Contribution of OATP2 (OATP1B1) and OATP8 (OATP1B3) to the hepatic uptake of pitavastatin in humans

Evaluation and prediction of potential drug-drug interactions of linagliptin using in vitro cell culture methods

Linagliptin is a highly potent dipeptidyl peptidase-4 (DPP-4) inhibitor approved for the treatment of type 2 diabetes. Unlike other DPP-4 inhibitors, linagliptin is cleared primarily via the bile and gut. We used a panel of stably and transiently transfected cell lines to elucidate the carrier-mediated transport processes that are involved in linagliptin disposition in vivo and to assess the potential for drug-drug interactions (DDIs). Our results demonstrate that linagliptin is a substrate of organic cation transporter 2 (OCT2) and P-glycoprotein (P-gp) but not of organic anion-transporting polypeptide 1B1 and 1B3; organic anion transporter 1, 3, and 4; OCT1; or organic cation/carnitine transporter 1 and 2, suggesting that OCT2 and P-gp play a role in the disposition of linagliptin in vivo. Linagliptin inhibits transcellular transport of digoxin by P-gp with an apparent IC(50) of 66.1 μM, but it did not inhibit activity of multidrug resistance-associated protein 2 and breast cancer resistance protein as represented by transport of probe substrate into membrane vesicles from respective transporter-expressing cells. In addition, the inhibitory effect of linagliptin on major solute carrier transporter isoforms was investigated. Linagliptin showed inhibitory potency against only OCT1 and OCT2 out of all major solute carrier transporter isoforms examined, and those inhibition potencies, evaluated using three different in vitro probe substrates, were substrate-specific. Considering the low therapeutic plasma concentration of linagliptin, our data clearly suggest a very low risk for transporter-mediated DDIs with comedications in clinical practice.

Drug-drug interactions mediated by hepatic transporters

Transporters are a class of functional membrane proteins that are extensively expressed in the liver and are involved in the transport of a wide variety of compounds including endogenous and xenobiotic substances from the bloodstream into hepatocytes. And then, these parent substances are metabolized in the liver and excreted into bile. Hepatic transporters play important roles in the hepatic transport process of drugs, including the uptake of drugs from blood into hepatocytes or the efflux of drugs into bile. The uptake, metabolism and excretion of drugs in the liver are related to hepatic transporters. Decline or loss of transporting function of transporters caused by gene mutations is related with a variety of liver diseases. Drug-drug interaction may be induced when one drug (as a substrate or inhibitor of hepatic transporters) is co-administered simultaneously with another in clinical practice, such as adverse events of cerivastatin. In this paper, we review the potential drug-drug interactions mediated by hepatic transporters.

Impact of ABCC2, ABCG2 and SLCO1B1 polymorphisms on the pharmacokinetics of pitavastatin in humans

Pitavastatin, a 3-hydroxyl-3-methylglutaryl-coenzyme A reductase inhibitor is distributed to the liver, a target organ of action and excreted mainly into the bile. To investigate the impact of influx (OATP1B1) and efflux (MRP2, BCRP) transporter alleles on its disposition, the pharmacokinetic (PK) parameters were compared among the following groups: SLCO1B1 (*15 carrier and non-carrier), ABCC2 (G1249A, C3972T, C-24T, G1549A, and G1774T), and ABCG2 (C421A) single nucleotide polymorphisms in 45 healthy Korean volunteers. Pitavastatin AUC(last) was higher in individuals carrying the SLCO1B1*15 allele than those not carrying it (144.1 ± 55.3 vs. 84.7 ± 25.7 h·ng/mL [mean ± SD], p = 0.002). The AUC(last) varied significantly according to the ABCC2 C-24T allele (103.4 ± 42.2, 80.2 ± 23.8, and 39.0 h·ng/mL in CC, CT and TT, respectively; p = 0.027). Other SNPs of ABCC2 and ABCG2 were not significant. The effect of these transporters and body weight on the AUC(last) and C(max) were tested, and only SLCO1B1 and ABCC2 C-24T genotypes were significant factors by analysis of covariance. These variants accounted for almost 50% of the variation in AUC(last) and C(max) of pitavastatin. Therefore, ABCC2 C-24T was significantly associated with pitavastatin human PK when the known effect of SLCO1B1*15 was also considered.

Transporter-mediated drug--drug interactions involving OATP substrates: predictions based on in vitro inhibition studies

Transporter-mediated drug–drug interactions (DDIs) are among the most important of the clinically relevant pharmacokinetic DDIs. We investigated the validity of a static prediction of area under the plasma concentration-time curve (AUC) ratios (AUCRs; AUC(with inhibitor)/AUC(control) using in vitro inhibition profiles, and selected the types of assumptions that improved the prediction accuracy with minimizing false-negative predictions. We used data from 58 DDI studies involving 12 substrates of hepatic organic anion–transporting polypeptides (OATPs). With original assumptions regarding the maximal increase in intestinal availability, maximum unbound concentration at the inlet to the liver, and inhibition of only the hepatic uptake process, the predicted AUCRs were comparable to those reported within a two/threefold error margin in 44/52 studies, whereas in 16 studies, the predictions were judged to be falsenegatives. When the inhibitory effects on both hepatic uptake and efflux/metabolisms were considered, the overall prediction accuracy became worse, although the false-negative prediction decreased to 11 studies. This illustrates that if appropriate assumptions are selected, unnecessary clinical DDI studies can be reasonably avoided.

OATPs, OATs and OCTs: the organic anion and cation transporters of the SLCO and SLC22A gene superfamilies

The human organic anion and cation transporters are classified within two SLC superfamilies. Superfamily SLCO (formerly SLC21A) consists of organic anion transporting polypeptides (OATPs), while the organic anion transporters (OATs) and the organic cation transporters (OCTs) are classified in the SLC22A superfamily. Individual members of each superfamily are expressed in essentially every epithelium throughout the body, where they play a significant role in drug absorption, distribution and elimination. Substrates of OATPs are mainly large hydrophobic organic anions, while OATs transport smaller and more hydrophilic organic anions and OCTs transport organic cations. In addition to endogenous substrates, such as steroids, hormones and neurotransmitters, numerous drugs and other xenobiotics are transported by these proteins, including statins, antivirals, antibiotics and anticancer drugs. Expression of OATPs, OATs and OCTs can be regulated at the protein or transcriptional level and appears to vary within each family by both protein and tissue type. All three superfamilies consist of 12 transmembrane domain proteins that have intracellular termini. Although no crystal structures have yet been determined, combinations of homology modelling and mutation experiments have been used to explore the mechanism of substrate recognition and transport. Several polymorphisms identified in members of these superfamilies have been shown to affect pharmacokinetics of their drug substrates, confirming the importance of these drug transporters for efficient pharmacological therapy. This review, unlike other reviews that focus on a single transporter family, briefly summarizes the current knowledge of all the functionally characterized human organic anion and cation drug uptake transporters of the SLCO and the SLC22A superfamilies.

Current understanding of hepatic and intestinal OATP-mediated drug-drug interactions

At present, many patients are medicated with various drugs, which are, at the same time, associated with an increased risk of drug-drug interactions (DDIs). Detailed analysis of mechanisms underlying DDIs is the basis of a better prediction of adverse drug events caused by drug interactions. In the last few decades, an involvement of transporters in such processes has been more and more recognized. Indeed, uptake transporters belonging to the organic anion-transporting polypeptide (OATP) family have been shown to interact with a variety of drugs in clinical use. Particularly, the subfamily of OATP1B transporters has been extensively studied, identifying several clinical significant DDIs based on those hepatic uptake transporters. By contrast, the role of OATP2B1 in this context is rather underestimated. Therefore, in addition to known interactions based on OATP1B transporters, we have focused on DDIs probably based on OATP2B1 inhibition in the liver and those possibly owing to the inhibition of OATP2B1-mediated drug absorption in the intestine.

Effects of grapefruit juice and SLCO1B1 388A>G polymorphism on the pharmacokinetics of pitavastatin

Pitavastatin undergoes little hepatic metabolism but it is a substrate for uptake and efflux transporters, particularly OATP1B1 (gene SLCO1B1). A previous study in 8 Japanese healthy subjects showed that co-administration with grapefruit juice (GFJ) resulted in a small increase in systemic exposure to pitavastatin. We examined whether common polymorphisms in SLCO1B1 might influence the pharmacokinetics of pitavastatin or the interaction with GFJ. Twelve Chinese healthy male volunteers took pitavastatin 2 mg orally with water or with GFJ on separate occasions and plasma concentrations of pitavastatin acid and lactone were measured over 48 h. GFJ increased the mean area under the plasma concentration-time curve (AUC0-48 h) for both pitavastatin acid and lactone by 14% (p<0.05). Subjects with SLCO1B1 *1b/*1b haplotype (388GG-521TT) had 47% and 44% higher systemic exposure for pitavastatin acid and lactone than the SLCO1B1 *1a carriers (388AA/AG-521TT, p<0.05 and p=0.005, respectively). The SLCO1B1 388A>G polymorphism, which increases transporter activity for some statins, was associated with higher plasma levels of pitavastatin acid and lactone in subjects with the homozygous variant indicating decreased hepatic uptake. Co-administration of pitavastatin with GFJ resulted in a small but significant increase in plasma levels in healthy Chinese subjects.

Evaluation of organic anion-transporting polypeptide 1B1 and CYP3A4 activities in primary human hepatocytes and HepaRG cells cultured in a dynamic three-dimensional bioreactor system

The long-term stability of liver cell functions is a major challenge when studying hepatic drug transport, metabolism, and toxicity in vitro. The aim of the present study was to investigate organic anion-transporting polypeptide (OATP) 1B1 and CYP3A4 activities in fresh primary human hepatocytes and differentiated cryopreserved HepaRG cells when cultured in a three-dimensional (3D) bioreactor system. OATP1B1 activity was determined by loss from media experiments of [(3)H]estradiol-17β-D-glucuronide and atorvastatin acid (ATA) for up to 7 days in culture. ATA metabolite formation was determined at days 3 to 4 to evaluate CYP3A4 activity. Overall, the results showed that freshly isolated human hepatocytes inoculated in the bioreactor retained OATP1B1 activity for at least 7 days, whereas in HepaRG cells no OATP1B1 activity was observed beyond day 2. The activity data were in agreement with immunohistochemical stainings, which showed that OATP1B1 protein expression was preserved for at least 9 days in fresh human hepatocytes, whereas OATP1B1 was expressed markedly lower in HepaRG cells after 9 days in culture. Fresh human hepatocytes and HepaRG cells exhibited similar CYP3A4 activity in bioreactor culture, and immunohistochemical stainings supported these findings. Activity and mRNA expression of OATP1B1 and CYP3A4 in primary human hepatocytes compared with HepaRG cells in fresh suspensions were in agreement with data obtained in bioreactor culture. In conclusion, freshly isolated human hepatocytes cultured in a 3D bioreactor system preserve both OATP1B1 and CYP3A4 activities, allowing long-term in vitro studies on drug disposition and toxicity.

Localization and functional characterization of the rat Oatp4c1 transporter in an in vitro cell system and rat tissues

The organic anion transporting polypeptide 4c1 (Oatp4c1) was previously identified as a novel uptake transporter predominantly expressed at the basolateral membrane in the rat kidney proximal tubules. Its functional role was suggested to be a vectorial transport partner of an apically-expressed efflux transporter for the efficient translocation of physiological substrates into urine, some of which were suggested to be uremic toxins. However, our in vitro studies with MDCKII cells showed that upon transfection rat Oatp4c1 polarizes to the apical membrane. In this report, we validated the trafficking and function of Oatp4c1 in polarized cell systems as well as its subcellular localization in rat kidney. Using several complementary biochemical, molecular and proteomic methods as well as antibodies amenable to immunohistochemistry, immunofluorescence, and immunobloting we investigated the expression pattern of Oatp4c1 in polarized cell systems and in the rat kidney. Collectively, these data demonstrate that rat Oatp4c1 traffics to the apical cell surface of polarized epithelium and localizes primarily in the proximal straight tubules, the S3 fraction of the nephron. Drug uptake studies in Oatp4c1-overexpressing cells demonstrated that Oatp4c1-mediated estrone-3-sulfate (E3S) uptake was pH-dependent and ATP-independent. These data definitively demonstrate the subcellular localization and histological location of Oatp4c1 and provide additional functional evidence that reconciles expression-function reports found in the literature.

Pleiotropic effects of pitavastatin

3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are established first line treatments for hypercholesterolaemia. In addition to the direct effects of statins in reducing concentrations of atherogenic low density lipoprotein cholesterol (LDL-C), several studies have indicated that the beneficial effects of statins may be due to some of their cholesterol-independent, multiple (pleiotropic) effects which may differ between different members of the class. Pitavastatin is a novel synthetic lipophilic statin that has a number of pharmacodynamic and pharmacokinetic properties distinct from those of other statins, which may underlie its potential pleiotropic benefits in reducing cardiovascular risk factors. This review examines the principal pleiotropic effects of pitavastatin on endothelial function, vascular inflammation, oxidative stress and thrombosis. The article is based on a systematic literature search carried out in December 2010, together with more recent relevant publications where appropriate. The available data from clinical trials and in vitro and animal studies suggest that pitavastatin is not only effective in reducing LDL-C and triglycerides, but also has a range of other effects. These include increasing high density lipoprotein cholesterol, decreasing markers of platelet activation, improving cardiac, renal and endothelial function, and reducing endothelial stress, lipoprotein oxidation and, ultimately, improving the signs and symptoms of atherosclerosis. It is concluded that the diverse pleiotropic actions of pitavastatin may contribute to reducing cardiovascular morbidity and mortality beyond that achieved through LDL-C reduction.

Use of mechanistic modeling to assess interindividual variability and interspecies differences in active uptake in human and rat hepatocytes

Interindividual variability in activity of uptake transporters is evident in vivo, yet limited data exist in vitro, confounding in vitro-in vivo extrapolation. The uptake kinetics of seven organic anion-transporting polypeptide substrates was investigated over a concentration range in plated cryopreserved human hepatocytes. Active uptake clearance (CL(active, u)), bidirectional passive diffusion (P(diff)), intracellular binding, and metabolism were estimated for bosentan, pitavastatin, pravastatin, repaglinide, rosuvastatin, telmisartan, and valsartan in HU4122 donor using a mechanistic two-compartment model in Matlab. Full uptake kinetics of rosuvastatin and repaglinide were also characterized in two additional donors, whereas for the remaining drugs CL(active, u) was estimated at a single concentration. The unbound affinity constant (K(m, u)) and P(diff) values were consistent across donors, whereas V(max) was on average up to 2.8-fold greater in donor HU4122. Consistency in K(m, u) values allowed extrapolation of single concentration uptake activity data and assessment of interindividual variability in CL(active) across donors. The maximal contribution of active transport to total uptake differed among donors, for example, 85 to 96% and 68 to 87% for rosuvastatin and repaglinide, respectively; however, in all cases the active process was the major contributor. In vitro-in vivo extrapolation indicated a general underprediction of hepatic intrinsic clearance, an average empirical scaling factor of 17.1 was estimated on the basis of seven drugs investigated in three hepatocyte donors, and donor-specific differences in empirical factors are discussed. Uptake K(m, u) and CL(active, u) were on average 4.3- and 7.1-fold lower in human hepatocytes compared with our previously published rat data. A strategy for the use of rat uptake data to facilitate the experimental design in human hepatocytes is discussed.

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

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

Prediction of the in vivo OATP1B1-mediated drug-drug interaction potential of an investigational drug against a range of statins

To support drug development, the drug-drug interaction potential (DDI) of an investigational drug (AZX) was assessed against the probe estradiol 17β-glucuronide as well as against simvastatin acid, atorvastatin, pravastatin, pitavastatin, fluvastatin, rosuvastatin and estrone 3-sulfate. The inhibitory potentials of the OATP1B1 inhibitors rifamycin SV and gemfibrozil were assessed in parallel. Monolayer cellular uptake assays were used to determine inhibition of human OATP1B1. Apparent K(m) values for the OATP1B1-mediated transport of [(3)H] substrates were determined prior to their use as probes in inhibition studies, and ranged from 0.6 to 29 μM for statins. The K(m) of lipophilic simvastatin acid could not be determined due to its high passive permeability that masked OATP1B1 transport, and therefore this statin could not be used as a probe. Estrone 3-sulfate exhibited biphasic kinetics, whereas estradiol 17β-glucuronide demonstrated simple Michaelis-Menton kinetics. AZX moderately inhibited OATP1B1-mediated transport of all statins (IC(50)=4.6-9.7 μM), except fluvastatin, of estradiol 17β-glucuronide (IC(50)=5.3 μM), and weakly inhibited estrone 3-sulfate (IC(50)=79 μM). Rifamycin SV strongly, and gemfibrozil weakly, inhibited the OATP1B1-mediated transport of substrates. Estradiol 17β-glucuronide was identified as a good surrogate probe for statins when assessing OATP1B1 inhibitory potential using this test system. Inhibition data was used to predict the likelihood of a clinical DDI, using current draft US FDA guidance and recommendations of the International Transporter Consortium. Predictions for AZX indicated the potential for an OATP1B1-mediated DDI in vivo and that a clinical interaction study is warranted to confirm whether AZX is an OATP1B1 inhibitor in the clinic.

Are all statins the same? Focus on the efficacy and tolerability of pitavastatin

Pitavastatin is the newest member of the HMG-CoA reductase inhibitor family and is approved as adjunctive therapy to diet to reduce elevated levels of total cholesterol, low-density lipoprotein (LDL) cholesterol, apolipoprotein (Apo) B, and triglycerides and to increase levels of high-density lipoprotein (HDL) cholesterol in adult patients with primary hyperlipidemia or mixed dyslipidemia. Pitavastatin undergoes minimal metabolism by cytochrome P450 (CYP) enzymes and, therefore, has a low propensity for drug-drug interactions with drugs metabolized by CYP enzymes or the CYP3A4 substrate grapefruit juice. In clinical trials, pitavastatin potently and consistently reduced serum levels of total, LDL, and non-HDL cholesterol, and triglycerides in patients with primary hypercholesterolemia where diet and other non-pharmacological measures were inadequate. Mean reductions from baseline in serum total and LDL cholesterol and triglyceride levels were 21-32%, 30-45%, and 10-30%, respectively. Moreover, a consistent trend towards increased HDL cholesterol levels of 3-10% was seen. Long-term extension studies show that the beneficial effects of pitavastatin are maintained for up to 2 years. Pitavastatin produces reductions from baseline in serum total and LDL cholesterol levels to a similar extent to those seen with the potent agent atorvastatin and to a greater extent than those seen with simvastatin or pravastatin. In the majority of other studies comparing pitavastatin and atorvastatin, no significant differences in the favorable effects on lipid parameters were seen, although pitavastatin was consistently associated with trends towards increased HDL cholesterol levels. Pitavastatin also produces beneficial effects on lipids in patients with type 2 diabetes mellitus and metabolic syndrome without deleterious effects on markers of glucose metabolism, such as fasting blood glucose levels or proportion of glycosylated hemoglobin. Pitavastatin appears to exert a number of beneficial effects on patients at risk of cardiovascular events independent of lipid lowering. In the JAPAN-ACS (Japan Assessment of Pitavastatin and Atorvastatin in Acute Coronary Syndrome) study, pitavastatin was non-inferior to atorvastatin at reducing plaque volume in patients with ACS undergoing percutaneous coronary intervention. Further beneficial effects, including favorable effects on the size and composition of atherosclerotic plaques, improvements in cardiovascular function, and improvements in markers of inflammation, oxidative stress, and renal function, have been demonstrated in a number of small studies. Pitavastatin is generally well tolerated in hyperlipidemic patients with or without type 2 diabetes, with the most common treatment-related adverse events being musculoskeletal or gastrointestinal in nature. Increases in plasma creatine kinase levels were seen in <5% of pitavastatin recipients and the incidence of myopathy or rhabdomyolysis was extremely low. In summary, pitavastatin, the latest addition to the statin family, produces potent and consistent beneficial effects on lipids, is well tolerated, and has a favorable pharmacokinetic profile. The combination of a potent decrease in total and LDL cholesterol levels and increase in HDL cholesterol levels suggest that pitavastatin may produce substantial cardiovascular protection.

Association of SLCO1B1 1b with peptic ulcer amongst Japanese patients taking low-dose aspirin

BACKGROUND

In the recent case-control study, we showed an inverse association between peptic ulcer and angiotensin type 1 receptor (AT1R) blockers (ARBs) or HMG-Co A reductase inhibitors (statins). The aim was to evaluate whether the genotypes of uptake and efflux transporters of ARBs and statins relate to the presence of peptic ulcer and/or ulcer bleeding associated with aspirin use.

METHODS

Patients taking 100mg of enteric-coated aspirin for cardiovascular diseases who also participated in endoscopic surveillance were studied. SLCO1B, ABCC2, ABCG2, and MDR1 genotypes were determined by PCR or PCR-RFLP.

RESULTS

492 patients enrolled including 78 with peptic ulcer. The frequencies of the SLCO1B1 521TT genotype were significantly higher in the ulcer group (p=0.006) compared to the controls. After adjustment for significant factors, the SLCO1B1 1b haplotype was significantly associated with peptic ulcer (OR, 3.64; 95% CI, 1.81-7.29).

CONCLUSIONS

SLCO1B1 1b haplotype may identify patients at increased risk for aspirin-induced peptic ulcer.

Development of a multiplex UPLC-MRM MS method for quantification of human membrane transport proteins OATP1B1, OATP1B3 and OATP2B1 in in vitro systems and tissues

OATP1B1, OATP1B3 and OATP2B1 are important members of the organic anion transporting polypeptides (OATP) family and are implicated in the hepatic disposition of endobiotics and xenobiotics. Quantitating the expression levels of human OATP1B1, OATP1B3 and OATP2B1 in in vitro systems and tissue samples could significantly improve attempts to scale up in vitro data and result in more effective in vitro-in vivo correlation of transporter-mediated effects on drug disposition, such as hepatic clearance. In the present study, a quantification method was developed, characterized, and implemented for simultaneous determination of human OATP1B1, OATP1B3 and OATP2B1 in HEK cells transfected with OATP-expressing plasmid vectors (SLCO1B1, SLCO1B3, and SLCO2B1, respectively), human hepatocytes, human brain capillary endothelial cells, and humanized mouse liver tissue using UPLC-MRM MS. Purified membrane protein standards prepared and characterized as previously reported (Protein Expr. Purif. 2008, 57, 163-71) were first used as standards for absolute quantification of the expression levels of the three human OATP membrane proteins. The specificity of the optimized MRM transitions were characterized by analyzing the tryptic digests of the membrane protein fraction of wild type HEK cells and control mouse liver tissue using the herein reported UPLC-MRM MS method. The linearity of the calibration curve spanned from 0.2 μg mL(-1) (0.040 μg mg(-1)) to 20 μg mL(-1) (4.0 μg mg(-1)), with accuracy (% RE) within 15% at all concentrations examined for all three OATPs of interest in this study. The intra- and inter-day assay accuracy (% RE) and coefficient of variations (% CV) of triplicates are all within 15% for all levels of quality control samples prepared by mixing the membrane fraction of control mouse liver tissue with the required amount of purified human OATP1B1, OATP1B3 and OATP2B1.

In vivo biliary clearance should be predicted by intrinsic biliary clearance in sandwich-cultured hepatocytes

It has been reported that in vivo biliary clearance can be predicted using sandwich-cultured rat and human hepatocytes. The predicted apparent biliary clearance (CL(bile, app)) from sandwich- cultured rat hepatocytes (SCRH) based on medium concentrations correlates to in vivo CL(bile, app) based on plasma concentrations of angiotensin II receptor blockers (ARBs), HMG-CoA reductase inhibitors (statins), β-lactam antibiotics, and topotecan. However, the predicted biliary clearance from SCRH was 7- to 300-fold lower than in vivo biliary clearance. We speculated that the process of biliary excretion might not have been evaluated using sandwich-cultured hepatocytes. To evaluate this issue, intrinsic biliary clearance (CL(bile, int)) based on intracellular compound concentrations was evaluated to investigate the in vitro-in vivo correlation of this process among ARBs, statins, β-lactam antibiotics, and topotecan. Intrinsic biliary clearance in SCRH correlated to in vivo values obtained by constant intravenous infusion of six compounds, but not rosuvastatin and cefmetazole, to rats. Moreover, differences between SCRH and in vivo CL(bile, int) (0.7-6-fold) were much smaller than those of CL(bile, app) (7-300-fold). Therefore, in vivo CL(bile, int) is more accurately reflected using SCRH than CL(bile, app). In conclusion, to predict in vivo biliary clearance more accurately, CL(bile, int) should be evaluated instead of CL(bile, app) between SCRH and in vivo.

Genetic polymorphisms of OATP transporters and their impact on intestinal absorption and hepatic disposition of drugs

There is convincing evidence that many organic anion transporting polypeptide (OATP) transporters influence the pharmacokinetics and pharmacological efficacy of their substrate drugs. Each OATP family member has a unique combination of tissue distribution, substrate specificity and mechanisms of gene expression. Among them, OATP1B1, OATP1B3 and OATP2B1 have been considered as critical molecular determinants of the pharmacokinetics of a variety of clinically important drugs. Liver-specific expression of OATP1B1 and OATP1B3 contributes to the hepatic uptake of drugs from the portal vein, and OATP2B1 may alter their intestinal absorption as well as hepatic extraction. Accordingly, changes in function and expression of these three OATPs owing to genetic polymorphisms may lead to altered pharmacological effects, including decreased drug efficacy and increased risk of adverse effects. Association of genetic polymorphisms in OATP genes with alterations in the pharmacokinetic properties of their substrate drugs has been reported; however, there still exists a degree of discordance between the reported outcomes in different clinical settings. For better understanding of the clinical relevance of genetic polymorphisms of OATP1B1, OATP1B3 and OATP2B1, the present review focuses on the association of the genotypes of these OATPs with in vitro activity changes and in vivo clinical outcomes of substrate drugs.

Rare versus common variants in pharmacogenetics: SLCO1B1 variation and methotrexate disposition

Methotrexate is used to treat autoimmune diseases and malignancies, including acute lymphoblastic leukemia (ALL). Inter-individual variation in clearance of methotrexate results in heterogeneous systemic exposure, clinical efficacy, and toxicity. In a genome-wide association study of children with ALL, we identified SLCO1B1 as harboring multiple common polymorphisms associated with methotrexate clearance. The extent of influence of rare versus common variants on pharmacogenomic phenotypes remains largely unexplored. We tested the hypothesis that rare variants in SLCO1B1 could affect methotrexate clearance and compared the influence of common versus rare variants in addition to clinical covariates on clearance. From deep resequencing of SLCO1B1 exons in 699 children, we identified 93 SNPs, 15 of which were non-synonymous (NS). Three of these NS SNPs were common, with a minor allele frequency (MAF) >5%, one had low frequency (MAF 1%-5%), and 11 were rare (MAF <1%). NS SNPs (common or rare) predicted to be functionally damaging were more likely to be found among patients with the lowest methotrexate clearance than patients with high clearance. We verified lower function in vitro of four SLCO1B1 haplotypes that were associated with reduced methotrexate clearance. In a multivariate stepwise regression analysis adjusting for other genetic and non-genetic covariates, SLCO1B1 variants accounted for 10.7% of the population variability in clearance. Of that variability, common NS variants accounted for the majority, but rare damaging NS variants constituted 17.8% of SLCO1B1's effects (1.9% of total variation) and had larger effect sizes than common NS variants. Our results show that rare variants are likely to have an important effect on pharmacogenetic phenotypes.

Functional consequences of genetic variations in the human organic anion transporting polypeptide 1B3 (OATP1B3) in the Korean population

The objectives of this study were to investigate the allele frequencies and linkage disequilibrium (LD) in the organic anion transporting polypeptide 1B3 (OATP1B3) in the Korean population and to examine the functional consequences. Using samples from 48 Koreans, direct sequencing was carried out to determine the allele frequencies and LD of OATP1B3 in a representative Korean population. Thirty-six genetic variations in the transporter were found in Koreans; among them, five undocumented variations (i.e.,-6436G>C in the 5'-upstream region, 26A>C and 586A>G in the protein coding region, and IVS6-72A>T and IVS12-80A>T in intron regions) were identified. In the upstream region, -5035G>A was found to have lowered gene expression, as determined by a reporter gene assay, suggesting that this variation reduces the expression of OATP1B3 in humans. The functional relevance of the genetic variations in the protein coding region was determined by an uptake study involving representative substrates in human embryonic kidney 293 cells expressing wild type or variant forms. Variations involving 699G>A showed a reduced uptake activity for testosterone, but not for estradiol 17β-d-glucuronide or methotrexate, indicating that the functional impact of the variations is substrate specific. Considering the kinetic relevance of OATP1B3, the functionally affected variations may be therapeutically important.

The evolution of the OATP hepatic uptake transport protein family in DMPK sciences: from obscure liver transporters to key determinants of hepatobiliary clearance

Over the last two decades the impact on drug pharmacokinetics of the organic anion transporting polypeptides (OATPs: OATP-1B1, 1B3 and 2B1), expressed on the sinusoidal membrane of the hepatocyte, has been increasingly recognized. OATP-mediated uptake into the hepatocyte coupled with subsequent excretion into bile via efflux proteins, such as MRP2, is often referred to as hepatobiliary excretion. OATP transporter proteins can impact some drugs in several ways including pharmacokinetic variability, pharmacodynamic response and drug-drug interactions (DDIs). The impact of transporter mediated hepatic clearance is illustrated with case examples, from the literature and also from the Pfizer portfolio. The currently available in vitro techniques to study the hepatic transporter proteins involved in the hepatobiliary clearance of drugs are reviewed herein along with recent advances in using these in vitro data to predict the human clearance of compounds recognized by hepatic uptake transporters.

Contribution of organic anion transporting polypeptide (OATP) 1B1 and OATP1B3 to hepatic uptake of nateglinide, and the prediction of drug–drug interactions via these transporters

Objectives

We have investigated the contributions of organic anion transporting polypeptide (OATP) 1B1 and OATP1B3 to the hepatic uptake of nateglinide, and the possibility of drug–drug interactions via these transporters.

Methods

Uptake studies using transporter-expressing HEK293 cells and cryopreserved human hepatocytes were performed to examine the contributions of each transporter. Inhibition studies using cryopreserved human hepatocytes were performed to examine the possibility of drug–drug interactions.

Key findings

The rate of saturable hepatic uptake of nateglinide using human hepatocytes was 47.6%. A certain increase in uptake was observed in the examination using transporter-expressing HEK293 cells, indicating contributions of OATP1B1 and OATP1B3 to hepatic nateglinide uptake. The 50% inhibitory concentration (IC50) values of nateglinide using cryopreserved human hepatocytes for uptake of estrone 3-sulfate (substrate of OATP1B1), and cholecystokinin octapeptide (substrate of OATP1B3) were 168 and 17.4 µmol/l, respectively. Moreover, ciclosporin inhibited saturable hepatic uptake of nateglinide with an IC50 value of 6.05 µmol/l. The calculated 1 + Iin,max,u/IC50 values for inhibition of OATP1B1 and OATP1B3 by nateglinide, and the inhibition of saturable uptake of nateglinide by ciclosporin, were all close to 1, indicating a low clinical risk of drug–drug interaction with nateglinide taken up via OATP1B1 and OATP1B3.

Conclusions

OATP1B1 and OATP1B3 may have contributed to the hepatic uptake of nateglinide, but the possibility of drug–drug interactions appeared to be low.

Hepatic uptake in the dog: comparison of uptake in hepatocytes and human embryonic kidney cells expressing dog organic anion-transporting polypeptide 1B4

Although the dog is frequently used in pharmacological, pharmacokinetic, and drug safety studies, little is known about canine drug transporters. Dog organic anion-transporting polypeptide (Oatp1b4) has recently been cloned (Comp Biochem Physiol C Toxicol Pharmacol 151:393-399, 2010), but the contribution of Oatp1b4 to hepatic uptake has yet to be clarified. This study compares the transport characteristics of dog Oatp1b4 with those of human OATP1B1/1B3 and demonstrates the importance of Oatp1b4 in the uptake of anionic compounds in dog hepatocytes. Oatp1b4 is the predominant Oatp in dog liver with expression levels double and 30 times those of Oatp2b1 and Oatp1a2, respectively. Uptake of a range of typical OATP substrates by Oatp1b4-expressing HEK293 cells was compared with that in fresh dog hepatocytes. All compounds tested were transported by Oatp1b4 and uptake intrinsic clearance (CL(int, uptake)) in dog hepatocytes in sodium-free buffer was correlated significantly with CL(int, uptake) in Oatp1b4-expressing cells. Dog in vivo clearance for five substrates was predicted more accurately from CL(int, uptake) than from metabolic intrinsic clearance (CL(int, met)), indicating that uptake governs the overall in vivo hepatic clearance of these anionic compounds in dog. The substrate specificities of dog Oatp1b4 appear to be similar to those of human OATP1B1/OATP1B3, whereas the relative uptake clearance of substrates for Oatp1b4 correlate better with OATP1B3 than with the more abundant hepatic analog OATP1B1.

Interaction of fluvastatin with the liver-specific Na+ -dependent taurocholate cotransporting polypeptide (NTCP)

It has been reported that polymorphisms in the organic anion transporting polypeptide 1B1 (OATP1B1, SLCO1B1) result in decreased hepatic uptake of simvastatin carboxy acid, the active metabolite of simvastatin. This is not the case for fluvastatin and it has been hypothesized that for this drug other hepatic uptake pathways exist. Here, we studied whether Na(+)-dependent taurocholate co-transporting polypeptide (NTCP, SLC10A1) can be an alternative hepatic uptake route for fluvastatin. Chinese Hamster Ovary cells transfected with human NTCP (CHO-NTCP) were used to investigate the inhibitory effect of fluvastatin and other statins on [(3)H]-taurocholic acid uptake ([(3)H]-TCA). Statin uptake by CHO-NTCP and cryopreserved human hepatocytes was assessed via LC-MS/MS. Fluvastatin appeared to be a potent and competitive inhibitor of [(3)H]-TCA uptake (IC(50) of 40μM), pointing to an interaction at the level of the bile acid binding pocket of NTCP. The inhibitory action of other statins was also studied, which revealed that statin inhibitory potency increased with molecular descriptors of lipophilicity: calculated logP (r(2)=0.82, p=0.034), logD(7.4) (r(2)=0.77, p=0.0001). Studies in CHO-NTCP cells showed that fluvastatin was indeed an NTCP substrate (K(m) 250±30μM, V(max) 1340±50ng/mg total cell protein/min). However, subsequent studies revealed that at clinically relevant plasma concentrations, NTCP contributed minimally to overall accumulation in human hepatocytes. In conclusion, fluvastatin interacts with NTCP at the level of the bile acid binding pocket and is an NTCP substrate. However, under normal conditions, NTCP-mediated uptake of this drug seems not to be a significant hepatocellular uptake pathway.

Expression of organic anion-transporting polypeptides 1B1 and 1B3 in ovarian cancer cells: relevance for paclitaxel transport

PURPOSE

Ovarian cancer remains a deadly malignancy because most patients develop recurrent disease that is resistant to chemotherapy. Organic anion-transporting polypeptides (OATPs) mediate the uptake of clinically important drugs thereby effecting intracellular drug accumulation. In this study, we investigated whether OATPs may also contribute to paclitaxel transport in estrogen-responsive and estrogen-independent ovarian carcinoma cell lines and tumor tissue.

METHODS

Expression of all 11 human OATPs in human ovarian cancer tissue samples and in the ovarian carcinoma cell lines OVCAR-3 and SK-OV-3 was investigated using real-time RT-PCR. Kinetic analysis of paclitaxel uptake was characterized in both cell lines and in OATP-transfected Xenopus laevis oocytes. Cytotoxicity of paclitaxel in OVCAR-3, SK-OV-3 and OATP1B1- and OATP1B3-transfected SK-OV-3 cells was performed using the CellTiter-Glo assay.

RESULTS

OATP1B1 and OATP1B3 are active paclitaxel transporters in transfected X. laevis oocytes. Real-time RT-PCR analysis revealed expression of both OATPs in human ovarian cancer tissue specimens and in cancer cell lines. The higher mRNA levels for OATP1B1 and OATP1B3 found in SK-OV-3 cells correlated with higher initial uptake rates for paclitaxel. In addition, cytotoxicity studies with OATP1B1- and OATP1B3-transfected SK-OV-3 cells demonstrated lower IC(50) values compared to cells transfected with the empty vector.

CONCLUSIONS

Our results revealed OATP1B1 and OATP1B3 as high-affinity paclitaxel transporters expressed in ovarian cancer cell lines and tumor tissues, suggesting a role for these polypeptides in the disposition of paclitaxel during therapy.

A quantitative framework and strategies for management and evaluation of metabolic drug-drug interactions in oncology drug development: new molecular entities as object drugs

This article outlines general strategies for the management and evaluation of pharmacokinetic drug-drug interactions (DDIs) resulting from perturbation of clearance of investigational anticancer drug candidates by concomitantly administered agents in a drug development setting, with a focus on drug candidates that cannot be evaluated in first-in-human studies in healthy subjects. A risk level classification is proposed, based on quantitative integration of knowledge derived from preclinical drug-metabolism studies evaluating the projected percentage contribution [f(i)(%)] of individual molecular determinants (e.g. cytochrome P450 isoenzymes) to the overall human clearance of the investigational agent. The following classification is proposed with respect to susceptibility to DDIs with metabolic inhibitors: a projected maximum DDI expected to result in a ≤1.33-fold increase in exposure, representing a low level of risk; a projected maximum DDI expected to result in a >1.33-fold but <2-fold increase in exposure, representing a moderate level of risk; and a projected maximum DDI expected to result in a ≥2-fold increase in exposure, representing a potentially high level of risk. For DDIs with metabolic inducers, the following operational classification is proposed, based on the sum of the percentage contributions of enzymes that are inducible via a common mechanism to the overall clearance of the investigational drug: <<25%, representing a low level of risk; <50%, representing a moderate level of risk; and ≥50%, representing a potentially high level of risk. To ensure patient safety and to minimize bias in determination of the recommended phase II dose (RP2D), it is recommended that strong and moderate inhibitors and inducers of the major contributing enzyme are excluded in phase I dose-escalation studies of high-risk compounds, whereas exclusion of strong inhibitors and inducers of the contributing enzyme(s) is recommended as being sufficient for moderate-risk compounds. For drugs that will be investigated in diseases such as glioblastoma, where there may be relatively frequent use of enzyme-inducing antiepileptic agents (EIAEDs), a separate dose-escalation study in this subpopulation is recommended to define the RP2D. For compounds in the high-risk category, if genetic deficiencies in the activity of the major drug-metabolizing enzyme are known, it is recommended that poor metabolizers be studied separately to define the RP2D for this subpopulation. Whereas concomitant medication exclusion criteria that are utilized in the phase I dose-escalation studies will probably also need to be maintained for high-risk compounds in phase II studies unless the results of a clinical DDI study indicate the absence of a clinically relevant interaction, these exclusion criteria can potentially be relaxed beyond phase I for moderate-risk compounds, if supported by the nature of clinical toxicities and the understanding of the therapeutic index in phase I. Adequately designed clinical DDI studies will not only inform potential relaxation of concomitant medication exclusion criteria in later-phase studies but, importantly, will also inform the development of pharmacokinetically derived dose-modification guidelines for use in clinical practice when coupled with adequate safety monitoring, as illustrated in the prescribing guidance for many recently approved oncology therapeutics.

Organic anion transporting polypeptide 1B1: a genetically polymorphic transporter of major importance for hepatic drug uptake

The importance of membrane transporters for drug pharmacokinetics has been increasingly recognized during the last decade. Organic anion transporting polypeptide 1B1 (OATP1B1) is a genetically polymorphic influx transporter expressed on the sinusoidal membrane of human hepatocytes, and it mediates the hepatic uptake of many endogenous compounds and xenobiotics. Recent studies have demonstrated that OATP1B1 plays a major, clinically important role in the hepatic uptake of many drugs. A common single-nucleotide variation (coding DNA c.521T>C, protein p.V174A, rs4149056) in the SLCO1B1 gene encoding OATP1B1 decreases the transporting activity of OATP1B1, resulting in markedly increased plasma concentrations of, for example, many statins, particularly of active simvastatin acid. The variant thereby enhances the risk of statin-induced myopathy and decreases the therapeutic indexes of statins. However, the effect of the SLCO1B1 c.521T>C variant is different on different statins. The same variant also markedly affects the pharmacokinetics of several other drugs. Furthermore, certain SLCO1B1 variants associated with an enhanced clearance of methotrexate increase the risk of gastrointestinal toxicity by methotrexate in the treatment of children with acute lymphoblastic leukemia. Certain drugs (e.g., cyclosporine) potently inhibit OATP1B1, causing clinically significant drug interactions. Thus, OATP1B1 plays a major role in the hepatic uptake of drugs, and genetic variants and drug interactions affecting OATP1B1 activity are important determinants of individual drug responses. In this article, we review the current knowledge about the expression, function, substrate characteristics, and pharmacogenetics of OATP1B1 as well as its role in drug interactions, in parts comparing with those of other hepatocyte-expressed organic anion transporting polypeptides, OATP1B3 and OATP2B1.

Pitavastatin: a new 3-hydroxy-3-methylglutaryl coenzyme a reductase inhibitor for the treatment of hyperlipidemia

Statins have proven beneficial for reducing both primary and secondary events in patients with coronary heart disease. Tight control of serum lipid parameters in these patients is recommended by the most recent clinical guidelines. Although numerous lipid-lowering treatments are available, only a small percentage of eligible patients receive therapy and fewer achieve their lipid-lowering goals. Thus it is clear that new treatment strategies to manage patients with lipid abnormalities are warranted. Pitavastatin (Lival; Kowa Pharmaceuticals America, Montgomery, AL, USA) has been recently approved for the treatment of hypercholesterolemia and combined dyslipidemia. Pitavastatin 1-4 mg/day has shown similar low-density lipoprotein-reducing activity to other commercially available statins, including simvastatin and atorvastatin. Adverse events occurred at similar rates to other statins in clinical trials with favorable effects seen in patients with dyslipidemia and metabolic syndrome. Pharmacokinetic drug-drug interactions are minimized due to the lack of significant metabolism of pitavastatin by the cytochrome P450 enzyme system, although some drugs affect its uptake into hepatocytes and should be avoided. In addition to its higher acquisition cost, pitavastatin has not been shown to improve clinical outcomes in high-risk patient populations and thus may not be the agent of choice in many patients at this time in lieu of cheaper, clinically proven alternatives.

Uptake transporters of the human OATP family: molecular characteristics, substrates, their role in drug-drug interactions, and functional consequences of polymorphisms

Organic anion transporting polypeptides (OATPs, gene family: SLC21/SLCO) mediate the uptake of a broad range of substrates including several widely prescribed drugs into cells. Drug substrates for members of the human OATP family include HMG-CoA-reductase inhibitors (statins), antibiotics, anticancer agents, and cardiac glycosides. OATPs are expressed in a variety of different tissues including brain, intestine, liver, and kidney, suggesting that these uptake transporters are important for drug absorption, distribution, and excretion. Because of their wide tissue distribution and broad substrate spectrum, altered transport kinetics, for example, due to drug-drug interactions or due to the functional consequences of genetic variations (polymorphisms), can contribute to the interindividual variability of drug effects. Therefore, the molecular characteristics of human OATP family members, the role of human OATPs in drug-drug interactions, and the in vitro analysis of the functional consequences of genetic variations in SLCO genes encoding OATP proteins are the focus of this chapter.

Intestinal absorption of HMG-CoA reductase inhibitor pitavastatin mediated by organic anion transporting polypeptide and P-glycoprotein/multidrug resistance 1

The purpose of this study was to investigate the involvement of organic anion transporting polypeptide (OATP/Oatp) and P-glycoprotein (P-gp)/multidrug resistance 1 (MDR1/Mdr1) in intestinal absorption of pitavastatin. Pitavastatin was found to be a substrate for human OATP1A2, OATP2B1, and MDR1 and rat Oatp1a5, Oatp2b1, and Mdr1a in experiments using transporter-expressing Xenopus oocytes and LLC-PK1 cell systems. Naringin inhibited Oatp1a5- and Mdr1a-mediated transport of pitavastatin with IC(50) values of 18.5 and 541 µM, respectively. The difference in the IC(50) values of naringin for Oatp1a5 and Mdr1a-mediated pitavastatin transport may account for the complex concentration-dependent effect of naringin on the intestinal absorption of pitavastatin. Rat intestinal permeability of pitavastatin measured by the in situ closed-loop perfusion method was indeed significantly reduced by 200 µM naringin, but was significantly increased by 1000 µM naringin. Furthermore, the permeability was significantly increased by elacridar, a potent inhibitor of Mdr1, while the permeability was significantly decreased in the presence of both elacridar and naringin, suggesting that Oatp1a5 and Mdr1a are both involved in intestinal absorption of pitavastatin. Our present results indicate that OATP/Oatp and MDR1/Mdr1 play roles in the intestinal absorption of pitavastatin as influx and efflux transporters, respectively.

Pitavastatin for the treatment of primary hyperlipidemia and mixed dyslipidemia

Pitavastatin is a new, synthetic member of the statin class of lipid-lowering drugs. Compared with other available statins, it has a unique cyclopropyl group on its base structure that is believed to increase 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibition by a factor of five and to significantly increase the transcription and activity of LDL receptors. Pitavastatin is primarily metabolized via glucuronidation and is not a substrate for the cytochrome P450 3A4 enzyme, thus avoiding the potential for cytochrome P450-mediated drug-drug interactions. Clinical trials have shown that pitavastatin is comparable to atorvastatin and simvastatin in improving lipid measures, and more potent than pravastatin. Pitavastatin is effective in reducing triglycerides and increasing HDL-cholesterol, so it will be particularly beneficial in treating patients with mixed dyslipidemia. Its safety and adverse event profile is similar to that of other available statins, and it has an established history of use in Asia indicating tolerability and safety for treatment lasting up to 7 years.

Flavopiridol pharmacogenetics: clinical and functional evidence for the role of SLCO1B1/OATP1B1 in flavopiridol disposition

BACKGROUND

Flavopiridol is a cyclin-dependent kinase inhibitor in phase II clinical development for treatment of various forms of cancer. When administered with a pharmacokinetically (PK)-directed dosing schedule, flavopiridol exhibited striking activity in patients with refractory chronic lymphocytic leukemia. This study aimed to evaluate pharmacogenetic factors associated with inter-individual variability in pharmacokinetics and outcomes associated with flavopiridol therapy.

METHODOLOGY/PRINCIPAL FINDINGS

Thirty-five patients who received single-agent flavopiridol via the PK-directed schedule were genotyped for 189 polymorphisms in genes encoding 56 drug metabolizing enzymes and transporters. Genotypes were evaluated in univariate and multivariate analyses as covariates in a population PK model. Transport of flavopiridol and its glucuronide metabolite was evaluated in uptake assays in HEK-293 and MDCK-II cells transiently transfected with SLCO1B1. Polymorphisms in ABCC2, ABCG2, UGT1A1, UGT1A9, and SLCO1B1 were found to significantly correlate with flavopiridol PK in univariate analysis. Transport assay results indicated both flavopiridol and flavopiridol-glucuronide are substrates of the SLCO1B1/OATP1B1 transporter. Covariates incorporated into the final population PK model included bilirubin, SLCO1B1 rs11045819 and ABCC2 rs8187710. Associations were also observed between genotype and response. To validate these findings, a second set of data with 51 patients was evaluated, and overall trends for associations between PK and PGx were found to be consistent.

CONCLUSIONS/SIGNIFICANCE

Polymorphisms in transport genes were found to be associated with flavopiridol disposition and outcomes. Observed clinical associations with SLCO1B1 were functionally validated indicating for the first time its relevance as a transporter of flavopiridol and its glucuronide metabolite. A second 51-patient dataset indicated similar trends between genotype in the SLCO1B1 and other candidate genes, thus providing support for these findings. Further study in larger patient populations will be necessary to fully characterize and validate the clinical impact of polymorphisms in SLCO1B1 and other transporter and metabolizing enzyme genes on outcomes from flavopiridol therapy.