Substrate-Dependent Drug-Drug Interactions between Gemfibrozil, Fluvastatin and Other Organic Anion-Transporting Peptide (OATP) Substrates on OATP1B1, OATP2B1, and OATP1B3

Interactions of oleanane pentacyclic triterpenoids with human organic anion transporting polypeptide 1B1 and 1B3

Oleanane pentacyclic triterpenoids have been widely used in clinical practice. However, studies on their interactions with hepatic transporters remain limited. In this study, we systematically investigated the inhibitory effects of 14 oleanane pentacyclic triterpenoids on organic anion transporting polypeptide 1B1 and 1B3 (OATP1B1 and OATP1B3), two liver-specific uptake transporters. Through fluorescence-based cellular uptake assays, we identified three potent OATP1B1 inhibitors (saikosaponin B1, saikosaponin A and 18β-glycyrrhetinic acid) and five potent OATP1B3 inhibitors (echinocystic acid, 3-oxo-16α-hydroxy-olean-12-en-28β-oic acid, chikusetsu saponin IVa, saikosaponin B1 and 18β-glycyrrhetinic acid). Structural analysis revealed that free oleanane triterpenoids inhibited OATP1B1/1B3 more potently than triterpene glycosides. Despite their similar structures, 18β-glycyrrhetinic acid exhibited much stronger inhibition on OATP1B1/1B3 than 18α-glycyrrhetinic acid, while both were substrates of OATP1B3. Interestingly, OATP1B3 overexpression significantly increased reactive oxygen species (ROS) levels in HepG2 cells after treatment with 18β-glycyrrhetinic acid. To conclude, this study highlights the potential interactions of oleanane pentacyclic triterpenoids with OATP1B1/1B3, and provides novel insights into the anti-cancer activity of 18β-glycyrrhetinic acid.

Oligomerization of drug transporters: Forms, functions, and mechanisms

Drug transporters are essential players in the transmembrane transport of a wide variety of clinical drugs. The broad substrate spectra and versatile distribution pattern of these membrane proteins infer their pharmacological and clinical significance. With our accumulating knowledge on the three-dimensional structure of drug transporters, their oligomerization status has become a topic of intense study due to the possible functional roles carried out by such kind of post-translational modification (PTM). In-depth studies of oligomeric complexes formed among drug transporters as well as their interactions with other regulatory proteins can help us better understand the regulatory mechanisms of these membrane proteins, provide clues for the development of novel drugs, and improve the therapeutic efficacy. In this review, we describe different oligomerization forms as well as their structural basis of major drug transporters in the ATP-binding cassette and solute carrier superfamilies, summarize our current knowledge on the influence of oligomerization for protein expression level and transport function of these membrane proteins, and discuss the regulatory mechanisms of oligomerization. Finally, we highlight the challenges associated with the current oligomerization studies and propose some thoughts on the pharmaceutical application of this important drug transporter PTM.

Screening oral drugs for their interactions with the intestinal transportome via porcine tissue explants and machine learning

In vitro systems that accurately model in vivo conditions in the gastrointestinal tract may aid the development of oral drugs with greater bioavailability. Here we show that the interaction profiles between drugs and intestinal drug transporters can be obtained by modulating transporter expression in intact porcine tissue explants via the ultrasound-mediated delivery of small interfering RNAs and that the interaction profiles can be classified via a random forest model trained on the drug-transporter relationships. For 24 drugs with well-characterized drug-transporter interactions, the model achieved 100% concordance. For 28 clinical drugs and 22 investigational drugs, the model identified 58 unknown drug-transporter interactions, 7 of which (out of 8 tested) corresponded to drug-pharmacokinetic measurements in mice. We also validated the model's predictions for interactions between doxycycline and four drugs (warfarin, tacrolimus, digoxin and levetiracetam) through an ex vivo perfusion assay and the analysis of pharmacologic data from patients. Screening drugs for their interactions with the intestinal transportome via tissue explants and machine learning may help to expedite drug development and the evaluation of drug safety.

Cryo-EM structures of human organic anion transporting polypeptide OATP1B1

Members of the solute carrier organic anion transporting polypeptide (OATPs) family function as transporters for a large variety of amphipathic organic anions including endogenous metabolites and clinical drugs, such as bile salts, steroids, thyroid hormones, statins, antibiotics, antivirals, and anticancer drugs. OATP1B1 plays a vital role in transporting such substances into the liver for hepatic clearance. FDA and EMA recommend conducting in vitro testing of drug-drug interactions (DDIs) involving OATP1B1. However, the structure and working mechanism of OATPs still remains elusive. In this study, we determined cryo-EM structures of human OATP1B1 bound with representative endogenous metabolites (bilirubin and estrone-3-sulfate), a clinical drug (simeprevir), and a fluorescent indicator (2',7'-dichlorofluorescein), in both outward- and inward-open states. These structures reveal major and minor substrate binding pockets and conformational changes during transport. In combination with mutagenesis studies and molecular dynamics simulations, our work comprehensively elucidates the transport mechanism of OATP1B1 and provides the structural basis for DDI predictions involving OATP1B1, which will greatly promote our understanding of OATPs.

Utilization of OATP1B Biomarker Coproporphyrin-I to Guide Drug-Drug Interaction Risk Assessment: Evaluation by the Pharmaceutical Industry

Drug-drug interactions (DDIs) involving hepatic organic anion transporting polypeptides 1B1/1B3 (OATP1B) can be substantial, however, challenges remain for predicting interaction risk. Emerging evidence suggests that endogenous biomarkers, particularly coproporphyrin-I (CP-I), can be used to assess in vivo OATP1B activity. The present work under the International Consortium for Innovation and Quality in Pharmaceutical Development was aimed primarily at assessing CP-I as a biomarker for informing OATP1B DDI risk. Literature and unpublished CP-I data along with pertinent in vitro and clinical DDI information were collected to identify DDIs primarily involving OATP1B inhibition and assess the relationship between OATP1B substrate drug and CP-I exposure changes. Static models to predict changes in exposure of CP-I, as a selective OATP1B substrate, were also evaluated. Significant correlations were observed between CP-I area under the curve ratio (AUCR) or maximum concentration ratio (Cmax R) and AUCR of substrate drugs. In general, the CP-I Cmax R was equal to or greater than the CP-I AUCR. CP-I Cmax R < 1.25 was associated with absence of OATP1B-mediated DDIs (AUCR < 1.25) with no false negative predictions. CP-I Cmax R < 2 was associated with weak OATP1B-mediated DDIs (AUCR < 2). A correlation was identified between CP-I exposure changes and OATP1B1 static DDI predictions. Recommendations for collecting and interpreting CP-I data are discussed, including a decision tree for guiding DDI risk assessment. In conclusion, measurement of CP-I is recommended to inform OATP1B inhibition potential. The current analysis identified changes in CP-I exposure that may be used to prioritize, delay, or replace clinical DDI studies.

Preincubation Time-Dependent, Long-Lasting Inhibition of Drug Transporters and Impact on the Prediction of Drug-Drug Interactions

Transporter-mediated drug-drug interaction (DDI) is of clinical concern, and the quantitative prediction of DDIs is an indispensable part of drug development. Cell-based inhibition assays, in which a representative probe substrate and a potential inhibitor are coincubated, are routinely performed to assess the inhibitory potential of new molecular entities on drug transporters. However, the inhibitory effect of cyclosporine A (CsA) on organic anion transporting polypeptide (OATP) 1B1 is substantially potentiated with CsA preincubation, and this effect is both long-lasting and dependent on the preincubation time. This phenomenon has also been reported with transporters other than OATP1Bs, but it is considered more prevalent among OATP1Bs and organic cation transporters. Regulatory agencies have also noted this preincubation effect and have recommended that pharmaceutical companies consider inhibitor preincubation when performing in vitro OATP1B1 and OATP1B3 inhibition studies. Although the underlying mechanisms responsible for the preincubation effect are not fully understood, a trans-inhibition mechanism was recently demonstrated for OATP1B1 inhibition by CsA, in which CsA inhibited OATP1B1 not only extracellularly (cis-inhibition) but also intracellularly (trans-inhibition). Furthermore, the trans-inhibition potency of CsA was much greater than that of cis-inhibition, suggesting that trans-inhibition might be a key driver of clinical DDIs of CsA with OATP1B substrate drugs. Although confidence in transporter-mediated DDI prediction is generally considered to be low, the predictability might be further improved by incorporating the trans-inhibition mechanism into static and dynamic models for preincubation-dependent inhibitors of OATP1Bs and perhaps other transporters. SIGNIFICANCE STATEMENT: Preincubation time-dependent, long-lasting inhibition has been observed for OATP1B1 and other solute carrier transporters in vitro. Recently, a trans-inhibition mechanism for the preincubation effect of CsA on OATP1B1 inhibition was identified, with the trans-inhibition potency being greater than that of cis-inhibition. The concept of trans-inhibition may allow us to further understand the mechanism of transporter-mediated DDIs not only for OATP1B1 but also for other transporters and to improve the accuracy and confidence of DDI predictions.

Reimagining the Framework Supporting the Static Analysis of Transporter Drug Interaction Risk; Integrated Use of Biomarkers to Generate Pan‐Transporter Inhibition Signatures

Solute carrier (SLC) transporters present as the loci of important drug-drug interactions (DDIs). Therefore, sponsors generate in vitro half-maximal inhibitory concentration (IC₅₀ ) data and apply regulatory agency-guided "static" methods to assess DDI risk and the need for a formal clinical DDI study. Because such methods are conservative and high false-positive rates are likely (e.g., DDI study triggered when liver SLC R value ≥ 1.04 and renal SLC maximal unbound plasma (C_max,u )/IC₅₀ ratio ≥ 0.02), investigators have attempted to deploy plasma- and urine-based SLC biomarkers in phase I studies to de-risk DDI and obviate the need for drug probe-based studies. In this regard, it was possible to generate in-house in vitro SLC IC₅₀ data for various clinically (biomarker)-qualified perpetrator drugs, under standard assay conditions, and then estimate "% inhibition" for each SLC and relate it empirically to published clinical biomarker data (area under the plasma concentration vs. time curve (AUC) ratio (AUCR, AUC_inhibitor /AUC_reference ) and % decrease in renal clearance (ΔCL_renal )). After such a "calibration" exercise, it was determined that only compounds with high R values (> 1.5) and C_max,u /IC₅₀ ratios (> 0.5) are likely to significantly modulate liver (AUCR > 1.25) and renal (ΔCL_renal  > 25%) biomarkers and evoke DDI risk. The % inhibition approach supports integration of liver and renal SLC data and allows one to generate pan-SLC inhibition signatures for different test perpetrators (e.g., SLC % inhibition ranking). In turn, such signatures can guide the selection of the most appropriate individual (or combinations of) biomarkers for testing in phase I studies.

Human Organic Anion Transporting Polypeptides 1B1, 1B3, and 2B1 Are Involved in the Hepatic Uptake of Phenolsulfonphthalein

Phenolsulfonphthalein (PSP or phenol red), a sulfonphthalein dye, has been used as a diagnostic agent and a pH indicator in cell culture medium. After administered into the body, PSP is excreted into urine and bile. The urinary excretion of PSP is mediated by organic anion transporter 1/3 (OAT1/3) and multidrug resistance protein 2 (MRP2). In biliary excretion, PSP is effluxed from hepatocytes into the bile via MRP2. However, so far, the molecular mechanism for PSP transport from the blood into hepatocytes is unclear. In the present study, six human major hepatic uptake transporters expressed on the basolateral membrane of hepatocytes, namely, organic anion transporting polypeptide 1B1 (OATP1B1), OATP1B3, OATP2B1, Na+/taurocholate cotransporting polypeptide (NTCP), organic cation transporter 1 (OCT1), and OAT2, have been investigated to see whether they are involved in the hepatic uptake of PSP. An in vitro cell-based study demonstrated that PSP is a substrate for OATP1B1, OATP1B3, and OATP2B1, with OATP1B3 showing the highest transport efficiency. The K m values for OATP1B1-, OATP1B3-, and OATP2B1-mediated PSP uptake were 11.3 ± 1.5, 7.0 ± 1.5, and 5.1 ± 1.0 μM, respectively. PSP interacts with known OATP substrates/inhibitors. However, the presence of PSP in cell culture medium has no significant effect on OATP's function. In vivo pharmacokinetic study in wild-type and Oatp1b2-knockout mice showed that Oatp1b2-knockout led to elevated plasma concentration and decreased liver accumulation of PSP. Taken together, the present study showed that in the liver, OATP1B1, OATP1B3, and OATP2B1 are involved in the uptake of PSP from the blood into hepatocytes, which, along with MRP2-mediated efflux of PSP from hepatocytes into the bile, constitute the vectorial transport of PSP from the blood to the bile and may play a critical role in the biliary excretion of PSP.

Effect of major components of Tripterygium wilfordii Hook. f on the uptake function of organic anion transporting polypeptide 1B1

Organic anion transporting polypeptide 1B1 (OATP1B1), which is specifically expressed at the basolateral membrane of human hepatocytes, is well recognized as the key determinant in the pharmacokinetics of a wide variety of drugs and considered as an important drug-drug interaction (DDI) site. Triptergium wilfordii Hook. f. (TWHF) is a traditional Chinese medicine that has a long history in treating diseases and more pharmacological effects were demonstrated recently. Components of TWHF mainly belong to the groups of alkaloids, diterpenoids, and triterpenoids. However, whether TWHF constituents are involved in herb-drug interaction (HDI) remains largely unknown. In the present study, we investigated the effect of four major components of TWHF, i.e. Triptolide (TPL), Celastrol (CL), and two alkaloids Wilforine (WFR) and Wilforgine (WFG) on the function of OATP1B1. It was found that co-incubation of these compounds greatly inhibited the uptake function of OATP1B1, with WFG (IC₅₀ = 3.63 ± 0.61 μM) and WFR (IC₅₀ = 3.91 ± 0.30 μM) showing higher inhibitory potency than TPL (IC₅₀ = 184 ± 36 μM) and CL (IC₅₀ = 448 ± 81 μM). Kinetic analysis revealed that co-incubation of WFG or WFR led to the reduction of both Km and Vmax of the DCF uptake. On the other hand, pre-incubation of WFG or WFR increased Km value of OATP1B1; while CL affected both Km and Vmax. In conclusion, co- and pre-incubation of the tested TWHF components inhibited OATP1B1 activity in different manners. Although co-incubation of WFG and WFR did not seem to directly compete with the substrates, pre-incubation of these alkaloids may alter the substrate-transporter interaction.

Potential drug-drug interaction between sodium-glucose co-transporter 2 inhibitors and statins: pharmacological and clinical evidence

INTRODUCTION

Recent case reports suggested that concomitant use of sodium-glucose co-transporter 2 (SGLT2) inhibitors with statins could lead to increased statin toxicity. We provide a comprehensive overview of the available pharmacological and clinical evidence on this potential drug-drug interaction (DDI).

AREAS COVERED

We searched MEDLINE PubMed until November 2020 for (i) pharmacokinetic studies on SGLT2 inhibitors, statins, and their potential interaction, and (ii) case reports and clinical studies assessing the safety of concomitant use of SGLT2 inhibitors and statins. We also searched regulatory documents submitted to the United States Food and Drug Administration for unpublished data on this potential DDI.

EXPERT OPINION

SGLT2 inhibitors are increasingly used for type 2 diabetes, chronic heart failure, and chronic kidney disease, and concomitant use with statins is common given the comorbidity of indications. While pharmacokinetic studies in healthy subjects showed no clinically relevant changes in statin levels during SGLT2 inhibitor co-administration, the published case reports and pharmacologic reasoning support the possibility of an interaction. Underlying mechanisms could be pharmacokinetic or pharmacodynamic, and canagliflozin appears to be the SGLT2 inhibitor with the highest interaction potential. Further research including 'real-world' pharmacoepidemiologic studies is needed to better understand the clinical significance of this DDI.

OATP2B1 - The underrated member of the organic anion transporting polypeptide family of drug transporters?

The organic anion transporting polypeptide 2B1 (OATP2B1) was one of the first cloned members of the SLCO family. However, its physiological and pharmacological role is still poorly understood, and object of a current debate on the transporter's relevance. Within this commentary, we summarize the data currently available on the transporter's expression and its substrates and highlight the strength and difficulties of the methods that have been applied to gather these data. The conclusion drawn from these findings was that OATP2B1 due to its intestinal expression is most likely involved in oral drug absorption of its substrate and therefore prone for interactions. This has been tested in in vivo drug interaction and/or pharmacogenetic studies. While some of these support the notion of OATP2B1 being of relevance in drug absorption, the pharmacogenetic findings are rather inconclusive. We will explain our thoughts why OATP2B1 may not influence the general systemic pharmacokinetic of certain substrates, but possibly local distribution processes, like the transfer across the blood-brain-barrier. Besides the pharmacokinetic aspects, there are data on endogenous molecules like coproporphyrins and sulfated steroids. Therefore, we will also highlight possible physiological roles of OATP2B1, which are driven by its expression pattern in the tubular cells of the kidney as well as its expression in the blood brain barrier. Finally we also deal with the advantages and disadvantages in the use of animal models to decipher the role of OATP2B1 in pharmacokinetics of its substrates and beyond.

Leucine heptad motifs within transmembrane domains affect function and oligomerization of human organic anion transporting polypeptide 1B1

Organic anion transporting polypeptides (OATPs) are transmembrane proteins responsible for the uptake of a wide range of endogenous compounds and clinically important drugs. The liver-specific OATP1B1 serves crucial roles in the removal of many orally administered drugs. The proper function of the transporter hence is essential for the pharmacokinetics of various therapeutic agents. Membrane proteins tend to form oligomers that are important for their stability, targeting and/or interactions with the substrates. Previous study in our laboratory revealed that OATP1B1 may form homo-oligomers and that a GXXXG motif localized at transmembrane domain 8 (TM8) may affect its oligomerization. In the current study, three short-form leucine heptad repeats within the transmembrane domains of OATP1B1 were investigated. It was found that the disruption of leucine heptad repeats within TM3 dramatically reduced the uptake function and protein-protein association of OATP1B1; while within TM8, only L378 is essential for the function of OATP1B1 and alanine replacement of L378 exhibited no effect on the oligomerization. The fragmental expression of TM3 interfered with the association of OATP1B1 homo-oligomers as well as its association with OATP1B3, which is also selectively expressed at human hepatocytes, suggesting that the region may be shared by both transporters for their protein-protein interactions.

Effects of Cangfu Daotan Decoction on obese polycystic ovary syndrome and its mechanism

BACKGROUND

Polycystic ovary syndrome (PCOS) is one of the most common endocrine disorders affecting women of reproductive age. Cangfu Daotan Decoction (CFDTT) is one of the prescriptions in the stagnation of obesity-type PCOS. Our previous study showed that CFDTT treatment of obese PCOS was correlated with organic anion transporting polypeptides (OATPs).

METHODS

Here, we studied the effects of CFDTT on obese PCOS and its underlying mechanism. We built an obese PCOS rat model and treated the rats with CFDTT. Then, we detected the serum levels of TCHO, TG, LDL-c, HDL-c, luteinizing hormone (LH), follicle stimulating growth hormone (FSH), testosterone (T), estradiol (E₂), IL-1β, IL-6, and TNF-α in each group and adopted RT-PCR, western blotting and immunohistochemical staining to investigate the effects of CFDTT treatment on the expression of OATP2B1 and OATP3A1 in ovarian and uterine tissues. In addition, we compared the pregnancy outcomes of each group.

RESULTS

We found that CFDTT decreased the serum levels of TCHO, TG, LDL-c, LH, T, IL-1β, IL-6, and TNF-α and increased the levels of HDL-c, FSH and E₂ in a dose-dependent manner. CFDTT could induce the expression of OATP2B1 and OATP3A1 in ovarian and uterine tissues. Moreover, CFDTT could improve pregnancy outcomes.

CONCLUSION

These data suggested that the therapeutic mechanism of Cangfu Daotan Decoction on PCOS may be correlated with regulating lipid metabolism, sex hormone secretion and the inflammatory response and increasing OATP2B1 and OATP3A1 expression. Cangfu Daotan Decoction can be developed as a PCOS treatment drug.

Improved Prediction of the Drug-Drug Interactions of Pemafibrate Caused by Cyclosporine A and Rifampicin via PBPK Modeling: Consideration of the Albumin-Mediated Hepatic Uptake of Pemafibrate and Inhibition Constants With Preincubation Against OATP1B

Pemafibrate (PMF) is highly albumin-bound (>99.8%) and a substrate for hepatic uptake transporters (OATP1B) and CYP enzymes. Here, we developed a PBPK model of PMF to capture drug-drug interactions (DDI) incurred by cyclosporine (CsA) and rifampicin (RIF), the two OATP1B inhibitors. Initial PBPK modeling of PMF utilized in vitro hepatic uptake clearance (PS_inf) obtained in the absence of albumin, but failed in capturing the blood PMF pharmacokinetic (PK) profiles. Based on the results that in vitro PS_inf of unbound PMF was enhanced in the presence of albumin, we applied the albumin-facilitated dissociation model and the resulting PS_inf parameters improved the prediction of the blood PMF PK profiles. In refining our PBPK model toward improved prediction of the observed DDI data (PMF co-administered with single dosing of CsA or RIF; PMF following multiple RIF dosing), we adjusted the previously obtained in vivo OATP1B inhibition constants (K_i,OATP1B) of CsA or RIF for pitavastatin by correcting for substrate-dependency. We also incorporated the induction of OATP1B and CYP enzymes after multiple RIF dosing. Sensitivity analysis informed that the higher gastrointestinal absorption rate constant could further improve capturing the observed DDI data, suggesting the possible inhibition of intestinal ABC transporter(s) by CsA or RIF.

Synergistic transport of a fluorescent coumarin probe marks coumarins as pharmacological modulators of Organic anion-transporting polypeptide, OATP3A1

Organic anion-transporting polypeptide 3A1 (OATP3A1) is a membrane transporter mediating the cellular uptake of various hormones such as estrone-3-sulfate, prostaglandins E1 and E2 and thyroxine. OATP3A1 is widely expressed in the human body and its presence in tissue-blood barriers, neurons and muscle cells marks it as a potential pharmacological target. Herein we demonstrate that an otherwise membrane impermeant, zwitterionic fluorescent coumarin probe, bearing a sulfonate function is a potent substrate of human OATP3A1, thus readily transported into HEK-293-OATP3A1 cells allowing functional investigation and the screen of drug interactions of the OATP3A1 transporter. At the same time, dyes lacking either the sulfonate motif or the coumarin scaffold showed a dramatic decrease in affinity or even a complete loss of transport. Furthermore, we observed a distinct inhibition/activation pattern in the OATP3A1-mediated uptake of closely related fluorescent coumarin derivatives differing only in the presence of the sulfonate moiety. Additionally, we detected a synergistic effect between one of the probes tested and the endogenous OATP substrate estrone-3-sulfate. These data, together with docking results indicate the presence of at least two cooperative substrate binding sites in OATP3A1. Besides providing the first sensitive probe for testing OATP3A1 substrate/inhibitor interactions, our results also help to understand substrate recognition and transport mechanism of the poorly characterized OATP3A1. Moreover, coumarins are good candidates for OATP3A1-targeted drug delivery and as pharmacological modulators of OATP3A1.

Novel insights into the organic solute transporter alpha/beta, OSTα/β: From the bench to the bedside

Organic solute transporter alpha/beta (OSTα/β) is a heteromeric solute carrier protein that transports bile acids, steroid metabolites and drugs into and out of cells. OSTα/β protein is expressed in various tissues, but its expression is highest in the gastrointestinal tract where it facilitates the recirculation of bile acids from the gut to the liver. Previous studies established that OSTα/β is upregulated in liver tissue of patients with extrahepatic cholestasis, obstructive cholestasis, and primary biliary cholangitis (PBC), conditions that are characterized by elevated bile acid concentrations in the liver and/or systemic circulation. The discovery that OSTα/β is highly upregulated in the liver of patients with nonalcoholic steatohepatitis (NASH) further highlights the clinical relevance of this transporter because the incidence of NASH is increasing at an alarming rate with the obesity epidemic. Since OSTα/β is closely linked to the homeostasis of bile acids, and tightly regulated by the nuclear receptor farnesoid X receptor, OSTα/β is a potential drug target for treatment of cholestatic liver disease, and other bile acid-related metabolic disorders such as obesity and diabetes. Obeticholic acid, a semi-synthetic bile acid used to treat PBC, under review for the treatment of NASH, and in development for the treatment of other metabolic disorders, induces OSTα/β. Some drugs associated with hepatotoxicity inhibit OSTα/β, suggesting a possible role for OSTα/β in drug-induced liver injury (DILI). Furthermore, clinical cases of homozygous genetic defects in both OSTα/β subunits resulting in diarrhea and features of cholestasis have been reported. This review article has been compiled to comprehensively summarize the recent data emerging on OSTα/β, recapitulating the available literature on the structure-function and expression-function relationships of OSTα/β, the regulation of this important transporter, the interaction of drugs and other compounds with OSTα/β, and the comparison of OSTα/β with other solute carrier transporters as well as adenosine triphosphate-binding cassette transporters. Findings from basic to more clinically focused research efforts are described and discussed.

OATP1B3 Expression and Function is Modulated by Coexpression with OCT1, OATP1B1, and NTCP

Organic anion transporting polypeptide (OATP) 1B3 is a drug transporter expressed at the basolateral membrane of human hepatocytes. Along with other transporters, including OATP1B1, Na⁺/taurocholate cotransporting polypeptide (NTCP), and organic cation transporter (OCT) 1, it is responsible for the uptake of endo- and xenobiotics into hepatocytes. Our previous studies demonstrated that OATP1B3 can form hetero-oligomers with OATP1B1 in human embryonic kidney 293T (HEK293) cells and with NTCP in both HEK293 cells and frozen human liver sections. To further characterize the hetero-oligomerization of OATP1B3, we investigated OCT1 as a potential interacting partner and determined the functional consequences of OATP1B3 hetero-oligomerization. We demonstrated interactions between OATP1B3 and OCT1 by coimmunoprecipitation with an anti-OATP1B3 antibody from human hepatocytes. In addition, we visualized the interaction using the proximity ligation assay in both HEK293 cells and in frozen human liver sections. We investigated the functional consequences of OATP1B3 hetero-oligomerization by measuring the OATP1B3 plasma membrane expression and the uptake of the OATP1B3 selective substrate cholecystokinin-8 (CCK-8) in the absence and presence of OATP1B1, NTCP, and OCT1. A significant decrease of OATP1B3 plasma membrane expression was observed after coexpression with OCT1, whereas coexpression with OATP1B1 or NTCP resulted in an increase of plasma membrane expression. With respect to transport, coexpression of OCT1 increased the apparent turnover rate of OATP1B3, whereas coexpression of OATP1B1 or NTCP decreased it. These findings demonstrated that coexpression of OATP1B3 with OATP1B1, NTCP, and OCT1 in HEK293 cells results in a transporter-dependent modification of OATP1B3-mediated CCK-8 transport and suggest that functional results obtained in single transporter overexpressing cell lines over- or underestimate OATP1B3 function in human hepatocytes. SIGNIFICANCE STATEMENT: Coexpression of organic anion transporting polypeptide (OATP) 1B3 with organic cation transporter (OCT) 1, Na⁺/taurocholate cotransporting polypeptide, or OATP1B1 in human embryonic kidney 293T cells affects its expression level and function. When OCT1 is knocked down in human hepatocytes, function of OATP1B3 goes up. These results suggest that protein-protein interactions can affect the expression and function of the involved proteins, and thus single transporter expression systems might lead to over- or underestimation of drug-drug interactions.

The influence of genetic polymorphisms in drug metabolism enzymes and transporters on the pharmacokinetics of different fluvastatin formulations

The purpose of the present study was to investigate the impact of genetic polymorphism on fluvastatin pharmacokinetics. In addition, we compared the fluvastatin pharmacokinetics differences between extended-release (ER) 80 mg tablet and immediate-release (IR) 40 mg capsule in terms of drug metabolism enzyme and transporter genetic polymorphisms. In this open-label, randomized, two-period, two-treatment, crossover study (n = 24), effects of ABCG2, SLCO1B1, ABCB1, CYP2C9 and CYP3A5 polymorphisms on the pharmacokinetics of fluvastatin were analyzed. The administration dosage for IR 40 mg and ER 80 mg were twice and once daily, respectively, for total 7 d. Blood samples for pharmacokinetic evaluation were taken on the 1st and 7th d. The lower exposure following ER was observed. For ER tablets, SLCO1B1 T521C genotype correlated with AUC_0-24 of repeat doses (P = 0.010). SLCO1B1 T521C genotype had no statistically significant effect on AUC_0-24 of IR capsule of fluvastatin after single or repeated doses. In vitro study demonstrated that when the concentration of fluvastatin was low (< 1 µmol/l), the uptake of fluvastatin in the HEK293-OATP1B1 with SLCO1B1 521TT (K_m =0.18 µmol/l) was faster than that with SLCO1B1 521CC (K_m =0.49 µmol/l), On the other hand, when concentration reached to higher level (> 1 µmol/l), transport velocity of fluvastatin by HEK293-OATP1B1 with SLCO1B1 521TT (K_m  = 11.4 µmol/l) and with SLCO1B1 521TCC (K_m =15.1 µmol/l) tend to be the same. It suggests that the increased effect of SLCO1B1 T521C genotype on ER formulation of fluvastatin was mainly caused by lower blood concentrations. We recommend that formulation should be incorporated into future pharmacogenomics studies.

Investigation of the interactions between flavonoids and human organic anion transporting polypeptide 1B1 using fluorescent substrate and 3D-QSAR analysis

Organic anion transporting polypeptide 1B1 (OATP1B1) is a key hepatic uptake transporter whose inhibition could lead to adverse drug-drug and drug-food interactions. Flavonoids are widely distributed in food and beverages and thus our bodies are frequently exposed to them. Therefore, investigation of the interactions between OATP1B1 and flavonoids could be of great significance. In the present study, 25 common flavonoids were investigated for their interactions with OATP1B1 using the fluorescent substrate 2',7'-dichlorofluorescein (DCF) and three-dimensional quantitative structure-activity relationship (3D-QSAR) analysis. Kinetic study showed that OATP1B1-mediated DCF uptake exhibited a monophasic saturation kinetics with a K_m value of 9.7 ± 2.4 μM. Inhibition assay for flavonoids on OATP1B1-mediated DCF uptake was performed and their IC₅₀ values were determined upon which reliable and predictive CoMFA (q² = 0.604, r² = 0.841) and CoMSIA (q² = 0.534, r² = 0.807) models were developed. Our experimental and computational results showed that flavonoid aglycones interacted with OATP1B1 much stronger than their glycosides such as 3-O- and 7-O-glycosides as bulky hydrophilic and hydrogen-bond forming substituents at C-3 and C-7 positions on rings A and C were unfavorable for their binding. On the other hand, the presence of hydrogen-bond forming groups on ring B was beneficial as long as the number of hydroxyl groups was not >2. Our results also indicated that flavones usually interacted with OATP1B1 much stronger than their 3-hydroxyflavone counterparts (flavonols). The obtained information and 3D-QSAR models could be useful for elucidating and predicting the interactions between flavonoids and human OATP1B1.

Drug interaction study of flavonoids toward OATP1B1 and their 3D structure activity relationship analysis for predicting hepatoprotective effects

Organic anion transporting polypeptide 1B1 (OATP1B1), a liver-specific uptake transporter, was associated with drug induced liver injury (DILI). Screening and identifying potent OATP1B1 inhibitors with little toxicity is of great value in reducing OATP1B1-mediated DILI. Flavonoids are a group of polyphenols ubiquitously present in vegetables, fruits and herbal products, some of them were reported to produce transporter-mediated DDI. Our objective was to investigate potential inhibitors of OATP1B1 from 99 flavonoids, and to assess the hepatoprotective effects on bosentan induced liver injury. Eight flavonoids, including biochanin A, hispidulin, isoliquiritigenin, isosinensetin, kaempferol, licochalcone A, luteolin and sinensetin exhibited significant inhibition (>50 %) on OATP1B1 in OATP1B1-HEK293 cells, which reduced the OATP1B1-mediated influx of methotrexate, accordingly decreased its cytotoxicity in OATP1B1-HEK293 cells and increased its AUC_0-t in different extents in rats, from 28.27%-82.71 %. In bosentan-induced rat liver injury models, 8 flavonoids reduced the levels of serum total bile acid (TBA) and the liver concentration of bosentan in different degrees. Among them, kaempferol decreased the concentration most significantly, by 54.17 %, which indicated that flavonoids may alleviate bosentan-induced liver injury by inhibiting OATP1B1-mediated bosentan uptake. Furthermore, the pharmacophore model indicated the hydrogen bond acceptors and hydrogen bond donors may play critical role in the potency of flavonoids inhibition on OATP1B1. Taken together, our findings would provide helpful information for predicting the potential risks of flavonoid-containing food/herb-drug interactions in humans and alleviating bosentan -induced liver injury by OATP1B1 regulation.

Role of Oatp2b1 in Drug Absorption and Drug-Drug Interactions

The organic anion transporting polypeptide (OATP)2B1 is localized on the basolateral membrane of hepatocytes and is expressed in enterocytes. Based on its distribution pattern and functional similarity to OATP1B-type transporters, OATP2B1 might have a role in the absorption and disposition of a range of xenobiotics. Although several prescription drugs, including hydroxymethylglutaryl-coenzyme A-CoA reductase inhibitors (statins) such as fluvastatin, are OATP2B1 substrates in vitro, evidence supporting the in vivo relevance of this transporter remains limited, and most has relied on substrate-inhibitor interactions resulting in altered pharmacokinetic properties of the victim drugs. To address this knowledge deficit, we developed and characterized an Oatp2b1-deficient mouse model and evaluated the impact of this transporter on the absorption and disposition of fluvastatin. Consistent with the intestinal localization of Oatp2b1, we found that the genetic deletion or pharmacological inhibition of Oatp2b1 was associated with decreased absorption of fluvastatin by 2- to 3-fold. The availability of a viable Oatp2b1-deficient mouse model provides an opportunity to unequivocally determine the contribution of this transporter to the absorption and drug-drug interaction potential of drugs. SIGNIFICANCE STATEMENT: The current investigation suggests that mice deficient in Oatp2b1 provide a valuable tool to study the in vivo importance of this transporter. In addition, our studies have identified novel potent inhibitors of OATP2B1 among the class of tyrosine kinase inhibitors, a rapidly expanding class of drugs used in various therapeutic areas that may cause drug-drug interactions with OATP2B1 substrates.

Black tea extract and theaflavin derivatives affect the pharmacokinetics of rosuvastatin by modulating organic anion transporting polypeptide (OATP) 2B1 activity

Theaflavins (TFs) are derived from black tea, an important source of dietary polyphenols. Although the potential interactions between dietary polyphenols and drugs have been demonstrated through in vitro and in vivo studies, little information is available concerning the influence of TFs on drug disposition. Organic anion transporting polypeptide 2B1 (OATP2B1) is expressed in human enterocytes and plays a role in the intestinal absorption of numerous drugs. The current study evaluated the effects of black tea extracts on the pharmacokinetics of rosuvastatin in rats, and investigated the effect of four major TFs (theaflavin, theaflavin-3-gallate, theaflavin-3'-gallate and theaflavin-3,3'-digallate) on the transport activity of OATP2B1. Black tea extracts significantly decreased the maximum plasma concentration (C_max ) and area under the plasma concentration-time curve (AUC₀ -₈ ) of rosuvastatin by 48% and 37%, respectively (p < 0.001 and p < 0.01, respectively). Moreover, OATP2B1-mediated rosuvastatin and estrone-3-sulfate uptake was significantly reduced in the presence of TFs. A kinetic study revealed that the uptake efficiency (in terms of V_max /K_m ) of rosuvastatin was decreased following treatment with TFs. Black tea extracts also reduced OATP2B1-mediated rosuvastatin uptake. These results suggest that black tea reduces the plasma concentrations of rosuvastatin by inhibiting the intestinal OATP2B1-mediated transport of rosuvastatin.

Association between rs4149056 variant in SLCO1B1 and early discontinuation of statin after acute myocardial infarction

Data from two French surveys were used to analyze the association between in-hospital statin discontinuation and SLCO1B1 polymorphism (rs4149056) in patients with acute myocardial infarction. Using TaqMan allelic discrimination assay, 1674 and 1708 patients were genotyped for SLCO1B1 in 2005 and 2010, respectively. The association with in-hospital statin discontinuation was assessed after adjusting for confounding factors. In 2005, homozygosity for the reduced-function allele was associated with an increased risk of in-hospital statin discontinuation (OR: 3.68; p = 0.004) compared with the wild-type allele but this association disappeared in 2010. However, statin type and intensity-dose differed significantly between the surveys. SLCO1B1 polymorphism (rs4149056) does not seem to be a major determinant of early ‘in-hospital’ statin discontinuation after acute myocardial infarction.

Transport of Bupropion and its Metabolites by the Model CHO and HEK293 Cell Lines

BACKGROUND

Bupropion (BUP) is widely used as an antidepressant and smoking cessation aid. There are three major pharmacologically active metabolites of BUP, Erythrohydrobupropion (EB), Hydroxybupropion (OHB) and Threohydrobupropion (TB). At present, the mechanisms underlying the overall disposition and systemic clearance of BUP and its metabolites have not been well understood, and the role of transporters has not been studied.

OBJECTIVE

The goal of this study was to investigate whether BUP and its active metabolites are substrates of the major hepatic uptake and efflux transporters.

METHOD

CHO or HEK293 cell lines or plasma membrane vesicles that overexpress OATP1B1, OATP1B3, OATP2B1, OATP4A1, OCT1, BCRP, MRP2 or P-gp were used in cellular or vesicle uptake and inhibition assays. Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) was used to quantify transport activity.

RESULTS

BUP and its major active metabolites were actively transported into the CHO or HEK293 cells overexpressing OATP1B1, OATP1B3 or OATP2B1; however, such cellular active uptake could not be inhibited at all by prototypical inhibitors of any of the OATP transporters. These compounds were not transported by OCT1, BCRP, MRP2 or P-gp either. These results suggest that the major known hepatic transporters likely play a minor role in the overall disposition and systemic clearance of BUP and its active metabolites in humans. We also demonstrated that BUP and its metabolites were not transported by OATP4A1, an uptake transporter on the apical membrane of placental syncytiotrophoblasts, suggesting that OATP4A1 is not responsible for the transfer of BUP and its metabolites from the maternal blood to the fetal compartment across the placental barrier in pregnant women.

CONCLUSION

BUP and metabolites are not substrates of the major hepatic transporters tested and thus these hepatic transporters likely do not play a role in the overall disposition of the drug. Our results also suggest that caution should be taken when using the model CHO and HEK293 cell lines to evaluate potential roles of transporters in drug disposition.

Genomewide Association Study of Statin-Induced Myopathy in Patients Recruited Using the UK Clinical Practice Research Datalink

Statins can be associated with myopathy. We have undertaken a genomewide association study (GWAS) to discover and validate genetic risk factors for statin‐induced myopathy in a “real‐world” setting. One hundred thirty‐five patients with statin myopathy recruited via the UK Clinical Practice Research Datalink were genotyped using the Illumina OmniExpress Exome version 1.0 Bead Chip and compared with the Wellcome Trust Case‐Control Consortium (n = 2,501). Nominally statistically significant single nucleotide polymorphism (SNP) signals in the GWAS (P < 5 × 10⁻⁵) were further evaluated in several independent cohorts (comprising 332 cases and 449 drug‐tolerant controls). Only one (rs4149056/c.521C>T in the SLCO1B1 gene) SNP was genomewide significant in the severe myopathy (creatine kinase > 10 × upper limit of normal or rhabdomyolysis) group (P = 2.55 × 10⁻⁹; odds ratio 5.15; 95% confidence interval 3.13–8.45). The association with SLCO1B1 was present for several statins and replicated in the independent validation cohorts. The data highlight the role of SLCO1B1 c.521C>T SNP as a replicable genetic risk factor for statin myopathy. No other novel genetic risk factors with a similar effect size were identified.

A Comprehensive Framework for Physiologically-Based Pharmacokinetic Modeling in Matlab

Physiologically-based pharmacokinetic (PBPK) models are useful tools to predict clinical scenarios for special populations for whom there are high hurdles to conduct clinical trials such as children or the elderly. However, the coding of a PBPK model in a mathematical programming language can be challenging. This tutorial illustrates how to build a whole-body PBPK model in Matlab to answer specific pharmacological questions involving drug disposition and magnitudes of drug-drug interactions in different patient populations.

Amino-terminal region of human organic anion transporting polypeptide 1B1 dictates transporter stability and substrate interaction

Organic anion transporting polypeptides (OATPs) are key players of drug absorption, distribution and excretion due to their broad substrate specificity, wide tissue distribution and the involvement in drug-drug interaction. OATP1B1 is specifically localized at the basolateral membrane of human hepatocytes and serves a crucial role in the drug clearance from the body. Previous studies have shown that transmembrane domains (TMs) are essential for proper functions of OATPs. In the present study, site-directed mutagenesis was performed to study the TM1 and amino-terminus of OATP1B1. Two positively charged residues, K41 and K49, as well as a hydrophobic residue I46, in TM1 were identified to be important for the proper function of the transporter. K41A and K49A exhibited altered Km value at the high and low affinity binding sites of estrone-3- sulfate (ES), respectively; while alanine substitution of I46 showed altered Km and Vmax values for both binding components of ES. Additional replacement of K41 revealed that the positively charged property at this position is important for maintaining OATP1B1 protein level and function; while the specific side-group structure of lysine at position 49 is irreplaceable for the transporter activity. Conservative replacement of I46 with leucine also recovered the function of the transporter. In addition, studies of the amino-terminus of OATP1B1 revealed that residues ranging from 19 to 27 are essential for protein stability and substrate interaction. Therefore, the amino-terminal region, which includes TM1 and the amino-terminus of OATP1B1, is important for proper function of the membrane protein.

The intracellular NPxY motif is critical in maintaining the function and expression of human organic anion transporting polypeptide 1B1

Organic anion transporting polypeptides (OATPs, gene symbol SLCO) mediate sodium-independent transport of endogenous compounds such as bile salts, hormones and their conjugates as well as toxins and drugs. OATP1B1 is the major OATP specifically expressed at the basolateral membrane of human hepatocytes and many clinically important drugs have been shown to be substrates of the transporter. According to the computer-based hydropathy analysis, a large intracellular loop 3 (IL3) is situated between transmembrane domain 6 and 7 of OATPs, in which a conserved NPxY motif is found. In the current study, HEK293 cells expressing the HA-tagged OATP1B1 was utilized to investigate the role of the NPxY motif for the function and expression of the transporter. Alanine replacement of N335 or P336 retained substantial uptake function; while simultaneous mutation of these residues resulted in a double mutant that lost almost all the transport activity. On the other hand, Y338A showed >80% reduction for estrone-3-sulfate uptake. Plasma membrane protein analysis revealed that N335/P336A completely lost its cell surface protein expression; while that of Y338A is dramatically reduced. Further investigation with pharmacological inhibitors and immunocytochemistry demonstrated that N335/336A is detained in the Golgi apparatus and Y338A exhibited accelerated protein degradation rate compared to that of the wild-type. Conservative replacement of Y338 with phenylalanine fully recovered uptake and expression of the transporter. In summary, a new role was observed for the NPxY motif located in the IL3 of OATP1B1, which may affect processing and stability of the transporter.

Multiple circulating saponins from intravenous ShenMai inhibit OATP1Bs in vitro: potential joint precipitants of drug interactions

ShenMai, an intravenous injection prepared from steamed Panax ginseng roots (Hongshen) and Ophiopogon japonicus roots (Maidong), is used as an add-on therapy for coronary artery disease and cancer; saponins are its bioactive constituents. Since many saponins inhibit human organic anion-transporting polypeptides (OATP)1B, this investigation determined the inhibition potencies of circulating ShenMai saponins on the transporters and the joint potential of these compounds for ShenMai-drug interaction. Circulating saponins and their pharmacokinetics were characterized in rats receiving a 30-min infusion of ShenMai at 10 mL/kg. Inhibition of human OATP1B1/1B3 and rat Oatp1b2 by the individual saponins was investigated in vitro; the compounds' joint inhibition was also assessed in vitro and the data was processed using the Chou-Talalay method. Plasma protein binding was assessed by equilibrium dialysis. Altogether, 49 saponins in ShenMai were characterized and graded into: 10-100 μmol/day (compound doses from ShenMai; 7 compounds), 1-10 μmol/day (17 compounds), and <1 μmol/day (25 compounds, including Maidong ophiopogonins). After dosing, circulating saponins were protopanaxadiol-type ginsenosides Rb₁, Rb₂, Rc, Rd, Ra₁, Rg₃, Ra₂, and Ra₃, protopanaxatriol-type ginsenosides Rg₁, Re, Rg₂, and Rf, and ginsenoside Ro. The protopanaxadiol-type ginsenosides exhibited maximum plasma concentrations of 2.1-46.6 μmol/L, plasma unbound fractions of 0.4-1.0% and terminal half-lives of 15.6-28.5 h (ginsenoside Rg₃, 1.9 h), while the other ginsenosides exhibited 0.1-7.7 μmol/L, 20.8-99.2%, and 0.2-0.5 h, respectively. The protopanaxadiol-type ginsenosides, ginsenosides without any sugar attachment at C-20 (except ginsenoside Rf), and ginsenoside Ro inhibited OATP1B3 more potently (IC₅₀, 0.2-3.5 µmol/L) than the other ginsenosides (≥22.6 µmol/L). Inhibition of OATP1B1 by ginsenosides was less potent than OATP1B3 inhibition. Ginsenosides Rb₁, Rb₂, Rc, Rd, Ro, Ra₁, Re, and Rg₂ likely contribute the major part of OATP1B3-mediated ShenMai-drug interaction potential, in an additive and time-related manner.

The association between the SLCO1B1, apolipoprotein E, and CYP2C9 genes and lipid response to fluvastatin: a meta-analysis

OBJECTIVE

The aim of this study was to determine the impact of the SLCO1B1, apolipoprotein E (ApoE), and CYP2C9 genotypes on the lipid-lowering efficacy of fluvastatin.

METHODS

We performed electronic searches on the PubMed, Embase, and Cochrane Library databases to identify studies published through October 2017. Studies that reported the effect estimates with 95% confidence intervals (CIs) of total cholesterol (TC), triglyceride, low-density lipoprotein (LDL), and high-density lipoprotein were included so that the different genotype categories could be compared. Weighted mean difference (WMD) was used to summarize the effect estimates.

RESULTS

Six studies, involving a total of 1171 individuals, were included in the final analysis. We noted that the patient carrier SLCO1B1 521TT was associated with greater change in TC (WMD: -2.98; 95% CI: -5.12 to -0.84; P=0.006) and LDL (WMD: -5.58; 95% CI: -10.64 to -0.52; P=0.031) compared with 521TC or CC. Furthermore, the patient carrier ApoE*2/*3 showed more change in high-density lipoprotein compared with ApoE*3/*3 (WMD: 18.76; 95% CI: 8.97-28.55; P<0.001) and ApoE*3/*4 or *4/*4 (WMD: 22.51; 95% CI: 0.98-44.04; P=0.040). Finally, the CYP2C9 genotypes showed no correlation with the effects of fluvastatin on TC, triglyceride, and LDL.

CONCLUSION

The findings of this study suggested that the SLCO1B1 and ApoE polymorphisms could influence the lipid-lowering effect of fluvastatin, whereas the CYP2C9 genotypes were not associated with the therapeutic effects of fluvastatin.

Novel in Vitro Method Reveals Drugs That Inhibit Organic Solute Transporter Alpha/Beta (OSTα/β)

Drug interactions with the organic solute transporter alpha/beta (OSTα/β) are understudied even though OSTα/β is an important transporter that is expressed in multiple human tissues including the intestine, kidneys, and liver. In this study, an in vitro method to identify novel OSTα/β inhibitors was first developed using OSTα/β-overexpressing Flp-In 293 cells. Incubation conditions were optimized using previously reported OSTα/β inhibitors. A method including a 10 min preincubation step with the test compound was used to screen for OSTα/β inhibition by 77 structurally diverse compounds and fixed-dose combinations. Seven compounds and one fixed-dose combination (100 μM final concentration) inhibited OSTα/β-mediated dehydroepiandrosterone sulfate (DHEAS) uptake by >25%. Concentration-dependent OSTα/β inhibition was evaluated for all putative inhibitors (atorvastatin, ethinylestradiol, fidaxomicin, glycochenodeoxycholate, norgestimate, troglitazone, and troglitazone sulfate). Ethinylestradiol, fidaxomicin, and troglitazone sulfate yielded a clear concentration-inhibition response with IC50 values <200 μM. Among all tested compounds, there was no clear association between physicochemical properties, the severity of hepatotoxicity, and the degree of OSTα/β inhibition. This study utilized a novel in vitro method to identify OSTα/β inhibitors and, for the first time, provided IC50 values for OSTα/β inhibition. These data provide evidence that several drugs, some of which are associated with cholestatic drug-induced liver injury, may impair the function of the OSTα/β transporter.

Current Research Method in Transporter Study

Transporters play an important role in the absorption, distribution, metabolism, and excretion (ADME) of drugs. In recent years, various in vitro, in situ/ex vivo, and in vivo methods have been established for studying transporter function and drug-transporter interaction. In this chapter, the major types of in vitro models for drug transport studies comprise membrane-based assays, cell-based assays (such as primary cell cultures, immortalized cell lines), and transporter-transfected cell lines with single transporters or multiple transporters. In situ/ex vivo models comprise isolated and perfused organs or tissues. In vivo models comprise transporter gene knockout models, natural mutant animal models, and humanized animal models. This chapter would be focused on the methods for the study of drug transporters in vitro, in situ/ex vivo, and in vivo. The applications, advantages, or limitations of each model and emerging technologies are also mentioned in this chapter.

Hepatic Organic Anion Transporting Polypeptide-Mediated Clearance in the Beagle Dog: Assessing In Vitro-In Vivo Relationships and Applying Cross-Species Empirical Scaling Factors to Improve Prediction of Human Clearance

In the present study, the beagle dog was evaluated as a preclinical model to investigate organic anion transporting polypeptide (OATP)-mediated hepatic clearance. In vitro studies were performed with nine OATP substrates in three lots of plated male dog hepatocytes ± OATP inhibitor cocktail to determine total uptake clearance (CL_uptake) and total and unbound cell-to-medium concentration ratio (Kp_uu). In vivo intrinsic hepatic clearances (CL_int,H) were determined following intravenous drug administration (0.1 mg/kg) in male beagle dogs. The in vitro parameters were compared with those previously reported in plated human, monkey, and rat hepatocytes; the ability of cross-species scaling factors to improve prediction of human in vivo clearance was assessed. CL_uptake in dog hepatocytes ranged from 9.4 to 135 µl/min/10⁶ cells for fexofenadine and telmisartan, respectively. Active process contributed >75% to CL_uptake for 5/9 drugs. Rosuvastatin and valsartan showed Kp_uu > 10, whereas cerivastatin, pitavastatin, repaglinide, and telmisartan had Kp_uu < 5. The extent of hepatocellular binding in dog was consistent with other preclinical species and humans. The bias (2.73-fold) obtained from comparison of predicted versus in vivo dog CL_int,H was applied as an average empirical scaling factor (ESF_av) for in vitro-in vivo extrapolation of human CL_int,H The ESF_av based on dog reduced underprediction of human CL_int,H for the same data set (geometric mean fold error = 2.1), highlighting its utility as a preclinical model to investigate OATP-mediated uptake. The ESF_av from all preclinical species resulted in comparable improvement of human clearance prediction, in contrast to drug-specific empirical scalars, rationalized by species differences in expression and/or relative contribution of particular transporters to drug hepatic uptake.

Ligand-dependent modulation of hOCT1 transport reveals discrete ligand binding sites within the substrate translocation channel

The human hepatic organic cation transporter 1 (hOCT1) is a well-known transporter of both xenobiotic and endogenous cations. The substrates and inhibitors of hOCT1 are structurally and physiochemically diverse and include some widely prescribed drugs (metformin and imatinib), vitamins (thiamine), and neurotransmitters (serotonin). It has been demonstrated that the closely related renal isoform, hOCT2, is subject to ligand-dependent modulation, wherein one ligand may enhance or inhibit transport of a second, chemically unrelated, ligand. This phenomenon has important implications for drug-drug interactions due to the ubiquity of polypharmacy and the large number of drugs that are present as cations under physiological conditions. Therefore, the objective of this study was to determine if hOCT1 is subject to the same ligand-dependent modulation as hOCT2, and to identify unique putative ligand binding sites in the translocation channel for a sub-set of ligands using computational modeling. The competitive counter flow (CCF) assay was employed to examine ligand-dependent effects by utilizing four different radiolabeled probe substrates: MPP+, serotonin, metformin, and TEA. We identified 20 ligands that modulated the transport of the four test substrates examined. One of the putative ligands identified, BSP, is an anion at physiological pH. Direct uptake studies of radiolabeled BSP suggested that it is a hOCT1 substrate with a Km of 13.6 ± 2.6 µM and Vmax of 55.1 ± 4.1 pmol/mg protein/min. Each ligand identified was computationally docked into a homology model of hOCT1 using the UCSF DOCK software package. The docking study revealed three separate ligand binding pockets within the hOCT1 translocation pathway, defined by their interactions with three prototypical substrates: MPP+, TEA, and acyclovir. Our results suggest that hOCT1 is not only subject to ligand-dependent modulation, but also that individual ligand binding occurs at discrete sites within the hOCT1 translocation pathway which may influence ligand binding at the other sites.

A Strategy to Refine the Phenotyping Approach and Its Implementation to Predict Drug Clearance: A Physiologically Based Pharmacokinetic Simulation Study

The phenotyping approach to predict drug metabolism activity is often hampered by a lack of correlation between the probe and the drug of interest. In this article, we present a strategy to refine the phenotyping approach based on a physiologically based pharmacokinetic simulation (implemented in Simcyp Simulator version 17) using previously published models. The apparent clearance (CL/F) of erlotinib was better predicted by the sum of caffeine and i.v. midazolam CL/F (r²  = 0.60) compared to that of either probe drug alone. The clearance of atorvastatin and repaglinide had a strong correlation (r²  = 0.70 and 0.63, respectively) with that of pitavastatin (a SLCO1B1 probe). Use of multiple probes for drugs that are predominantly metabolized by more than one cytochrome P450 (CYP) enzyme should be considered. In a case in which hepatic uptake transporters play a significant role in the disposition of a drug, the pharmacokinetic of a transporter probe will provide better predictions of the drug clearance.

In vivo hepatic clearance of lipophilic drugs predicted by in vitro uptake data into cryopreserved hepatocytes suspended in sera of rats, guinea pigs, monkeys and humans

Evaluation of uptake of lipophilic acid compounds into hepatocytes was an unresolved drug development issue because of their adsorption to cells and materials and low analytical sensitivity and accuracy in assessment of protein bindings. Uptake assays of compounds using hepatocytes suspended in serum were expected to solve these problems for prediction of in vivo hepatic clearance. Here, for compounds with high protein binding (>99%), diflunisal, montelukast, cerivastatin, telmisartan, fluvastatin and six new drug candidates, in vivo hepatic clearance predicted based on hepatic depletion and uptake (CL_{h, uptake, predicted}) data using hepatocytes in the absence and presence of sera was investigated. In vitro hepatic uptake results with hepatocytes suspended in serum improved prediction of human hepatic clearance values for highly lipophilic montelukast and telmisartan. In vivo CL_{h, uptake, predicted} values of six new highly lipophilic acid drug candidates (protein binding >99.97%) and diflunisal, montelukast and cerivastatin predicted based on hepatocytes suspended in serum were within threefold differences of their total clearance in vivo in rats, guinea pigs or monkeys, except for montelukast in monkeys (5.8-fold). These results suggest that the human hepatic uptake in hepatocytes suspended in serum is useful for prediction of CL_{h, uptake, predicted}, especially for highly lipophilic/protein binding acid compounds.

Glycyrrhizin has a high likelihood to be a victim of drug-drug interactions mediated by hepatic organic anion-transporting polypeptide 1B1/1B3

BACKGROUND AND PURPOSE

Intravenous glycyrrhizin, having anti-inflammatory and hepatoprotective properties, is incorporated into the management of liver diseases in China. This investigation was designed to elucidate the molecular mechanism underlying hepatobiliary excretion of glycyrrhizin and to investigate its potential for drug-drug interactions on organic anion-transporting polypeptide (OATP)1B.

EXPERIMENTAL APPROACH

Human transporters mediating hepatobiliary excretion of glycyrrhizin were characterized at the cellular and vesicular levels and compared with rat hepatic transporters. The role of Oatp1b2 in glycyrrhizin's elimination and pharmacokinetics was evaluated in rats using the inhibitor rifampin. A physiologically based pharmacokinetic (PBPK) model for glycyrrhizin, incorporating transporter-mediated hepatobiliary excretion, was established and applied to predict potential drug-drug interactions related to glycyrrhizin in humans.

KEY RESULTS

Hepatobiliary excretion of glycyrrhizin involved human OATP1B1/1B3 (Oatp1b2 in rats)-mediated hepatic uptake from blood and human multidrug resistance-associated protein (MRP)2/breast cancer resistance protein (ABCP)/bile salt export pump (BSEP)/multidrug resistance protein 1 (Mrp2/Abcp/Bsep in rats)-mediated hepatic efflux into bile. In rats, rifampin impaired hepatic uptake of glycyrrhizin significantly increasing its systemic exposure. Glomerular-filtration-based renal excretion of glycyrrhizin was slow due to extensive protein binding in plasma. Quantitative analysis using the PBPK model demonstrated that OATP1B1/1B3 have critical roles in the pharmacokinetics of glycyrrhizin, which is highly likely to be a victim of drug-drug interactions when co-administered with potent dual inhibitors of these transporters.

CONCLUSIONS AND IMPLICATIONS

Transporter-mediated hepatobiliary excretion governs glycyrrhizin's elimination and pharmacokinetics. Understanding glycyrrhizin's potential drug-drug interactions on OATP1B1/1B3 should enhance the therapeutic outcome of glycyrrhizin-containing drug combinations on liver diseases.

An update on efflux and uptake transporters as determinants of statin response

INTRODUCTION

Statins are used in the treatment of dyslipidemia promoting primary and secondary prevention against detrimental cardiovascular events. ATP-binding cassette (ABC) and solute carrier (SLC) membrane transporters transport statins across the cell membrane. Differences in drug transporter tissue expression and activity contribute to variability in statin pharmacokinetics (PK) and response. Areas covered: The purpose of this review is to discuss factors impacting transporter expression and the effect this has on statin efficacy and safety. Previous studies have demonstrated that genetic polymorphisms, drug-drug interactions (DDI), nuclear receptors, and microRNAs affect statin PK and pharmacodynamics. Expert opinion: Genetic variants of ABCG2 and SLCO1B1 transporters affect statin PK and, as a result, the intended lipid-lowering response. However, the effect size is small, limiting its applicability in clinical practice. Furthermore, genetic variants do not totally explain the observed intervariability in statin response. Thus, it is likely that transcriptional and post-transcriptional regulation of drug transporters are also highly involved. Further studies are required to understand the contribution of each of these new factors in statin disposition and toxicity.

Simultaneous Assessment of Transporter-Mediated Drug-Drug Interactions Using a Probe Drug Cocktail in Cynomolgus Monkey

We aim to establish an in vivo preclinical model to enable simultaneous assessment of inhibition potential of an investigational drug on clinically relevant drug transporters, organic anion-transporting polypeptide (OATP)1B, breast cancer resistance protein (BCRP), P-glycoprotein (P-gp), and organic anion transporter (OAT)3. Pharmacokinetics of substrate cocktail consisting of pitavastatin (OATP1B substrate), rosuvastatin (OATP1B/BCRP/OAT3), sulfasalazine (BCRP), and talinolol (P-gp) were obtained in cynomolgus monkey-alone or in combination with transporter inhibitors. Single-dose rifampicin (30 mg/kg) significantly (P < 0.01) increased the plasma exposure of all four drugs, with a marked effect on pitavastatin and rosuvastatin [area under the plasma concentration-time curve (AUC) ratio ∼21-39]. Elacridar, BCRP/P-gp inhibitor, increased the AUC of sulfasalazine, talinolol, as well as rosuvastatin and pitavastatin. An OAT1/3 inhibitor (probenecid) significantly (P < 0.05) impacted the renal clearance of rosuvastatin (∼8-fold). In vitro, rifampicin (10 µM) inhibited uptake of pitavastatin, rosuvastatin, and sulfasalazine by monkey and human primary hepatocytes. Transport studies using membrane vesicles suggested that all probe substrates, except talinolol, are transported by cynoBCRP, whereas talinolol is a cynoP-gp substrate. Elacridar and rifampicin inhibited both cynoBCRP and cynoP-gp in vitro, indicating potential for in vivo intestinal efflux inhibition. In conclusion, a probe substrate cocktail was validated to simultaneously evaluate perpetrator impact on multiple clinically relevant transporters using the cynomolgus monkey. The results support the use of the cynomolgus monkey as a model that could enable drug-drug interaction risk assessment, before advancing a new molecular entity into clinical development, as well as providing mechanistic insights on transporter-mediated interactions.