Interaction between dolutegravir and folate transporters and receptor in human and rodent placenta

Background

Due to the critical role of folates in neurodevelopment, it is important to understand potential interactions between anti-HIV drugs used during pregnancy, and folate delivery pathways in the placenta. This study investigates the effect of dolutegravir (DTG) exposure on the functional expression of the reduced folate carrier (RFC), proton-coupled folate transporter (PCFT), and folate receptor-α (FRα) in the placenta.

Methods

Human placental cell lines, human placental explants, and a pregnant mouse model treated with clinically relevant concentrations of DTG were used. Gene and protein expression were assessed by qPCR, immunoblot and immunohistochemical assays. Folate transport function was measured by applying radioisotope-based transport assays.

Findings

In placental cells, clinically relevant DTG exposure for 3h or 6h was associated with a modest but significant reduction in the expression of RFC and PCFT both at the mRNA and protein levels, as well as decreased uptake of RFC and PCFT substrates [³H]-methotrexate and [³H]-folic acid, respectively. In pregnant mice, DTG administration was associated with an increase in both placental RFC and PCFT mRNA expression, accompanied by a decrease in placental FRα mRNA under folate-deficient dietary conditions.

Interpretation

These findings demonstrate a potential interaction between DTG and folate transport pathways in the placenta, particularly in vivo, under folate deficient conditions, potentially impacting folate delivery to the foetus in the context of DTG-based ART during pregnancy.

Funding

Funded by Ontario HIV Treatment Network, grant #506657; and Eunice Kennedy Shriver National Institute of Child Health & Human Development of the National Institutes of Health, award #R01HD104553.

Interactions of Potential Anti-COVID-19 Compounds with Multispecific ABC and OATP Drug Transporters

During the COVID-19 pandemic, several repurposed drugs have been proposed to alleviate the major health effects of the disease. These drugs are often applied with analgesics or non-steroid anti-inflammatory compounds, and co-morbid patients may also be treated with anticancer, cholesterol-lowering, or antidiabetic agents. Since drug ADME-tox properties may be significantly affected by multispecific transporters, in this study, we examined the interactions of the repurposed drugs with the key human multidrug transporters present in the major tissue barriers and strongly affecting the pharmacokinetics. Our in vitro studies, using a variety of model systems, explored the interactions of the antimalarial agents chloroquine and hydroxychloroquine; the antihelmintic ivermectin; and the proposed antiviral compounds ritonavir, lopinavir, favipiravir, and remdesivir with the ABCB1/Pgp, ABCG2/BCRP, and ABCC1/MRP1 exporters, as well as the organic anion-transporting polypeptide (OATP)2B1 and OATP1A2 uptake transporters. The results presented here show numerous pharmacologically relevant transporter interactions and may provide a warning on the potential toxicities of these repurposed drugs, especially in drug combinations at the clinic.

Effects of perfluoroalkyl carboxylic acids on the uptake of sulfobromophthalein via organic anion transporting polypeptides in human intestinal Caco-2 cells

We performed a detailed investigation of the uptake of sulfobromophthalein (BSP) from the apical membrane of Caco-2 cells, which is a substrate for organic anion transporting polypeptides (OATPs), and calculated the kinetic parameters of BSP uptake as follows: K_m = 13.9 ± 1.3 μM, V_max = 1.15 ± 0.07 nmol (mg protein)^-1 (5 min)^-1, and k_d = 38.2 ± 0.53 μL (mg protein)^-1 (5 min)^-1. Coincubation with medium-chain (C7-C11) perfluoroalkyl carboxylic acids (PFCAs), such as perfluoroheptanoic acid (PFHpA, C7), perfluorooctanoic acid (PFOA, C8), perfluorononanoic acid (PFNA, C9), perfluorodecanoic acid (PFDA, C10) and perfluoroundecanoic acid (PFUnDA, C11), significantly decreased BSP uptake by 27-55%, while coincubation with short- (C3-C6) and long-chain (C12-C14) PFCAs decreased the uptake only slightly. Dixon plotting suggested that PFOA, PFNA and PFDA competitively inhibited the BSP uptake with inhibition constant (K_i) values of 62.2 ± 1.3 μM, 35.3 ± 0.1 μM and 43.2 ± 0.3 μM, respectively. PFCAs with medium-chains could be substrates for OATPs, probably OATP2B1, which is the most abundantly expressed OATP isoform in Caco-2 cells.

HIV-1 Sanctuary Sites-the Role of Membrane-Associated Drug Transporters and Drug Metabolic Enzymes

Despite significant advances in the treatment of human immunodeficiency virus-1 (HIV) infection with highly active antiretroviral drug therapy, the persistence of the virus in cellular and anatomic reservoirs is a major obstacle preventing total HIV eradication. Viral persistence could result from a variety of contributing factors including, but not limited to, non-adherence to treatment and adverse drug reactions, latently infected cells carrying replication-competent virus, drug-drug interactions, and inadequate antiretroviral drug (ARV) concentrations reached in several anatomic sites such as the brain, testis, and gut-associated lymphoid tissues. The distribution of ARVs at specific sites of infection is primarily dependent on drug physicochemical properties and drug plasma protein binding, as well as drug efflux, influx, and metabolic processes. A thorough understanding of the functional roles of drug transporters and metabolic enzymes in the disposition of ARVs in immune cell types and tissues that are characterized as HIV reservoirs and sanctuaries is critical to overcome the challenge of suboptimal drug distribution at sites of persistent HIV infection. This review summarizes the current knowledge related to the expression and function of drug transporters and metabolic enzymes in HIV cellular and anatomic reservoirs, and their potential contribution to drug-drug interactions and insufficient drug concentration at these sites.

Pharmacokinetics, Placental and Breast Milk Transfer of Antiretroviral Drugs in Pregnant and Lactating Women Living with HIV

BACKGROUND

Remarkable progress has been achieved in the identification of HIV infection in pregnant women and in the prevention of vertical HIV transmission through maternal antiretroviral treatment (ART) and neonatal antiretroviral drug (ARV) prophylaxis in the last two decades. Millions of women globally are receiving combination ART throughout pregnancy and breastfeeding, periods associated with significant biological and physiological changes affecting the pharmacokinetics (PK) and pharmacodynamics (PD) of ARVs. The objective of this review was to summarize currently available knowledge on the PK of ARVs during pregnancy and transport of maternal ARVs through the placenta and into the breast milk. We also summarized main safety considerations for in utero and breast milk ARVs exposures in infants.

METHODS

We conducted a review of the pharmacological profiles of ARVs in pregnancy and during breastfeeding obtained from published clinical studies. Selected maternal PK studies used a relatively rich sampling approach at each ante- and postnatal sampling time point. For placental and breast milk transport of ARVs, we selected the studies that provided ratios of maternal to the cord (M:C) plasma and breast milk to maternal plasma (M:P) concentrations, respectively.

RESULTS

We provide an overview of the physiological changes during pregnancy and their effect on the PK parameters of ARVs by drug class in pregnancy, which were gathered from 45 published studies. The PK changes during pregnancy affect the dosing of several protease inhibitors during pregnancy and limit the use of several ARVs, including three single tablet regimens with integrase inhibitors or protease inhibitors co-formulated with cobicistat due to suboptimal exposures. We further analysed the currently available data on the mechanism of the transport of ARVs from maternal plasma across the placenta and into the breast milk and summarized the effect of pregnancy on placental and of breastfeeding on mammal gland drug transporters, as well as physicochemical properties, C:M and M:P ratios of individual ARVs by drug class. Finally, we discussed the major safety issues of fetal and infant exposure to maternal ARVs.

CONCLUSIONS

Available pharmacological data provide evidence that physiological changes during pregnancy affect maternal, and consequently, fetal ARV exposure. Limited available data suggest that the expression of drug transporters may vary throughout pregnancy and breastfeeding thereby possibly impacting the amount of ARV crossing the placenta and secreted into the breast milk. The drug transporter's role in the fetal/child exposure to maternal ARVs needs to be better understood. Our analysis underscores the need for more pharmacological studies with innovative study design, sparse PK sampling, improved study data reporting and PK modelling in pregnant and breastfeeding women living with HIV to optimize their treatment choices and maternal and child health outcomes.

Food, Acid Supplementation and Drug Absorption - a Complicated Gastric Mix: a Randomized Control Trial

PURPOSE

The purpose of this study was to determine the impact of food on gastric pH and the ability of over the counter betaine hydrochloride (BHCl) acid to reacidify gastric pH after food-induced elevations in gastric pH.

METHODS

This open-label cross over clinical study (NCT02758015) included 9 subjects who were randomly assigned to one of 16 possible, 4-period cross-over sequences to determine the impact and relationship of food and gastric pH with acid supplementation. Subjects were administered various doses (1500 mg, 3000 mg and 4500 mg) of betaine hydrochloride (BHCl) to determine the ability of acid supplementation to reacidify gastric pH after the elevation of gastric pH caused by the ingestion of food.

RESULTS

Following the administration of food and the resulting elevation in gastric pH, time to return to baseline gastric pH levels without acid supplementation was 49.7 ± 14.0 min. Administering 4500 mg of BHCl acid in capsules was able to reacidify gastric pH levels back to baseline following the administration of food in approximately 17.3 ± 5.9 min. AUC_pH of each treatment were similar and not statistically different. Mean max pH following the administration of food was 3.20 ± 0.55.

CONCLUSION

The ability of food to elevate and maintain gastric pH levels in the presence of acid supplementation was made evident throughout the study. A 4500 mg dose of BHCl was required to reacidify gastric pH after the administration of food. This study details the difficulty faced by clinicians in dosing a poorly soluble, weakly basic drug to patients receiving acid reducing agents where administration with food is recommended to avoid gastric side effects.

TRIAL REGISTRATION

https://clinicaltrials.gov/ct2/show/NCT02758015.

Drug Transport at the Brain and Endothelial Dysfunction in Preeclampsia: Implications and Perspectives

Transport of drugs across biological barriers has been a subject of study for decades. The discovery and characterization of proteins that confer the barrier properties of endothelia and epithelia, including tight junction proteins and membrane transporters belonging to the ATP-binding cassette (ABC) and Solute Carrier (SLC) families, represented a significant step forward into understanding the mechanisms that govern drug disposition. Subsequently, numerous studies, including both pre-clinical approaches and clinical investigations, have been carried out to determine the influence of physiological and pathological states on drug disposition. Importantly, there has been increasing interest in gaining a better understanding of drug disposition during pregnancy, since epidemiological and clinical studies have demonstrated that the use of medications by pregnant women is significant and this condition embodies a series of significant anatomical and physiological modifications, particularly at excretory organs and barrier sites (e.g., placenta, breast) expressing transporter proteins which influence pharmacokinetics. Currently, most of the research in this field has focused on the expression profiling of transporter proteins in trophoblasts and endothelial cells of the placenta, regulation of drug-resistance mechanisms in disease states and pharmacokinetic studies. However, little attention has been placed on the influence that the cerebrovascular dysfunction present in pregnancy-related disorders, such as preeclampsia, might exert on drug disposition in the mother’s brain. This issue is particularly important since recent findings have demonstrated that preeclamptic women suffer from long-term alterations in the integrity of the blood-brain barrier (BBB). In this review we aim to analyze the available evidence regarding the influence of pregnancy on the expression of transporters and TJ proteins in brain endothelial cells, as well the mechanisms that govern the pathophysiological alterations in the BBB of women who experience preeclampsia. Future research efforts should be focused not only on achieving a better understanding of the influence of preeclampsia-associated endothelial dysfunction on drug disposition, but also in optimizing the pharmacological treatments of women suffering pregnancy-related disorders, its comorbidities and to develop new therapies aiming to restore the integrity of the BBB.

Transmembrane Domain 1 of Human Organic Anion Transporting Polypeptide 2B1 Is Essential for Transporter Function and Stability

Organic anion transporting polypeptides (OATPs, gene symbol SLCO) are important membrane transporter proteins that mediate the uptake of wide ranges of endogenous and exogenous compounds. OATP2B1 has been found in multiple organs and tissues, including the liver, small intestine, kidney, brain, placenta, heart, skin, as well as skeletal muscle, and is proposed to be involved in the uptake of orally administered drugs. Quite a few reports have demonstrated that transmembrane domains (TMs) are crucial for proper functions of OATP family members. Comparative modeling proposed that TM1, along with TM2, 4, and 5 of the N-terminal half of OATP2B1, may be localized within the substrate interaction pocket and are important for uptake function of the transporter. Alanine scanning of the putative transmembrane domain 1 of OATP2B1 revealed that substitution of L58 with alanine dramatically altered the K_m value, and mutation of V52, H55, Q59, and L69 resulted in significantly reduced substrate turnover number, whereas A61V, Q62A, and S66A exhibited significant change in both K_m and V_max values. In addition, phenylalanine at position 51 seems to play an important role in maintaining proper folding of OATP2B1 because alanine replacement of F51 caused accelerated degradation of the transporter protein. Although proteasome and lysosome inhibitors could partially recover protein level, the mutant transporter remained nonfunctional. Taken together, the identification of nine essential amino acid residues within TM1 of OATP2B1 suggested that the transmembrane domain is important for maintaining proper function of the transporter.

Regulation of Reduced Folate Carrier (RFC) by Vitamin D Receptor at the Blood-Brain Barrier

Folates are essential for brain development and function. Folate transport in mammalian tissues is mediated by three major folate transport systems, i.e., reduced folate carrier (RFC), proton-coupled folate transporter (PCFT), and folate receptor alpha (FRα), known to be regulated by ligand-activated nuclear receptors, such as vitamin D receptor (VDR). Folate uptake at the choroid plexus, which requires the actions of both FRα and PCFT, is critical to cerebral folate delivery. Inactivating FRα or PCFT mutations cause severe cerebral folate deficiency resulting in early childhood neurodegeneration. The objective of this study was to investigate the role of RFC in folate uptake at the level of the blood-brain barrier (BBB) and its potential regulation by VDR. We detected robust expression of RFC in different in vitro BBB model systems, particularly in immortalized cultures of human cerebral microvascular endothelial cells (hCMEC/D3) and isolated mouse brain capillaries. [³H]-methotrexate uptake by hCMEC/D3 cells at pH 7.4 was inhibited by PT523 and pemetrexed, antifolates with high affinity for RFC. We also showed that activation of VDR through calcitriol (1,25-dihydroxyvitamin D₃) exposure up-regulates RFC mRNA and protein expression as well as function in hCMEC/D3 cells and isolated mouse brain capillaries. We further demonstrated that RFC expression could be down-regulated by VDR-targeting siRNA, further confirming the role of VDR in the direct regulation of this folate transporter. Together, these data suggest that augmenting RFC functional expression could constitute a novel strategy for enhancing brain folate delivery for the treatment of neurometabolic disorders caused by loss of FRα or PCFT function.

Single Nucleotide Polymorphisms in Cellular Drug Transporters Are Associated with Intolerance to Antiretroviral Therapy in Brazilian HIV-1 Positive Individuals

Adverse reactions are the main cause of treatment discontinuation among HIV+ individuals. Genes related to drug absorption, distribution, metabolism and excretion (ADME) influence drug bioavailability and treatment response. We have investigated the association between single nucleotide polymorphisms (SNPs) in 29 ADME genes and intolerance to therapy in a case-control study including 764 individuals. Results showed that 15 SNPs were associated with intolerance to nucleoside and 11 to non-nucleoside reverse transcriptase inhibitors (NRTIs and NNRTIs), and 8 to protease inhibitors (PIs) containing regimens under alpha = 0.05. After Bonferroni adjustment, two associations remained statistically significant. SNP rs2712816, at SLCO2B1 was associated to intolerance to NRTIs (OR_GA/AA = 2.37; p = 0.0001), while rs4148396, at ABCC2, conferred risk of intolerance to PIs containing regimens (OR_CT/TT = 2.64; p = 0.00009). Accordingly, haplotypes carrying rs2712816A and rs4148396T alleles were also associated to risk of intolerance to NRTIs and PIs, respectively. Our data reinforce the role of drug transporters in response to HIV therapy and may contribute to a future development of personalized therapies.

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.

Coadministration of ticagrelor and ritonavir: Toward prospective dose adjustment to maintain an optimal platelet inhibition using the PBPK approach

Ticagrelor is a potent antiplatelet drug metabolized by cytochrome (CYP)3A. It is contraindicated in patients with human immunodeficiency virus (HIV) because of the expected CYP3A inhibition by most protease inhibitors, such as ritonavir and an increased bleeding risk. In this study, a physiologically based pharmacokinetic (PBPK) model was created for ticagrelor and its active metabolite (AM). Based on the simulated interaction between ticagrelor 180 mg and ritonavir 100 mg, a lower dose of ticagrelor was calculated to obtain, when coadministered with ritonavir, the same pharmacokinetic (PK) and platelet inhibition as ticagrelor administered alone. A clinical study was thereafter conducted in healthy volunteers. Observed PK profiles of ticagrelor and its AM were successfully predicted with the model. Platelet inhibition was nearly complete in both sessions despite administration of a fourfold lower dose of ticagrelor in the second session. This PBPK model could be prospectively used to broaden the usage of ticagrelor in patients with ritonavir-treated HIV regardless of the CYP3A inhibition.

Potent and Selective Inhibition of Plasma Membrane Monoamine Transporter by HIV Protease Inhibitors

Plasma membrane monoamine transporter (PMAT) is a major uptake-2 monoamine transporter that shares extensive substrate and inhibitor overlap with organic cation transporters 1-3 (OCT1-3). Currently, there are no PMAT-specific inhibitors available that can be used in in vitro and in vivo studies to differentiate between PMAT and OCT activities. In this study, we showed that IDT307 (4-(4-(dimethylamino)phenyl)-1-methylpyridinium iodide), a fluorescent analog of 1-methyl-4-phenylpyridinium (MPP+), is a transportable substrate for PMAT and that IDT307-based fluorescence assay can be used to rapidly identify and characterize PMAT inhibitors. Using the fluorescent substrate-based assays, we analyzed the interactions of eight human immunodeficiency virus (HIV) protease inhibitors (PIs) with human PMAT and OCT1-3 in human embryonic kidney 293 (HEK293) cells stably transfected with individual transporters. Our data revealed that PMAT and OCTs exhibit distinct sensitivity and inhibition patterns toward HIV PIs. PMAT is most sensitive to PI inhibition whereas OCT2 and OCT3 are resistant. OCT1 showed an intermediate sensitivity and a distinct inhibition profile from PMAT. Importantly, lopinavir is a potent PMAT inhibitor and exhibited >120 fold selectivity toward PMAT (IC₅₀ = 1.4 ± 0.2 µM) over OCT1 (IC₅₀ = 174 ± 40 µM). Lopinavir has no inhibitory effect on OCT2 or OCT3 at maximal tested concentrations. Lopinavir also exhibited no or much weaker interactions with uptake-1 monoamine transporters. Together, our results reveal that PMAT and OCTs have distinct specificity exemplified by their differential interaction with HIV PIs. Further, we demonstrate that lopinavir can be used as a selective PMAT inhibitor to differentiate PMAT-mediated monoamine and organic cation transport from those mediated by OCT1-3.

Identification of natural products as modulators of OATP2B1 using LC-MS/MS to quantify OATP-mediated uptake

CONTEXT

Organic anion-transporting polypeptide 2B1 (OATP2B1) which is highly expressed in enterocytes and hepatocytes could be a key determinant for the intestinal absorption and hepatic uptake of its substrate drugs. Natural products are commonly used in traditional Chinese medicine, foods, and beverages.

OBJECTIVE

The objective of this study is to determine the OATP2B1-mediated drug interactions that could occur between natural products and OATP2B1 substrate drugs.

MATERIALS AND METHODS

Human OATP2B1 was transiently expressed in human embryonic kidney (HEK293) cells and characterized by immunofluorescence, Western blot, and uptake assay. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods for detecting OATP2B1 substrates estrone-3-sulfate (E3S) and three statins had been developed and were employed to investigate the effects of 27 frequently used natural products on the function of OATP2B1. Uptake of 5 μM E3S and 1 μM statins in the absence or presence of natural products was measured at 37 °C for 2 min with empty vector- and OATP2B1-transfected HEK293 cells. The IC50 values of inhibitors for OATP2B1-mediated 5 μM E3S uptake were determined.

RESULTS

Our results showed that mulberrin, scutellarin, quercetin, and glycyrrhetinic acid were strong inhibitors of OATP2B1-mediate E3S uptake with IC50 values being 1.8, 2.0, 7.5, and 13.0 μM, which were comparable with their plasma concentrations in clinical trials. They also inhibited OATP-mediated uptake of atorvastatin, fluvastatin, and rosuvastatin. These results indicated that clinically relevant drug interactions could occur between these natural compounds and OATP2B1 substrate drugs.

DISCUSSION AND CONCLUSION

The information obtained from this study might be helpful to predict and to avoid potential OATP2B1-mediated drug interactions.

Organic anion transporting polypeptide 2B1 expression correlates with uptake of estrone-3-sulfate and cell proliferation in estrogen receptor-positive breast cancer cells

Estrone-3-sulfate (E1S) is thought to be a major estrogen precursor in estrogen receptor (ER)-positive breast cancer. Since E1S is a hydrophilic compound, the uptake of E1S into cancer cells is probably mediated by transporters, such as organic anion-transporting polypeptide (OATP, SLCO) family. In this study, we investigated the relationship between expression of OATP2B1 and cell proliferation in ER-positive breast cancer. Cell-based assays were carried out in MCF-7 cells both with and without overexpression of OATP2B1. Normal breast and tumor tissues were collected and used in this study. Cell proliferation, ER-mediated transcriptional activities and estradiol secretion were stimulated by addition of E1S to the culture medium of MCF-7 cells. These stimulatory effects were significantly greater in MCF-7 cells overexpressing OATP2B1 than in control cells. The expression level of SLCO2B1 mRNA was significantly correlated with histological grade, Ki-67 labelling index and mRNA expression of steroid sulfatase. The expression level of SLCO2B1 mRNA in luminal B-like cancers was higher than that in luminal A-like cancers. Uptake of E1S resulted in down-regulation of ERα protein and induction of Ki-67 in MCF-7 cells. The present study suggests that OATP2B1 is involved in cell proliferation by increasing the amount of estrogen in ER-positive breast cancer cells.

(125)I-Labelled 2-Iodoestrone-3-sulfate: synthesis, characterization and OATP mediated transport studies in hormone dependent and independent breast cancer cells

INTRODUCTION

Organic Anion Transporting Polypeptides (OATP) are a family of membrane associated transporters that facilitate estrone-3-sulphate (E3S) uptake by hormone dependent, post-menopausal breast cancers. We have established E3S as a potential ligand for targeting hormone dependent breast cancer cells, and in this study sought to prepare and investigate radioiodinated E3S as a tool to study the OATP system.

METHODS

2- and 4-Iodoestrone-3-sulfates were prepared from estrone via aromatic iodination followed by a rapid and high yielding sulfation procedure. The resulting isomers were separated by preparative HPLC and verified by (1)H NMR and analytical HPLC. Transport studies of 2- and 4-[(125)I]-E3S were conducted in hormone dependent (i.e. MCF-7) and hormone independent (i.e. MDA-MB-231) breast cancer cells in the presence or absence of the specific transport inhibitor, bromosulfophthalein (BSP). Cellular localization of OATP1A2, OATP2B1, OATP3A1 and OATP4A1 were determined by immunofluorescence analysis using anti-Na(+)/K(+) ATPase-α (1:100 dilution) and DAPI as plasma membrane and nuclear markers, respectively.

RESULTS

Significantly (p<0.01) higher total accumulation of 2-[(125)I]-E3S was observed in hormone dependent MCF-7 as compared to hormone independent MDA-MB-231 breast cancer cells. In contrast 4-[(125)I]-E3S did not show cellular accumulation in either case. The efficiency of 2-[(125)I]-E3S transport (expressed as a ratio of Vmax/Km) was 2.4 times greater in the MCF-7 as compared to the MDA-MB-231 breast cancer cells. OATP1A2, OATP3A1 and OATP4A1 expression was localized in plasma membranes of MCF-7 and MDA-MB-231 cells confirming the functional role of these transporters in radioiodinated E3S cellular uptake.

CONCLUSION

An efficient method for the preparation of 2- and 4-[(125)I]-E3S was developed and where the former demonstrated potential as an in vitro probe for the OATP system. The new E3S probe can be used to study the OATP system and as a platform to create radiopharmaceuticals for imaging breast cancer.

Gastric reacidification with betaine HCl in healthy volunteers with rabeprazole-induced hypochlorhydria

Previous studies have demonstrated that increased gastric pH from the use of acid-reducing agents, such as proton-pump inhibitors or H2-receptor antagonists, can significantly impact the absorption of weakly basic drugs that exhibit pH-dependent solubility. Clinically practical strategies to mitigate this interaction have not been developed. This pilot study evaluated the extent and time course of gastric reacidification after a solid oral dosage form of anhydrous betaine HCl in healthy volunteers with pharmacologically induced hypochlorhydria. Six healthy volunteers with baseline normochlorhydria (fasting gastric pH < 4) were enrolled in this single period study. Hypochlorhydria was induced via 20 mg oral rabeprazole twice daily for four days. On the fifth day, an additional 20 mg dose of oral rabeprazole was given and gastric pH was monitored continuously using the Heidelberg pH capsule. After gastric pH > 4 was confirmed for 15 min, 1500 mg of betaine HCl was given orally with 90 mL of water and gastric pH was continuously monitored for 2 h. Betaine HCl significantly lowered gastric pH by 4.5 (± 0.5) units from 5.2 (± 0.5) to 0.6 (± 0.2) (P < 0.001) during the 30 min interval after administration. The onset of effect of betaine HCl was rapid, with a mean time to pH < 3 of 6.3 (± 4.3) min. The reacidification period was temporary with a gastric pH < 3 and < 4 lasting 73 (± 33) and 77 (± 30) min, respectively. Betaine HCl was well tolerated by all subjects. In healthy volunteers with pharmacologically induced hypochlorhydria, betaine HCl was effective at temporarily lowering gastric pH. The rapid onset and relatively short duration of gastric pH reduction gives betaine HCl the potential to aid the absorption of orally administered weakly basic drugs that exhibit pH-dependent solubility when administered under hypochlorhydric conditions.

Estrone-3-sulphate, a potential novel ligand for targeting breast cancers

The current study investigates the potential of estrone-3-sulphate (E3S) as a ligand for targeting Organic Anion Transporting Polypeptides (OATP), a family of membrane associated uptake transporters, for detection and diagnosis of hormone dependent breast cancers. E3S, an OATP substrate, is a predominant source of tumour estradiol in post-menopausal patients. To assess the potential of E3S as a ligand, distribution of exogenous E3S was determined at the whole body, tumour and cellular levels in murine models of hormone-dependent (MCF-7) and independent (MDA-MB-231) breast cancers. The highest levels of tumour uptake were observed at 6 h post injection (p.i) with significant difference (p = 0.04) between the level in MCF-7 (13.9±3.1%ID/g) and MDA-MB-231 (10.4±1.1%ID/g) (%ID/g: percentage of the total injected dose per gram tissue). The highest tumour-to-blood ratios (MCF-7∶7.4±1.2; MDA-MB-231∶9.1±2.1) were observed at 48 p.i., and highest tumour-to-muscle ratios (MCF-7∶10.7±1.5; MDA-MB-231∶3.8±0.7) were observed at 6 h p.i. Analogous to total tumour uptake, ex vivo tumour cell uptake at 2 h p.i. was 6 fold higher in MCF-7 in comparison to MDA-MB-231 tumour cells. Blocking studies, conducted by pre-administration of 100-fold excess E3S, resulted in significantly lower (MCF-7: p = 0.01; MDA-MB-231: p = 0.02) tumour uptake in both xenograft models, suggesting the involvement of an active carrier-mediated process. The expression of OATP1A2 was detected in tumour sections from both xenografts, with significantly higher expression (p = 0.002) in the MCF-7 xenografts. Overall, the higher tumour uptake and tumour-to-muscle ratio, alongside the higher expression of OATP1A2, in the MCF-7 xenograft model suggests the potential of E3S to serve as a novel ligand for targeting hormone dependent breast cancers.

Transport function and transcriptional regulation of a liver-enriched human organic anion transporting polypeptide 2B1 transcriptional start site variant

Human organic anion transporting polypeptide 2B1 (OATP2B1) is a membrane transporter that facilitates the cellular uptake of a number of endogenous compounds and drugs. OATP2B1 is widely expressed in tissues including the small intestine, liver, kidney, placenta, heart, skeletal muscle, and platelets. It was recently shown that differential promoter usage in tissues results in expression of five OATP2B1 transcriptional start site variants that use distinct first exons but share common subsequent exons. These variants are expected to encode either a full-length (OATP2B1-FL) or shortened protein lacking 22 N terminus amino acids (OATP2B1-Short). Little is known regarding the transport activity and regulation of OATP2B1 variants with N terminus truncation. Here, using absolute quantitative polymerase chain reaction, we find the full-length variant is the major form expressed in duodenum but the short variant predominates in liver. Using a transient heterologous cell expression system, we find that the transport activities of the short OATP2B1 variant toward substrates estrone sulfate and rosuvastatin are similar to the well characterized full-length variant. Transcriptional activity screening of the liver-enriched OATP2B1 variant promoter identified hepatocyte nuclear factor 4 α (HNF4α) as a novel transacting factor. With a combination of in silico screening, promoter mutation in cell-based reporter assays, small interfering RNA knockdown, and chromatin immunoprecipitation (ChIP) studies, we identified a functional HNF4α binding site close to the transcription start site (-17 to -4 bp). We conclude that the major OATP2B1 protein form in liver is transport competent and its hepatic expression is regulated by HNF4α.

Interaction between HIV protease inhibitors (PIs) and hepatic transporters in sandwich cultured human hepatocytes: implication for PI-based DDIs

Although HIV protease inhibitors (PIs) produce profound metabolic interactions through inactivation/inhibition of CYP3A enzymes, their role as victims of transporter-based drug-drug interactions (DDIs) is less well understood. Therefore, this study investigated if the PIs, nelfinavir (NFV), ritonavir (RTV), lopinavir (LPV) or amprenavir (APV) were transported into sandwich-cultured human hepatocytes (SCHH), and whether OATPs contributed to this transport. The findings showed that, except for (3) H-APV, no significant decrease in the total hepatocyte accumulation of the (3) H-PIs was detected in the presence of the corresponding unlabeled PI, indicating that the uptake of the other PIs was not mediated. Further, hepatocyte biliary efflux studies using (3) H-APV and unlabeled APV confirmed this decrease to be due to inhibition of sinusoidal influx transporter(s) and not the canalicular efflux transporters. Moreover, this sinusoidal transport of APV was not OATP-mediated. The results indicate that the hepatic uptake of NFV, RTV or LPV was primarily mediated by passive diffusion. The hepatic uptake of APV was mediated by an unidentified sinusoidal transporter(s). Therefore, NFV, RTV or LPV will not be victims of DDIs involving inhibition of hepatic influx transporters; however, the disposition of APV may be affected if its sinusoidal transport is inhibited.

Cynomolgus monkey as a potential model to assess drug interactions involving hepatic organic anion transporting polypeptides: in vitro, in vivo, and in vitro-to-in vivo extrapolation

Organic anion-transporting polypeptides (OATP) 1B1, 1B3, and 2B1 can serve as the loci of drug-drug interactions (DDIs). In the present work, the cynomolgus monkey was evaluated as a potential model for studying OATP-mediated DDIs. Three cynomolgus monkey OATPs (cOATPs), with a high degree of amino acid sequence identity (91.9, 93.5, and 96.6% for OATP1B1, 1B3, and 2B1, respectively) to their human counterparts, were cloned, expressed, and characterized. The cOATPs were stably transfected in human embryonic kidney cells and were functionally similar to the corresponding human OATPs (hOATPs), as evident from the similar uptake rate of typical substrates (estradiol-17β-d-glucuronide, cholecystokinin octapeptide, and estrone-3-sulfate). Moreover, six known hOATP inhibitors exhibited similar IC(50) values against cOATPs. To further evaluate the appropriateness of the cynomolgus monkey as a model, a known hOATP substrate [rosuvastatin (RSV)]-inhibitor [rifampicin (RIF)] pair was examined in vitro; the monkey-derived parameters (RSV K(m) and RIF IC(50)) were similar (within 3.5-fold) to those obtained with hOATPs and human primary hepatocytes. In vivo, the area under the plasma concentration-time curve of RSV (3 mg/kg, oral) given 1 hour after a single RIF dose (15 mg/kg, oral) was increased 2.9-fold in cynomolgus monkeys, consistent with the value (3.0-fold) reported in humans. A number of in vitro-in vivo extrapolation approaches, considering the fraction of the pathways affected and free versus total inhibitor concentrations, were also explored. It is concluded that the cynomolgus monkey has the potential to serve as a useful model for the assessment of OATP-mediated DDIs in a nonclinical setting.

Rapid stimulating effect of the antiarrhythmic agent amiodarone on absorption of organic anion compounds

In a clinical setting, changes in pharmacokinetics due to drug-drug interactions can often directly affect the therapeutic safety and efficacy of drugs. Recently, interest has been shown in drug-drug interactions in the intestine. It is now recognized that changes in the functions of drug transporters substantially influence the absorption of administered drugs from the intestine. Amiodarone (AMD) is a potent drug used in the treatment of serious supraventricular and ventricular tachyarrhythmias. Despite its potent pharmacological effects, its wide clinical use is precluded by drug-drug interactions. In this study, we characterized the transporter function between AMD and various compounds in human intestinal model Caco-2 cells. AMD significantly and rapidly increased the uptake of [(3)H]estrone-3-sulfate (E-3-S) for 5 min. The apical-to-basal transport of [(3)H]E-3-S was significantly increased by AMD. The AMD-stimulated [(3)H]E-3-S uptake was inhibited by organic anion transporting polypeptide (OATP) substrates. Caco-2 cells treated with AMD showed increased OATP2B1 expression on the cell surface. AMD also increased the absorption of sulfobromophthalein (BSP), which is a typical organic anion compound, and the expression level of Oatp2b1 at the membrane in in vivo experiments. The results indicate that AMD induces OATP2B1/Oatp2b1 expression at the membrane in the intestine and enhances absorption of organic anion compounds.

Differential role of organic anion-transporting polypeptides in estrone-3-sulphate uptake by breast epithelial cells and breast cancer cells

The purpose of this study was to investigate the differential expression and function of organic anion-transporting polypeptides (OATPs) in breast epithelial and breast cancer cells. Estrone-3-sulfate (E3S), a substrate for 7 of 11 OATPs, is a predominant source of tumor estrogen in postmenopausal, hormone-dependent patients with breast cancer. Overexpression of certain OATPs (e.g., OATP1A2) reported in breast tumor tissues compared with surrounding normal tissues could contribute toward two to three times higher tumoral E3S concentration. Little is known about expression and function of other OATP family members among breast epithelial and breast cancer cells. We therefore compared gene and protein expression of seven OATPs (OATP1A2, OATP1B1, OATP1B3, OATP1C1, OATP2B1, OATP3A1, and OATP4A1) in immortalized breast epithelial cells (MCF10A), hormone-dependent breast cancer cells (MCF7), and hormone-independent breast cancer cells (MDA/LCC6-435, MDA-MB-231, and MDA-MB-468) by quantitative polymerase chain reaction and immunoblotting, respectively. Expression of solute carrier superfamily encoding for OATPs (SLCO) 1A2, 1B1, 1B3, 2B1, and 3A1 is exclusive, similar, or significantly higher in cancer cells compared with MCF10A cells. Protein expression of OATPs is found to be either exclusive or higher in cancer cells compared with MCF10A cells. Specificity of OATP-mediated E3S uptake is observed only in cancer cells, with the highest total uptake in MCF7 cells. Transport kinetics of E3S uptake demonstrates transport efficiency that is 10 times greater in the MCF7 cells than in the hormone-independent cells. These data suggest that OATPs could be a novel therapeutic target for hormone-dependent breast cancers, particularly in postmenopausal patients, where the major source of tumor estrogen is E3S.

Classification of inhibitors of hepatic organic anion transporting polypeptides (OATPs): influence of protein expression on drug-drug interactions

The hepatic organic anion transporting polypeptides (OATPs) influence the pharmacokinetics of several drug classes and are involved in many clinical drug–drug interactions. Predicting potential interactions with OATPs is, therefore, of value. Here, we developed in vitro and in silico models for identification and prediction of specific and general inhibitors of OATP1B1, OATP1B3, and OATP2B1. The maximal transport activity (MTA) of each OATP in human liver was predicted from transport kinetics and protein quantification. We then used MTA to predict the effects of a subset of inhibitors on atorvastatin uptake in vivo. Using a data set of 225 drug-like compounds, 91 OATP inhibitors were identified. In silico models indicated that lipophilicity and polar surface area are key molecular features of OATP inhibition. MTA predictions identified OATP1B1 and OATP1B3 as major determinants of atorvastatin uptake in vivo. The relative contributions to overall hepatic uptake varied with isoform specificities of the inhibitors.

Oral drug delivery utilizing intestinal OATP transporters

Transporters play important roles in tissue distribution and urinary- and biliary-excretion of drugs and transporter molecules involved in those processes have been elucidated well. Furthermore, an involvement of efflux transporters such as P-glycoproteins, multidrug resistance associated protein 2, and breast cancer resistance protein as the intestinal absorption barrier and/or intestinal luminal secretion mechanisms has been demonstrated. However, although there are many suggestions for the contribution of uptake/influx transporters in intestinal absorption of drugs, information on the transporter molecules responsible for the intestinal absorptive process is limited. Among them, most studied absorptive drug transporter is peptide transporter PEPT1. However, utilization of PEPT1 for oral delivery of drugs may not be high due to the chemical structural requirement of PEPT1 limited to peptide-mimetics. Recently, organic anion transporting polypeptide (OATP) family such as OATP1A2 and OATP2B1 has been suggested to mediate intestinal absorption of several drugs. Since OATPs exhibit species difference in expressed tissues and functional properties between human and animals, human studies are essential to clarify the intestinal absorption mechanisms of drugs via OATPs. Recent pharmacogenomic studies demonstrated that OATP2B1 is involved in the drug absorption in human. In addition, information of drug-juice interaction in the intestine also uncovered the contribution of OATP1A2 and OATP2B1 in drug absorption. Since OATP1A2 and OATP2B1 exhibit broader substrate selectivity compared with PEPT1, their potential to be applied for oral delivery should be high. In this review, current understanding of characteristics and contribution as the absorptive transporters of OATPs in small intestine in human is described. Now, it is getting clearer that OATPs have significant roles in intestinal absorption of drugs, therefore, there are higher possibility to utilize OATPs as the tools for oral delivery.

Intestinal transporters for endogenic and pharmaceutical organic anions: the challenges of deriving in-vitro kinetic parameters for the prediction of clinically relevant drug-drug interactions

OBJECTIVES

This review provides an overview of intestinal human transporters for organic anions and stresses the need for standardization of the various in-vitro methods presently employed in drug-drug interaction (DDI) investigations.

KEY FINDINGS

Current knowledge on the intestinal expression of the apical sodium-dependent bile acid transporter (ASBT), the breast cancer resistance protein (BCRP), the monocarboxylate transporters (MCT) 1, MCT3-5, the multidrug resistance associated proteins (MRP) 1-6, the organic anion transporting polypetides (OATP) 2B1, 1A2, 3A1 and 4A1, and the organic solute transporter α/β (OSTα/β) has been covered along with an overview of their substrates and inhibitors. Furthermore, the many challenges in predicting clinically relevant DDIs from in-vitro studies have been discussed with focus on intestinal transporters and the various methods for deducting in-vitro parameters for transporters (K(m) /K(i) /IC50, efflux ratio). The applicability of using a cut-off value (estimated based on the intestinal drug concentration divided by the K(i) or IC50) has also been considered.

SUMMARY

A re-evaluation of the current approaches for the prediction of DDIs is necessary when considering the involvement of other transporters than P-glycoprotein. Moreover, the interplay between various processes that a drug is subject to in-vivo such as translocation by several transporters and dissolution should be considered.

Functional pleiotropy of organic anion transporting polypeptide OATP2B1 due to multiple binding sites

The purpose of this study was to examine whether the presence of multiple binding sites can explain the pleiotropy of substrate recognition by OATP2B1, using Xenopus oocytes expressing OATP2B1. OATP2B1-mediated uptake of estrone-3-sulfate apparently exhibited biphasic saturation kinetics, with Km values of 0.10 ± 0.05 and 29.9 ± 12.1 µM and Vmax values of 14.1 ± 6.4 and 995 ± 273 fmol/min/oocyte for high- and low-affinity sites, respectively. Contribution analysis revealed that transport of estrone-3-sulfate mediated by high- and low-affinity sites on OATP2B1 could be evaluated at the concentrations of 0.005 and 50 µM, respectively. pH-dependence study of OATP2B1-mediated estrone-3-sulfate uptake suggested that high- and low-affinity sites show different pH sensitivity. When the inhibitory effect of 12 compounds on estrone-3-sulfate uptake by high- and low-affinity sites on OATP2B1 was examined, 4 compounds appeared to be inhibitors of the high-affinity site on OATP2B1. Two other compounds appeared to be inhibitors for the low-affinity site and four others were inhibitory at both sites. These results indicated the presence of multiple binding sites on OATP2B1 with different affinity for drugs. Accordingly, it is likely that drug-drug and drug-beverage interactions occur only when two drugs share the same binding site on OATP2B1.

Drug-permeability and transporter assays in Caco-2 and MDCK cell lines

The human colon adenocarcinoma Caco-2 and Madin–Darby canine kidney epithelial cell lines provide in vitro tools to assess a drug’s permeability and transporter interactions during discovery and development. The cells, when cultured on semiporous filters, form confluent monolayers that model the intestinal epithelial barrier for permeability, transporter and drug-interaction assays. The applications of these assays in pharmaceutical research include qualitative prediction and ranking of absorption, determining mechanism(s) of permeability, formulation effects on drug permeability, and the potential for transporter-mediated drug–drug interactions. This review focuses on recent examples of Caco-2 and Madin–Darby canine kidney cells assays for drug permeability including transfected and knock-down cells, miniaturization and automation, and assay combinations to better understand and predict intestinal drug absorption.

Expression of ATP-binding cassette membrane transporters in rodent and human sertoli cells: relevance to the permeability of antiretroviral therapy at the blood-testis barrier

The blood-testis barrier (BTB), composed primarily of Sertoli cells, is responsible for protecting developing germ cells from xenobiotic exposure. ATP-binding cassette (ABC) membrane-associated drug efflux transporters, P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and the multidrug resistance-associated proteins (Mrps), have been shown to restrict antiretroviral drug permeability at blood-tissue barriers such as the blood-brain barrier. However, it remains unclear whether these transporters are functional at the level of Sertoli cells and can regulate anti-HIV drug permeability at the BTB. This study investigated the functional expression of ABC transporters in a mouse Sertoli cell line system (TM4) and in primary cultures of human Sertoli cells (HSECs). Expression of multidrug resistance Mdr1a/1b/MDR1/P-gp, Mrp1/MRP1, and Mrp4/MRP4 is confirmed by quantitative polymerase chain reaction and immunoblotting analysis in TM4 cells and HSECs. Immunofluorescence studies revealed plasma membrane localization of P-gp, Mrp1/MRP1, and Mrp4/MRP4 in both cell systems. However, Bcrp expression and localization was only detected in rodent cells. Accumulation of 1) rhodamine-6G (R-6G), a fluorescent P-gp substrate, 2) [³H]atazanavir, a HIV protease inhibitor and known P-gp substrate, 3) 2'7'-bis-(2-carboxyethyl)-5-(and-6)carboxyfluorescein (BCECF), a fluorescent Mrp substrate, and 4) [³H]mitoxantrone, a BCRP substrate, by TM4 monolayer cells in the presence of established inhibitors demonstrates that these transporters are functional. In addition, several anti-HIV drugs significantly enhance the accumulation of R-6G, [³H]atazanavir, BCECF, and [³H]mitoxantrone by TM4 cells. This study provides the first evidence of ABC transporter expression and activity in Sertoli cells and suggests that these transporters could play an important role in restricting antiretroviral drug permeability at the BTB.

pH-sensitive interaction of HMG-CoA reductase inhibitors (statins) with organic anion transporting polypeptide 2B1

The human organic anion transporting polypeptide 2B1 (OATP2B1, SLCO2B1) is ubiquitously expressed and may play an important role in the disposition of xenobiotics. The present study aimed to examine the role of OATP2B1 in the intestinal absorption and tissue uptake of 3-hydroxy-3-methylglutaryl-Coenzyme A (HMG-CoA) reductase inhibitors (statins). We first investigated the functional affinity of statins to the transporter as a function of extracellular pH, using OATP2B1-transfeced HEK293 cells. The results indicate that OATP2B1-mediated transport is significant for rosuvastatin, fluvastatin and atorvastatin, at neutral pH. However, OATP2B1 showed broader substrate specificity as well as enhanced transporter activity at acidic pH. Furthermore, uptake at acidic pH was diminished in the presence of proton ionophore, suggesting proton gradient as the driving force for OATP2B1 activity. Notably, passive transport rates are predominant or comparable to active transport rates for statins, except for rosuvastatin and fluvastatin. Second, we studied the effect of OATP modulators on statin uptake. At pH 6.0, OATP2B1-mediated transport of atorvastatin and cerivastatin was not inhibitable, while rosuvastatin transport was inhibited by E-3-S, rifamycin SV and cyclosporine with IC(50) values of 19.7 ± 3.3 μM, 0.53 ± 0.2 μM and 2.2 ± 0.4 μM, respectively. Rifamycin SV inhibited OATP2B1-mediated transport of E-3-S and rosuvastatin with similar IC(50) values at pH 6.0 and 7.4, suggesting that the inhibitor affinity is not pH-dependent. Finally, we noted that OATP2B1-mediated transport of E-3-S, but not rosuvastatin, is pH sensitive in intestinal epithelial (Caco-2) cells. However, uptake of E-3-S and rosuvastatin by Caco-2 cells was diminished in the presence of proton ionophore. The present results indicate that OATP2B1 may be involved in the tissue uptake of rosuvastatin and fluvastatin, while OATP2B1 may play a significant role in the intestinal absorption of several statins due to their transporter affinity at acidic pH.

Influence of drug transport proteins on the pharmacokinetics and drug interactions of HIV protease inhibitors

Protease inhibitors, a class of antiretroviral agents frequently used in the treatment of HIV infection, interact with numerous transport proteins resulting in clinically significant drug-drug interactions (DDIs). This review focuses on the proteins that transport protease inhibitors and directly influence the pharmacokinetics of these drugs, as well as the transport proteins that are inhibited or induced by protease inhibitors. Clinically relevant DDIs involving drug transporters and protease inhibitors, either as "victim" drugs or as "perpetrator" drugs, and the pharmacokinetic consequences of such interactions are highlighted. A summary of transporter-mediated processes underlying the toxicity of protease inhibitors is provided. Finally, the effect of HIV infection or co-infection on drug transport proteins, and the implications for protease inhibitor pharmacokinetics is discussed. Transport proteins significantly influence the pharmacokinetics, efficacy and toxicity profiles of this important class of drugs.

A novel method using confocal laser scanning microscopy for sensitive measurement of P-glycoprotein-mediated transport activity in Caco-2 cells

OBJECTIVES

The aim of this study was to use time-lapse confocal laser scanning microscopy to establish a more sensitive and specific method for evaluating P-glycoprotein activity in Caco-2 cells.

METHODS

The change in the fluorescence of residual rhodamine 123 at the apical and central regions of Caco-2 cells was measured in the presence of digoxin or St John's wort by using time-lapse confocal laser scanning microscopy. The data were compared with measurements made using conventional techniques, a fluorescence microplate reader and a fluorescence microscope.

KEY FINDINGS

The percentage decrease of rhodamine 123 caused by 10 µm digoxin or 0.1 µg/ml St John's wort was significantly larger in the apical region of the Caco-2 cell than in the central region or in the whole cell. The digoxin-induced inhibition in the apical region as measured by time-lapse confocal laser scanning microscopy was greater than that measured in the whole cell by a microplate reader or a fluorescence microscope.

CONCLUSIONS

The assay of residual rhodamine 123 in the apical region of Caco-2 cells by confocal laser scanning microscopy was more sensitive than the conventional methods using a microplate reader or fluorescence microscopy. It will be a valuable screening tool for studying both the inhibition and induction of P-glycoprotein activity.