This review summarizes current knowledge on the role of drug transporters in regulating ARV transport in the context of HIV-1 infection.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

Expression of membrane drug efflux transporters in the sigmoid colon of HIV-infected and uninfected men

The use of antiretroviral therapy (ART) as pre-exposure prophylaxis (PrEP) has gained global attention as a promising HIV prevention strategy in men who have sex with men. Permeability of these agents in the rectal mucosa may be partially regulated by interactions with drug efflux transporters, P-glycoprotein (P-gp), multidrug resistance-associated proteins (MRPs) and/or breast cancer resistance protein (BCRP). The objective of this work was to investigate the expression of drug efflux transporters in recto-sigmoid colon tissues of HIV-infected and uninfected men, and evaluate the association of ART and/or HIV infection with drug transporter expression. MDR1/P-gp, MRPs (1-4) and BCRP mRNA and protein expression were detected in sigmoid colon biopsies of HIV-uninfected individuals. Biopsies from HIV-infected, ART-naïve participants revealed a significant downregulation of P-gp and MRP2 protein levels compared to HIV-uninfected individuals. Biopsies from HIV-infected ART-treated patients showed 1.9-fold higher P-gp protein expression and 1.5-fold higher MRP2 protein expression compared to the ones obtained from the HIV-infected ART-naïve patients. This is a first report demonstrating that HIV infection or ART could alter expression of drug efflux transporters in gut mucosa which in turn could affect the permeability of PrEP antiretroviral agents across this barrier, a highly vulnerable site of HIV transmission.

Induction of P-glycoprotein by antiretroviral drugs in human brain microvessel endothelial cells

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.

Functional impact of ABCB1 variants on interactions between P-glycoprotein and methadone

Methadone is a widely used substitution therapy for opioid addiction. Large inter-individual variability has been observed in methadone maintenance dosages and P-glycoprotein (P-gp) was considered to be one of the major contributors. To investigate the mechanism of P-gp's interaction with methadone, as well as the effect of genetic variants on the interaction, Flp-In™-293 cells stably transfected with various genotypes of human P-gp were established in the present study. The RNA and protein expression levels of human P-gp were confirmed by real-time quantitative RT-PCR and western blot, respectively. Utilizing rhodamine 123 efflux assay and calcein-AM uptake study, methadone was demonstrated to be an inhibitor of wild-type human P-gp via non-competitive kinetic (IC50 = 2.17±0.10 µM), while the variant-type human P-gp, P-gp with 1236T-2677T-3435T genotype and P-gp with 1236T-2677A-3435T genotype, showed less inhibition potency (IC50 = 2.97±0.09 µM and 4.43±1.10 µM, respectively) via uncompetitive kinetics. Methadone also stimulated P-gp ATPase and inhibited verapamil-stimulated P-gp ATPase activity under therapeutic concentrations. These results may provide a possible explanation for higher methadone dosage requirements in patients carrying variant-type of P-gp and revealed the possible drug-drug interactions in patients who receive concomitant drugs which are also P-gp substrates.

Efflux pump-mediated resistance in chemotherapy

Efflux pump mechanisms perform important physiological functions such as prevention of toxin absorption from the gastrointestinal tract, elimination of bile from the hepatocytes, effective functioning of the blood-brain barrier and placental barrier, and renal excretion of drugs. They exist in all living cells, but those in the bacterial and mammalian cells are more important to the clinician and pharmacologist, as they constitute an important cause of antimicrobial drug resistance, which contributes to treatment failure, high medical bills, and increased mortality / morbidity. This review was aimed at highlighting the role of efflux pump mechanisms in microbial resistance to chemotherapeutic agents. It was also aimed to elucidate their structure and mechanisms of action so as to integrate the efflux pump mechanisms in the design and development of novel antimicrobial agents. Findings from previous studies and research on this subject assessed through Google search, Pubmed, Hinari websites, as well as standard textbooks on chemotherapy, provided the needed information in the process of this review. Efflux pump inhibitors are promising strategies for preventing and reverting efflux-mediated resistance to chemotherapeutic agents. They are usually employed as adjuncts in antimicrobial and cancer chemotherapy. Toxicity, more common with the older-generation inhibitors such as verapamil and reserpine, constitutes the greatest impediment to their clinical applications. No efflux pump inhibitor has been approved for routine clinical use, as a result of doubtful clinical efficacy and unacceptably high incidence of adverse effects, particularly inhibition of the P-450 drug metabolizing enzyme. At present, their applications are mainly restricted to epidemiological studies. Nonetheless, the search for efficacious and tolerable efflux pump inhibitors continues because of the potential benefits. There is a need to consider efflux pump substrate selectivity in the design and development of novel chemotherapeutic agents.

Inhibition of P-glycoprotein by HIV protease inhibitors increases intracellular accumulation of berberine in murine and human macrophages

BACKGROUND

HIV protease inhibitor (PI)-induced inflammatory response in macrophages is a major risk factor for cardiovascular diseases. We have previously reported that berberine (BBR), a traditional herbal medicine, prevents HIV PI-induced inflammatory response through inhibiting endoplasmic reticulum (ER) stress in macrophages. We also found that HIV PIs significantly increased the intracellular concentrations of BBR in macrophages. However, the underlying mechanisms of HIV PI-induced BBR accumulation are unknown. This study examined the role of P-glycoprotein (P-gp) in HIV PI-mediated accumulation of BBR in macrophages.

METHODOLOGY AND PRINCIPAL FINDINGS

Cultured mouse RAW264.7 macrophages, human THP-1-derived macrophages, Wild type MDCK (MDCK/WT) and human P-gp transfected (MDCK/P-gp) cells were used in this study. The intracellular concentration of BBR was determined by HPLC. The activity of P-gp was assessed by measuring digoxin and rhodamine 123 (Rh123) efflux. The interaction between P-gp and BBR or HIV PIs was predicated by Glide docking using Schrodinger program. The results indicate that P-gp contributed to the efflux of BBR in macrophages. HIV PIs significantly increased BBR concentrations in macrophages; however, BBR did not alter cellular HIV PI concentrations. Although HIV PIs did not affect P-gp expression, P-gp transport activities were significantly inhibited in HIV PI-treated macrophages. Furthermore, the molecular docking study suggests that both HIV PIs and BBR fit the binding pocket of P-gp, and HIV PIs may compete with BBR to bind P-gp.

CONCLUSION AND SIGNIFICANCE

HIV PIs increase the concentration of BBR by modulating the transport activity of P-gp in macrophages. Understanding the cellular mechanisms of potential drug-drug interactions is critical prior to applying successful combinational therapy in the clinic.

Exposure to HIV-protease inhibitors selects for increased expression of P-glycoprotein (ABCB1) in Kaposi's sarcoma cells

Background:

Given that HIV-protease inhibitors (HIV-PIs) are substrates/inhibitors of the multidrug transporter ABCB1, can induce ABCB1 expression, and are used in combination with doxorubicin for AIDS-Kaposi's Sarcoma (KS) treatment, the role that ABCB1 plays in mediating multidrug resistance (MDR) in a fully transformed KS cell line (SLK) was explored.

Methods:

The KS cells were exposed to both acute and chronic treatments of physiological concentrations of different HIV-PIs (indinavir, nelfinavir, atazanavir, ritonavir, or lopinavir), alone or together with doxorubicin. The ABCB1 mRNA and protein expression levels were then assessed by qRT–PCR and western blotting, flow cytometry, and immunofluorescence.

Results:

Chronic treatment of SLK cells with one of the five HIV-PIs alone or together resulted in increased resistance to doxorubicin. Co-treatment with one of the HIV-PIs in combination with doxorubicin resulted in a synergistic increase in resistance to doxorubicin, and the degree of resistance was found to correlate with the expression of ABCB1. The SLK cells were also revealed to be cross-resistant to the structurally unrelated drug paclitaxel.

Conclusion:

These studies suggest that ABCB1 is primarily responsible for mediating MDR in SLK cells selected with either HIV-PIs alone or in combination with doxorubicin. Therefore, the roles that ABCB1 and drug cocktails play in mediating MDR in KS in vivo should be evaluated.

Impact of drug transporters on cellular resistance towards saquinavir and darunavir

OBJECTIVES

Highly active antiretroviral therapy is complicated by drug-drug interactions and the development of viral resistance. Drug interactions involve transporters that may critically affect the pharmacokinetics of many antiretroviral drugs and contribute to the formation of functional sanctuary sites. We therefore investigated the effect of saquinavir and darunavir on drug transporter expression and functional consequences for cellular resistance towards these compounds.

METHODS

Induction of transporters was investigated in LS180 cells over a period of 4 weeks by means of RT-PCR, and for some transporters also at the protein and functional levels. Cellular resistance was measured by growth inhibition assays.

RESULTS

Incubation with 10 µM darunavir for 1 week significantly increased mRNA expression of P-glycoprotein (P-gp/MDR1/ABCB1) 3.8-fold and of organic anion-transporting polypeptide 2B1 (SLCO2B1) 1.9-fold. In contrast, 10 µM saquinavir significantly increased mRNA expression of P-gp 5.7-fold, multidrug resistance-associated protein 1 (MRP1/ABCC1) 2.3-fold, MRP2/ABCC2 4.5-fold, MRP3/ABCC3 2.0-fold, MRP4/ABCC4 1.8-fold, MRP5/ABCC5 3.8-fold, breast cancer resistance protein (BCRP/ABCG2) 4.1-fold, SLCO1B1 4.6-fold, SLCO2B1 1.8-fold and SLCO3A1 1.8-fold. P-gp induction was also confirmed at the protein and functional levels. Induction by darunavir caused an increase in cellular resistance towards this compound, as measured in growth inhibition assays; however, saquinavir treatment did not cause reduced sensitivity of cells, indicating unchanged intracellular concentration. Hence, induction by darunavir increased drug efflux and might therefore lead to a suboptimal intracellular concentration of darunavir.

CONCLUSIONS

The study revealed substantial induction of several drug transporters by saquinavir and darunavir, possibly leading to decreased efficacy of antiretrovirals and drugs used to treat co-morbidity.

ABC transporters in human lymphocytes: expression, activity and role, modulating factors and consequences for antiretroviral therapies

IMPORTANCE OF THE FIELD

ATP-binding cassette (ABC) transporters are a superfamily of efflux pumps that transport numerous compounds across cell membranes. These transporters are located in various human tissues including peripheral blood cells, in particular lymphocytes, and present a high variability of expression and activity. This variability may affect the intracellular concentrations and efficacy of drugs acting within lymphocytes, such as antiretroviral drugs.

AREAS COVERED IN THIS REVIEW

This review focuses on the current knowledge about the expression, activity, roles and variability of ABC drug transporters in human lymphocytes. The identified modulating factors and their impact on the intracellular pharmacokinetics and efficacy of antiretroviral drugs are also detailed.

WHAT THE READER WILL GAIN

Controversial data regarding the expression, activity and sources of variability of ABC transporters in lymphocytes are discussed. The modulating factors and their pharmacological consequences regarding antiretroviral therapies are also provided.

TAKE HOME MESSAGE

Numerous studies have reported conflicting results regarding the expression and activity of ABC drug transporters in lymphocytes. Despite these discrepancies, which may partly result from heterogeneous analytical methods, ABCC1 appears to have the highest expression in lymphocytes and may thus play a predominant role in the resistance to antiretroviral drugs, particularly to protease inhibitors.

Lack of P-glycoprotein induction by rifampicin and phenobarbital in human lymphocytes

The efficacy of drugs acting within lymphocytes depends on their intracellular concentrations, which could be modulated by membrane efflux transporters including P-glycoprotein (P-gp), encoded by the MDR1 gene. In particular, P-gp induction may compromise the efficacy of its substrates. Rifampicin and phenobarbital have been shown to induce P-gp in hepatic and intestinal cells through the activation of the nuclear receptors PXR and CAR. However, controversial data exist in human lymphocytes. We investigated the effect of these drugs on P-gp activity and expression in lymphocytes in vitro and ex vivo. CCRF-CEM cells and peripheral blood mononuclear cells (PBMCs) from healthy volunteers were incubated in the presence of rifampicin, phenobarbital, or without any drug. P-gp activity was measured by flow cytometry using DiOC(6) efflux. MDR1, PXR and CAR mRNA expression were measured by quantitative RT-PCR. Neither P-gp activity nor MDR1 mRNA expression were modified by rifampicin or phenobarbital both in CCRF-CEM cells and PBMCs. Moreover, P-gp protein expression at the membrane was neither detectable nor induced. The very weak PXR and CAR mRNA expression levels in these cells could partly explain these results. Therefore, P-gp induction by rifampicin and phenobarbital may play a negligible role in drug interactions occurring within lymphocytes.

Concentration-dependent effects and intracellular accumulation of HIV protease inhibitors in cultured CD4 T cells and primary human lymphocytes

BACKGROUND

The intracellular and plasma concentrations of HIV protease inhibitors (HPIs) vary widely in vivo. It is unclear whether there is a concentration-dependent effect of HPIs such that at increasing concentration they may either block their own efflux (leading to 'autoboosting') or influx (leading to saturability/decreased intracellular accumulation).

METHOD

The effects of various concentrations (0-30 microM) of lopinavir, saquinavir, ritonavir and atazanavir on the accumulation of [(14)C]lopinavir, [(3)H]saquinavir, [(3)H]ritonavir and [(3)H]atazanavir, respectively, were investigated in CEM(parental), CEM(VBL) [P-glycoprotein (ABCB1) overexpressing], CEM(E1000) (MRP1 overexpressing) and in peripheral blood mononuclear cells (PBMCs). We also investigated the effects of inhibitors of ABCB1/ABCG2 (tariquidar), ABCC (MK571) and ABCC1/2 (frusemide), singly and in combination with HPIs, on cellular accumulation.

RESULTS

In all the cell lines, with increasing concentration of lopinavir, saquinavir and ritonavir, there was a significant increase in the cellular accumulation of [(14)C]lopinavir, [(3)H]saquinavir and [(3)H]ritonavir. Tariquidar, MK571 and frusemide (alone and in combination with lopinavir, saquinavir and ritonavir) significantly increased the accumulation of [(14)C]lopinavir, [(3)H]saquinavir and [(3)H]ritonavir. Ritonavir (alone or in combination with tariquidar) decreased the intracellular accumulation of [(3)H]ritonavir in PBMCs. Atazanavir decreased the accumulation of [(3)H]atazanavir in a concentration-dependent manner in all of the cells tested.

CONCLUSIONS

There are complex and variable drug-specific rather than class-specific effects of the HPIs on their own accumulation.

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.

The accumulation and metabolism of zidovudine in 3T3-F442A pre-adipocytes

BACKGROUND AND PURPOSE

Cultured pre-adipocytes accumulate and metabolize zidovudine (ZDV), but its mode of accumulation into these cells is unclear. We investigated the mode of accumulation of [(3)H]-ZDV, and the impact of changes in external pH and modulators of drug transporters on its accumulation and metabolism.

EXPERIMENTAL APPROACH

The initial rate and steady-state accumulation of [(3)H]-ZDV were measured in 3T3-F442A cells. P-glycoprotein (P-gp) expression was detected by Western blotting. External pH was varied, and modulators of intracellular pH and drug transporters were used to study the mode of accumulation of ZDV. Phosphorylated ZDV metabolites were detected by high-performance liquid chromatography.

KEY RESULTS

Intracellular accumulation of ZDV was rapid, reaching equilibrium within 20 min; nigericin increased accumulation by 1.9-fold, but this did not alter the generation of ZDV mono-, di- and triphosphate. The accumulation and metabolism were pH dependent, being maximal at pH 7.4 and least at pH 5.1. Monensin, carbonyl cyanide p-trifluoromethoxy) phenyl hydrazone, brefeldin A, bafilomycin A1 and concanamycin A increased accumulation; 2-deoxyglucose, dipyridamole, thymidine and tetraphenylphosphonium inhibited accumulation. The accumulation was saturable; the derived K(d) and capacity of binding were 250 nmol per 10(6) cells and 265 nM respectively. 3T3-F442A cells express P-gp; inhibitors of P-gp (XR9576 and verapamil), P-gp/BCRP (GF120918), multidrug resistance protein (MRP) (MK571) and MRP/OATP (probenecid) increased the accumulation of ZDV. Saquinavir, ritonavir, amprenavir and lopinavir increased accumulation.

CONCLUSIONS AND IMPLICATIONS

The accumulation of ZDV in 3T3-F442A cells was rapid, energy dependent, saturable and pH sensitive. Western blot analysis showed that 3T3-F442A cells express P-gp, and direct inhibition assays suggest that ZDV is a substrate of P-gp and MRP.

Methadone metabolism and clearance are induced by nelfinavir despite inhibition of cytochrome P4503A (CYP3A) activity

BACKGROUND

Methadone plasma concentrations are decreased by nelfinavir. Methadone clearance and the drug interactions have been attributed to CYP3A4, but actual mechanisms of methadone clearance and the nelfinavir interaction are unknown. We assessed nelfinavir effects on methadone pharmacokinetics and pharmacodynamics, intestinal and hepatic CYP3A4/5 activity, and intestinal P-glycoprotein transport activity. CYP3A4/5 and transporters were assessed using alfentanil and fexofenadine, respectively.

METHODS

Twelve healthy HIV-negative volunteers underwent a sequential crossover. On three consecutive days they received oral alfentanil plus fexofenadine, intravenous alfentanil, and intravenous plus oral methadone. This was repeated after nelfinavir. Plasma and urine analytes were measured by mass spectrometry. Opioid effects were measured by pupil diameter change (miosis).

RESULTS

Nelfinavir decreased intravenous and oral methadone plasma concentrations 40-50%. Systemic clearance, hepatic clearance, and hepatic extraction all increased 1.6- and 2-fold, respectively, for R- and S-methadone; apparent oral clearance increased 1.7- and 1.9-fold. Nelfinavir stereoselectively increased (S>R) methadone metabolism and metabolite formation clearance, and methadone renal clearance. Methadone bioavailability and P-glycoprotein activity were minimally affected. Nelfinavir decreased alfentanil systemic and apparent oral clearances 50 and 76%, respectively. Nelfinavir appeared to shift the methadone plasma concentration-effect (miosis) curve leftward and upward.

CONCLUSIONS

Nelfinavir induced methadone clearance by increasing renal clearance, and more so by stereoselectively increasing hepatic metabolism, extraction and clearance. Induction occurred despite 50% inhibition of hepatic CYP3A4/5 activity and more than 75% inhibition of first-pass CYP3A4/5 activity, suggesting little or no role for CYP3A in clinical methadone disposition. Nelfinavir may alter methadone pharmacodynamics, increasing clinical effects.

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 (3)H-atazanavir or (3)H-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.

Induction of P-glycoprotein expression and function in human intestinal epithelial cells (T84)

Intestinal induction of Pgp is known to limit the oral availability of certain drug compounds and give rise to detrimental drug-drug interactions. We have investigated the induction of P-glycoprotein (Pgp; MDR1) activity in a human intestinal epithelial cell line (T84) following pre-exposure to a panel of drug compounds, reported to be Pgp substrates, inhibitors or inducers. Human MDR1-transfected MDCKII epithelial monolayers were used to assess Pgp substrate interactions and inhibition of digoxin secretion by the selected drug compounds. The T84 cell line was used to assess induction of Pgp-mediated digoxin secretion following pre-exposure to the same compounds. Changes in gene expression (MDR1, MRP2, PXR and CAR) were determined by quantitative RT-PCR. Net transepithelial digoxin secretion was increased (1.3 fold, n=6, P<0.05) following pre-exposure to the PXR activator hyperforin (100nM, 72h), as was MDR1 mRNA expression (3.0 fold, n=4, P<0.05). A number of Pgp substrates (quinidine, amprenavir, irinotecan, topotecan, atorvastatin and erythromycin) induced net digoxin secretion, as did the non-Pgp substrate artemisinin. Various non-Pgp substrates demonstrated inhibition of digoxin secretion (verapamil, mifepristone, clotrimazole, mevastatin, diltiazem and isradipine) but did not induce Pgp-mediated digoxin secretion. Of the compounds that increased Pgp secretion, quinidine, topotecan, atorvastatin and amprenavir pre-exposure also elevated MDR1 mRNA levels, whereas erythromycin, irinotecan and artemisinin displayed no change in transcript levels. This indicates possible post-translational regulation of digoxin secretion. Finally, a strong correlation between drug modulation of MRP2 and PXR mRNA expression levels was evident.

Intestinal human colon adenocarcinoma cell line LS180 is an excellent model to study pregnane X receptor, but not constitutive androstane receptor, mediated CYP3A4 and multidrug resistance transporter 1 induction: studies with anti-human immunodeficiency virus protease inhibitors

Lack of an established cell line model to study induction of cytochromes P450 (P450s) and drug transporters poses a challenge in predicting in vivo drug-drug interactions. Although not well characterized, LS180 cells could be an excellent cell line to study induction of P450s and transporters because they express pregnane X receptor (PXR). Therefore, as part of a larger study of in vitro to in vivo prediction of inductive drug interactions, we determined induction of various P450s and drug transporters by the anti-human deficiency virus protease inhibitors (PIs) and the prototypic inducer, rifampin, in LS180 cells. Among these proteins, the various PIs significantly induced (n = 3-5) only CYP3A4 and multidrug resistance transporter 1 (MDR1) transcripts (2- to 50-fold). CYP3A4 activity (1'-hydroxymidazolam formation) was increased (2-fold) by rifampin (10 microM) but was reduced by the PIs (1.5- to 7-fold). Surprisingly, constitutive androstane receptor 1 (CAR1) was not found to be expressed in these cells. Additionally, using a reporter assay, we found that PIs did not activate CAR3 (the natural splice variant of CAR1) but significantly activated PXR (2- to 24-fold), which correlated well with induction of CYP3A4 and MDR1 transcripts (approximately r = 0.9). Furthermore, in a PXR-knockdown stable LS180 cell line, induction of CYP3A4 and MDR1 mRNA after treatment with PIs and rifampin was significantly reduced (1.4- to 5-fold) compared with that in PXR nonsilenced cells. Based on these data, we conclude that LS180 cells could be used as a readily available, high-throughput cell line to screen for PXR-mediated induction of CYP3A4 and MDR1 transcripts. These data also indicate that the majority of the PIs are likely to produce intestinal drug-drug interactions by inactivating or inhibiting CYP3A enzymes even though they induce CYP3A4 and MDR1 transcripts via PXR.

Induction of P-glycoprotein expression and activity by ritonavir in bovine brain microvessel endothelial cells

Extended treatment with human immunodeficiency virus (HIV) protease inhibitors (HPIs) is standard in HIV/AIDS therapy. While these drugs have helped decrease the overall incidence of AIDS defining illnesses, the relative prevalence of HIV/AIDS dementia has increased. HPIs may cause induction of blood-brain barrier (BBB) drug transporters (P-glycoprotein; P-gp) and thereby limit entry of HPIs into brain tissue, increasing the probability that the brain could become an HIV sanctuary site. Using bovine brain microvessel endothelial cells (BMEC) as an in-vitro model of the BBB, the potential for the HIV protease inhibitor ritonavir to cause induction of P-gp activity and expression was examined. BMEC were isolated from fresh cow brain by enzymatic digest and density centrifugation. Primary culture BMEC were co-incubated with ritonavir or vehicle control for 120 h. Quantitative drug accumulation of rhodamine 123 (Rh123) and fluorescence microscopy were used as measures of P-gp activity. P-gp expression was assessed using quantitative Western blotting. Ritonavir decreased Rh123 cell accumulation and increased P-gp immunoreactive protein in a concentration-dependent manner. Fluorescent microscopy mirrored Rh123 quantitative studies. In BMEC pretreated with 30 microM ritonavir, Rh123 accumulation was decreased 40% and immunoreactive P-gp protein increased 2-fold. Collectively, a strong correlation between decreased Rh123 BMEC accumulation and increased P-gp immunoreactive protein was observed (Spearman r2 = 0.77, P < 0.0001). Thus extended exposure of BMEC to ritonavir caused a concentration-dependent increase in P-gp activity and expression. Similar findings may occur at the clinical level with prolonged HIV protease inhibitor use, giving insight into the central nervous system as an HIV sanctuary site and eventual development of HIV dementia.

Effects of human immunodeficiency virus protease inhibitors on the intestinal absorption of tenofovir disoproxil fumarate in vitro

Human immunodeficiency virus protease inhibitors (PIs) modestly affect the plasma pharmacokinetics of tenofovir (TFV; -15% to +37% change in exposure) following coadministration with the oral prodrug TFV disoproxil fumarate (TDF) by a previously undefined mechanism. TDF permeation was found to be reduced by the combined action of ester cleavage and efflux transport in vitro. Saturable TDF efflux observed in Caco-2 cells suggests that at pharmacologically relevant intestinal concentrations, transport has only a limited effect on TDF absorption, thus minimizing the magnitude of potential intestinal drug interactions. Most tested PIs increased apical-to-basolateral TDF permeation and decreased secretory transport in MDCKII cells overexpressing P-glycoprotein (Pgp; MDCKII-MDR1 cells) and Caco-2 cells. PIs were found to cause a multifactorial effect on the barriers to TDF absorption. All PIs showed similar levels of inhibition of esterase-dependent degradation of TDF in an intestinal subcellular fraction, except for amprenavir, which was found to be a weaker inhibitor. All PIs caused a dose-dependent increase in the accumulation of a model Pgp substrate in MDCKII-MDR1 cells. Pgp inhibition constants ranged from 10.3 microM (lopinavir) to >100 microM (amprenavir, indinavir, and darunavir). Analogous to hepatic cytochrome P450-mediated drug interactions, we propose that the relative differences in perturbations in TFV plasma levels when TDF is coadministered with PIs are based in part on the net effect of inhibition and induction of intestinal Pgp by PIs. Combined with prior studies, these findings indicate that intestinal absorption is the mechanism for changes in TFV plasma levels when TDF is coadministered with PIs.

Minimal effect of MDR1 and CYP3A5 genetic polymorphisms on the pharmacokinetics of indinavir in HIV-infected patients

AIMS

The protease inhibitor indinavir is characterized by an important interindividual pharmacokinetic variability, which results from the actions of the metabolizing enzymes cytochrome P450 (CYP) 3A and the multidrug efflux pump P-glycoprotein (P-gp), encoded by MDR1. Using a population pharmacokinetic approach, we investigated the effect of several MDR1 and CYP3A5 polymorphisms on the pharmacokinetic parameters of indinavir in HIV-infected patients.

METHODS

Twenty-eight patients receiving indinavir alone or together with ritonavir were included. Indinavir pharmacokinetics were studied over a 12 h interval. Genetic polymorphisms were assessed by real-time PCR assays and direct sequencing for MDR1 and by PCR-SSCP analysis for CYP3A5.

RESULTS

The pharmacokinetics of indinavir were best described by a one-compartment model with first-order absorption. In the final model, the MDR1 C3435T genotype and ritonavir were identified as statistically significant covariates (P </= 0.001) for the absorption rate constant (95% confidence interval on the difference between CC and CT genotype 0.37, 5.53) and for clearance (95% confidence interval on the difference 5.8, 26.2), respectively. Patients with the CYP3A5*3/*3 genotype receiving indinavir alone had a 31% decrease in the indinavir clearance rate compared with patients carrying the CYP3A5*1/*3 genotype.

CONCLUSIONS

The MDR1 C3435T genotype affects the absorption constant of indinavir suggesting that P-gp may be implicated in its pharmacokinetic variability. Through its inhibition of CYP3A and P-gp, ritonavir could attenuate the pharmacokinetic variability linked to genetic differences, reducing significantly the interindividual variability of indinavir. However, genotyping MDR1 and/or CYP3A5 to optimize protease inhibitor boosted regimens does not seem clinically relevant.

Interindividual variability in the effect of atazanavir and saquinavir on the expression of lymphocyte P-glycoprotein

OBJECTIVES

ABCB1 encodes the efflux transporter P-glycoprotein (P-gp), which regulates the intracellular concentration of many xenobiotics, including several HIV protease inhibitors (PIs). Exposure to some xenobiotics, such as the antibiotic rifampicin, increases P-gp expression. In the present study, we investigated the effect of the HIV PIs saquinavir and atazanavir on the expression and function of ABCB1 and P-gp in primary and cultured lymphocytes, as well as the molecular interactions between these drugs and P-gp. ABCB1 and P-gp expression and function were examined in lymphocyte samples from healthy subjects before and after atazanavir-boosted saquinavir treatment. Expression and function were also studied in CEM cells following exposure to atazanavir and saquinavir. The inhibitory effects of these drugs were investigated in ABCB1-transfected HEK293T cells.

METHODS

P-gp expression and function were measured by flow cytometry. ABCB1 mRNA expression was evaluated using quantitative RT-PCR.

RESULTS

There were no overall changes in ABCB1 or P-gp expression or function after saquinavir-atazanavir treatment in primary lymphocyte samples. However, there was considerable interindividual variability in baseline lymphocyte ABCB1 expression, as well as in the degree of change in ABCB1 expression after saquinavir-atazanavir administration. In cell culture, 5 microM saquinavir increased ABCB1 levels, although it did not affect P-gp expression. Atazanavir inhibited P-gp function at concentrations above therapeutic levels.

CONCLUSIONS

Differences in lymphocyte ABCB1 expression, which may be caused by genetic polymorphisms in ABCB1 or its regulatory partners, are a likely cause of interindividual variation in the disposition and efficacy of clinically relevant P-gp substrates, including HIV PIs.

Lack of sex-related differences in saquinavir pharmacokinetics in an HIV-seronegative cohort

AIMS

To examine the influence of sex on steady-state saquinavir pharmacokinetics in HIV-seronegative volunteers administered saquinavir without a concomitant protease inhibitor.

METHODS

Thirty-eight healthy volunteers (14 female) received saquinavir soft-gel capsules 1200 mg three times daily for 3 days to achieve steady-state conditions. Following administration of the 10th dose, blood was collected serially over 8 h for measurement of saquinavir plasma concentrations. Saquinavir pharmacokinetic parameter values were determined using noncompartmental methods and compared between males and females. CYP3A phenotype (using oral midazolam) and MDR-1 genotypes at positions 3435 and 2677 were determined for all subjects in order to characterize possible mechanisms for any observed sex-related differences.

RESULTS

There was no significant difference in saquinavir AUC(0-8) or any other pharmacokinetic parameter value between the sexes. These findings persisted after mathematically correcting for total body weight. The mean weight-normalized AUC(0-8) was 29.9 (95% confidence interval 15.5, 44.3) and 29.8 (18.6, 40.9) ng h(-1) ml(-1) kg(-1) for males and females, respectively. No significant difference in CYP3A phenotype was observed between the groups; likewise, the distribution of MDR-1 genotypes was similar for males and females.

CONCLUSION

In contrast to previous study findings, results from this investigation showed no difference in saquinavir pharmacokinetics between males and females. The discrepancy between our findings and those previously reported may be explained by the fact that we evaluated HIV-seronegative volunteers and administered saquinavir in the absence of concomitant protease inhibitors such as ritonavir. Caution must be exercised when extrapolating pharmacokinetic data from healthy volunteer studies (including sex-based pharmacokinetic differences) to HIV-infected populations or to patients receiving additional concurrent medications.

Expression and function of efflux drug transporters in the intestine

A variety of drug transporters expressed in the body control the fate of drugs by affecting absorption, distribution, and elimination processes. In the small intestine, transporters mediate the influx and efflux of endogenous or exogenous substances. In clinical pharmacotherapy, ATP-dependent efflux transporters (ATP-binding cassette [ABC] transporters) expressed on the apical membrane of the intestinal epithelial cells determine oral bioavailability, intestinal efflux clearance, and the site of drug-drug interaction of certain drugs. The expression and functional activity of efflux transporters exhibit marked interindividual variation and are relatively easily modulated by factors such as therapeutic drugs and daily foods and beverages. In this article, we will summarize the recent findings regarding the intestinal efflux transporters, especially P-glycoprotein (P-gp or human multidrug resistance gene [MDR] 1), multidrug resistance-associated protein 2 (MRP2), and breast cancer resistance protein (BCRP).

Modulation of the intracellular accumulation of saquinavir in peripheral blood mononuclear cells by inhibitors of MRP1, MRP2, P-gp and BCRP

BACKGROUND

The efflux transporters P-glycoprotein (P-gp), multidrug resistance-associated proteins (MRP) and breast cancer resistance protein (BCRP) limit the accumulation of antiretrovirals in cell lines but it is more important to know whether the expression of these transporters in peripheral blood mononuclear cells (PBMC) impacts cellular drug concentrations.

OBJECTIVES

To study the transport and accumulation of saquinavir (SQV) in PBMC and the effects of specific inhibitors of MRP1, MRP2, P-gp and BCRP.

METHODS

Transport and accumulation of [H]-SQV was measured in PBMC in the absence or presence of specific and non-specific inhibitors of MRP1, MRP2, P-gp and BCRP. Flow cytometric, western blot and real-time PCR assays were used to examine the relative expression of the drug efflux transporters in the same batches of PBMC.

RESULTS

MRP2 is present in PBMC. The expression of P-gp, MRP1, MRP2 (mRNA) and BCRP all displayed batch-to-batch variability. Specific and non-specific inhibitors of MRP1, P-gp and MRP2 significantly increased the baseline accumulation of SQV. Accumulation of SQV was not correlated with the expression of any single transporter.

CONCLUSIONS

Multiple drug efflux transporters are important in the intracellular accumulation of SQV in PBMC. If drug efflux contributes towards virological failure, then all contributing transporters will need to be inhibited.

Functional assessment of multiple P-glycoprotein (P-gp) probe substrates: influence of cell line and modulator concentration on P-gp activity

Compounds known to modulate P-glycoprotein (P-gp) activity were evaluated in cell monolayers expressing P-gp for their effects on the secretory transport of P-gp substrates paclitaxel, vinblastine, and digoxin. Paclitaxel has been proposed to selectively interact with a binding site on P-gp that is distinct from the vinblastine and digoxin-binding site. Using Madin-Darby canine kidney (MDCK)-multidrug resistance-1 (MDR1), MDCK-wild-type (WT), and Caco-2 cell monolayers, the basal-to-apical (BL-AP) apparent permeability (Papp) of [3H]paclitaxel, [3H]vinblastine, and [3H]digoxin in the presence of various concentrations of a series of structurally diverse P-gp substrates and modulators of P-gp function were determined. MDCK-WT cell monolayers demonstrated active secretory transport of all P-gp substrate probes, although the sensitivity to inhibition by verapamil was lower than that demonstrated in MDCK-MDR1 cell monolayers. When evaluated as competitive inhibitors, several known P-gp substrates had no effect or only a slight modulatory effect on the BL-AP Papp of all probe substrates in MDCK-MDR1 cells. The secretory transport of P-gp substrates in MDCK-WT cells was more sensitive to inhibition by known P-gp modulators compared with MDCK-MDR1 cells. Low concentrations of ketoconazole (1-3 microM) activated the BL-AP Papp of [3H]vinblastine and [3H]digoxin in MDCK-MDR1 cells but not in MDCK-WT or Caco-2 cells. Determination of secretory transport in P-gp expressing cell monolayers, such as MDCK-MDR1 and Caco-2, may be complicated by substrate cooperativity and allosteric binding, which may result in the activation of P-gp. In addition, expression of other efflux transporters in these cell lines introduces additional complexity in distinguishing which transporter is responsible for substrate recognition and transport.

Increased expression of MDR1 mRNAs and P-glycoprotein in placentas from HIV-1 infected women

P-glycoprotein transports several compounds including protease inhibitors, actually used in the clinical management of HIV-1 infection. Since P-glycoprotein is expressed in placental trophoblasts, its efflux activity could interfere with placental transfer of antiretrovirals. The purpose of this study was to investigate the expression of P-gp-encoding MDR1 gene and P-gp itself in full-term placentas from uninfected (n=35) and HIV-1 infected women (n=24). MDR1 transcripts were quantified by real-time PCR using relative (MDR1 normalized upon 28S levels) and absolute (copy number) determinations. P-glycoprotein localization and expression were evaluated by immunohistochemistry and western blot analysis, respectively. Relative or absolute PCR quantification showed a significant 3.3-fold (p<0.0009) or 3.7-fold (p<0.0002) mean increase in MDR1 placental transcription in HIV-infected compared to non-infected women, respectively. Ratios of individual HIV-positive values to HIV-negative mean ranged from 0.1 to 21.8. Moreover a significant 2.5-fold increased expression of immunoreactive P-glycoprotein was evidenced in placentas from HIV-infected women (p<0.0001). This MDR1 overexpression was observed in a similar extent in placentas from pregnant women treated with Zidovudine alone or in combination with Nelfinavir and/or Lamivudine. Our findings suggest that P-glycoprotein in placentas from HIV-infected women would contribute to modulate the materno-fetal transport of antiretrovirals across the placental barrier and consequently diminish fetal exposure to these compounds.

The implications of P-glycoprotein in HIV: friend or foe?

P-glycoprotein (P-gp), coded by the ABCB1 gene, has a wide tissue distribution. The drug transporter is known to limit the bioavailability of a plethora of drugs and xenobiotics including the human immunodeficiency virus (HIV) protease inhibitors. There remains a considerable degree of debate in the literature with respect to the role of ABCB1 polymorphisms in HIV-treatment outcome and some studies have also implicated antiretroviral drugs as inducers of P-gp. Recent evidence indicates a role for P-gp in the inhibition of viral infectivity and/or release and cellular relationships with other infection-related proteins (and cholesterol). It is becoming increasingly clear that future studies on P-gp in HIV should consider both pharmacological and virological issues.

Atazanavir: effects on P-glycoprotein transport and CYP3A metabolism in vitro

The effect of atazanavir on P-glycoprotein (P-gp) expression and activity, as well as its inhibitory potency against CYP3A activity, was evaluated in vitro. Induction of P-gp activity and expression was studied using LS180V cells. P-gp inhibition was studied using both LS180V cells and Caco-2 cells. P-gp activity was assessed by measuring P-gp-mediated rhodamine 123 (Rh123) transport, and P-gp expression was determined using SDS-polyacrylamide gel electrophoresis/Western blot analysis. CYP3A inhibition was tested using triazolam hydroxylation in human liver microsomes (HLM). Extended (3-day) exposure of LS180V cells to 30 microM atazanavir caused a 2.5-fold increase in immunoreactive P-gp expression as well as a concentration-dependent decrease of intracellular Rh123 to a mean 45% (S.D. 5.2%) of control. Acute exposure (2 h) of LS180V cells to atazanavir increased intracellular Rh123 concentrations up to 300% of control at 100 microM atazanavir. At 30 microM and above, acute atazanavir exposure reversed P-gp induction caused by 3-day pretreatment with 10 microM ritonavir. P-gp inhibition was also observed in Caco-2 cells, causing an effect comparable to that observed for the known P-gp inhibitor verapamil (50% of control). In HLM, atazanavir was an inhibitor of triazolam hydroxylation, with inhibitory potency greatly increased by preincubation. IC50 values with and without preincubation were 0.31 microM (S.D. 0.13) and 5.7 microM (S.D. 4.1), respectively. Thus, atazanavir is an inhibitor and inducer of P-gp as well as a potent inhibitor of CYP3A in vitro, suggesting a potential for atazanavir to cause drug-drug interactions in vivo.

Differential bidirectional transfer of indinavir in the isolated perfused human placenta

The protease inhibitor (PI) indinavir may be used in the management of human immunodeficiency virus (HIV) infection during pregnancy. Poor maternal-to-fetal transfer of indinavir has been reported previously, but the mechanisms of transfer remain unknown. The bidirectional transfer of indinavir was assessed in dually perfused, isolated human placentae. Term placentae (n = 5) were obtained from non-HIV-infected pregnant women. To investigate transport mechanisms, the steady-state transfer of indinavir was compared to those of antipyrine (a marker of passive diffusion) and [(3)H]vinblastine (a marker of P-glycoprotein [P-gp] transport) in the maternal-to-fetal and fetal-to-maternal directions in each placenta. Indinavir and antipyrine perfusate concentrations were determined by using reverse-phase, high-performance liquid chromatography; [(3)H]vinblastine concentrations were measured by liquid scintillation. The antipyrine transfer clearance in each direction did not differ (P = 0.76), a finding consistent with passive diffusion. However, the maternal-to-fetal transfer clearance of vinblastine, normalized to that of antipyrine (clearance index) (0.31 +/- 0.05), was significantly lower than the fetal-to-maternal clearance index of vinblastine (0.67 +/- 0.17; P = 0.017), suggesting the involvement of placental P-gp. Similarly, the maternal-to-fetal clearance index of indinavir (0.39 +/- 0.09) was significantly lower than its fetal-to-maternal clearance index (0.97 +/- 0.12; P < 0.001). These results represent the first evidence for differential transfer of a xenobiotic in the intact human placenta. The use of transport modulators to increase the maternal-to-fetal transfer of PIs as a possible strategy to reduce mother-to-child transmission of HIV warrants investigation.

The intracellular pharmacology of antiretroviral protease inhibitors

Therapeutic drug monitoring (TDM) of antiretroviral protease inhibitors (PIs) has been suggested to have the potential to both reduce toxicity and optimize individual therapy. However, the major target of PIs is within cells infected with HIV. Therefore clinical outcome ultimately must be related to intracellular drug concentrations since antiviral activity of PIs is highly correlated with intracellular concentrations in vitro. Intracellular pharmacokinetics provides information regarding drug disposition in a compartment where HIV replication occurs and combined with plasma data may be useful in understanding therapeutic failure in relation to cellular resistance. In order to improve therapeutic efficacy, it is therefore important that the intracellular pharmacokinetics of drugs, such as PIs, is studied in addition to plasma pharmacokinetics. Multidrug resistance transporters may result in a lower cellular concentration of drug via an efflux mechanism, thus contributing to sanctuary site formation. However, conclusive proof that transporters contribute to clinical drug resistance is still lacking, although recent studies have attempted to address this issue. In relation to host and cellular factors, this review considers several issues involved in influencing intracellular drug concentrations and discusses the intracellular levels of PIs recently published from cellular studies.

Rapid induction of P-glycoprotein expression by high permeability compounds in colonic cells in vitro: a possible source of transporter mediated drug interactions?

P-glycoprotein (PGP) substrates with high membrane permeability, such as propranolol and verapamil, are considered to be essentially "transparent" to PGP since the transporter does not significantly limit their absorption or elimination. However, the question of whether such compounds can modulate PGP expression in epithelial cells following short-term exposure, with potential consequences for drug interactions, has not been addressed. LS180 colonic epithelial cells were exposed to propranolol or verapamil at concentrations (50-300 microM) consistent with those likely to be present in the gut lumen during oral dosing. Both compounds stimulated four to six-fold increases in MDR1 mRNA and PGP protein expression measured by quantitative real-time PCR and immunoblotting, respectively. These changes were accompanied by an induction in transporter activity measured by rhodamine 123 efflux. In contrast, metoprolol, a compound with similar permeability but no affinity for PGP had no effect on PGP expression. The induction of PGP by propranolol and verapamil was rapid with significant increases occurring within 3h with maximal stimulation after 6h exposure. Rifampicin, shown to cause clinical drug interactions via a PXR-mediated increase in PGP expression, exhibited a very similar time-course and extent of induction. In conclusion, verapamil and propranolol, whose trans-epithelial permeability are unaffected by PGP, appear to be effective inducers of PGP expression in gut epithelial cells in vitro. While the in vivo significance of these observations is unknown, this questions whether high permeability, "PGP-transparent" compounds, currently favoured in drug selection strategies, should be evaluated in terms of their potential for transporter-mediated drug interactions.

Protease inhibitors potentiate chemotherapy-induced neutropenia

Pharmacokinetic interactions between chemotherapy and highly active antiretroviral therapy (HAART) are described, but there are few data on their clinical relevance. Patients with systemic AIDS-related non-Hodgkin lymphoma (ARL) were treated with concomitant HAART and infusional cyclophosphamide-doxorubicin-etoposide (CDE) chemotherapy. We compared neutropenia according to whether patients received protease inhibitor (PI)-based HAART or non-PI regimens. Differences in survival, response rates, immunologic parameters, and virologic parameters were also investigated. The day-10 (Mann-Whitney U test; P = .012) and day-14 (P = .025) neutrophil counts were significantly lower in patients receiving PIs, though there were no differences in the number of days of granulocyte colony-stimulating factor (G-CSF) administered between groups (P = .16). Grade 3 or 4 infections requiring hospitalization were recorded for a total of 58 (31%) of 190 cycles of CDE: 23 (48%) of 48 when prescribed PIs and 35 (25%) of 142 with concomitant PI-sparing HAART (chi(2) test; P = .0025). There were no statistically significant differences in the response rates, relapse-free survival, or disease-free survival between patients receiving PIs and those not receiving PIs. PI-based HAART appears to significantly potentiate the myelotoxicity of CDE chemotherapy. This potentiation may be a consequence of microsomal enzyme inhibition reducing the metabolism of cytotoxics in this regimen.

Ritonavir Decreases the Nonrenal Clearance of Digoxin in Healthy Volunteers with Known MDR1 Genotypes

Our objective was to examine the influence of ritonavir on P-glycoprotein (P-gp) activity in humans by characterizing the effect of ritonavir on the pharmacokinetics of the P-gp substrate digoxin in individuals with known MDR1 genotypes. Healthy volunteers received a single dose of digoxin 0.4 mg orally before and after 14 days of ritonavir 200 mg twice daily. After each digoxin dose blood and urine were collected over 72 hours and analyzed for digoxin. Digoxin pharmacokinetic parameter values were determined using noncompartmental methods. MDR1 genotypes at positions 3435 and 2677 in exons 26 and 21, respectively, were determined using PCR-RFLP analysis. Ritonavir increased the digoxin AUC(0-72) from 26.20 +/- 8.67 to 31.96 +/- 11.24 ng x h/mL (P = 0.03) and the AUC(0-8) from 6.25 +/- 1.8 to 8.04 +/- 2.22 ng x h/mL (P = 0.02) in 12 subjects. Digoxin oral clearance decreased from 149 +/- 101 mL/h x kg to 105 +/- 57 mL/h x kg (P = 0.04). Other digoxin pharmacokinetic parameter values, including renal clearance, were unaffected by ritonavir. Overall, 75% (9/12) of subjects had higher concentrations of digoxin after ritonavir administration. The majority of subjects were heterozygous at position 3435 (C/T) (6 subjects) and position 2677 (G/T,A) (7 subjects); although data are limited, the effect of ritonavir on digoxin pharmacokinetics appears to occur across all tested MDR1 genotypes. Concomitant low-dose ritonavir reduced the nonrenal clearance of digoxin, thereby increasing its systemic availability. The most likely mechanism for this interaction is ritonavir-associated inhibition of P-gp. Thus, ritonavir can alter the pharmacokinetics of coadministered medications that are P-gp substrates.

Antiretrovirals, Part 1: Overview, History, and Focus on Protease Inhibitors

This column is the first in a series on HIV/AIDS antiretroviral drugs. This first review summarizes the history of HIV/AIDS and the development of highly active antiretroviral therapy (HAART) and highlights why it is important for non-HIV specialists to know about these drugs. There are four broad classes of HIV medications used in varying combinations in HAART: the protease inhibitors, nucleoside analogue reverse transcriptase inhibitors, the non-nucleoside reverse transcriptase inhibitors, and cell membrane fusion inhibitors. This paper reviews the mechanism of action, side effects, toxicities, and drug interactions of the protease inhibitors.

The effect of quinidine, used as a probe for the involvement of P-glycoprotein, on the intestinal absorption and pharmacodynamics of methadone

AIMS

There is considerable unexplained interindividual variability in the methadone dose-effect relationship. The efflux pump P-glycoprotein (P-gp) regulates brain access and intestinal absorption of many drugs. Evidence suggests that methadone is a P-gp substrate in vitro, and P-gp affects methadone analgesia in animals. However the role of P-gp in human methadone disposition and pharmacodynamics is unknown. This investigation tested the hypothesis that the intestinal absorption and pharmacodynamics of oral and intravenous methadone are greater after inhibition of intestinal and brain P-gp, using the P-gp inhibitor quinidine as an in vivo probe.

METHODS

Two randomized, double-blind, placebo-controlled, balanced crossover studies were conducted in healthy subjects. Pupil diameters and/or plasma concentrations of methadone and the primary metabolite EDDP were measured after 10 mg intravenous or oral methadone HCl, dosed 1 h after oral quinidine (600 mg) or placebo.

RESULTS

Quinidine did not alter the effects of intravenous methadone. Miosis t(max) (0.3 +/- 0.3 vs 0.3 +/- 0.2 h (-0.17, 0.22)), peak (5.3 +/- 0.8 vs 5.1 +/- 1.0 mm (0.39, 0.84)) and AUC vs time (25.0 +/- 5.7 vs 26.8 +/- 7.1 mm h (-6.1, 2.5)) were unchanged (placebo vs quinidine (95% confidence interval on the difference)). Quinidine increased (P < 0.05) plasma methadone concentrations during the absorptive phase, decreased t(max) (2.4 +/- 0.7 vs 1.6 +/- 0.9 h (0.33, 1.2)), and increased peak miosis (3.2 +/- 1.5 vs 4.3 +/- 1.6 mm (-1.96, -0.19)) after oral methadone. The C(max) (55.6 +/- 10.3 vs 59.4 +/- 14.1 ng ml(-1) (-8.5, 0.65)) and AUC of methadone (298 +/- 46 vs 316 +/- 74 ng ml(-1) h (-54, 18)) were unchanged, as were the EDDP : methadone AUC ratios. Quinidine had no effect on the rate constant for transfer of methadone between plasma and effect compartment (k(e0)) (2.6 +/- 2.6 vs 2.5 +/- 1.4 h(-1) (-3.5, 4.2)).

CONCLUSIONS

Quinidine increased the plasma concentrations of oral methadone in the absorptive phase and the miosis caused by methadone, suggesting that intestinal P-gp affects oral methadone absorption and hence its clinical effects. Quinidine had no effect on methadone pharmacodynamics after intravenous administration, suggesting that if quinidine is an effective inhibitor of brain P-gp, then P-gp does not appear to be a determinant of the access of methadone to the brain.

Cellular issues relating to the resistance of HIV to antiretroviral agents

It has been proposed that the declining efficiency of antiretroviral agents in human immunodeficiency virus (HIV) infection may also depend on cellular factors at their site of action. Two in particular have been proposed: (i) the defective intracellular metabolism of NRTI in target cells and the altered uptake; and (ii) efflux of nucleoside reverse transcriptase inhibitors (NRTI) and protease inhibitors (PI) by cellular transporter molecules. Several studies have shown that: changes in the activities of various purine and pyrimidine biosynthetic enzymes may occur in lymphocytes of HIV-infected patients; HIV-infected patients on prolonged treatment with nucleoside analogues, e.g. zidovudine, show significantly decreased activity of thymidine kinase (TK) compared with untreated HIV-infected people; and NRTI and PI are substrates for the multidrug membrane transporters. With regard to the latter issue, it is known that the ATP-binding cassette transporter proteins such as the P-glycoprotein (MDR), and the newly discovered family of multidrug resistance-associated proteins (MRP1-6), promote the active extracellular efflux of a wide variety of therapeutics drugs and overexpression of some of them lowers intracellular concentration of PI. In the very near future such mechanisms, also called 'cellular drug resistance', might be taken into account, together with other immunological, virological and behavioural factors, to explain the 'drug failure' and/or the variability of response in HIV patients undergoing antiretroviral treatment.

Rifampicin and verapamil induce the expression of P-glycoprotein in vivo in Ehrlich ascites tumor cells

The effect of an in vivo treatment with two commonly employed drugs that are P-glycoprotein substrates, verapamil and rifampicin, on Ehrlich ascites carcinoma cells, was evaluated. Ehrlich ascites carcinoma cells were inoculated i.p. in CD-1 mice and animals were orally treated for 10 days with rifampicin (60 mg/kg/day) or verapamil (6 mg/kg/day). In the harvested cells the transcripts for mdr1a and mrp1, but not those for mdr1b, mrp2 and CYP3A, were detected, and treatment with verapamil or rifampicin did not modify the levels of the transcripts. On the contrary, an increased expression of P-glycoprotein was observed at the protein level with Western blot. The intracellular uptake of doxorubicin, a P-glycoprotein and MRP substrate, was significantly lower in cells obtained from treated animals in comparison with cells obtained from controls; in addition, the uptake was increased by a pretreatment with verapamil. The survival time of control animals implanted with untreated cells was similar to that of animals inoculated with cells obtained from rifampicin treated animals, however, the antineoplastic effect of doxorubicin was significanly higher in control animals. A treatment with rifampicin or verapamil in Ehrlich ascites tumor confers resistance to the antineoplastic drug doxorubicin, probably through an increased expression of P-glycoprotein.