Effects of organic anion transporting polypeptide 1B1 haplotype on pharmacokinetics of pravastatin, valsartan, and temocapril

Toward Prospective Prediction of Pharmacokinetics in OATP1B1 Genetic Variant Populations

Physiologically based pharmacokinetic (PBPK) models are increasingly being used to provide human pharmacokinetic (PK) predictions for organic anion-transporting polypeptide (OATP) substrates based on in vitro assay data. As a natural extension in the application of these models, in this study, we incorporated in vitro information of three major OATP1B1 genetic variants into a previously reported PBPK model to predict the impact of OATP1B1 polymorphisms on human PK. Using pravastatin and rosuvastatin as examples, we showed that the predicted plasma concentration-time profiles in groups carrying different OATP1B1 genetic variants reasonably matched the clinical observations from multiple studies. This modeling and simulation approach may aid decision making in early pharmaceutical research and development as well as patient-specific dose adjustment in clinical practice.

The pharmacogenomics of statins

The statin class of cholesterol-lowering drugs have been used for decades to successfully lower plasma cholesterol concentrations and cardiovascular risk. Adverse effects of statins are generally considered mild, but increase with age of patients and polypharmacy. One aspect of statin therapy that is still difficult for prescribers to predict is the individual's response to statin therapy. Recent advances in the field of pharmacogenomics have indicated variants of candidate genes that affect statin efficacy and safety. In this review, a number of candidates that affect statin pharmacokinetics and pharmacodynamics are discussed. Some of these candidates, in particular those involved in import and efflux of statins, have now been linked to increased risk of side effects. Furthermore, pharmacogenomic studies continue to reveal new players that are involved in the fine-tuning of the complex regulation of cholesterol homeostasis and response to statins.

Drug metabolizing enzyme and transporter protein profiles of hepatocytes derived from human embryonic and induced pluripotent stem cells

Human embryonic and induced pluripotent stem cell-derived hepatocytes (hESC-Hep and hiPSC-Hep) have the potential to provide relevant human in vitro model systems for toxicity testing and drug discovery studies. In this study, the expression and function of important drug metabolizing cytochrome P450 (CYP) enzymes and transporter proteins in hESC-Hep and hiPSC-Hep were compared to cryopreserved human primary hepatocytes (hphep) and HepG2 cells. Overall, CYP activities in hESC-Hep and hiPSC-Hep were much lower than in hphep cultured for 4 h, but CYP1A and 3A activities were comparable to levels in hphep cultured for 48h (CYP1A: 35% and 26% of 48 h hphep, respectively; CYP3A: 80% and 440% of 48 h hphep, respectively). Importantly, in hESC-Hep and hiPSC-Hep, CYP activities were stable or increasing for at least one week in culture which was in contrast to the rapid loss of CYP activities in cultured hphep between 4 and 48 h after plating. With regard to transporters, in hESC-Hep and hiPSC-Hep, pronounced NTCP activity (17% and 29% of 4 h hphep, respectively) and moderate BSEP activity (6% and 8% of 4 h hphep, respectively) were observed. Analyses of mRNA expression and immunocytochemistry supported the observed CYP and transporter activities and showed expression of additional CYPs and transporters. In conclusion, the stable expression and function of CYPs and transporters in hESC-Hep and hiPSC-Hep for at least one week opens up the possibility to reproducibly perform long term and extensive studies, e.g. chronic toxicity testing, in a stem cell-derived hepatic system.

Clinical significance of organic anion transporting polypeptides (OATPs) in drug disposition: their roles in hepatic clearance and intestinal absorption

Organic anion transporting polypeptide (OATP) family transporters accept a number of drugs and are increasingly being recognized as important factors in governing drug and metabolite pharmacokinetics. OATP1B1 and OATP1B3 play an important role in hepatic drug uptake while OATP2B1 and OATP1A2 might be key players in intestinal absorption and transport across blood-brain barrier of drugs, respectively. To understand the importance of OATPs in the hepatic clearance of drugs, the rate-determining process for elimination should be considered; for some drugs, hepatic uptake clearance rather than metabolic intrinsic clearance is the more important determinant of hepatic clearances. The importance of the unbound concentration ratio (liver/blood), K(p,uu) , of drugs, which is partly governed by OATPs, is exemplified in interpreting the difference in the IC(50) of statins between the hepatocyte and microsome systems for the inhibition of HMG-CoA reductase activity. The intrinsic activity and/or expression level of OATPs are affected by genetic polymorphisms and drug-drug interactions. Their effects on the elimination rate or intestinal absorption rate of drugs may sometimes depend on the substrate drug. This is partly because of the different contribution of OATP isoforms to clearance or intestinal absorption. When the contribution of the OATP-mediated pathway is substantial, the pharmacokinetics of substrate drugs should be greatly affected. This review describes the estimation of the contribution of OATP1B1 to the total hepatic uptake of drugs from the data of fold-increases in the plasma concentration of substrate drugs by the genetic polymorphism of this transporter. To understand the importance of the OATP family transporters, modeling and simulation with a physiologically based pharmacokinetic model are helpful.

A systematic comparison of the properties of clinically used angiotensin II type 1 receptor antagonists

Angiotensin II type 1 receptor antagonists (ARBs) have become an important drug class in the treatment of hypertension and heart failure and the protection from diabetic nephropathy. Eight ARBs are clinically available [azilsartan, candesartan, eprosartan, irbesartan, losartan, olmesartan, telmisartan, valsartan]. Azilsartan (in some countries), candesartan, and olmesartan are orally administered as prodrugs, whereas the blocking action of some is mediated through active metabolites. On the basis of their chemical structures, ARBs use different binding pockets in the receptor, which are associated with differences in dissociation times and, in most cases, apparently insurmountable antagonism. The physicochemical differences between ARBs also manifest in different tissue penetration, including passage through the blood-brain barrier. Differences in binding mode and tissue penetration are also associated with differences in pharmacokinetic profile, particularly duration of action. Although generally highly specific for angiotensin II type 1 receptors, some ARBs, particularly telmisartan, are partial agonists at peroxisome proliferator-activated receptor-γ. All of these properties are comprehensively reviewed in this article. Although there is general consensus that a continuous receptor blockade over a 24-hour period is desirable, the clinical relevance of other pharmacological differences between individual ARBs remains to be assessed.

Drug-transporter interaction testing in drug discovery and development

The human body consists of several physiological barriers that express a number of membrane transporters. For an orally absorbed drug the intestinal, hepatic, renal and blood-brain barriers are of the greatest importance. The ATP-binding cassette (ABC) transporters that mediate cellular efflux and the solute carrier transporters that mostly mediate cellular uptake are the two superfamilies responsible for membrane transport of vast majority of drugs and drug metabolites. The total number of human transporters in the two superfamilies exceeds 400, and about 40-50 transporters have been characterized for drug transport. The latest Food and Drug Administration guidance focuses on P-glycoprotein, breast cancer resistance protein, organic anion transporting polypeptide 1B1 (OATP1B1), OATP1B3, organic cation transporter 2 (OCT2), and organic anion transporters 1 (OAT1) and OAT3. The European Medicines Agency’s shortlist additionally contains the bile salt export pump, OCT1, and the multidrug and toxin extrusion transporters, multidrug and toxin extrusion protein 1 (MATE1) and MATE2/MATE2K. A variety of transporter assays are available to test drug-transporter interactions, transporter-mediated drug-drug interactions, and transporter-mediated toxicity. The drug binding site of ABC transporters is accessible from the cytoplasm or the inner leaflet of the plasma membrane. Therefore, vesicular transport assays utilizing inside-out vesicles are commonly used assays, where the directionality of transport results in drugs being transported into the vesicle. Monolayer assays utilizing polarized cells expressing efflux transporters are the test systems suggested by regulatory agencies. However, in some monolayers, uptake transporters must be coexpressed with efflux transporters to assure detectable transport of low passive permeability drugs. For uptake transporters mediating cellular drug uptake, utilization of stable transfectants have been suggested. In vivo animal models complete the testing battery. Some issues, such as in vivo relevance, gender difference, age and ontogeny issues can only be addressed using in vivo models. Transporter specificity is provided by using knock-out or mutant models. Alternatively, chemical knock-outs can be employed. Compensatory changes are less likely when using chemical knock-outs. On the other hand, specific inhibitors for some uptake transporters are not available, limiting the options to genetic knock-outs.

Mechanisms of pharmacokinetic enhancement between ritonavir and saquinavir; micro/small dosing tests using midazolam (CYP3A4), fexofenadine (p-glycoprotein), and pravastatin (OATP1B1) as probe drugs

We investigated the mechanisms of ritonavir-mediated enhancement effect on the pharmacokinetics of saquinavir using in vivo probes for CYP3A4 (midazolam), p-glycoprotein (fexofenadine), and OATP1B1 (pravastatin) following oral micro/small dosing. A cocktail of the drugs (2 mg of saquinavir, 100 µg of each probe) was administered to eight healthy volunteers (phase 1), and then coadministered with 20 mg (phase 2) and 100 mg (phase 3) of ritonavir. Plasma concentrations of the drugs were measured by validated LC-MS/MS methods. The mean plasma AUC0-24 (pg hour/mL) of saquinavir at phases 1, 2, and 3 was 101, 2 540, and 23 900 (P < .01), respectively. The relative area under the plasma concentration-time curve (AUC)0-24 ratios of midazolam and fexofenadine at phases 1, 2, and 3 were 1:5.9:14.7 (P < .01), and 1:1.4:2.2 (P < .01-.05), respectively. In contrast, there was no difference in the pharmacokinetics of pravastatin. Inhibition of intestinal and hepatic CYP3A-mediated metabolism, and intestinal p-glycoprotein-mediated efflux of saquinavir, but not OATP1B1, is involved in the enhancement mechanism. Micro/small dosing is useful for examining the mechanism of drug interactions without safety concern.

Genetics is a major determinant of expression of the human hepatic uptake transporter OATP1B1, but not of OATP1B3 and OATP2B1

BACKGROUND

Organic anion transporting polypeptide (OATP) 1B1, OATP1B3, and OATP2B1 (encoded by SLCO1B1, SLCO1B3, SLCO2B1) mediate the hepatic uptake of endogenous compounds like bile acids and of drugs, for example, the lipid-lowering atorvastatin, thereby influencing hepatobiliary elimination. Here we systematically elucidated the contribution of SLCO variants on expression of the three hepatic OATPs under consideration of additional important covariates.

METHODS

Expression was quantified by RT-PCR and immunoblotting in 143 Caucasian liver samples. A total of 109 rare and common variants in the SLCO1B3-SLCO1B1 genomic region and the SLCO2B1 gene were genotyped by MALDI-TOF mass spectrometry and genome-wide SNP microarray technology. SLCO1B1 haplotypes affecting hepatic OATP1B1 expression were associated with pharmacokinetic data of the OATP1B1 substrate atorvastatin (n = 82).

RESULTS

Expression of OATP1B1, OATP1B3, and OATP2B1 at the mRNA and protein levels showed marked interindividual variability. All three OATPs were expressed in a coordinated fashion. By a multivariate regression analysis adjusted for non-genetic and transcription covariates, increased OATP1B1 expression was associated with the coding SLCO1B1 variant c.388A > G (rs2306283) even after correction for multiple testing (P = 0.00034). This held true for haplotypes harboring c.388A > G but not the functional variant c.521T > C (rs4149056) associated with statin-related myopathy. c.388A > G also significantly affected atorvastatin pharmacokinetics. SLCO variants and non-genetic and regulatory covariates together accounted for 59% of variability of OATP1B1 expression.

CONCLUSIONS

Our results show that expression of OATP1B1, but not of OATP1B3 and OATP2B1, is significantly affected by genetic variants. The SLCO1B1 variant c.388A > G is the major determinant with additional consequences on atorvastatin plasma levels.

The promiscuous binding of pharmaceutical drugs and their transporter-mediated uptake into cells: what we (need to) know and how we can do so

A recent paper in this journal sought to counter evidence for the role of transport proteins in effecting drug uptake into cells, and questions that transporters can recognize drug molecules in addition to their endogenous substrates. However, there is abundant evidence that both drugs and proteins are highly promiscuous. Most proteins bind to many drugs and most drugs bind to multiple proteins (on average more than six), including transporters (mutations in these can determine resistance); most drugs are known to recognise at least one transporter. In this response, we alert readers to the relevant evidence that exists or is required. This needs to be acquired in cells that contain the relevant proteins, and we highlight an experimental system for simultaneous genome-wide assessment of carrier-mediated uptake in a eukaryotic cell (yeast).

Ethnic variability in the plasma exposures of OATP1B1 substrates such as HMG-CoA reductase inhibitors: a kinetic consideration of its mechanism

Because the plasma exposure levels of rosuvastatin in Asians are generally twice those in Caucasians, the starting dose for Asians in the United States is set to half of that for non-Asians. However, the precise role of ethnicity in the clearance of rosuvastatin has not yet been clarified. This review focuses on ethnic variability in the clinical pharmacokinetics of 3-hydroxy-3-methylglutaryl co-enzyme A (HMG-CoA) reductase inhibitors (statins) and angiotensin II receptor antagonists. The mechanisms of such variability are discussed quantitatively, with building a hypothetical model for pravastatin, and validated against other statins. Our analyses suggest that the ethnic variability in the plasma exposure of statins cannot be explained only by the difference in the allele frequencies of organic anion-transporting polypeptide (OATP)1B1 and breast cancer resistance protein (BCRP), and the intrinsic ethnic variability in the activity of OATP1B1 (the ratio of Japanese/Caucasians is 0.584) must be considered. Further work and validation with additional data will clarify the applicability of this model to other OATP1B1 substrates.

The pharmacokinetics and pharmacodynamics of valsartan in the post-myocardial infarction population

INTRODUCTION

The most common risk factors for heart failure are hypertension and myocardial infarction. Angiotensin receptor blockers (ARBs) attenuate the deleterious effects of angiotensin II. Valsartan is a once or twice daily ARB that is FDA-approved for hypertension, LV dysfunction post-myocardial infarction and congestive heart failure as both an adjunct in ACE-inhibitor tolerant, and alternative in ACE-I intolerant patients.

AREAS COVERED

This article presents a comprehensive review of the literature regarding the pharmacokinetics and pharmacodynamics of valsartan, with particular attention paid to the post-myocardial infarction population.

EXPERT OPINION

Valsartan is a safe, well-tolerated and readily titratable ARB. In addition to its vasodilatory effects there are pleotropic effects associated with the ARB such as modulation of a number of neurohormonal regulators, cytokines and small molecules. Given the clear evidence-based benefits above and beyond its hypertensive properties, it has the potential, if priced appropriately, to grow in its impact as a pharmacotherapeutic long after its patent expires.

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.

Influence of serum in hemodialysis patients on the expression of intestinal and hepatic transporters for the excretion of pravastatin

It is known that the lipid-lowering agent pravastatin, which is not metabolized by cytochrome P450, is eliminated as an unchanged drug in bile and urine. It is interesting to note that the non-renal clearance of pravastatin in end-stage renal failure patients is decreased compared with that of healthy volunteers. This study investigated the influence of uremic serum and toxins on the transport mechanisms of pravastatin to elucidate the cause of decreased non-renal clearance in end-stage renal failure patients. Caco-2 and Hep3B cells were used as models of intestinal epithelial cells and hepatocytes respectively. Normal and uremic serum were deproteinized by treatment with methanol. 3-Carboxy-4-methyl-5propyl-2-furanpropanoic acid (CMPF), hippuric acid, indole-3-acetic acid, 3-indoxyl sulfate, and p-cresol were chosen as uremic toxins. Uremic serum-treated Caco-2 cells exhibited significantly increased accumulation of pravastatin and significantly decreased expression of MRP2 mRNA compared with normal serum-treated Caco-2 cells. In addition, the expression of MRP2 mRNA tended to decrease in cells treated with CMPF, indole-3-acetic acid, or 3-indoxyl sulfate. Uremic serum-treated Hep3B cells showed a significantly decreased initial uptake rate of pravastatin; furthermore, the expressions of OATP1B1 and OATP2B1 mRNA were decreased compared to normal serum-treated Hep3B cells. These results suggest that the decrease in the non-renal clearance of pravastatin in end-stage renal failure patients is partly induced by the downregulation of intestinal MRP2 and hepatic OATP1B1 and/or OATP2B1 by various uremic toxins in end-stage renal failure patients.

Functional characterization and substrate specificity of a novel gene encoding zinc finger-like protein, ZfLp, in Xenopus laevis oocytes

In the present study, we isolated and determined the pharmacological characteristics of a novel gene encoding the zinc finger-like protein (ZfLp). The isolated cDNA consisted of 1,581 base pairs that encoded a 526-amino acid protein. The amino acid sequence of ZfLp is 96% identical to that of zinc finger protein 415 isoform 5 (ZNF415-5). Reverse-transcription (RT)-polymerase chain reaction (PCR) analysis revealed that the ZfLp mRNA is expressed in the breast, lung, stomach, small intestine colon and ovary, but not in the liver. When expressed in Xenopus laevis oocytes, ZfLp mediated the high affinity transport of [(3)H]paclitaxel (taxol) in a sodium-independent manner (K(m) = 336.7 ± 190.0 nM). The uptake of [(3)H]paclitaxel (taxol) by ZfLp was trans-stimulated by glutarate and glutathione (GSH). A cis-inhibition experiment revealed that ZfLp-mediated transport of [(3)H]paclitaxel (taxol) is inhibited by several organic solutes specifically clotrimazole. Using several clotrimazole derivatives, we found that N-benzylimidazole would be a minimum unit for producing the inhibition of ZfLp-mediated drug uptake. Our results may provide insights into the novel role of soluble protein, such as ZNF, in the human body. Our results, therefore, would be expected to facilitate research on the novel role of ZNFs and on the discovery of novel drugs for targeting ZNF-related proteins such as ZfLp.

Effect of CYP2C9 and SLCO1B1 polymorphisms on the pharmacokinetics and pharmacodynamics of nateglinide in healthy Chinese male volunteers

PURPOSE

Nateglinide is commonly used in the treatment of patients with type 2 diabetes mellitus. Our objective was to assess the association between CYP2C9 and SLCO1B1 polymorphisms and the metabolism of nateglinide in healthy Chinese male volunteers.

METHODS

A total of 35 healthy Chinese male volunteers with different CYP2C9 and SLCO1B1 genotypes were given a single oral dose of 120 mg nateglinide. Plasma concentrations of nateglinide and blood glucose level were measured up to 8 h.

RESULTS

In subjects with the CYP2C9*1/*3 & 521TT, CYP2C9*1/*1 & 521TC/CC and CYP2C9*1/*3 & 521TC genotype, AUC(0-∞) of nateglinide was 56 %, 34 % and 56 % higher (P = 0.002, P = 0.041 and P = 0.013, respectively), and the CL/F of nateglinide was 35 %, 11 % and 36 % lower (P = 0.000, P = 0.003 and P = 0.002, respectively) than that in the reference group. When only considering 521 T>C polymorphism, it had no significant association with the pharmacokinetics of nateglinide. CYP2C9*3 and 521 T>C polymorphisms were the significant predictors of the AUC(0-∞) and CL/F of nateglinide (adjusted multiple R(2) = 34 % and 43 %, respectively) according to multiple linear regression analyses, but they have no significant association with changes in the blood glucose-lowering effect of nateglinide.

CONCLUSIONS

Both SLCO1B1 521 T>C and the CYP2C9*3 polymorphisms can significantly affect the pharmacokinetics of nateglinide, but they could only partially explain the interindividual variability of plasma concentration of nateglinide. Moreover, 521 T>C and the CYP2C9*3 polymorphisms have no effect on pharmacodynamics of nateglinide in healthy Chinese male subjects.

Impact of drug transporter pharmacogenomics on pharmacokinetic and pharmacodynamic variability - considerations for drug development

INTRODUCTION

Drug transporter proteins are expressed on the cell membrane, regulating substrate exposure in systemic circulation and/or peripheral tissues. Genetic polymorphism of drug transporter genes encoding these proteins could alter the functional activity and/or protein expression, having effects on absorption, distribution, metabolism and excretion (ADME), efficacy and adverse effects.

AREAS COVERED

The authors provide the reader with an overview of the pharmacogenetics (PGx) of 12 membrane transporters. The clinical literature is summarized as to the quantitative significance on pharmacokinetics (PK) and implications on pharmacodynamics (PD) and adverse effects, due to transporter influence on intracellular drug concentrations.

EXPERT OPINION

Unlike polymorphisms for cytochrome P450s (CYPs) resulting in large magnitude of PK variation, genetic mutations for membrane transporters are typically less than threefold alteration in systemic PK for drugs with a few exceptions. However, substantially greater changes in intracellular drug levels may result. We are aware of 1880 exome variants in 12 of the best-studied transporters to date, and nearly 40% of these change the amino acid. However, the functional consequences of most of these variants remain to be determined, and have only been empirically evaluated for a handful. To the extent that genetic polymorphisms impact ADME, it is a variable that will contribute to ethnic differences due to substantial frequency differences for the known variants.

Genetics or environment in drug transport: the case of organic anion transporting polypeptides and adverse drug reactions

INTRODUCTION

Organic anion transporting polypeptide (OATP) uptake transporters are important for the disposition of many drugs and perturbed OATP activity can contribute to adverse drug reactions (ADRs). It is well documented that both genetic and environmental factors can alter OATP expression and activity. Genetic factors include single nucleotide polymorphisms (SNPs) that change OATP activity and epigenetic regulation that modify OATP expression levels. SNPs in OATPs contribute to ADRs. Environmental factors include the pharmacological context of drug-drug interactions and the physiological context of liver diseases. Liver diseases such as non-alcoholic fatty liver disease, cholestasis and hepatocellular carcinoma change the expression of multiple OATP isoforms. The role of liver diseases in the occurrence of ADRs is unknown.

AREAS COVERED

This article covers the roles OATPs play in ADRs when considered in the context of genetic or environmental factors. The reader will gain a greater appreciation for the current evidence regarding the salience and importance of each factor in OATP-mediated ADRs.

EXPERT OPINION

A SNP in a single OATP transporter can cause changes in drug pharmacokinetics and contribute to ADRs but, because of overlap in substrate specificities, there is potential for compensatory transport by other OATP isoforms. By contrast, the expression of multiple OATP isoforms is decreased in liver diseases, reducing compensatory transport and thereby increasing the probability of ADRs. To date, most research has focused on the genetic factors in OATP-mediated ADRs while the impact of environmental factors has largely been ignored.

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.

Influence of SLCO1B1 polymorphisms on the drug-drug interaction between darunavir/ritonavir and pravastatin

The authors investigated whether SLCO1B1 polymorphisms contribute to variability in pravastatin pharmacokinetics when pravastatin is administered alone versus with darunavir/ritonavir. HIV-negative healthy participants were prospectively enrolled on the basis of SLCO1B1 diplotype: group 1 (*1A/*1A, n = 9); group 2 (*1A/*1B, n = 10; or *1B/*1B, n = 2); and group 3 (*1A/*15, n = 1; *1B/*15, n = 5; or *1B/*17, n = 1). Participants received pravastatin (40 mg) daily on days 1 through 4, washout on days 5 through 11, darunavir/ritonavir (600/100 mg) twice daily on days 12 through 18, with pravastatin 40 mg added back on days 15 through 18. Pharmacokinetic studies were conducted on day 4 (pravastatin alone) and day 18 (pravastatin + darunavir/ritonavir). Pravastatin area under the plasma concentration-time curve (AUC(tau)) was 21% higher during administration with darunavir/ritonavir compared with pravastatin alone; however, this difference was not statistically significant (P = .11). Group 3 variants had 96% higher pravastatin AUC(tau) on day 4 and 113% higher pravastatin AUC(tau) on day 18 compared with group 1. The relative change in pravastatin pharmacokinetics was largest in group 3 but did not differ significantly between diplotype groups. In sum, the influence of SLCO1B1*15 and *17 haplotypes on pravastatin pharmacokinetics was maintained in the presence of darunavir/ritonavir. Because OATP1B1 inhibition would be expected to be greater in carriers of normal or high-functioning SLCO1B1 haplotypes, these findings suggest that darunavir/ritonavir is not a potent inhibitor of OATP1B1-mediated pravastatin transport in vivo.

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.

Isolation of modulators of the liver-specific organic anion-transporting polypeptides (OATPs) 1B1 and 1B3 from Rollinia emarginata Schlecht (Annonaceae)

Organic anion-transporting polypeptides 1B1 and 1B3 (OATP1B1 and OATP1B3) are liver-specific transporters that mediate the uptake of a broad range of drugs into hepatocytes, including statins, antibiotics, and many anticancer drugs. Compounds that alter transport by one or both of these OATPs could potentially be used to target drugs to hepatocytes or improve the bioavailability of drugs that are cleared by the liver. In this study, we applied a bioassay-guided isolation approach to identify such compounds from the organic extract of Rollinia emarginata Schlecht (Annonaceae). Fractions of the plant extract were screened for effects on OATP1B1- and OATP1B3-mediated transport of the model substrates estradiol-17β-glucuronide and estrone-3-sulfate. We isolated three compounds, ursolic acid, oleanolic acid, and 8-trans-p-coumaroyloxy-α-terpineol, which inhibited estradiol-17β-glucuronide uptake by OATP1B1 but not OATP1B3. In addition, a rare compound, quercetin 3-O-α-l-arabinopyranosyl(1→2) α-L-rhamnopyranoside, was identified that had distinct effects on each OATP. OATP1B1 was strongly inhibited, as was OATP1B3-mediated transport of estradiol-17β-glucuronide. However, OATP1B3-mediated uptake of estrone-3-sulfate was stimulated 4- to 5-fold. Kinetic analysis of this stimulation revealed that the apparent affinity for estrone-3-sulfate was increased (decreased K(m)), whereas the maximal rate of transport (V(max)) was significantly reduced. These results demonstrate a mechanism through which the hepatic uptake of drug OATP substrates could be stimulated.

Effect of organic anion-transporting polypeptide 1B1 (OATP1B1) polymorphism on the single- and multiple-dose pharmacokinetics of enalapril in healthy Chinese adult men

BACKGROUND

Enalapril is an angiotensin-converting enzyme (ACE) inhibitor approved for the treatment of mild to severe hypertension and congestive heart failure. There is evidence that enalapril may be an organic anion-transporting polypeptide 1B1 (OATP1B1) substrate, suggesting that genetic polymorphisms of the OATP1B1 gene may play a role in causing the interindividual pharmacokinetic differences of this drug.

OBJECTIVE

The purpose of this study was to investigate the functional significance of the OATP1B1 genetic polymorphism on the pharmacokinetics of enalapril and its active metabolite enalaprilat in healthy Chinese adult male participants.

METHODS

This was a single-center, open-label, single- and multiple-dose study conducted in healthy Chinese male participants. Each participant received a single oral dose of 10 mg enalapril under fasting conditions, followed by enalapril 10 mg/d for 7 days. In the single-dose phase, sequential blood samples were collected from 0 to 24 hours after drug administration. In the multiple-dose phase, samples were obtained before drug administration on days 4, 5, 6, and 7; on day 7, samples were collected from 0 to 72 hours after drug administration. An HPLC-MS/MS method was used to determine plasma concentrations of enalapril and enalaprilat. A polymerase chain reaction technique was used for genotyping of 2 single nucleotide polymorphisms (SNPs) of the OATP1B1 gene: T521C and A388G. The pharmacokinetic parameters of enalapril and enalaprilat were then compared according to genotype groups, using 1-way ANOVA, except for T(max) in which the Mann-Whitney test or Kruskal-Wallis test was used.

RESULTS

The study included 32 healthy Han Chinese male participants (age range, 18-28 years; weight range, 50.0-80.0 kg; height range,159-182.0 cm). Twenty-six were OATP1B1*15 noncarriers (homozygous for 521TT), the others were *15 carriers with at least one 521 T>C mutant allele. After single and multiple oral doses of 10 mg enalapril, plasma concentrations of enalapril in *15 noncarriers were lower than that in *15 carriers, with significant difference in area under the curve at steady state (AUC(ss)) between *15 noncarriers and *15 carriers (P = 0.048) in the multiple-dose phase. There were no significant differences in enalapril's AUC(0-24), C(max), or the ratio of the AUC(0-24h) in the single-dose study to the AUC(ss) (R(ac)) between the *15 carriers and noncarriers. In contrast to enalapril, the mean AUC(0-24h) and C(max) of enalaprilat in *15 noncarriers was significantly higher than those in *15 carriers (P = 0.040 and P = 0.027, respectively) in the single-dose phase. There were no significant differences in enalaprilat's AUC(ss) or C(maxss) between the 2 groups in the multiple-dose phase. For the 3 groups classified according to the effect of A388G variant in all subjects homozygous for 521T (TT), *1a/*1a, *1a/*1b, and *1b/*1b, no significant difference was found in AUC(0-24h), C(max), and T(max) of enalapril and enalaprilat.

CONCLUSIONS

In this small population of healthy Chinese men, the OATP1B1*15 allele and T521C variant appeared to be an important determinant of the pharmacokinetics of enalapril. There were significant differences between the *15 carriers and noncarriers in enalapril's AUC(ss) and enalaprilat's AUC(0-24h), C(max), and R(ac). However, there were no significant differences in enalapril's AUC(0-24), C(max), or enalaprilat's AUC(ss), C(maxss) between the *15 carriers and noncarriers.

Drug transporters in drug efficacy and toxicity

Drug transporters are now widely acknowledged as important determinants governing drug absorption, excretion, and, in many cases, extent of drug entry into target organs. There is also a greater appreciation that altered drug transporter function, whether due to genetic polymorphisms, drug-drug interactions, or environmental factors such as dietary constituents, can result in unexpected toxicity. Such effects are in part due to the interplay between various uptake and efflux transporters with overlapping functional capabilities that can manifest as marked interindividual variability in drug disposition in vivo. Here we review transporters of the solute carrier (SLC) and ATP-binding cassette (ABC) superfamilies considered to be of major importance in drug therapy and outline how understanding the expression, function, and genetic variation in such drug transporters will result in better strategies for optimal drug design and tissue targeting as well as reduce the risk for drug-drug interactions and adverse drug responses.

Frequency of the SLCO1B1 388A>G and the 521T>C polymorphism in Tanzania genotyped by a new LightCycler®-based method

PURPOSE

The 388A>G and the 521T>C polymorphism of the SLCO1B1 gene affect the activity of the uptake transporter OATP1B1, thus influencing kinetics, safety, and efficacy of substrate drugs. To evaluate the impact of these polymorphisms in populations of different ethnic origins, it is important to know their frequencies and to develop fast and reliable methods for genotyping. We therefore established a new, high-throughput method and determined the genotype and allelic frequencies of these polymorphisms in Tanzanians, for which the frequencies were unknown thus far.

METHODS

New LightCycler® 480-based methods with hybridization probes were established and used to genotype 366 Tanzanian and 236 European individuals for 388A>G (rs2306283) and 521T>C (rs4149056) in the SLCO1B1 gene. The methods were validated by direct sequencing of the polymerase chain reaction (PCR) products of heterozygous individuals and checked for intrarun precision repeatability, interrun precision reproducibility, robustness, and deviations from the Hardy-Weinberg equilibrium.

RESULTS

The new methods allow unambiguous identification of the corresponding genotypes. There was a clear difference in allelic distribution between Tanzanians and Europeans, with the 388A>G (rs2306283) being much more prevalent in Tanzanians (87% vs 41%) and the 521T>C (rs4149056) being very rare in this African population (6% vs 17%).

CONCLUSIONS

We developed robust and high-throughput methods to genotype common SLCO1B1 allelic variants with the LightCycler® 480. In Tanzanians, we identified the highest frequency of the 388A>G and 521T>C polymorphisms ever reported from black populations. The clinical relevance of SLCO1B1 genetic variation in the African population remains to be investigated.

Ethnic differences in the metabolism, toxicology and efficacy of three anticancer drugs

INTRODUCTION

Large inter-individual and inter-ethnic differences are observed in efficacies and toxicities of medical drugs. To improve the predictability of these differences, pharmacogenetic information has been applied to clinical situations. Expanding pharmacogenetic information would be a valuable tool to the medical community as well as the patient to fulfill the promise of personalized anticancer drug therapy.

AREAS COVERED

This review highlights genetic polymorphisms and ethnic differences of genes, UGT1As, CYP3A4, CES1As, ABCB1, ABCC2, ABCG2, SLCO1B1, CDA and CYP2D6, involved in metabolism and disposition of three anticancer drugs: irinotecan, gemcitabine and tamoxifen.

EXPERT OPINION

Recent pharmacogenetic studies have successfully identified distinct ethnic differences in genetic polymorphisms that are potentially involved in efficacies and toxicities of anticancer drugs. This achievement has led to personalized irinotecan therapy, reflecting ethnic differences in UGT1A1 genotypes, and possible benefits of genetic testing have also been suggested for gemcitabine and tamoxifen therapy, which still requires further validation. The ultimate goal for patients is a high rate or even perfect prediction of efficacies and toxicities of anticancer drugs in each ethnic population. For this challenge, more clinical studies combined with comprehensive omics approaches are necessary to further advance the field.

Role of organic cation transporter 1, OCT1 in the pharmacokinetics and toxicity of cis-diammine(pyridine)chloroplatinum(II) and oxaliplatin in mice

PURPOSE

The goal of this study was to test the hypothesis that by controlling intracellular uptake, organic cation transporter 1, Oct1 is a key determinant of the disposition and toxicity of cis-diammine(pyridine)chloroplatinum(II)(CDPCP) and oxaliplatin.

METHODS

Pharmacokinetics, tissue accumulation and toxicity of CDPCP and oxaliplatin were compared between Oct1-/- and wild-type mice.

RESULTS

After intravenous administration, hepatic and intestinal accumulation of CDPCP was 2.7-fold and 3.9-fold greater in Oct1 wild-type mice (p < 0.001). Deletion of Oct1 resulted in a significantly decreased clearance (0.444 ± 0.0391 ml/min*kg versus 0.649 ± 0.0807 ml/min*kg in wild-type mice, p < 0.05) and volume distribution (1.90 ± 0.161 L/kg versus 3.37 ± 0.196 L/kg in wild-type mice, p < 0.001). Moreover, Oct1 deletion resulted in more severe off-target toxicities in CDPCP-treated mice. Histologic examination of the liver and measurements of liver function indicated that the level of hepatic toxicity was mild and reversible, but was more apparent in the wild-type mice. In contrast, the effect of Oct1 on the pharmacokinetics and toxicity of oxaliplatin in the mice was minimal.

CONCLUSIONS

Our study suggests that Oct1 plays an important role in the pharmacokinetics, tissue distribution and toxicity of CDPCP, but not oxaliplatin.

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.

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.

Pharmacogenetic interactions between ABCB1 and SLCO1B1 tagging SNPs and the effectiveness of statins in the prevention of myocardial infarction

AIMS

Genetic variability within the SLCO1B1 and ABCB1 transporter genes has been associated with modification of statin effectiveness in cholesterol management.

MATERIALS & METHODS

We conducted a case-control study using a population-based registry of pharmacy records linked to the hospital discharge records. Within a hypercholesterolemic cohort, we included 668 myocardial infarction cases and 1217 controls.

RESULTS

We tested 24 tagging SNPs and found two SNPs within ABCB1 (rs3789244, p = 0.01; rs1922242, p = 0.01) to interact with statin treatment. In addition, we found a nonsignificant haplotype-treatment interaction (p = 0.054). The odds ratio for subjects homozygous for SLCO1B1*1A was 0.49 (95% CI: 0.34-0.71) compared with 0.31 (95% CI: 0.24-0.41) for heterozygous or noncarriers of the *1A allele.

CONCLUSION

This is the first study to demonstrate that common genetic variability within the SLCO1B1 and ABCB1 genes is associated with the modification of the effectiveness of statins in the prevention of the clinical outcome, myocardial infarction.

Hepatic OATP and OCT uptake transporters: their role for drug-drug interactions and pharmacogenetic aspects

Uptake transporters in the basolateral membrane of hepatocytes are important for the hepatobiliary elimination of drugs. Further, since drug-metabolizing enzymes are located intracellularly, uptake into hepatocytes is a prerequisite for their subsequent metabolism. Therefore, alteration of uptake transporter function (e.g., by concomitantly administered drugs or due to functional consequences of genetic variations, leading to reduced transport function) may result in a change in drug pharmacokinetics. In this review, we focus on the hepatocellularly expressed members of the OATP and OCT family, their impact on transport-mediated drug-drug interactions, and on the functional consequences of variations in genes encoding these transporters.

Several conserved positively charged amino acids in OATP1B1 are involved in binding or translocation of different substrates

OATP1B1 and 1B3 are related transporters mediating uptake of numerous compounds into hepatocytes. A putative model of OATP1B3 with a "positive binding pocket" containing conserved positively charged amino acids was predicted (Meier-Abt et al. J Membr Biol 208:213-227, 2005). Based on this model, we tested the hypothesis that these positive amino acids are important for OATP1B1 function. We made mutants and measured surface expression and uptake of estradiol-17β-glucuronide, estrone-3-sulfate and bromosulfophthalein in HEK293 cells. Two of the mutants had low surface expression levels: R181K at 10% and R580A at 30% of wild-type OATP1B1. A lysine at position 580 (R580K) rescued the expression of R580A. Mutations of several amino acids resulted in substrate-dependent effects. The largest changes were seen for estradiol-17β-glucuronide, while estrone-3-sulfate and bromosulfophthalein transport were less affected. The wild-type OATP1B1 K (m) value for estradiol-17β-glucuronide of 5.35 ± 0.54 μM was increased by R57A to 30.5 ± 3.64 μM and decreased by R580K to 0.52 ± 0.18 μM. For estrone-3-sulfate the wild-type high-affinity K (m) value of 0.55 ± 0.12 μM was increased by K361R to 1.8 ± 0.47 μM and decreased by R580K to 0.1 ± 0.04 μM. In addition, R580K reduced the V (max) values for all three substrates to <25% of wild-type OATP1B1. Mutations at intracellular K90, H92 and R93 mainly affected V (max) values for estradiol-17β-glucuronide uptake. In conclusion, the conserved amino acids R57, K361 and R580 seem to be part of the substrate binding sites and/or translocation pathways in OATP1B1.

Drug transporters and renal drug disposition in the newborn

The individual response to a drug in terms of drug efficacy and toxicity is highly variable; this represents a major problem in clinical practice. Potential causes for such variability include pathogenesis and severity of the disease being treated, drug interactions, patient age, nutritional status, renal and liver function and concomitant illness. Inherited differences in drug metabolism and genetic polymorphism of targets of drug therapy can have even greater influence on the efficacy and toxicity of medications. We will discuss the role of drug transporters (organic anion transporting polypeptides and Pgp), drug-related gene polymorphisms and pathologies, renal function and drug metabolism in a very special patient population, the newborn. Reliable predictions of drug pharmacokinetics in the newborn, derived from an understanding of the transport mechanisms, should allow therapeutic agents to be used more safely in this special population.

HIV protease inhibitors are substrates for OATP1A2, OATP1B1 and OATP1B3 and lopinavir plasma concentrations are influenced by SLCO1B1 polymorphisms

OBJECTIVE

OATP1B1 and OATP1B3 are major hepatic drug transporters whilst OATP1A2 is mainly located in the brain but is also located in liver and several other organs. These transporters affect the distribution and clearance of many endobiotics and xenobiotics and have been reported to have functional single nucleotide polymorphisms (SNPs). We have assessed the substrate specificities of these transporters for a panel of antiretrovirals and investigated the effects of SNPs within these transporters on the pharmacokinetics of lopinavir.

METHODS

SLCO1A2, SLCO1B1 and SLCO1B3 were cloned, verified and used to generate cRNA for use in the Xenopuslaevis oocyte transport system. Using the oocyte system, antiretrovirals were tested for their substrate specificities. Plasma samples (n=349) from the Liverpool therapeutic drug monitoring registry were genotyped for SNPs in SLCO1A2, SLCO1B1 and SLCO1B3 and associations between SNPs and lopinavir plasma concentrations were analysed.

RESULT

Antiretroviral protease inhibitors, but not non-nucleoside reverse transcriptase inhibitors, are substrates for OATP1A2, OATP1B1 and OATP1B3. Furthermore, ritonavir was not an inhibitor of OATP1B1. The 521T>C polymorphism in SLCO1B1 was significantly associated with higher lopinavir plasma concentrations. No associations were observed with functional variants of SLCO1A2 and SLCO1B3.

CONCLUSION

These data add to our understanding of the factors that contribute to variability in plasma concentrations of protease inhibitors. Further studies are now required to confirm the association of SLCO1B1 521T>C with lopinavir plasma concentrations and to assess the influence of other polymorphisms in the SLCO family.

Genetic polymorphisms of uptake (OATP1B1, 1B3) and efflux (MRP2, BCRP) transporters: implications for inter-individual differences in the pharmacokinetics and pharmacodynamics of statins and other clinically relevant drugs

Recent pharmacogenomic/pharmacogenetic studies have disclosed important roles of drug transporters in the pharmacokinetic/pharmacodynamic (PK/PD) profiles of some clinically relevant drugs. It has concurrently been explained that variations in the drug transporter genes are associated with not only inter-individual but also inter-ethnic differences in PK/PD profiles of these drugs. This review focuses on two uptake and two efflux transporters. Organic anion transporting polypeptide (OATP) 1B1 and OATP1B3 are uptake transporters, specifically expressed in the liver, and considered important for drugs, particularly as their pharmacological target organ is the liver. Two ATP-binding cassette transporters, multi-drug resistance-associated protein 2 and breast cancer resistance protein, are efflux transporters, expressed in various human tissues, and considered particularly important for intestinal drug absorption and hepatic drug elimination. All 3-hydroxyl-3-methylglutaryl-CoA reductase inhibitors (statins) except fluvastatin are substrates for OATP1B1, but hepatobiliary (canalicular) efflux transporters differ among statins. In this review, we update the pharmacogenomic/pharmacogenetic properties of these transporters and their effects on PK/PD profiles of statins and other clinically relevant drugs. In addition, we describe a physiologically-based pharmacokinetic model for predicting the effects of changes in transporter activities on systemic and hepatic exposure to pravastatin.

Hepatic OATP1B transporters and nuclear receptors PXR and CAR: interplay, regulation of drug disposition genes, and single nucleotide polymorphisms

Drug uptake transporters are now increasingly recognized as clinically relevant determinants of variable drug responsiveness and unexpected drug-drug interactions. Emerging evidence strongly suggests members of the organic anion transporting polypeptide (OATP) family appear to be particularly important to the disposition of many drugs in clinical use today. Specifically, the liver-enriched OATP1B subfamily members OATP1B1 and OATP1B3 exhibit broad substrate specificity and the ability to transport drugs which are ligands for xenobiotic sensing nuclear receptors such as the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Accordingly, OATP1B transporters may indirectly regulate expression of drug metabolism genes via modulation of the intracellular concentration of PXR and CAR ligands. Moreover, a number of functionally important single nucleotide polymorphisms (SNPs) in OATP1B transporters have been described. In this review, a brief summary of known SNPs in PXR and CAR will be followed by an in-depth outline of OATP1B1 and OATP1B3 transporters particularly in relation to the known SNPs in these OATPs and the interplay between OATP1B transporters with PXR and CAR, both in vitro and in vivo.

Prediction of pharmacokinetic profile of valsartan in humans based on in vitro uptake-transport data

The aim of this study was to evaluate a physiologically based pharmacokinetic (PBPK) model for predicting PK profiles in humans based on a model refined in rats and humans in vitro uptake-transport data using valsartan as a probe substrate. Valsartan is eliminated unchanged, mostly through biliary excretion, both in humans and rats. It was, therefore, chosen as model compound to predict in vivo elimination based on in vitro hepatic uptake-transport data using a fully mechanistic PBPK model. Plated rat and human hepatocytes, and cell lines overexpressing human OATP1B1 and OATP1B3 were used for in vitro uptake experiments. A mechanistic two-compartment model was used to derive the active and passive transport parameters, namely uptake Michaelis-Menten parameters (V(max) and K(m,u)) together with passive diffusion (P(dif)). These transport parameters were then used as input in a whole body physiologically based pharmacokinetic (PBPK) model. The uptake rate of valsartan was higher for rat hepatocytes (K(m,u)=28.4+/-3.7 microM, V(max)=1320+/-180 pmol/mg/min, and P(dif) =1.21+/-0.42 microl/mg/min) compared to human hepatocytes (K(m,u)=44.4+/-14.6 microM, V(max)=304+/-85 pmol/mg/min, and P(dif)=0.724+/-0.271 microl/mg/min). OATP1B1 and -1B3 parameters were correlated to human hepatocyte data, using experimentally established relative activity factors (RAF). Resulting PBPK simulations were compared for plasma- (humans and rats) and bile- (rats) concentration-time profiles following iv bolus administration of valsartan. Plasma clearances (CL(P)) for rats and humans were predicted within twofold relative to predictions based on respective in vitro data. The simulations were extended to simulate the impact of either OATP1B1 or -1B3 inhibition on plasma profile. The limited data set indicates that the mechanistic model allowed for accurate evaluation of in vitro transport data; and the resulting hepatic uptake transport kinetic parameters enabled the prediction of in vivo PK profiles and plasma clearances, using PBPK modelling. Moreover, the interspecies difference in elimination rate observed in vivo was correctly reflected in the transport parameters determined in vitro.