P-Glycoprotein and transporter MRP1 reduce HIV protease inhibitor uptake in CD4 cells: potential for accelerated viral drug resistance?

Mitigating COVID-19 Mortality and Morbidity in China's Aging Population: A Focus on Available Medications and Future Developments

The COVID-19 pandemic, often referred to as the geropandemic, has put immense pressure on global healthcare systems worldwide, leading to a rush in the development and approval of medications for the treatment of the viral infection. Clinical trials on efficacy and safety had a limited spectrum on inclusion and endpoints because of the urgent need for fast results. The chronologically and biologically aged population is especially at risk for severe or lethal disease, as well as treatment-associated toxicity. In China, the growing elderly population segment has been a focus in public health measurements of COVID-19, guiding towards herd immunity with a mild variant, thus minimizing overall deaths and morbidity. While the COVID-19 pandemic has now been reclassified and the virus weakened, there is a clear need for novel therapies to protect the elderly. This paper reviews the current safety and efficacy of available COVID-19 medications in China, with a specific focus on 3CL protease inhibitors and the aging population. The current COVID wave in China has demonstrated a significant impact on the elderly and the need for new drugs that are effective at low doses and can be used alone, without harmful side effects, generation of viral resistance, and drug-drug interactions. The rush to develop and approve COVID-19 medications has brought up important questions about the balance between speed and caution, resulting in a pipeline of novel therapies now moving through clinical trials, including third-generation 3CL protease inhibitors. A majority of those therapeutics are being developed in China.

ABCB1, ABCG2, ABCC1, ABCC2, and ABCC3 drug transporter polymorphisms and their impact on drug bioavailability: what is our current understanding?

INTRODUCTION

Interindividual differences in drug response are a frequent clinical challenge partly due to variation in pharmacokinetics. ATP-binding cassette (ABC) transporters are crucial determinants of drug disposition. They are subject of gene regulation and drug-interaction; however, it is still under debate to which extend genetic variants in these transporters contribute to interindividual variability of a wide range of drugs.

AREAS COVERED

This review discusses the current literature on the impact of genetic variants in ABCB1, ABCG2 as well as ABCC1, ABCC2, and ABCC3 on pharmacokinetics and drug response. The aim was to evaluate if results from recent studies would increase the evidence for potential clinically relevant pharmacogenetic effects.

EXPERT OPINION

Although enormous efforts have been made to investigate effects of ABC transporter genotypes on drug pharmacokinetics and response, the majority of studies showed only weak if any associations. Despite few unique results, studies mostly failed to confirm earlier findings or still remained inconsistent. The impact of genetic variants on drug bioavailability is only minor and other factors regulating the transporter expression and function seem to be more critical. In our opinion, the findings on the so far investigated genetic variants in ABC efflux transporters are not suitable as predictive biomarkers.

Validation of a UHPLC-MS/MS Method to Quantify Twelve Antiretroviral Drugs within Peripheral Blood Mononuclear Cells from People Living with HIV

Recently, anti-HIV treatment has achieved high efficacy and tolerability. Nevertheless, few data are available about the intracellular penetration of antiretrovirals, partly due to the technical challenges related to intracellular quantification. This work aimed to validate an ultra-high performance liquid chromatography (UHPLC) tandem mass spectrometry (MS/MS) method for the simultaneous quantification of maraviroc, nevirapine, rilpivirine, dolutegravir, raltegravir, cobicistat, darunavir, ritonavir, atazanavir, efavirenz, elvitegravir, and etravirine within peripheral blood mononuclear cells (PBMCs) and apply it to samples from patients. PBMCs were isolated by density gradient on cell preparation tubes (CPT). Samples were prepared by addition of internal standards (IS), sonication, centrifugation, and drying. Reconstituted extracts underwent chromatographic separation by reversed phase UHPLC and detection was performed by electrospray ionization and multiple reaction monitoring. Method validation followed FDA and EMA guidelines, showing acceptable accuracy, precision, recovery and IS-normalized matrix effect. The application to 56 samples from patients undergoing antiretroviral treatment provided description of intracellular penetration, showing method eligibility for future studies.

Pharmacogenomics of Antiretroviral Drug Metabolism and Transport

Antiretroviral therapy has markedly reduced morbidity and mortality for persons living with human immunodeficiency virus (HIV). Individual tailoring of antiretroviral regimens has the potential to further improve the long-term management of HIV through the mitigation of treatment failure and drug-induced toxicities. While the mechanisms underlying anti-HIV drug adverse outcomes are multifactorial, the application of drug-specific pharmacogenomic knowledge is required in order to move toward the personalization of HIV therapy. Thus, detailed understanding of the metabolism and transport of antiretrovirals and the influence of genetics on these pathways is important. To this end, this review provides an up-to-date overview of the metabolism of anti-HIV therapeutics and the impact of genetic variation in drug metabolism and transport on the treatment of HIV. Future perspectives on and current challenges in pursuing personalized HIV treatment are also discussed.

Interactions of protease inhibitors atazanavir and ritonavir with ABCB1, ABCG2, and ABCC2 transporters: Effect on transplacental disposition in rats

Atazanavir and ritonavir are preferred protease inhibitors frequently used in combination antiretroviral therapy for prevention of HIV mother-to-child transmission. Although their use is associated with higher risk of congenital anomalies, factors affecting atazanavir and ritonavir placental transfer are not known. This study is the first attempt to evaluate whether the placental drug efflux ATP-binding cassette (ABC) transporters, p-glycoprotein (ABCB1), breast cancer resistance protein (ABCG2), and/or multidrug resistance-associated proteins 2 (ABCC2), affect placental pharmacokinetics of atazanavir or ritonavir. Transport experiments across MDCKII cells expressing respective human ABC carrier showed that atazanavir is a substrate of ABCB1 and dual perfusion studies in a rat placenta confirmed this finding. In conclusion, we suggest that placental ABCB1 might reduce ATV maternal-to-fetal transfer and therefore represent a site for pharmacokinetic drug-drug interactions of ATV. Further studies in human placenta models are necessary to provide additional data closer to clinical environment.

Understanding of human ATP binding cassette superfamily and novel multidrug resistance modulators to overcome MDR

Indeed, multi-drug resistance (MDR) is a significant obstacle to effective chemotherapy. The overexpression of ATP-binding cassette (ABC) membrane transporters is a principal cause of enhanced cytotoxic drug efflux and treatment failure in various types of cancers. At cellular level, the pumps of ABC family regulate the transportation of numerous substances including drugs in and out of the cells. In past, the overexpression of ABC pumps suggested a well-known mechanism of drug resistance in cancers as well as infectious diseases. In oncology, the search for new compounds for the inhibition of these hyperactive ABC pumps either genetically or functionally, growing interest to reverse multi-drug resistance and increase chemotherapeutic effects. Several ABC pump inhibitor/modulators has been explored to address the cancer associated MDR. However, the clinical results are still disappointing and conventional chemotherapies are constantly failed in successful eradication of MDR tumors. In this context, the structural and functional understanding of different ATP pumps is most important. In this concise review, we elaborated basic crystal structure of ABC transporter proteins as well as its critical elements such as different domains, motifs as well as some important amino acids which are responsible for ATP binding and drug efflux as well as demonstrated an ATP-switch model employed by various ABC membrane transporters. Furthermore, we briefly summarized different newly identified MDR inhibitors/modulators, deployed alone or in combination with cytotoxic agents to deal with MDR in different types of cancers.

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.

The Role of Eukaryotic and Prokaryotic ABC Transporter Family in Failure of Chemotherapy

Over the years chemotherapy failure has been a vital research topic as researchers have been striving to discover reasons behind it. The extensive studies carried out on chemotherapeutic agents confirm that resistance to chemotherapy is a major reason for treatment failure. “Resistance to chemotherapy,” however, is a comprehensive phrase that refers to a variety of different mechanisms in which ATP-binding cassette (ABC) mediated efflux dominates. The ABC is one of the largest gene superfamily of transporters among both eukaryotes and prokaryotes; it represents a variety of genes that code for proteins, which perform countless functions, including drug efflux – a natural process that protects cells from foreign chemicals. Up to date, chemotherapy failure due to ABC drug efflux is an active research topic that continuously provides further evidence on multiple drug resistance (MDR), aiding scientists in tackling and overcoming this issue. This review focuses on drug resistance by ABC efflux transporters in human, viral, parasitic, fungal and bacterial cells and highlights the importance of the MDR permeability glycoprotein being the mutual ABC transporter among all studied organisms. Current developments and future directions to overcome this problem are also discussed.

Placental ABC Transporters: Biological Impact and Pharmaceutical Significance

The human placenta fulfills a variety of essential functions during prenatal life. Several ABC transporters are expressed in the human placenta, where they play a role in the transport of endogenous compounds and may protect the fetus from exogenous compounds such as therapeutic agents, drugs of abuse, and other xenobiotics. To date, considerable progress has been made toward understanding ABC transporters in the placenta. Recent studies on the expression and functional activities are discussed. This review discusses the placental expression and functional roles of several members of ABC transporter subfamilies B, C, and G including MDR1/P-glycoprotein, the MRPs, and BCRP, respectively. Since placental ABC transporters modulate fetal exposure to various compounds, an understanding of their functional and regulatory mechanisms will lead to more optimal medication use when necessary in pregnancy.

Intracellular accumulation of Praziquantel in T lymphoblastoid cell lines, CEM (parental) and CEMVBL(P-gp-overexpressing)

Background

Praziquantel (PZQ) is an antihelminthic drug whose P-glycoprotein (P-gp) substrate specificity has not been conclusively characterized. We investigated its specificity by comparing its in vitro intracellular accumulation in CEM (parental), and CEMvbl cells which over express P-gp, a drug efflux transporter. Saquinavir (SQV), a known substrate of efflux transporters was used as control.

Methods

A reversed phase liquid chromatography method was developed to simultaneously quantify PZQ and SQV in cell culture media involving involved a liquid - liquid extraction followed by ultra-high performance liquid chromatography using a Hypurity C₁₈ column and ultraviolet detection set at a wavelength of 215 nm. The mobile phase consisted of ammonium formate, acetonitrile and methanol (57:38:5 v/v). Separation was facilitated via isocratic elution at a flow rate of 1.5 ml/min, with clozapine (CLZ) as internal standard. This was validated over the concentration range of 1.6 to 25.6 μM for all analytes. Intracellular accumulation of SQV in CEMvbl was significantly lower compared to that in CEM cells (0.1 ± 0.031 versus 0.52 ± 0.046, p = 0.03 [p <0.05]).

Results

Accumulation of PZQ in both cell lines cells were similar (0.05 ± 0.005 versus 0.04 ± 0.009, p = 0.4) suggesting that it is not a substrate of P-gp in CEM cells. In presence tariquidar, a known inhibitor of P-gp, the intracellular accumulation of SQV in CEMvbl cells increased (0.52 ± 0.068 versus 0.61 ± 0.102, p = 0.34 in CEM cells and 0.09 ± 0.015 versus 0.56 ± 0.089, p = 0.029 [p < 0.05] in CEMvbl cells). PZQ did not significantly affect the accumulation of SQV in either CEM (0.52 ± 0.068 versus 0.54 ± 0.061, p = 0.77), or in CEMvbl cells (0.09 ± 0.015 versus 0.1 ± 0.031, p = 0.89) cells compared to tariquidar, implying that PZQ is not an inhibitor of P-gp in CEMvbl cells.

Conclusions

PZQ is neither a substrate nor an inhibitor of the efflux drug transporter P-gp in T-lymphoblastoid cells, CEM and CEMvbl. We also report a simple, accurate and precise method for simultaneous quantification of PZQ and SQV.

P-glycoprotein (ABCB1) activity decreases raltegravir disposition in primary CD4+P-gphigh cells and correlates with HIV-1 viral load

Objectives

To evaluate the role of P-glycoprotein (P-gp) and multidrug-resistant-protein 1 (MRP1) on raltegravir intracellular drug disposition in CD4+ T cells, investigate the effect of HIV-1 infection on P-gp expression and correlate HIV-1 viraemia with P-gp activity in primary CD4+ T cell subsets.

Methods

The cellular accumulation ratio of [³H]raltegravir was quantified in CD4+ T cell lines overexpressing either P-gp (CEM-P-gp) or MRP1 (CEM-MRP1) and in primary CD3+CD4+ T cells with high (P-gp^high) and low P-gp activity (P-gp^low); inhibition of efflux transporters was confirmed by the intracellular retention of calcein-AM. The correlation of P-gp activity with HIV-1 viraemia was assessed in naive and memory T cell subsets from 21 HIV-1-infected treatment-naive subjects.

Results

[³H]Raltegravir cellular accumulation ratio decreased in CEM-P-gp cells (P < 0.0001). XR9051 (a P-gp inhibitor) and HIV-1 PIs reversed this phenomenon. Primary CD4+P-gp^high cells accumulated less raltegravir (38.4% ± 9.6%) than P-gp^low cells, whereas XR9051 also reversed this effect. In vitro HIV-1 infection of PBMCs and stimulation of CD4+ T cells increased P-gp mRNA and P-gp activity, respectively, while primary CD4+P-gp^high T cells sustained a higher HIV-1 replication than P-gp^low cells. A significant correlation between HIV-1 viraemia and P-gp activity was found in different CD4+ T cell subsets, particularly memory CD4+ T cells (r = 0.792, P < 0.0001).

Conclusions

Raltegravir is a substrate of P-gp in CD4+ T cells. Primary CD4+P-gp^high T cells eliminate intracellular raltegravir more readily than P-gp^low cells and HIV-1 viraemia correlates with P-gp overall activity. Specific CD4+P-gp^high T cell subsets could facilitate the persistence of viral replication in vivo and ultimately promote the appearance of drug resistance.

Drug transporters in tissues and cells relevant to sexual transmission of HIV: Implications for drug delivery

Efflux and uptake transporters of drugs are key regulators of the pharmacokinetics of many antiretroviral drugs. A growing body of literature has revealed the expression and functionality of multiple transporters in female genital tract (FGT), colorectal tissue, and immune cells. Drug transporters could play a significant role in the efficacy of preventative strategies for HIV-1 acquisition. Pre-exposure prophylaxis (PrEP) is a promising strategy, which utilizes topically (vaginally or rectally), orally or other systemically administered antiretroviral drugs to prevent the sexual transmission of HIV to receptive partners. The drug concentration in the receptive mucosal tissues and target immune cells for HIV is critical for PrEP effectiveness. Hence, there is an emerging interest in utilizing transporter information to explain tissue disposition patterns of PrEP drugs, to interpret inter-individual variability in PrEP drug pharmacokinetics and effectiveness, and to improve tissue drug exposure through modulation of the cervicovaginal, colorectal, or immune cell transporters. In this review, the existing literature on transporter expression, functionality and regulation in the transmission-related tissues and cells is summarized. In addition, the relevance of transporter function for drug delivery and strategies that could exploit transporters for increased drug concentration at target locales is discussed. The overall goal is to facilitate an understanding of drug transporters for PrEP optimization.

Ritonavir-boosted protease inhibitor based therapy: a new strategy in chronic hepatitis C therapy

Chronic hepatitis C virus (HCV) infection is a worldwide health issue. All oral therapies are quickly replacing peg-interferon-based treatment regimens. Developing effective, well tolerated, treatments accessible for difficult to treat populations remains an unmet need. Ritonavir, an HIV-1 protease inhibitor, has pharmacokinetic properties that enhance the activity of concomitantly administered direct acting antivirals against HCV. Ritonavir inhibits Cytochrome P450 isozyme 3A4, diminishing first pass effect and hepatic metabolism, changing the pharmacokinetic parameters of Cytochrome P450 isozyme 3A4 substrates. When combined with the HCV protease inhibitor paritaprevir, ritonavir increases mean area under the curve, allowing once daily dosing. While Phase II and III clinical trials with ritonavir-boosted paritaprevir, ombitasvir, and dasabuvir demonstrated high efficacy in those with HCV infection, drug-drug interactions warrant cautious use of ritonavir in specific patient populations. Consideration of the patients' full medication list is imperative due to the ubiquitous nature of the Cytochrome P450 isozyme 3A4 system.

In vitro activity of rifampicin and verapamil combination in multidrug-resistant mycobacterium tuberculosis

The aim of the present study was to evaluate the effect of the combination of rifampicin (RIF) and verapamil (VP) against the Mycobacterium tuberculosis H37Rv reference strain and six multidrug-resistant (MDR) M. tuberculosis clinical isolates by determining Time-Kill Curves and the ability to efflux drug by fluorometry. The RIF+VP combination showed synergism in one MDR clinical isolate. For the other five MDR clinical isolates, the drug combination showed no interaction. The MDR clinical isolate had lower ethidium bromide (EtBr) accumulation when exposed to the RIF+VP combination, compared with RIF and VP exposure alone. The other MDR clinical isolates showed no significant difference in EtBr accumulation. These results suggest greater efflux action in one of the MDR clinical isolates compared with the M. tuberculosis H37Rv reference strain. The other five MDR isolates may have additional mechanisms of drug resistance to RIF. The use of the RIF+VP combination made one MDR bacillus more susceptible to RIF probably by inhibiting efflux pumps, and this combination therapy, in some cases, may contribute to a reduction of resistance to RIF in M. tuberculosis.

Expression of breast cancer resistance protein in peripheral T cell subsets from HIV‑1‑infected patients with antiretroviral therapy

The aim of the present study was to investigate the expression of breast cancer resistance protein (BCRP) in peripheral T cell subsets of human immunodeficiency virus 1 (HIV‑1)‑infected patients, and to analyze the association between the levels of BCRP expression and disease progression in HIV‑1 infection. Peripheral blood mononuclear cells (PBMCs) were obtained from HIV‑1‑infected patients (n=118), including 92 patients with antiretroviral therapy (ART) and 26 patients without a history of ART. Control samples from 30 healthy donors were also analyzed. The expression levels of BCRP in T cells were evaluated by flow cytometry. A high inter‑individual variability was observed in CD4+ and CD8+ T cells in the HIV‑1‑infected patients and healthy donors; however, the analyzed expression levels of BCRP were significantly higher in the HIV‑1‑infected group with ART than those in the group with no history of ART (P<0.01). Furthermore, the frequency of BCRP‑expressing T cells was inversely correlated with CD4+ and CD8+ T cell counts in HIV‑1‑infected patients with ART. The results suggested that BCRP expression varied among HIV‑1‑infected patients and healthy donors but was significantly higher in HIV‑1 patients undergoing ART. In conclusion, the present study suggested that overexpression of BCRP may be involved in disease progression of the HIV‑1 infection and may participate in drug resistance to ART, thus contributing to the failure of highly active ART in HIV‑1 therapeutics.

Expression of six drug transporters in vaginal, cervical, and colorectal tissues: Implications for drug disposition in HIV prevention

Effective antiretroviral (ARV)-based HIV prevention strategies require optimizing drug exposure in mucosal tissues; yet factors influencing mucosal tissue disposition remain unknown. We hypothesized drug transporter expression in vaginal, cervical, and colorectal tissues is a contributing factor and selected 3 efflux (ABCB1/MDR1, ABCC2/MRP2, ABCC4/MRP4) and 3 uptake (SLC22A6/OAT1, SLC22A8/OAT3, SLCO1B1/OATP1B1) transporters to further investigate based on their affinity for 2 ARVs central to prevention (tenofovir, maraviroc). Tissue was collected from 98 donors. mRNA and protein expression were quantified using qPCR and immunohistochemistry (IHC). Hundred percent of tissues expressed efflux transporter mRNA. IHC localized them to the epithelium and/or submucosa. Multivariable analysis adjusted for age, smoking, and co-medications revealed significant (P < 0.05) differences in efflux transporter mRNA between tissue types (vaginal ABCB1 3.9-fold > colorectal; vaginal ABCC2 2.9-fold > colorectal; colorectal ABCC4 2.0-fold > cervical). In contrast, uptake transporter mRNA was expressed in <25% of tissues. OAT1 protein was detected in 0% of female genital tissues and in 100% of colorectal tissues, but only in rare epithelial cells. These data support clinical findings of higher maraviroc and tenofovir concentrations in rectal tissue compared to vaginal or cervical tissue after oral dosing. Quantifying mucosal transporter expression and localization can facilitate ARV selection to target these tissues.

P-glycoprotein and drug resistance in systemic autoimmune diseases

Autoimmune diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and psoriatic arthritis (PsA) are chronic inflammatory disorders of unknown etiology characterized by a wide range of abnormalities of the immune system that may compromise the function of several organs, such as kidney, heart, joints, brain and skin. Corticosteroids (CCS), synthetic and biologic immunosuppressive agents have demonstrated the capacity to improve the course of autoimmune diseases. However, a significant number of patients do not respond or develop resistance to these therapies over time. P-glycoprotein (P-gp) is a transmembrane protein that pumps several drugs out of the cell, including CCS and immunosuppressants; thus, its over-expression or hyper-function has been proposed as a possible mechanism of drug resistance in patients with autoimmune disorders. Recently, different authors have demonstrated that P-gp inhibitors, such as cyclosporine A (CsA) and its analogue Tacrolimus, are able to reduce P-gp expression and or function in SLE, RA and PsA patients. These observations suggest that P-gp antagonists could be adopted to revert drug resistance and improve disease outcome. The complex inter-relationship among drug resistance, P-gp expression and autoimmunity still remains elusive.

Reversal of multidrug resistance by the inhibition of ATP-binding cassette pumps employing "Generally Recognized As Safe" (GRAS) nanopharmaceuticals: A review

Pumps of the ATP-binding cassette superfamily (ABCs) regulate the access of drugs to the intracellular space. In this context, the overexpression of ABCs is a well-known mechanism of multidrug resistance (MDR) in cancer and infectious diseases (e.g., viral hepatitis and the human immunodeficiency virus) and is associated with therapeutic failure. Since their discovery, ABCs have emerged as attractive therapeutic targets and the search of compounds that inhibit their genetic expression and/or their functional activity has gained growing interest. Different generations of pharmacological ABC inhibitors have been explored over the last four decades to address resistance in cancer, though clinical results have been somehow disappointing. "Generally Recognized As Safe" (GRAS) is a U.S. Food and Drug Administration designation for substances that are accepted as safe for addition in food. Far from being "inert", some amphiphilic excipients used in the production of pharmaceutical products have been shown to inhibit the activity of ABCs in MDR tumors, emerging as a clinically translatable approach to overcome resistance. The present article initially overviews the classification, structure and function of the different ABCs, with emphasis on those pumps related to drug resistance. Then, the different attempts to capitalize on the activity of GRAS nanopharmaceuticals as ABC inhibitors are discussed.

Microglial activation decreases retention of the protease inhibitor saquinavir: implications for HIV treatment

Background

Active HIV infection within the central nervous system (CNS) is confined primarily to microglia. The glial cell compartment acts as a viral reservoir behind the blood-brain barrier. It provides an additional roadblock to effective pharmacological treatment via expression of multiple drug efflux transporters, including P-glycoprotein. HIV/AIDS patients frequently suffer bacterial and viral co-infections, leading to deregulation of glial cell function and release of pro-inflammatory mediators including cytokines, chemokines, and nitric oxide.

Methods

To better define the role of inflammation in decreased HIV drug accumulation into CNS targets, accumulation of the antiretroviral saquinavir was examined in purified cultures of rodent microglia exposed to the prototypical inflammatory mediator lipopolysaccharide (LPS).

Results

[³H]-Saquinavir accumulation by microglia was rapid, and was increased up to two-fold in the presence of the specific P-glycoprotein inhibitor, PSC833. After six or 24 hours of exposure to 10 ng/ml LPS, saquinavir accumulation was decreased by up to 45%. LPS did not directly inhibit saquinavir transport, and did not affect P-glycoprotein protein expression. LPS exposure did not alter RNA and/or protein expression of other transporters including multidrug resistance-associated protein 1 and several solute carrier uptake transporters.

Conclusions

The decrease in saquinavir accumulation in microglia following treatment with LPS is likely multi-factorial, since drug accumulation was attenuated by inhibitors of NF-κβ and the MEK1/2 pathway in the microglia cell line HAPI, and in primary microglia cultures from toll-like receptor 4 deficient mice. These data provide new pharmacological insights into why microglia act as a difficult-to-treat viral sanctuary site.

Developmental regulation of P-glycoprotein activity within thymocytes results in increased anti-HIV protease inhibitor activity

The thymus harbors HIV-1 and supports its replication. Treatment with PI-containing ART restores thymic output of naïve T cells. This study demonstrates that CXCR4-using WT viruses are more sensitive to PI in fetal thymcocytes than mature T cells with average IC(50) values for two PIs, RTV and IDV, of 1.5 nM (RTV) and 4.4 nM (IDV) in thymocytes versus 309.4 nM (RTV) and 27.3 nM (IDV) in mature T cells. P-gp activity, as measured using Rh123 efflux and quantitation of P-gp mRNA, increased with thymocyte maturation into CD4 and CD8 lineage T cells. P-gp activity is developmentally regulated in the thymus. Thymocytes developed increased levels of P-gp activity as maturation from DP to SP CD4 or CD8 T cells occurred, although CD4 T cells acquired activity more rapidly. Reduced P-gp activity in thymocytes is one mechanism for effectiveness of PI therapy in suppressing viral replication in the thymus and in reconstitution of naïve T cells, particularly among children receiving PI-containing ART.

Potential of novel antiretrovirals to modulate expression and function of drug transporters in vitro

OBJECTIVES

The chemokine receptor antagonists maraviroc and vicriviroc and the integrase inhibitors elvitegravir and raltegravir are novel antiretroviral agents for the treatment of HIV-1 infections. ATP-binding cassette (ABC) transporters as modulators of the effectiveness and safety of therapy can mediate viral resistance and drug-drug interactions. To expand knowledge on drug-drug interactions of these antiretrovirals we investigated whether these compounds are substrates, inhibitors or inducers of important ABC transporters.

METHODS

We evaluated P-glycoprotein (P-gp/ABCB1) inhibition by the calcein assay in P388/dx and L-MDR1 cells, breast cancer resistance protein (BCRP/ABCG2) inhibition in MDCKII-BCRP cells by pheophorbide A efflux, and inhibition of the multidrug resistance-associated protein 2 (MRP2/ABCC2) by using the MRP2 PREDIVEZ™ Vesicular Transport Kit. Substrate characteristics were evaluated by growth inhibition assays in MDCKII cells overexpressing particular ABC transporters. Induction of transporters was quantified by real-time RT-PCR in LS180 cells and for ABCB1 also at the functional level.

RESULTS

Elvitegravir and vicriviroc inhibited ABCB1 in P388/dx and L-MDR1 cells (f2 values 1.9±0.2 µmol/L and 8.5±3.6 µmol/L, respectively). The IC50 for ABCG2 inhibition was 15.7±5.7 µmol/L for elvitegravir and 236.7±93.3 µmol/L for vicriviroc. Raltegravir and maraviroc showed no evidence of ABCB1 or ABCG2 inhibition. Maraviroc and vicriviroc stimulated ABCC2 transport function. Growth inhibition assays suggest that elvitegravir, raltegravir and vicriviroc are substrates of ABCB1. Induction assays demonstrate that mRNA expression of several ABC transporters is induced by these antiretrovirals in LS180 cells.

CONCLUSIONS

The new antiretrovirals bear the potential to modulate expression and function of several ABC transporters, with elvitegravir revealing the highest interaction potential.

Effect of tipranavir/ritonavir combination on the pharmacokinetics of tadalafil in healthy volunteers

This study evaluated the effects of single-dose administration and steady-state concentrations of tipranavir 500 mg and ritonavir 200 mg (TPV/r) combination on the pharmacokinetics of tadalafil 10 mg (TAD) in an open-label study. Seventeen healthy male volunteers received sequential dosing of the studied product: TAD (day 1) alone in a single dose for 7 days followed by TAD (day 8) in a single dose with TPV/r (500/200 mg twice daily, days 8-18). Pharmacokinetic parameters were determined in a noncompartmental analysis. The geometric mean ratio and 90% confidence interval were used to evaluate drug interactions. The effect of a single dose of TAD on the pharmacokinetics of TPV/r resulted in a small decrease in exposure after either first-dose or steady-state TPV/r (geometric mean ratios [90% confidence interval]: area under the concentration-time curve, 0.85 [0.74-0.97]). In contrast, coadministration of TAD exposure was increased significantly (2.33 [2.02-2.69]) when administered with the first dose of TPV/r but not when TPV/r steady state was reached (1.01 [0.83-1.21]). Antiretroviral activity may not be reduced, but the dose of TAD should be reduced at the start of TPV/r therapy and then a full dose can be resumed after steady state is reached.

P-glycoprotein: tissue distribution, substrates, and functional consequences of genetic variations

P-glycoprotein (ABCB1, MDR1) belongs to the ABC transporter family transporting a wide range of drugs and xenobiotics from intra- to extracellular at many biological interfaces such as the intestine, liver, blood-brain barrier, and kidney. The ABCB1 gene is highly polymorphic. Starting with the observation of lower duodenal protein expression and elevated digoxin bioavailability in relation to the 3435C>T single nucleotide polymorphism, hundreds of pharmacokinetic and outcome studies have been performed, mostly genotyping 1236C>T, 2677G>T/A, and 3435C>T. Though some studies pointed out that intracellular concentrations of anticancer drugs, for example, within lymphocytes, might be affected by ABCB1 variants resulting in differential outcome, current knowledge of the functional significance genetic variants of ABC membrane transporters does not allow selection of a particular SNP to predict an individual's pharmacokinetics.

Interaction potential of etravirine with drug transporters assessed in vitro

Etravirine is a novel nonnucleoside reverse transcriptase inhibitor (NNRTI) for the treatment of HIV-1 infections. ABC transporters potentially mediate clinically relevant drug-drug interactions. We assessed substrate characteristics and the inhibitory and inductive potential of etravirine on ABC transporters. Etravirine did not inhibit P-gp/ABCB1 and was not transported by the tested ABC transporters but was a potent inhibitor of BCRP/ABCG2. Etravirine induced several ABC transporters, especially BCRP/ABCG2. These data demonstrate that etravirine has the potential for drug-drug interactions by modulation of expression and function of several ABC transporters.

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.

No evidence for induction of ABC transporters in peripheral blood mononuclear cells in humans after 14 days of efavirenz treatment

Intracellular concentrations of antiretroviral drugs in peripheral blood mononuclear cells (PBMCs) are an important determinant of therapeutic success. In vitro data indicate that efavirenz induces several ATP-binding cassette (ABC) transporters, and pharmacogenetic studies found an association between ABCB1(C3435T) and efavirenz exposure and between this polymorphism and improved virological outcomes. We therefore aimed to clarify whether efavirenz also induces ABC transporters in vivo in PBMCs and whether intracellular concentrations might be altered after induction. Twelve healthy individuals received multiple oral doses of efavirenz over 14 days (400 mg once daily). Blood samples were drawn on study days 1 (single dose) and 14 (multiple dose), and efavirenz concentrations were analyzed by liquid chromatography-tandem mass spectrometry. Expression of P glycoprotein (P-gp) and of the multidrug resistance-associated proteins 1 and 2 as well as P-gp activity was analyzed in PBMCs on day 1 and day 14 using real-time reverse transcription-PCR (RT-PCR) and rhodamine 123 efflux. Although a clear autoinduction could be confirmed by a significant decrease of efavirenz exposure from day 1 to day 14, efavirenz did not change expression of the ABC transporters or P-gp activity in PBMCs. Moreover, intracellular concentrations of efavirenz were 1.3- to 1.8-fold higher than the corresponding plasma concentrations, and the intracellular/plasma concentration ratio remained constant during the treatment and did not correlate with ABC transporter expression or function. In conclusion, our study confirmed that intracellular concentrations of efavirenz are independent from these efflux transporters and demonstrated for the first time that the transporters are not induced in PBMCs in vivo after 2 weeks of treatment with efavirenz.

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.

Modulation of endocrine and transport functions in human trophoblasts by saquinavir and nelfinavir

OBJECTIVES

The distribution of drugs to the maternal-fetal interface is influenced by the expression of various efflux transporters. Among these transporters, P-glycoprotein (P-gp) is responsible for the efflux of a great number of drugs such as protease inhibitors of the human immunodeficiency virus, thus reducing the chemical exposure of the fetus.

STUDY DESIGN

The effects of saquinavir and nelfinavir were evaluated on human trophoblast functions and integrity by investigating their effect on human chorionic gonadotropin (hCG) secretion and on P-gp expression and functionality.

RESULTS

Nelfinavir significantly reduced hCG secretion by 30% after a 48-h treatment but it had no effect on syncytia formation. Saquinavir had no effect on hCG secretion but significantly increased both expression (to a 2-fold extent) and functionality (by 17.9%) of P-gp, whereas nelfinavir only increased functionality (by 23.1%) with a dissociation of P-gp from caveolin-1.

CONCLUSION

These results suggest that the effects of saquinavir and nelfinavir differ on trophoblast functions.

Protease inhibitors atazanavir, lopinavir and ritonavir are potent blockers, but poor substrates, of ABC transporters in a broad panel of ABC transporter-overexpressing cell lines

OBJECTIVES

A possible mechanism for HIV therapy failure is the efflux of HIV drugs from viral target cells or certain body compartments by ATP-binding cassette (ABC) transporters, allowing ongoing viral replication. Here, we investigated the interaction between protease inhibitors (PIs) and ABC transporters.

METHODS

To explore the potential blocking capacity of PIs, we exposed cells overexpressing multidrug resistance 1 P-glycoprotein (MDR1 P-gp), multidrug resistance protein 1 (MRP1) and breast cancer resistance protein (BCRP) to established cytotoxic substrates with or without one of the PIs atazanavir, lopinavir or ritonavir. Furthermore, to assess whether PIs serve as substrates, cell growth-inhibitory effects of these PIs were evaluated on cells overexpressing 1 of 11 ABC transporters and their parental counterparts.

RESULTS

Atazanavir, lopinavir and ritonavir were highly effective in reversing resistance against established substrates in cells overexpressing MDR1 P-gp and MRP1, and, to a lesser extent, BCRP. Concurrently, however, PIs appeared to be relatively poor substrates for ABC transporters. Only a moderate level of resistance to atazanavir was observed in cells overexpressing MRP6 and MRP9 [resistance factor (RF): 2.0-2.6]. Cells overexpressing MDR1 P-gp, MRP3, MRP4 and MRP5 displayed low levels of resistance to atazanavir (RF: 1.3-1.7); MRP7- and MRP9-overexpressing cells to lopinavir (RF: 1.4-1.5); and MRP9-overexpressing cells to ritonavir (RF: 1.4).

CONCLUSIONS

PIs can act as potent blockers of MDR1 P-gp, MRP1 and BCRP, but they are poor substrates for 11 ABC transporters. Consequently, ABC transporters are unlikely to play a major role in PI failure, but still may contribute to drug-specific adverse events and drug-drug interactions.

Impact of ATP-binding cassette transporters on human immunodeficiency virus therapy

Even though potent antiretrovirals are available against human immunodeficiency virus (HIV)-1 infection, therapy fails in a significant fraction of patients. Among the most relevant reasons for treatment failure are drug toxicity and side effects, but also the development of viral resistance towards the drugs applied. Efflux by ATP-binding cassette (ABC-) transporters represents one major mechanism influencing the pharmacokinetics of antiretroviral drugs and particularly their distribution, thus modifiying the concentration within the infected cells, that is, at the site of action. Moreover, drug-drug interactions may occur at the level of these transporters and modulate their activity or expression thus influencing the efficacy and toxicity of the substrate drugs. This review summarizes current knowledge on the interaction of antiretrovirals used for HIV-1 therapy with ABC-transporters and highlights the impact of ABC-transporters for cellular resistance and therapeutic success. Moreover, the suitability of different cell models for studying the interaction of antiretrovirals with ABC-transporters is discussed.

Nanotechnology applications for improved delivery of antiretroviral drugs to the brain

Human immunodeficiency virus (HIV) can gain access to the central nervous system during the early course of primary infection. Once in the brain compartment the virus actively replicates to form an independent viral reservoir, resulting in debilitating neurological complications, latent infection and drug resistance. Current antiretroviral drugs (ARVs) often fail to effectively reduce the HIV viral load in the brain. This, in part, is due to the poor transport of many ARVs, in particular protease inhibitors, across the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSBF). Studies have shown that nanocarriers including polymeric nanoparticles, liposomes, solid lipid nanoparticles (SLN) and micelles can increase the local drug concentration gradients, facilitate drug transport into the brain via endocytotic pathways and inhibit the ATP-binding cassette (ABC) transporters expressed at the barrier sites. By delivering ARVs with nanocarriers, significant increase in the drug bioavailability to the brain is expected to be achieved. Recent studies show that the specificity and efficiency of ARVs delivery can be further enhanced by using nanocarriers with specific brain targeting, cell penetrating ligands or ABC-transporters inhibitors. Future research should focus on achieving brain delivery of ARVs in a safe, efficient, and yet cost-effective manner.

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.

Organic cation transporters and their roles in antiretroviral drug disposition

BACKGROUND

Understanding metabolising processes and drug-transporter interactions is particularly crucial to the management of patients with HIV infection, given the several antiretroviral drugs that must be taken lifelong and the use of other medications for HIV-related and non-HIV-related conditions. Several interactions of antiretroviral drugs with metabolising enzymes, especially cytochrome P450 and ATP-dependent transporter P-glycoprotein, have been described but the role of the organic cation transporters (OCTs) is less clearly defined.

OBJECTIVE

To review the relevance of the OCTs for antiretroviral drug disposition.

METHODS

Interactions of OCTs with antiretroviral drugs and evidence for clinical relevance are discussed.

RESULTS/CONCLUSION

Several antiretroviral drugs show relevant interactions with the OCTs in cell-based experiments and the OCTs are highly upregulated in HIV-infected patients. For evaluating the clinical significance, interaction studies in HIV patients and reliable in vitro models for delineation of in vivo effects are needed.

P-glycoprotein, multidrug resistance-associated proteins and human organic anion transporting polypeptide influence the intracellular accumulation of atazanavir

BACKGROUND

Drug efflux (for example, P-glycoprotein [P-gp], multidrug resistance-associated proteins [MRPs] and breast cancer resistance protein [BCRP]) and influx (for example, human organic anion transporting polypeptide [hOCTP] or human organic anion transporting polypeptide [hOATP]) transporters alter the cellular concentrations of some HIV protease inhibitors (HPIs). Here, we studied the lipophilicity and uptake of [(3)H]-atazanavir (ATV) in CEM (parental), CEM(VBL) (P-gp-overexpressing), CEM(E1000) (MRP1-overexpressing) and peripheral blood mononuclear cells (PBMCs), and evaluate the effects of modulators of drug transporters on uptake.

METHODS

Lipophilicity was measured by octanol/saline partition method. The influence of influx/efflux transporters on uptake was evaluated in the absence and presence of inhibitors of P-gp (GPV031), P-gp/BCRP (tariquidar and GF120918), P-gp/MRP1 (dipyridamole and daidzein), MRP1/2 (frusemide and genistein), hOATP/hOCTP (estrone-3-sulfate [E-3-S]) and hOATP/hOCTP/MRP (probenecid). The effects of a number of HPIs on uptake were also evaluated. Data from digitonin permeabilized cells allowed the evaluation of the contribution of cellular binding to total drug uptake, whereas the inhibitory effect of ATV on P-gp was assessed by daunomycin efflux/uptake assays.

RESULTS

[(3)H]-ATV is lipophilic and accumulates in the cultured cells as follows: CEM>CEM(E1000)>CEM(VBL). Tariquidar, GF120918 and daidzein significantly increased the uptake of [(3)H]-ATV in the cultured cells. By contrast, only daidzein and tipranavir significantly increased uptake in PBMCs, with tariquidar and frusemide devoid of effects, whereas dipyridamole, E-3-S, GPV031 and genistein significantly decreased accumulation. ATV inhibits P-gp activity; manipulation of uptake with digitonin suggests binding of [(3)H]-ATV to P-gp.

CONCLUSIONS

[(3)H]-ATV is lipophilic, a P-gp, MRP and hOATP substrate and an inhibitor of P-gp. Concomitant administration of ATV with drugs and dietary components (for example, daidzein and genistein) that interact with these transporters could alter its pharmacokinetics.

Identification of a potential pharmacological sanctuary for HIV type 1 in a fraction of CD4(+) primary cells

We have identified a subset of HIV-susceptible CD4(+)CCR5(+) cells in human PBMCs that can efficiently exclude protease inhibitors (PI) due to high P-glycoprotein (P-gp) efflux activity. Phenotypically these cells are heterogeneous, include both T and non-T cells, and some display markers of memory cells. Cells with high P-gp represent 16-56% (median = 37.3) of all CD4(+)CCR5(+) cells in healthy donors, and are selectively depleted in HIV-1-infected individuals (4.1-33%, median = 10.1). A fraction of primary cells productively infected by HIV-1, in vitro, have high P-gp pump activity, demonstrating that infection does not inhibit P-gp function. In agreement with these data, HIV-susceptible cells expressing high levels of P-gp require higher levels of PI for complete inhibition of virus spread. We conclude that the PI concentrations achieved in plasma could be suboptimal for full inhibition of virus spread in high P-gp cells, indicating that they may represent a pharmacological sanctuary for HIV-1.

Oral cyclosporin A inhibits CD4 T cell P-glycoprotein activity in HIV-infected adults initiating treatment with nucleoside reverse transcriptase inhibitors

PURPOSE

P-glycoprotein limits the tissue penetration of many antiretroviral drugs. The aim of our study was to characterize the effects of the P-glycoprotein substrate cyclosporin A on T cell P-glycoprotein activity in human immunodeficiency virus-infected participants in the AIDS Clinical Trials Group study A5138.

METHODS

We studied P-glycoprotein activity on CD4 and CD8 T cells in 16 participants randomized to receive oral cyclosporin A (n=9) or not (n=7) during initiation antiretroviral therapy (ART) that did not include protease or non-nucleoside reverse transcriptase inhibitors.

RESULTS

CD4 T cell P-glycoprotein activity decreased by a median of 8 percentage points with cyclosporin A/ART (difference between cyclosporin A/ART vs. ART only, P= 0.001). Plasma trough cyclosporin A concentrations correlated with the change in P-glycoprotein activity in several T cell subsets.

CONCLUSIONS

Oral cyclosporin A can inhibit peripheral blood CD4 T cell P-glycoprotein activity. Targeted P-glycoprotein inhibition may enhance the delivery of ART to T cells.

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.

Induction of multiple drug transporters by efavirenz

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.

Cultured CD4T cells and primary human lymphocytes express hOATPs: intracellular accumulation of saquinavir and lopinavir

BACKGROUND AND PURPOSE

Drug efflux tranporters (P-glycoprotein (P-gp), multidrug resistance-associated protein (MRP)) limit the cellular uptake of human immunodeficiency virus protease inhibitors but the contribution of influx transporters in cells that (over)express P-gp or MRP is less clear. Here, we studied the expression of one influx transporter system, human organic anion-transporting polypeptide (hOATP), in some T-cell lines (CEM, CEM(VBL), CEM(E1000)) and in peripheral blood mononuclear cells (PBMCs) and examined the effects of manipulation of influx/efflux transporters on the uptake of saquinavir and lopinavir.

EXPERIMENTAL APPROACH

The expression of hOATPs was studied by PCR. We used hOATP substrate or inhibitor (estrone-3-sulphate (E-3-S) or montelukast, respectively) and inhibitors of P-gp (XR9576) and MRP (MK571 and frusemide) to study functional interactions between influx and efflux transporters in the uptake of saquinavir and lopinavir. Lipophilicity of the drugs was measured by octanol/saline partition coefficient.

KEY RESULTS

CEM cells, their variants and PBMCs express various hOATP isoforms, with OATP3A1 detected in all of the cells. MK571, XR9576 and frusemide increased the uptake of saquinavir and lopinavir. E-3-S and montelukast reduced the uptake of saquinavir and lopinavir in some, but not all, of the cells. Pretreatment of the cells with MK571, XR9576 or frusemide, followed by E-3-S co-incubation reduced the cellular accumulation of saquinavir and lopinavir. Lopinavir is much more lipophilic than saquinavir.

CONCLUSIONS AND IMPLICATIONS

Human OATPs, MRP, P-gp and lipophilicity determine the cellular uptake and retention of saquinavir and lopinavir. These data may have important implications for drug-drug interactions, drug safety and efficacy.