Knocking down breast cancer resistance protein (Bcrp) by adenoviral vector-mediated RNA interference (RNAi) in sandwich-cultured rat hepatocytes: a novel tool to assess the contribution of Bcrp to drug biliary excretion

Development of a Rat Sandwich-Cultured Hepatocytes Model Expressing Functional Human Organic Anion Transporting Polypeptide (OATP) 1B3: A Potential Screening Tool for Liver-Targeting Compounds

PURPOSE

Organic anion transporting polypeptide (OATP) 1B3 transports many clinically important drugs, including statins, from blood into the liver. It exclusively expresses in human liver under normal physiological conditions. There is no rodent ortholog of human OATP1B3. Tissue targeting of therapeutic molecules mediated by transporters, including liver-targeting via liver-specific OATPs, is an emerging area in drug development. Sandwich-cultured primary hepatocytes (SCH) are a well characterized in vitro model for assessment of hepatic drug uptake and biliary excretion. The current study was designed to develop a novel rat SCH model expressing human OATP1B3 to study the hepatic disposition of OATP1B3 substrates.

METHODS

Primary rat hepatocytes transduced with adenoviral vectors expressing FLAG-tagged OATP1B3 (Ad-OATP1B3), a control vector Ad-LacZ, or that were non-transduced were cultured in a sandwich configuration. FLAG immunoblot and immunofluorescence-staining determined expression and localization of OATP1B3. Uptake of [3H]-cholecystokinin octapeptide (CCK-8), a specific OATP1B3 substrate, was determined. Taurocholate (TC) is a substrate routinely used in SCH to assess biliary excretion via bile canaliculi (BC) and is also a substrate of OATP1B3. [3H]-TC accumulation in cells+BC, cells, biliary excretion index (BEI) and in vitro Clbiliary were determined using B-CLEAR® technology.

RESULTS

OATP1B3 protein was extensively expressed and primarily localized on the plasma membrane in day 4 Ad-OATP1B3-transduced rat SCH. [3H]-CCK-8 accumulation in cells+BC was significantly greater (~5-13 folds, p<0.001) in day 4 SCH with vs. without Ad-OATP1B3-transduction. Expressing OATP1B3 in rat SCH significantly increased [3H]-TC accumulation in cells+BC and cells, without affecting BEI and in vitro Clbiliary.

CONCLUSIONS

Rat SCH expressing human OATP1B3-is a novel in vitro model allowing simultaneous assessment of hepatic uptake, hepatocellular accumulation and biliary excretion process of a human OATP1B3 substrate. This model could be a potential tool for screening for liver-targeting compounds mediated by OATP1B3.

Expanding the Efflux In Vitro Assay Toolbox: A CRISPR-Cas9 Edited MDCK Cell Line with Human BCRP and Completely Lacking Canine MDR1

The Breast Cancer Resistance Protein (BCRP) is a key transporter in drug efflux and drug-drug interactions. However, endogenous expression of Multidrug Resistance Protein 1 (MDR1) confounds the interpretation of BCRP-mediated transport in in vitro models. Here we used a CRISPR-Cas9 edited Madin-Darby canine kidney (MDCK) II cell line (MDCK^cMDR1-KO) for stable expression of human BCRP (hBCRP) with no endogenous canine MDR1 (cMDR1) expression (MDCK-hBCRP^cMDR1-KO). Targeted quantitative proteomics verified expression of hBCRP, and global analysis of the entire proteome corroborated no or very low background expression of other drug transport proteins or metabolizing enzymes. This new cell line, had similar proteome like MDCK^cMDR1-KO and a previously established, corresponding cell line overexpressing human MDR1 (hMDR1), MDCK-hMDR1^cMDR1-KO. Functional studies with MDCK-hBCRP^cMDR1-KO confirmed high hBCRP activity. The MDCK-hBCRP^cMDR1-KO cell line together with the MDCK-hMDR1^cMDR1-KO easily and accurately identified shared or specific substrates of the hBCRP and the hMDR1 transporters. These cell lines offer new, improved in vitro tools for the assessment of drug efflux and drug-drug interactions in drug development.

Sandwich-Cultured Hepatocytes as a Tool to Study Drug Disposition and Drug-Induced Liver Injury

Sandwich-cultured hepatocytes (SCH) are metabolically competent and have proper localization of basolateral and canalicular transporters with functional bile networks. Therefore, this cellular model is a unique tool that can be used to estimate biliary excretion of compounds. SCH have been used widely to assess hepatobiliary disposition of endogenous and exogenous compounds and metabolites. Mechanistic modeling based on SCH data enables estimation of metabolic and transporter-mediated clearances, which can be used to construct physiologically based pharmacokinetic models for prediction of drug disposition and drug-drug interactions in humans. In addition to pharmacokinetic studies, SCH also have been used to study cytotoxicity and perturbation of biological processes by drugs and hepatically generated metabolites. Human SCH can provide mechanistic insights underlying clinical drug-induced liver injury (DILI). In addition, data generated in SCH can be integrated into systems pharmacology models to predict potential DILI in humans. In this review, applications of SCH in studying hepatobiliary drug disposition and bile acid-mediated DILI are discussed. An example is presented to show how data generated in the SCH model were used to establish a quantitative relationship between intracellular bile acids and cytotoxicity, and how this information was incorporated into a systems pharmacology model for DILI prediction.

Hepatic drug transporters: the journey so far

INTRODUCTION

The key role of transporter biology in both the manifestation and treatment of disease is now firmly established. Experiences of sub-optimal drug exposure due to drug-transporter interplay have supported incorporation of studies aimed at understanding the interactions between compounds and drug transporters much earlier in drug discovery. While drug transporters can impact the most pivotal pharmacokinetic parameter with respect to human dose and exposure projections, clearance, at a renal or hepatobiliary level, the latter will form the focus of this perspective.

AREAS COVERED

A synopsis of guidelines on which transporters to study together with an overview of the currently available toolkit is presented. A perspective on when to conduct studies with various hepatic transporters is also provided together with structural "alerts" which should prompt early investigation.

EXPERT OPINION

Great progress has been made in individual laboratories and via consortia to understand the role of drug transporters in disease, drug disposition, drug-drug interactions and toxicity. A systematic analysis of the value posed by the available approaches and an inter-lab comparison now seems warranted. The emerging ability to use physico-chemical properties to guide future screening cascades promises to revolutionise the efficiency of early drug discovery.

IPEC-J2 MDR1, a Novel High-Resistance Cell Line with Functional Expression of Human P-glycoprotein (ABCB1) for Drug Screening Studies

The P-glycoprotein (P-gp) efflux pump has been shown to affect drug distribution and absorption in various organs and to cause drug resistance in cancer therapy. The aim of this work was to develop a cell line to serve as a screening system for potential substrates of P-gp. This requires a cell line with high paracellular tightness, low expression of nonhuman ABC transporters, and high expression of functional human P-gp (ABCB1). The porcine intestinal epithelial cell line, IPEC-J2, was selected as a transfection host, due to its ability to form extremely high-resistance monolayers (>10,000 Ω·cm(2)) and its low endogenous expression of ABC-type efflux transporters. The IPEC-J2 cells were transfected with a plasmid that contained the sequence of the human MDR1 gene, which encodes P-gp, followed by a selection of successfully transfected cells with geneticin and puromycin. The resulting cell line, IPEC-J2 MDR1, retained its high transepithelilal resistance (>15,000 Ω·cm(2)), which translated into low permeability of the small hydrophilic tracer, mannitol (P < 10(-7) cm·s(-1)). The lipophilic compound, diazepam, displayed high permeability resulting in a dynamic range of 1500 (PDiazepam/Pmannitol) to separate high and low permeability compounds. Human P-gp was expressed predominantly in the apical membrane, as demonstrated by immunocytochemistry, Western blots, and a high efflux ratios (Pbasolateral-apical/Papical-basolateral) of known P-gp substrates. P-gp was demonstrated to be responsible for the efflux transport by substrate profiling, combined with application of P-gp and BCRP inhibitors. Furthermore, the compounds atenolol, citalopram, and mitoxantrone were identified as P-gp substrates. Functional P-gp expression was shown to be stable through at least 10 cell passages. In conclusion, the IPEC-J2 MDR1 cell line displays high paracellular tightness combined with high expression of human P-gp and low expression of porcine ABC transporters, and it may serve as a useful tool in drug development studies.

In vitro model systems to investigate bile salt export pump (BSEP) activity and drug interactions: A review

The bile salt export pump protein (BSEP), expressed on the canalicular membranes of hepatocytes, is primarily responsible for the biliary excretion of bile salts. The inhibition of BSEP transport activity can lead to an increase in intracellular bile salt levels and liver injury. This review discusses the various in vitro assays currently available for assessing the effect of drugs or other chemical entities to modulate BSEP transport activity. BSEP transporter assays use one of the following platforms: Xenopus laevis oocytes; canalicular membrane vesicles (CMV); BSEP-expressed membrane vesicles; cell lines expressing BSEP; sandwich cultured hepatocytes (SCH); and hepatocytes in suspension. Two of these, BSEP-expressed insect membrane vesicles and sandwich cultured hepatocytes, are the most commonly used assays. BSEP membrane vesicles prepared from transfected insect cells are useful for assessing BSEP inhibition or substrate specificity and exploring mechanisms of BSEP-associated genetic diseases. This model can be applied in a high-throughput format for discovery-drug screening. However, experimental results from use of membrane vesicles may lack physiological relevance and the model does not allow for investigation of in situ metabolism in modulation of BSEP activity. Hepatocyte-based assays that use the SCH format provide results that are generally more physiologically relevant than membrane assays. The SCH model is useful in detailed studies of the biliary excretion of drugs and BSEP inhibition, but due to the complexity of SCH preparation, this model is used primarily for determining biliary clearance and BSEP inhibition in a limited number of compounds. The newly developed hepatocyte in suspension assay avoids many of the complexities of the SCH method. The use of pooled cryopreserved hepatocytes in suspension minimizes genetic variance and individual differences in BSEP activity and also provides the opportunity for higher throughput screening and cross-species comparisons.

Small interfering RNA delivery by polyethylenimine-functionalised porous silicon nanoparticles

Polyethyleneimine-coated mesoporous silicon nanoparticles efficiently deliver siRNA in glioblastoma cells, subsequently reducing the protein expression of a chemotherapy resistance gene by 70% within 72 hours.

Hepatocellular exposure of troglitazone metabolites in rat sandwich-cultured hepatocytes lacking Bcrp and Mrp2: interplay between formation and excretion

Inhibition of bile acid transport by troglitazone (TGZ) and its major metabolite, TGZ sulfate (TS), may lead to hepatocellular accumulation of toxic bile acids; TS accumulation and hepatotoxicity may be associated with impaired TS biliary excretion. This study evaluated the impact of impaired transport of breast cancer resistance protein (Bcrp) and multidrug resistance-associated protein 2 (Mrp2) on the hepatobiliary disposition of generated metabolites, TS and TGZ glucuronide (TG). Sandwich-cultured hepatocytes (SCH) from Mrp2-deficient (TR(-)) rats in combination with Bcrp knockdown using RNA interference were employed. The biliary excretion index (BEI) of generated TS was not significantly altered by impaired Bcrp (20.9 to 21.1%) and/or Mrp2 function (24.4% and 17.5% in WT and TR(-) rat SCH, respectively). Thus, loss-of-function of Mrp2 and/or Bcrp do not appear to be risk factors for increased hepatocellular TS accumulation in rats, potentially because of a compensatory transporter(s) that excretes TS into bile. Further investigations revealed that the compensatory TS biliary transporter was not the bile salt export pump (Bsep) or P-glycoprotein (P-gp). Interestingly, TGZ sulfation was significantly decreased in TR(-) compared with WT rat SCH (total recovery: 2.8 versus 5.0% of TGZ dose), resulting in decreased hepatocellular TS accumulation, even though sulfotransferase activity in TR(-) rat hepatocyte S9 fraction was similar. Hepatocellular TG accumulation was significantly increased in TR(-) compared with WT rat SCH due to increased glucuronidation and negligible TG biliary excretion. These data emphasize that the interplay between metabolite formation and excretion determines hepatocellular exposure to generated metabolites such as TS and TG.

An electrically tight in vitro blood-brain barrier model displays net brain-to-blood efflux of substrates for the ABC transporters, P-gp, Bcrp and Mrp-1

Efflux transporters of the ATP-binding cassette superfamily including breast cancer resistance protein (Bcrp/Abcg2), P-glycoprotein (P-gp/Abcb1) and multidrug resistance-associated proteins (Mrp's/Abcc's) are expressed in the blood-brain barrier (BBB). The aim of this study was to investigate if a bovine endothelial/rat astrocyte in vitro BBB co-culture model displayed polarized transport of known efflux transporter substrates. The co-culture model displayed low mannitol permeabilities of 0.95 ± 0.1 · 10(-6) cm·s(-1) and high transendothelial electrical resistances of 1,177 ± 101 Ω·cm(2). Bidirectional transport studies with (3)H-digoxin, (3)H-estrone-3-sulphate and (3)H-etoposide revealed polarized transport favouring the brain-to-blood direction for all substrates. Steady state efflux ratios of 2.5 ± 0.2 for digoxin, 4.4 ± 0.5 for estrone-3-sulphate and 2.4 ± 0.1 for etoposide were observed. These were reduced to 1.1 ± 0.08, 1.4 ± 0.2 and 1.5 ± 0.1, by addition of verapamil (digoxin), Ko143 (estrone-3-sulphate) or zosuquidar + reversan (etoposide), respectively. Brain-to-blood permeability of all substrates was investigated in the presence of the efflux transporter inhibitors verapamil, Ko143, zosuquidar, reversan and MK 571 alone or in combinations. Digoxin was mainly transported via P-gp, estrone-3-sulphate via Bcrp and Mrp's and etoposide via P-gp and Mrp's. The expression of P-gp, Bcrp and Mrp-1 was confirmed using immunocytochemistry. The findings indicate that P-gp, Bcrp and at least one isoform of Mrp are functionally expressed in our bovine/rat co-culture model and that the model is suitable for investigations of small molecule transport.

An experimental approach to evaluate the impact of impaired transport function on hepatobiliary drug disposition using Mrp2-deficient TR- rat sandwich-cultured hepatocytes in combination with Bcrp knockdown

Breast cancer resistance protein (BCRP) and multidrug resistance-associated protein 2 (MRP2) are members of the ATP binding cassette (ABC) transporter family located in the canalicular membrane of hepatocytes that mediate biliary excretion of many drugs and endogenous compounds. BCRP and MRP2 have overlapping substrate profiles. Predicting drug disposition in the setting of altered transport function has important clinical significance. This investigation was designed to establish an in vitro model system to evaluate the impact of impaired Mrp2 and Bcrp function on hepatobiliary drug disposition. To achieve Bcrp knockdown by RNA interference (RNAi), sandwich-cultured hepatocytes (SCH) from Mrp2-deficient (TR(-)) and wild-type (WT) rats were infected with adenoviral vectors to express shRNA targeting Bcrp (Ad-siBcrp) at multiplicity of infection (MOI) of 1-10. MOI of 5 was identified as optimal. At MOI of 5, viral infection as well as WT or TR(-) status was statistically significant predictors of the rosuvastatin (RSV) biliary excretion index (BEI), consistent with the known role of Bcrp and Mrp2 in the biliary excretion of RSV in vivo in rats. Relative to WT rat SCH, marginal mean BEI (%) of RSV in TR(-) rat SCH decreased by 28.6 (95% CI: 5.8-51.3). Ad-siBcrp decreased marginal mean BEI (%) of RSV by 13.3 (7.5-9.1) relative to SCH infected with adenoviral vectors expressing a nontargeting shRNA (Ad-siNT). The BEI of RSV was almost ablated in TR(-) rat SCH with Bcrp knockdown (5.9 ± 3.0%) compared to Ad-siNT-infected WT rat SCH (45.4 ± 6.6%). These results demonstrated the feasibility of Bcrp knockdown in TR(-) rat SCH as an in vitro system to assess the impact of impaired Bcrp and Mrp2 function. At MOI of 5, viral infection had minimal effects on RSV total accumulation, but significantly decreased marginal mean taurocholate total accumulation (pmol/mg of protein) and BEI (%) by 9.9 (7.0-12.8) and 7.5 (3.7-11.3), respectively, relative to noninfected SCH. These findings may be due to off-target effects on hepatic bile acid transporters, even though no changes in protein expression levels of the hepatic bile acid transporters were observed. This study established a strategy for optimization of the knockdown system, and demonstrated the potential use of RNAi in SCH as an in vitro tool to predict altered hepatobiliary drug disposition when canalicular transporters are impaired.

Role of hepatic efflux transporters in regulating systemic and hepatocyte exposure to xenobiotics

Hepatic efflux transporters include numerous well-known and emerging proteins localized to the canalicular or basolateral membrane of the hepatocyte that are responsible for the excretion of drugs into the bile or blood, respectively. Altered function of hepatic efflux transporters due to drug-drug interactions, genetic variation, and/or disease states may lead to changes in xenobiotic exposure in the hepatocyte and/or systemic circulation. This review focuses on transport proteins involved in the hepatocellular efflux of drugs and metabolites, discusses mechanisms of altered transporter function as well as the interplay between multiple transport pathways, and highlights the importance of considering intracellular unbound concentrations of transporter substrates and/or inhibitors. Methods to evaluate hepatic efflux transport and predict the effects of impaired transporter function on systemic and hepatocyte exposure are discussed, and the sandwich-cultured hepatocyte model to evaluate comprehensively the role of hepatic efflux in the hepatobiliary disposition of xenobiotics is characterized.

Differential expression of ABC transporters (MDR1, MRP1, BCRP) in developing human embryos

Three ABC transporters (MDR1, MRP1, BCRP), belonging to the family of multidrug resistance (MDR) proteins, play a crucial role in the protection mechanisms during embryogenesis and mediate drug resistance in cancer cells. The distribution of these transporters in the series of human embryonal/fetal intestine, liver and kidneys of various stages of intrauterine development (IUD) by indirect two-step immunohistochemical method was investigated. The organ- and age-specific expression patterns of these transporters were depicted and compared with the expression in adult organs. The evaluation of intestine and liver samples demonstrate differences in expression pattern of ABC transporters during IUD. On the contrary, in kidneys the age-specific localization was not observed. However, the increasing positivity from the kidney surface towards deeper, more differentiated parts was found. Hopefully, our study may contribute to elucidation of the role of multidrug resistance (MDR) pathways during IUD in man.

Decreased hepatic breast cancer resistance protein expression and function in multidrug resistance-associated protein 2-deficient (TR⁻) rats

Multidrug resistance-associated protein (Mrp) 2-deficient (TR(-)) Wistar rats have been used to elucidate the role of Mrp2 in drug disposition. Decreased breast cancer resistance protein (Bcrp) levels were reported in sandwich-cultured hepatocytes (SCH) from TR(-) rats compared with those from wild-type (WT) rats. This study was designed to characterize hepatic Bcrp expression and function in TR(-) rats, using nitrofurantoin and pitavastatin as substrates. Bcrp was knocked down by RNA interference in rat SCH. Antibody BXP53, but not BXP21, specifically detected Bcrp knockdown in SCH. Bcrp protein levels were decreased markedly in TR(-) but not Mrp2-deficient Sprague-Dawley [Eisai hyperbilirubinemic rats (EHBR)] rats. Bcrp mRNA levels were decreased significantly in TR(-) livers as determined by TaqMan real-time reverse transcriptase-polymerase chain reaction. Biliary excretion of nitrofurantoin, a specific Bcrp substrate, was decreased significantly in SCH and isolated perfused livers from TR(-) rats compared with those from WT controls, indicating that hepatic Bcrp function is decreased in TR(-) rats. In Bcrp knockdown SCH, the biliary excretion index and in vitro biliary clearance of pitavastatin were decreased significantly to ∼ 58 and ∼ 52% of control, respectively, indicating that Bcrp plays a role in pitavastatin biliary excretion. Pitavastatin biliary excretion was decreased significantly in perfused livers from TR(-) compared with those from WT rats. In conclusion, expression and function of hepatic Bcrp are decreased significantly in TR(-) rats. The potential role of both Bcrp and Mrp2 should be considered when data generated in TR(-) rats are interpreted. TR(-) and EHBR rats in combination may be useful in differentiating the role of Mrp2 and Bcrp in drug/metabolite disposition.

Evaluation of (99m)technetium-mebrofenin and (99m)technetium-sestamibi as specific probes for hepatic transport protein function in rat and human hepatocytes

PURPOSE

This study characterized 99mTc-Mebrofenin (MEB) and 99mTc-Sestamibi (MIBI) hepatic transport and preferential efflux routes (canalicular vs. basolateral) in rat and human sandwich-cultured hepatocytes (SCH).

METHODS

99mTc-MEB and 99mTc-MIBI disposition was determined in suspended hepatocytes and in SCH in the presence and absence of inhibitors and genetic knockdown of breast cancer resistance protein (Bcrp).

RESULTS

The general organic anion transporting polypeptide (Oatp/OATP) inhibitor rifamycin SV reduced initial 99mTc-MEB uptake in rat and human suspended hepatocytes. Initial 99mTc-MIBI uptake in suspended rat hepatocytes was not Na+-dependent or influenced by inhibitors. Multidrug resistance-associated protein (Mrp2/MRP2) inhibitors decreased 99mTc-MEB canalicular efflux in rat and human SCH. 99mTc-MEB efflux in human SCH was predominantly canalicular (45.8 +/- 8.6%) and approximately 3-fold greater than in rat SCH. 99mTc-MIBI canalicular efflux was similar in human and rat SCH; basolateral efflux was 37% greater in human than rat SCH. 99mTc-MIBI cellular accumulation, biliary excretion index and in vitro biliary clearance in rat SCH were unaffected by Bcrp knockdown.

CONCLUSION

99mTc-MEB hepatic uptake is predominantly Oatp-mediated with biliary excretion by Mrp2. 99mTc-MIBI appears to passively diffuse into hepatocytes; biliary excretion is mediated by P-gp. The SCH model is useful to investigate factors that may alter the route and/or extent of hepatic basolateral and canalicular efflux of substrates.

Inhibition of hepatic organic anion-transporting polypeptide by RNA interference in sandwich-cultured human hepatocytes: an in vitro model to assess transporter-mediated drug-drug interactions

Organic anion-transporting polypeptides (OATPs), members of the SLCO/SLC21 family, mediate the transport of various endo- and xenobiotics. In human liver, OATP1B1, 1B3, and 2B1 are located at the basolateral membrane of hepatocytes and are involved in hepatic drug uptake and biliary elimination. Clinically significant drug-drug interactions (DDIs) mediated by hepatic OATPs have drawn great attention from clinical practitioners and researchers. However, there are considerable challenges to prospectively understanding the extent of OATP-mediated DDIs because of the lack of specific OATP inhibitors or substrates and the limitations of in vitro tools. In the present study, a novel RNA interference knockdown sandwich-cultured human hepatocyte model was developed and validated. Quantitative polymerase chain reaction, microarray and immunoblotting analyses, along with uptake assays, illustrated that the expression and transport activity of hepatic OATPs were reduced by small interfering (siRNA) efficiently and specifically in this model. Although OATP siRNA decreased only 20 to 30% of the total uptake of cerivastatin into human hepatocytes, it caused a 50% reduction in cerivastatin metabolism, which was observed by monitoring the formation of the two major metabolites of cerivastatin. The results suggest that coadministration of a drug that is a hepatic OATP inhibitor could significantly alter the pharmacokinetic profile of cerivastatin in clinical studies. Further studies with this novel model demonstrated that OATP and cytochrome P450 have a synergistic effect on cerivastatin-gemfibrozil interactions. The siRNA knockdown sandwich-cultured human hepatocytes may provide a new powerful model for evaluating DDIs.

Activated PKC{delta} and PKC{epsilon} inhibit epithelial chloride secretion response to cAMP via inducing internalization of the Na+-K+-2Cl- cotransporter NKCC1

The basolateral Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) is a key determinant of transepithelial chloride secretion and dysregulation of chloride secretion is a common feature of many diseases including secretory diarrhea. We have previously shown that activation of protein kinase C (PKC) markedly reduces transepithelial chloride secretion in human colonic T84 cells, which correlates with both functional inhibition and loss of the NKCC1 surface expression. In the present study, we defined the specific roles of PKC isoforms in regulating epithelial NKCC1 and chloride secretion utilizing adenoviral vectors that express shRNAs targeting human PKC isoforms (α, δ, ε) (shPKCs) or LacZ (shLacZ, non-targeting control). After 72 h of adenoviral transduction, protein levels of the PKC isoforms in shPKCs-T84 cells were decreased by ∼90% compared with the shLacZ-control. Activation of PKCs by phorbol 12-myristate 13-acetate (PMA) caused a redistribution of NKCC1 immunostaining from the basolateral membrane to intracellular vesicles in both shLacZ- and shPKCα-T84 cells, whereas the effect of PMA was not observed in shPKCδ- and shPKCε- cells. These results were further confirmed by basolateral surface biotinylation. Furthermore, activation of PKCs by PMA inhibited cAMP-stimulated chloride secretion in the uninfected, shLacZ- and shPKCα-T84 monolayers, but the inhibitory effect was significantly attenuated in shPKCδ- and shPKCε-T84 monolayers. In conclusion, the activated novel isoforms PKCδ or PKCε, but not the conventional isoform PKCα, inhibits transepithelial chloride secretion through inducing internalization of the basolateral surface NKCC1. Our study reveals that the novel PKC isoform-regulated NKCC1 surface expression plays an important role in the regulation of chloride secretion.

Sandwich-cultured hepatocytes: an in vitro model to evaluate hepatobiliary transporter-based drug interactions and hepatotoxicity

Sandwich-cultured hepatocytes (SCH) are a powerful in vitro tool that can be utilized to study hepatobiliary drug transport, species differences in drug transport, transport protein regulation, drug-drug interactions, and hepatotoxicity. This review provides an up-to-date summary of the SCH model, including a brief history of, and introduction to, the use of SCH, as well as methodology to evaluate hepatobiliary drug disposition. A summary of the literature that has utilized this model to examine the interplay between drug-metabolizing enzymes and transport proteins, drug-drug interactions at the transport level, and hepatotoxicity as a result of altered hepatic transport also is provided.

Sulindac and its metabolites inhibit multiple transport proteins in rat and human hepatocytes

Sulindac is a commonly used nonsteroidal anti-inflammatory drug. This study tested the hypothesis that sulindac-mediated drug-drug interactions and/or hepatotoxicity may be caused, in part, by inhibition of proteins responsible for the hepatic transport of drugs and/or bile acids by sulindac and/or sulindac metabolites [sulindac sulfone (S-sulfone) and sulindac sulfide (S-sulfide)]. The uptake and excretion of model substrates, [(3)H]taurocholate (TC), [(3)H]estradiol 17-beta-glucuronide (E217G), and nitrofurantoin (NF), were investigated in rat and human suspended and sandwich-cultured hepatocytes (SCH). In suspended rat hepatocytes, S-sulfone and S-sulfide inhibited Na(+)-dependent TC initial uptake (IC(50) of 24.9 +/- 6.4 and 12.5 +/- 1.8 microM, respectively) and Na(+)-independent E217G initial uptake (IC(50) of 12.1 +/- 1.6 and 6.3 +/- 0.3 microM, respectively). In rat SCH, sulindac metabolites (100 microM) decreased the in vitro biliary clearance (Cl(biliary)) of TC, E217G, and NF by 38 to 83%, 81 to 97%, and 33 to 57%, respectively; S-sulfone and S-sulfide also decreased the TC and NF biliary excretion index by 39 to 55%. In suspended human hepatocytes, S-sulfone and S-sulfide inhibited Na(+)-dependent TC initial uptake (IC(50) of 42.2 and 3.1 microM, respectively); S-sulfide also inhibited the TC Cl(biliary) in human SCH. Sulindac/metabolites markedly inhibited hepatic uptake and biliary excretion of E217G by 51 to 100% in human SCH. In conclusion, sulindac and metabolites are potent inhibitors of the uptake and biliary clearance of bile acids in rat and human hepatocytes and also inhibit substrates of rat breast cancer resistance protein, rat and human organic anion-transporting polypeptides, and human multidrug resistance-associated protein 2. Inhibition of multiple hepatic transport proteins by sulindac/metabolites may play an important role in clinically significant sulindac-mediated drug-drug interactions and/or liver injury.

Effect of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) micelles on pharmacokinetics and intestinal toxicity of irinotecan hydrochloride: potential involvement of breast cancer resistance protein (ABCG2)

Objectives

Intestinal toxicity and low levels of systemic drug exposure are among the major problems associated with tumour therapy. We have developed poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPO-PEO) micelles loaded with irinotecan hydrochloride (CPT-11) hoping to decrease CPT-11-induced intestinal toxicity while increasing its systemic exposure. In addition, we have investigated the potential involvement of breast cancer resistance protein (BCRP) in biliary excretion, pharmacokinetics, and intestinal toxicity of CPT-11.

Methods

PEO-PPO-PEO micelles were prepared using PEO20-PPO70-PEO20 and lecithin. The effect of PEO-PPO-PEO micelles on BCRP-mediated cellular accumulation and transport efflux of CPT-11 was evaluated in MDCKII/BCRP cells. The biliary excretion, intestinal damage, and pharmacokinetic study of CPT-11-loaded PEO-PPO-PEO micelles were investigated in rats.

Key findings

The obtained micelles could effectively inhibit BCRP-mediated CPT-11 efflux in MDCKII/BCRP cells, and significantly decrease the drug biliary excretion in rats. Moreover, intestinal toxicity, assessed by microscopic examination of pathological damage, was ameliorated in rats injected with PEO-PPO-PEO micelles compared with rats injected with CPT-11 alone. Treatment with PEO-PPO-PEO micelles resulted in prolonged circulation time in blood and increased bioavailability of CPT-11 and SN-38 (7-ethyl-10-hydroxycamptothecin).

Conclusions

PEO-PPO-PEO micelles were identified as promising carriers able to reduce intestinal toxicity and increase antitumour therapeutic effect of CPT-11. The study indicated a potential involvement of BCRP in CPT-11 pharmacokinetics and CPT-11-induced intestinal toxicity.

Evaluation of in vivo P-glycoprotein phenotyping probes: a need for validation

Drug transporters are involved in clinically relevant drug-drug interactions. P-glycoprotein (P-gp) is an efflux transporter that displays genetic polymorphism. Phenotyping permits evaluation of real-time, in vivo P-gp activity and P-gp-mediated drug-drug interactions. Digoxin, fexofenadine, talinolol and quinidine are commonly used probe drugs for P-gp phenotyping. Although current regulatory guidance documents highlight methodologies for evaluating transporter-based drug-drug interactions, whether current probe drugs are suitable for phenotyping has not been established, and validation criteria are lacking. This review proposes validation criteria and evaluates P-gp probes to determine probe suitability. Based on these criteria, digoxin, fexofenadine, talinolol and quinidine have limitations to their use and are not recommended for P-gp phenotyping.

Influence of seeding density and extracellular matrix on bile Acid transport and mrp4 expression in sandwich-cultured mouse hepatocytes

This study was undertaken to examine the influence of seeding density, extracellular matrix and days in culture on bile acid transport proteins and hepatobiliary disposition of the model bile acid taurocholate. Mouse hepatocytes were cultured in a sandwich configuration on six-well Biocoat plates with an overlay of Matrigel (BC/MG) or gelled-collagen (BC/GC) for 3 or 4 days at seeding densities of 1.0, 1.25, or 1.5 x 10(6) cells/well. The lower seeding densities of 1.0 and 1.25 x 10(6) cells/well resulted in good hepatocyte morphology and bile canalicular network formation, as visualized by 5-(and 6)-carboxy-2',7'-dichlorofluorescein accumulation. In general, taurocholate cellular accumulation tended to increase as a function of seeding density in BC/GC; cellular accumulation was significantly increased in hepatocytes cultured in BC/MG compared to BC/GC at the same seeding density on both days 3 and 4 of culture. In general, in vitro intrinsic biliary clearance of taurocholate was increased at higher seeding densities. Levels of bile acid transport proteins on days 3 and 4 were not markedly influenced by seeding density or extracellular matrix except for multidrug resistance protein 4 (Mrp4), which was inversely related to seeding density. Mrp4 levels decreased approximately 2- to 3-fold between seeding densities of 1.0 x 10(6) and 1.25 x 10(6) cells/well regardless of extracellular matrix; an additional approximately 3- to 5-fold decrease in Mrp4 protein was noted in BC/GC between seeding densities of 1.25 x 10(6) and 1.5 x 10(6) cells/well. Results suggest that seeding density, extracellular matrix and days in culture profoundly influence Mrp4 expression in sandwich-cultured mouse hepatocytes. Primary mouse hepatocytes seeded in a BC/MG configuration at densities of 1.25 x 10(6) cells/well and 1.0 x 10(6), and cultured for 3 days, yielded optimal transport based on the probes studied. This work demonstrates the applicability of the sandwich-cultured model to mouse hepatocytes.

International Transporter Consortium, Giacomini KM, Huang SM, Tweedie DJ, Benet LZ, Brouwer KLR, Chu X, Dahlin A, Evers R, Fischer V, Hillgren KM, Hoffmaster KA, Ishikawa T, Keppler D, Kim RB, Lee CA, Niemi M, Polli JW, Sugiyama Y, Swaan PW, Ware JA, Wright SH, Yee SW, Zamek-Gliszczynski MJ, Zhang L. Membrane transporters in drug development

Membrane transporters can be major determinants of the pharmacokinetic, safety and efficacy profiles of drugs. This presents several key questions for drug development, including which transporters are clinically important in drug absorption and disposition, and which in vitro methods are suitable for studying drug interactions with these transporters. In addition, what criteria should trigger follow-up clinical studies, and which clinical studies should be conducted if needed. In this article, we provide the recommendations of the International Transporter Consortium on these issues, and present decision trees that are intended to help guide clinical studies on the currently recognized most important drug transporter interactions. The recommendations are generally intended to support clinical development and filing of a new drug application. Overall, it is advised that the timing of transporter investigations should be driven by efficacy, safety and clinical trial enrolment questions (for example, exclusion and inclusion criteria), as well as a need for further understanding of the absorption, distribution, metabolism and excretion properties of the drug molecule, and information required for drug labelling.

Investigation of the involvement of P-glycoprotein and multidrug resistance-associated protein 2 in the efflux of ximelagatran and its metabolites by using short hairpin RNA knockdown in Caco-2 cells

Liver and bile secretion can be an important first-pass and clearance route for drug compounds and also the site of several drug-drug interactions. In the clinical program for ximelagatran development, an unexpected effect of erythromycin on the pharmacokinetics of the direct thrombin inhibitor ximelagatran and its metabolites was detected. This interaction was believed to be mediated by inhibition of drug transporters, which normally extrude the drug into the bile. Previous Caco-2 cell experiments indicated the involvement of an active efflux mechanism for ximelagatran, hydroxy-melagatran, and melagatran possibly mediated by P-glycoprotein (P-gp). However, the inhibitors used may not have been specific enough and the possibility that transporters other than P-gp were important in the Caco-2 cell assay cannot be excluded. In this study we used RNA interference, a post-transcriptional gene silencing mechanism in which mRNA is degraded in a sequence-specific manner, to specifically knock down P-gp or multidrug resistance-associated protein 2 (MRP2) transporters in Caco-2 cells. The data obtained from bidirectional transport studies in these cells indicate a clear involvement of P-gp but not of MRP2 in the transport of ximelagatran, hydroxy-melagatran, and melagatran across the apical cell membrane. The present study shows that short hairpin RNA Caco-2 cells are a valuable tool to investigate the contribution of specific transporters in the transcellular transport of drug molecules and to predict potential sites of pharmacokinetic interactions. The results also suggest that inhibition of hepatic P-gp is involved in the erythromycin-ximelagatran interaction seen in clinical studies.