Impact of OATP transporters on pharmacokinetics

Abstract

Membrane transporters are now recognized as important determinants of the transmembrane passage of drugs. Organic anion transporting polypeptides (OATP) form a family of influx transporters expressed in various tissues important for pharmacokinetics. Of the 11 human OATP transporters, OATP1B1, OATP1B3 and OATP2B1 are expressed on the sinusoidal membrane of hepatocytes and can facilitate the liver uptake of their substrate drugs. OATP1A2 is expressed on the luminal membrane of small intestinal enterocytes and at the blood-brain barrier, potentially mediating drug transport at these sites. Several clinically used drugs have been identified as substrates of OATP transporters (e.g. many statins are substrates of OATP1B1). Some drugs may inhibit OATP transporters (e.g. cyclosporine) causing pharmacokinetic drug–drug interactions. Moreover, genetic variability in genes encoding OATP transporters can result in marked inter-individual differences in pharmacokinetics. For example, a single nucleotide polymorphism (c.521T > C, p.Val174Ala) in the SLCO1B1 gene encoding OATP1B1 decreases the ability of OATP1B1 to transport active simvastatin acid from portal circulation into the liver, resulting in markedly increased plasma concentrations of simvastatin acid and an enhanced risk of simvastatin-induced myopathy. SLCO1B1 polymorphism also affects the pharmacokinetics of many other, but not all (fluvastatin), statins and that of the antidiabetic drug repaglinide, the antihistamine fexofenadine and the endothelin A receptor antagonist atrasentan. This review compiles the current knowledge about the expression and function of human OATP transporters, their substrate and inhibitor specificities, as well as pharmacogenetics.

Role of transporters in pharmacokinetics

After an orally administered drug is dissolved, it crosses the intestinal wall, reaches the liver via portal blood flow and subsequently enters the systemic circulation, which distributes the drug to the various tissues of the body (Tozer and Rowland, 2006). The drug is eliminated from the body by metabolism, which occurs mainly in the liver, and by excretion into bile or into urine. During these pharmacokinetic processes, a drug molecule passes through several biological membranes. The extent of drug movement through these membranes is generally affected by the physicochemical properties of a drug, namely size, lipophilicity and charge (or degree of ionization). In addition, membrane transporters have a significant role in facilitating or preventing drug movement (Ho and Kim, 2005).

Transporters may be classified as influx (uptake into cell) and efflux (out of cell) transporters, which are typically located either at the basolateral or apical membrane in polarized cells. Interplay of influx and efflux transporters together with phase I and II metabolism may be required for the sequential traverse of the basolateral and apical membranes (Giacomini and Sugiyama, 2006). For example, in the liver, an uptake transporter such as organic anion transporting polypeptide 1B1 (OATP1B1) may extract its drug substrates from the portal blood into hepatocytes. After metabolism, other drug transporters, such as multidrug resistance protein 1 (MDR1, also known as P-glycoprotein) may be important in efflux of the metabolite from the hepatocyte (Ho and Kim, 2005) (Figure 1). Drug transporters can therefore be regarded as completing the phase I and II enzyme-based detoxification system; drug uptake delivers the drug to the detoxification system to facilitate metabolism, whereas drug efflux decreases the load on detoxification enzymes (Nies et al., 2008).

Figure 1.

Role of transporters affecting hepatic uptake and excretion of drugs and the interplay of hepatic transporters with phase I and phase II metabolism in the hepatic elimination of drugs. BCRP, breast cancer resistance protein; BSEP, bile salt export pump; MDR, multidrug resistance protein; MRP, multidrug resistance-associated protein; NTCP, sodium taurocholate co-transporting polypeptide; OAT, organic anion transporter; OATP, organic anion transporting polypeptide; OCT, organic cation transporter.

Organic anion transporting polypeptides (OATPs)

OATPs are membrane influx transporters that regulate cellular uptake of a number of endogenous compounds and clinically important drugs (Niemi, 2007). The first discovered human member of the OATP/Oatp family was OATP1A2, which was cloned as an ortholog of rat Oatp1a1 (Kullak-Ublick et al., 1995). OATPs/Oatps within the same family share ≥40% amino acid sequence identity and are designated by Arabic numbering (e.g. OATP1) (Hagenbuch and Meier, 2004). Individual subfamilies include OATPs/Oatps with amino acid sequence identities ≥60% and are designated by letters (e.g. OATP1B). Individual gene products (proteins) within the same subfamily are designated by additional Arabic numbering (e.g. OATP1B1).

The human OATP family consists of 11 members: OATP1A2, 1B1, 1B3, 1C1, 2A1, 2B1, 3A1, 4A1, 4C1, 5A1 and 6A1 (Hagenbuch and Meier, 2003; Mikkaichi et al., 2004a; König et al., 2006) (Table 1). Of these, the roles of OATP1B1, 1A2, 1B3 and 2B1 in drug pharmacokinetics are best characterized. OATP1A2 may facilitate the entry of its substrates through the duodenal wall into circulation (Glaeser et al., 2007). OATP1B1, 1B3 and 2B1 are mainly located at the sinusoidal membranes of human hepatocytes and mediate the influx of their substrates from blood into the hepatocytes, and may thus represent an important step preceding elimination of drugs by metabolism or biliary excretion (Niemi, 2007).

Table 1.

Human OATP transporters, their gene names, chromosomal localization and tissue distribution. Modified from Niemi, 2007

OATP	Gene name	Gene locus	Tissue distribution
OATP1A2	SLCO1A2	12p12	Brain, kidney, liver, intestine
OATP1B1	SLCO1B1	12p12	Liver
OATP1B3	SLCO1B3	12p12	Liver
OATP1C1	SLCO1C1	12p12	Brain, testis, ciliary body
OATP2A1	SLCO2A1	3q21	Ubiquitous
OATP2B1	SLCO2B1	11q13	Liver, placenta, intestine, heart, skin
OATP3A1	SLCO3A1	15q26	Ubiquitous
OATP4A1	SLCO4A1	20q13.1	Ubiquitous
OATP4C1	SLCO4C1	5q21	Kidney
OATP5A1	SLCO5A1	8q13.1	Unknown
OATP6A1	SLCO6A1	5q21	Testis

OATP, organic anion transporting polypeptide.

According to computer-based hydropathy analysis, all OATPs share a very similar transmembrane domain organization, with 12 predicted transmembrane domains and a large fifth extracellular loop (Hagenbuch and Meier, 2003) (Figure 2; OATP1B1). Based on a comparative analysis of OATPs from multiple species, the transport of all OATPs/Oatps has been suggested to occur through a central, positively charged pore in a so-called rocker-switch type of mechanism (Meier-Abt et al., 2005). However, the exact transport mechanism of OATPs/Oatps has not been established (Mahagita et al., 2007).

Figure 2.

Schematic representation of the secondary structure of human organic anion transporting polypeptide 1B1, depicting the positions of known amino acid exchanges. Reprinted from Kalliokoski, 2008 with permission of the copyright holder.

OATPs are encoded by genes of the SLCO family (previously SLC21) (Hagenbuch and Meier, 2004). The genes encoding human OATP1 family members are located in the short arm of chromosome 12, whereas genes encoding other OATPs are located in chromosomes 3, 5, 8, 11, 15 and 20. Numerous sequence variations, such as single nucleotide polymorphisms (SNPs), have been identified in SLCO genes (Tirona et al., 2001; Niemi et al., 2004; Lee et al., 2005; Niemi, 2007). These polymorphisms may lead to significant consequences on drug pharmacokinetics, for example by decreasing uptake activity of the corresponding OATP (König et al., 2006; Seithel et al., 2008).

Many studies investigating drug–drug interactions have focused on inhibition or induction of drug-metabolizing enzymes, most notably those of the cytochrome P450 (CYP) families. However, it has become evident that significant drug–drug interactions may result from inhibition and probably also from induction of transporter function (Ho and Kim, 2005). The estimation of the role of a single transporter in drug–drug interaction may be challenging, since, for example, many OATP1B1 substrates are also substrates of other drug transporters and often subject to metabolism by CYP enzymes (Smith et al., 2005a; Kivistö and Niemi, 2007).

OATP1B1

OATP1B1 (previously known as OATP2, OATP-C and LST-1) is mainly expressed on the sinusoidal membrane of human hepatocytes (Abe et al., 1999; Hsiang et al., 1999; König et al. 2000a). SLCO1B1 mRNA has been detected also in other tissues, including small intestinal enterocytes (Glaeser et al., 2007). In vitro, OATP1B1 has been shown to transport both unconjugated and conjugated bilirubin (Cui et al., 2001; Briz et al., 2003, 2006), although in one study, no differences in bilirubin transport existed between OATP1B1-transfected and non-transfected cells (Wang et al., 2003). Other endogenous OATP1B1 substrates include bile acids (cholate and taurocholate), conjugated steroids (estradiol-17β-glucuronide, estrone-3-sulfate and dehydroepiandrosterone-3-sulfate), eicosanoids (leukotrienes C4 and E4, prostaglandin E2 and thromboxane B2) and thyroid hormones (thyroxine and triiodothyronine) (Abe et al., 1999; Hsiang et al., 1999; König et al., 2000a; Tamai et al., 2000; Cui et al., 2001). Examples of in vitro OATP1B1 drug substrates include several HMG-CoA reductase inhibitors, or statins, angiotensin-converting enzyme (ACE) inhibitors, and angiotensin II receptor antagonists (Table 2).

Table 2.

Drug substrates for OATP1B1, OATP1A2, OATP1B3 and OATP2B1

OATP	Substrate	Km (µM)	Reference
OATP1B1	Atorvastatin	12.4	Kameyama et al., 2005
	Atrasentan	–	Katz et al., 2006
	Benzylpenicillin	–	Tamai et al., 2000
	Bosentan	44	Treiber et al., 2007
	Caspofungin	–	Sandhu et al., 2005
	Cerivastatin	–	Shitara et al., 2003
	Enalapril	262	Liu et al., 2006
	Ezetimibe glucuronide	–	Oswald et al., 2008
	Fexofenadine	–	Matsushima et al., 2008
	Fluvastatin	1.4–3.5	Kopplow et al., 2005; Noe et al., 2007
	Methotrexate	–	Abe et al., 2001
	Olmesartan	12.8–42.6	Nakagomi-Hagihara et al., 2006; Yamada et al., 2007
	Pitavastatin	3.0–6.7	Hirano et al., 2004; Deng et al., 2008
	Pravastatin	13.7–35	Hsiang et al., 1999; Nakai et al., 2001
	Rifampicin	1.5–13	Vavricka et al., 2002; Tirona et al., 2003
	Rosuvastatin	4.0–7.3	Simonson et al., 2004; Ho et al., 2006b
	SN-38	–	Nozawa et al., 2005
	Temocapril	–	Maeda et al., 2006
	Troglitazone sulfate	–	Nozawa et al., 2004b
	Valsartan	1.39	Maeda et al., 2006; Yamashiro et al., 2006
OATP1A2	Erythromycin	–	Franke et al., 2008
	Fexofenadine	6.4	Cvetkovic et al., 1999
	Imatinib	–	Hu et al., 2008
	Levofloxacin	136	Maeda et al., 2007
	Methotrexate	457	Badagnani et al., 2006
	Ouabain	5.5	Bossuyt et al., 1996
	Pitavastatin	3.4	Fujino et al., 2005
	Rocuronium	–	van Montfoort et al., 1999
	Rosuvastatin	2.6	Ho et al., 2006b
	Saquinavir	36.4	Su et al., 2004
	Thyroxine	8.0	Fujiwara et al., 2001
	Unaprostone	–	Gao et al., 2005
OATP1B3	Atrasentan	–	Katz et al., 2006
	Bosentan	141	Treiber et al., 2007
	Digoxin	–	Kullak-Ublick et al., 2001
	Docetaxel	–	Smith et al., 2005b
	Enalapril	–	Liu et al., 2006
	Erythromycin	–	Franke et al., 2008
	Fexofenadine	108	Shimizu et al., 2005
	Fluvastatin	7.0	Kopplow et al., 2005
	Imatinib	–	Hu et al., 2008
	Methotrexate	24.7	Abe et al., 2001
	Olmesartan	71.8	Nakagomi-Hagihara et al., 2006
	Ouabain	–	Kullak-Ublick et al., 2001
	Paclitaxel	6.8	Smith et al., 2005b, 2007
	Pitavastatin	3.3	Hirano et al., 2006
	Pravastatin	–	Seithel et al., 2007
	Rifampicin	2.3	Vavricka et al., 2002
	Rosuvastatin	9.8	Ho et al., 2006b
	Telmisartan	0.8	Ishiguro et al., 2006
	SN-38	–	Yamaguchi et al., 2008
	Thyroxine	–	Kullak-Ublick et al., 2001
	Valsartan	18.2	Yamashiro et al., 2006
OATP2B1	Atorvastatin	0.2	Grube et al., 2006
	Benzylpenicillin	–	Tamai et al., 2000
	Bosentan	202	Treiber et al., 2007
	Fexofenadine	–	Nozawa et al., 2004a
	Fluvastatin	0.7	Kopplow et al., 2005
	Glibenclamide	6.3	Satoh et al., 2005
	Pravastatin	2.3	Nozawa et al., 2004a
	Rosuvastatin	2.4	Ho et al., 2006b
	Unaprostone	–	Gao et al., 2005

–, not provided; K_m, Michaelis-Menten kinetic constant; OATP, organic anion transporting polypeptide.

SLCO1B1 (OATP1B1) pharmacogenetics

The OATP1B1 protein consisting of 691 amino acids is encoded by the SLCO1B1 gene (Abe et al., 1999; Hsiang et al., 1999) (Figure 2). A large number of SNPs and other sequence variations have been described in the SLCO1B1 gene, and their allele frequencies vary markedly between different populations (Tirona et al., 2001; Niemi, 2007; Pasanen et al., 2008). For example, the c.388A > G (p.Asn130Asp) SNP is quite common in all populations, with an allele frequency ranging from ∼40% in Europeans to ∼80% in Sub-Saharan Africans and East Asians, whereas the c.521T > C (p.Val174Ala) SNP, relatively common in Europeans and Asians (allele frequency ∼10–20%), is less frequent in Sub-Saharan Africans (∼2%) (Pasanen et al., 2008).

The two common SLCO1B1 SNPs, c.521T > C (p.Val174Ala) and c.388A > G (p.Asn130Asp), together form four functionally distinct haplotypes: SLCO1B1*1A (c.388A-c.521T, reference haplotype), *1B (c.388G-c.521T), *5 (c.388A-c.521C) and *15 (c.388G-c.521C) (Tirona et al., 2001; Nozawa et al., 2002; Nishizato et al., 2003; Niemi et al., 2004). The SLCO1B1*15 haplotype can be further subclassified on the basis of two promoter SNPs, g.-11187G > A and g.-10499A > C, forming the *15 (GAGC), *16 (GCGC), and *17 (AAGC) haplotypes (Niemi et al., 2004). The haplotype frequencies for the SLCO1B1*1A, *1B, *5, *15, *16 and *17 were 52, 27, 2.7, 2.4, 7.9 and 6.9%, respectively, in a population of 468 healthy Finns genotyped for 11 SLCO1B1 SNPs (Pasanen et al., 2006b).

SLCO1B1*5 and SLCO1B1*15 haplotypes have been associated with reduced transport activity of OATP1B1 in vitro in studies performed with several OATP1B1 substrates in different cell lines (Kameyama et al., 2005; Nozawa et al., 2005). The SLCO1B1*1B haplotype has, however, been associated with increased OATP1B1 transport activity in vitro in studies performed with bromosulfophthalein and estrone-3-sulfate (Michalski et al., 2002; Kameyama et al., 2005), whereas no change or reduced transport activity has been seen in other studies with different substrates (Tirona et al., 2003; Nozawa et al., 2005). The effect of the c.521T > C SNP appears to dominate over that of the c.388A > G SNP, as the SLCO1B1*15 haplotype (including both SNPs) has been consistently associated with reduced transport activity of OATP1B1 (Kameyama et al., 2005). In addition to the SLCO1B1 c.521T > C and c.388A > G SNPs, some of the other non-synonymous SLCO1B1 SNPs have been associated with altered (decreased) transport function of OATP1B1 in vitro (Tirona et al., 2001; Michalski et al., 2002), but their clinical significance is either negligible or unestablished, partly because most of them have low allele frequencies (Niemi, 2007; Pasanen et al., 2008).

Of the endogenous substrates of OATP1B1, only bilirubin transport has been reported to be altered (decreased) in vivo in subjects with the SLCO1B1 c.521T > C variant allele, but the findings have been controversial (Huang et al., 2004, 2005; Ieiri et al., 2004; Ho et al., 2007). More conclusive data are available on the effect of SLCO1B1 polymorphism on the pharmacokinetics of several drugs, notably the meglitinide analog oral antidiabetic drug repaglinide and HMG-CoA reductase inhibitors.

In one study in healthy volunteers, the AUC of 0.25 mg repaglinide was 188% larger in participants with the SLCO1B1 c.521CC than in those with the c.521TT genotype (Niemi et al., 2005b). In subsequent prospective genotype panel studies in healthy volunteers, the AUC of 0.5 mg repaglinide was ∼70% larger in SLCO1B1 c.521CC participants and ∼30% lower in SLCO1B1*1B/*1B (c.388GG-c.521TT) participants than in SLCO1B1*1A/*1A (c.388AA-c.521TT) participants (Kalliokoski et al., 2008a,c) (Figure 3A). In each study, after repaglinide administration, there was a tendency towards lower blood glucose concentrations in c.521CC participants than in SLCO1B1*1A/*1A participants and higher blood glucose concentrations in SLCO1B1*1B/*1B participants than in SLCO1B1*1A/*1A participants (Figure 2B), although these differences were not statistically significant. After ingestion of single repaglinide doses ranging from 0.25 to 2 mg, the AUC of repaglinide was 60–110% larger in participants with the c.521CC genotype than in those with the SLCO1B1*1A/*1A genotype (Kalliokoski et al., 2008b). Furthermore, the pharmacokinetics of repaglinide proved to be linear as a function of repaglinide dose in both SLCO1B1 genotype groups. Therefore, the effect of SLCO1B1 polymorphism persists over a wide dose range and patients with the c.521CC genotype may be more susceptible, than patients with the c.521TT or c.521TC genotype, for the blood glucose-lowering effect of repaglinide.

Figure 3.

Mean plasma concentrations (A) and mean % decrease (±s.d.) in blood glucose 0–3 h (B) after ingestion of a single oral 0.5 mg dose of repaglinide in healthy participants with the SLCO1B1 c.521CC (n= 4, solid triangles), c.521TC (n= 12, solid circles), *1A/*1A (c.388AA-c521TT; n= 16, open circles) and *1B/*1B (c.388GG-c.521TT; n= 8, solid squares) genotypes. Modified from Kalliokoski et al., 2008a,c;.

Although repaglinide appears to be transported via OATP1B1 in vivo, direct in vitro evidence of this is lacking. In vitro data on the role of OATP1B1 in hepatic uptake are also lacking for nateglinide, the concentrations of which were increased in individuals with the SLCO1B1 c.521TC and c.521CC genotypes when investigated in a small group of healthy Chinese subjects (Zhang et al., 2006). These results could, however, not be reproduced in a larger group of healthy Caucasian volunteers (Kalliokoski et al., 2008c). Moreover, the SLCO1B1*1B/1B genotype did not significantly affect pharmacokinetics of nateglinide (Kalliokoski et al., 2008a). The pharmacokinetics of rosiglitazone and pioglitazone also remained unaffected by the SLCO1B1 c.521T > C SNP in healthy Caucasian subjects (Kalliokoski et al., 2008d), although these drugs are potential OATP1B1 substrates (Nozawa et al., 2004b; Chang et al., 2005).

One of the most studied drugs with respect to the SLCO1B1 polymorphism in vivo in humans is pravastatin, the plasma concentrations of which have been markedly increased in individuals of different ethnic backgrounds carrying one or especially two SLCO1B1 c.521C allelles (Mwinyi et al., 2004; Niemi et al., 2004; Igel et al., 2006; Ho et al., 2007; Deng et al., 2008). In one study, SLCO1B1 c.521CC participants had a 91 and 74% larger pravastatin AUC than those with the c.521TT or c.521TC genotype respectively (Niemi et al., 2006a). Furthermore, the plasma concentrations of active simvastatin acid, pitavastatin, atorvastatin and rosuvastatin have been 221, 162, 144 and 65% higher in c.521CC homozygotes than in c.521TT homozygotes (Pasanen et al., 2006a, 2007; Deng et al., 2008). However, the SLCO1B1 genotype had no significant effect on the pharmacokinetics of fluvastatin (Niemi et al., 2006a).

In addition, the AUCs of atrasentan and fexofenadine have been higher and the non-renal clearance of torsemide has been lower in healthy volunteers with the SLCO1B1 c.521CC genotype than in those with the c.521TT genotype (Niemi et al., 2005a; Katz et al., 2006; Vormfelde et al., 2008). SLCO1B1 genotype may also affect the pharmacokinetics of ezetimibe glucuronide (Oswald et al., 2008).

Data on the relevance of SLCO1B1 polymorphism in the pharmacokinetics of different drugs in patients are emerging. For example, in Asian patients, the SLCO1B1*15 haplotype has been associated with increased concentrations of SN-38, an active metabolite of anticancer drug irinotecan, which may be reflected in a higher risk of toxicity (Xiang et al., 2006; Takane et al., 2007, 2009;Han et al., 2008). The most convincing data on the clinical significance of the SLCO1B1 polymorphism comes from a recent genome-wide association study investigating simvastatin-induced myopathy (Link et al., 2008). More than 300 000 genetic markers were determined in 85 patients who had developed myopathy while receiving 80 mg simvastatin daily and in 90 control patients without myopathy. A non-coding SLCO1B1 SNP, which is in nearly complete linkage disequilibrium with the SLCO1B1 c.521T > C SNP, was the only strong marker associated with myopathy. More than 60% of the myopathy cases could be attributed to the c.521C variant, and the odds ratio for myopathy was 4.5 per copy of the c.521C allele. This association was replicated in a study of 20 000 patients receiving 40 mg simvastatin daily. Furthermore, the c.521C allele was associated with a slightly reduced and the c.388G allele with a slightly increased cholesterol-lowering effect of simvastatin. Therefore, knowledge on the patient's SLCO1B1 genotype might be useful in tailoring of drug therapy.

Role of OATP1B1 in drug–drug interactions

Many drugs have been identified in vitro as OATP1B1 inhibitors (Table 3). There are some in vivo interactions where OATP1B1 inhibition can be regarded as an important mechanism (Table 4). Cyclosporine has increased the plasma concentrations of atorvastatin and several other statins, probably partly due to OATP1B1 inhibition (Neuvonen et al., 2006). Other mechanisms are likely also involved in these interactions since cyclosporine inhibits several influx and efflux transporters, such as OATP1B3, OATP2B1, MDR1 and MRP2, and CYP3A4. However, fluvastatin, pitavastatin, pravastatin and rosuvastatin are not significantly metabolized by CYP3A4, and cyclosporine has increased their mean AUC ∼4–20-fold in organ transplant patients (Regazzi et al., 1993; Park et al., 2001; Hasunuma et al., 2003; Simonson et al., 2004). In addition to statins, cyclosporine has increased the mean AUC of repaglinide ∼2.5-fold in healthy subjects (Kajosaari et al., 2005b). Interestingly, the increase was 42% lower in subjects with the SLCO1B1 c.521TC genotype than in those with the c.521TT genotype. This finding may be explained by the reduced activity of OATP1B1 in carriers of the variant SLCO1B1 c.521C allele. Cyclosporine has also modestly (up to twofold) increased the AUCs of other OATP1B1 substrates, including bosentan, caspofungin and methotrexate (Binet et al., 2000; Fox et al., 2003; Niemi, 2007).

Table 3.

Selected OATP1B1, OATP1A2, OATP1B3 and OATP2B1 inhibitors

OATP	Inhibitor	IC50 (µM)	Reference
OATP1B1	Atorvastatin acid	0.87	Hsiang et al., 1999; Chen et al., 2005
	Atorvastatin lactone	2.6	Chen et al., 2005
	Carbamazepine	188	Gui et al., 2008
	Caspofungin	–	Sandhu et al., 2005
	Clarithromycin	8.26a–96	Hirano et al., 2006; Seithel et al., 2007
	Clotrimazole	9.0	Gui et al., 2008
	Cyclosporine	0.2a–2.2	Shitara et al., 2003; Tirona et al., 2003; Campbell et al., 2004; Simonson et al., 2004; Hirano et al., 2006; Ho et al., 2006b; Treiber et al., 2007
	Digoxin	31.7a	Hirano et al., 2006
	Erythromycin	11.4a–217	Hirano et al., 2006; Seithel et al., 2007
	Gemfibrozil	4.0–72	Schneck et al., 2004; Shitara et al., 2004; Hirano et al., 2006; Ho et al., 2006b; Noe et al., 2007
	Gemfibrozil-1-O-glucuronide	22.6a–24	Shitara et al., 2004; Hirano et al., 2006
	Glibenclamide	0.746a	Hirano et al., 2006
	Hyperforin	0.82	Tirona et al., 2003
	Indinavir	5.84–18.4a	Tirona et al., 2003; Campbell et al., 2004; Hirano et al., 2006
	Irinotecan	–	Nozawa et al., 2005
	Ketoconazole	19.2a	Hirano et al., 2006
	Lovastatin	6.1	Hsiang et al., 1999; Sandhu et al., 2005; Gui et al., 2008
	Lovastatin acid	4.0	Chen et al., 2005
	Lovastatin lactone	28	Chen et al., 2005
	Metyrapone	–	Gui et al., 2008
	Mifepristone	3.3	Gui et al., 2008
	Nelfinavir	0.93	Tirona et al., 2003
	Paclitaxel	0.03	Gui et al., 2008
	Pioglitazone	–	Nozawa et al., 2004b
	Pravastatin	–	Hsiang et al., 1999
	Repaglinide	2.2	Bachmakov et al., 2008
	Rifampicin	0.477a–17a	Vavricka et al., 2002; Tirona et al., 2003; Hirano et al., 2006; Lau et al., 2007; Treiber et al., 2007; Gui et al., 2008
	Rifamycin SV	0.171a–2a	Vavricka et al., 2002; Campbell et al., 2004; Chen et al., 2005; Hirano et al., 2006; Seithel et al., 2007
	Ritonavir	0.71–0.781a	Tirona et al., 2003; Hirano et al., 2006
	Rosiglitazone	6.0	Nozawa et al., 2004b; Bachmakov et al., 2008
	Roxithromycin	153	Seithel et al., 2007
	Saquinavir	1.2a–1.56a	Tirona et al., 2003; Campbell et al., 2004; Hirano et al., 2006
	Sildenafil	1.5	Treiber et al., 2007
	Simvastatin	–	Hsiang et al., 1999
	Simvastatin acid	3.6	Nakai et al., 2001; Chen et al., 2005
	Simvastatin lactone	9.7	Chen et al., 2005
	SN-38	–	Nozawa et al., 2005
	Tacrolimus	0.611a	Hirano et al., 2006
	Telithromycin	121	Seithel et al., 2007
	Telmisartan	0.436a	Hirano et al., 2006
	Troglitazone	1.2	Gui et al., 2008
	Troglitazone sulfate	–	Nozawa et al., 2004b
	Valsartan	8.96a	Hirano et al., 2006
OATP1A2	Apple juice	–	Dresser et al., 2002
	Grapefruit juice	–	Dresser et al., 2002
	Hesperidin	2.7	Bailey et al., 2007
	Naringin	3.6	Bailey et al., 2007
	Orange juice	–	Dresser et al., 2002
	Rifampicin	51	Vavricka et al., 2002
	Rifamycin SV	11a	Vavricka et al., 2002
	Verapamil	2.6a	Bailey et al., 2007
OATP1B3	Clarithromycin	32	Seithel et al., 2007
	Cyclosporine	0.06	Ho et al., 2006b
	Erythromycin	34	Seithel et al., 2007
	Rifampicin	5a	Vavricka et al., 2002
	Rifamycin SV	3a	Vavricka et al., 2002
	Roxithromycin	37	Seithel et al., 2007
	Telithromycin	11	Seithel et al., 2007
OATP2B1	Cyclosporine	0.07	Ho et al., 2006b
	Gemfibrozil	8	Ho et al., 2006b

^aK_i provided instead of IC₅₀.

–, not provided; IC₅₀, inhibitor concentration producing 50% inhibition of transporter activity; OATP, organic anion transporting polypeptide; SN-38, an active metabolite of irinotecan.

Table 4.

Examples of the possible involvement of OATP1B1 inhibition in clinical drug-drug interactions

Inhibitor	Substrate	Fold AUC increase	Reference
Atorvastatin	Repaglinide	1.2	Kalliokoski, et al., 2008e
Cyclosporine	Bosentan	2	Binet et al., 2000
	Caspofungin	1.4	Cancidas prescribing information
	Atorvastatin	7.4a	Åsberg, et al., 2001
		8.7	Hermann et al., 2004
		15.3	Lemahieu et al., 2005
	Cerivastatin	3.8a	Mück et al., 1999
	Fluvastatin	3–4	Park et al., 2001
	Lovastatin	20a	Olbricht et al., 1997
	Methotrexate	1.2	Fox et al., 2003
	Pitavastatin	5	Hasunuma et al., 2003
	Pravastatin	10	Regazzi et al., 1993
		5–7	Olbricht et al., 1997
		9.9	Hedman et al., 2004
	Repaglinide	2.4a	Kajosaari et al., 2005b
	Rosuvastatin	7.1	Simonson et al., 2004
	Simvastatin	2.6a	Arnadottir et al., 1993
		8a	Ichimaru et al., 2001
Gemfibrozil	Atorvastatin	1.2	Backman et al., 2005
	Cerivastatin	4.4b	Backman et al., 2002
	Fluvastatin	no change	Spence et al., 1995
	Lovastatin	2.8	Kyrklund et al., 2001
	Pitavastatin	1.5	Mathew et al., 2004
	Pravastatin	2	Kyrklund et al., 2003
	Repaglinide	8.1b	Niemi et al., 2003a
	Rosuvastatin	1.9	Schneck et al., 2004
	Simvastatin	2.9	Backman et al., 2000
Rifampicinc	Atorvastatin	7.3	Lau et al., 2007

^aCYP3A4 inhibition also involved.

^bCYP2C8 inhibition is considered the main mechanism of interaction.

^c600 mg rifampicin administered as a 30-min iv infusion immediately before atorvastatin administration.

AUC, area under the plasma concentration-time curve; CYP, cytochrome P450; OATP, organic anion transporting polypeptide.

Gemfibrozil has also increased the plasma concentrations of several OATP1B1 substrates (Niemi, 2007). Although gemfibrozil and its 1-O-β-glucuronide metabolite both inhibit OATP1B1 in vitro (Shitara et al., 2004), the glucuronide metabolite is a very potent mechanism-based inhibitor of CYP2C8 in vitro and in vivo (Backman et al., 2002; Wang et al., 2002; Ogilvie et al. 2006; Tornio et al. 2008). Gemfibrozil has increased the AUC of repaglinide, which is metabolized by CYP2C8 and CYP3A4, up to eightfold in healthy subjects (Niemi et al., 2003a), and this interaction has been observed even when the last dose of gemfibrozil was ingested 12 h before repaglinide (Tornio et al., 2008). In a prospective genotype panel study, the mean increase in the repaglinide AUC by gemfibrozil has been ∼50% larger in SLCO1B1 c.521CC participants than in c.521TT participants, but the relative (seven- to eightfold) increases in the repaglinide AUC did not differ significantly between the genotype groups (Kalliokoski et al., 2008e). Gemfibrozil has also increased the AUC of drugs that are not at all or only partly metabolized by CYP2C8, including pravastatin, rosuvastatin and simvastatin, ∼2–3-fold in healthy subjects (Backman et al., 2000; Kyrklund et al., 2003; Schneck et al., 2004).

Rifampicin is a potent inducer of drug-metabolizing enzymes (Niemi et al., 2003b). Rifampicin 600 mg daily administered orally for 5 days has decreased the AUC of repaglinide by ∼60% (Niemi et al., 2000). At high concentrations, rifampicin also inhibits the elimination of repaglinide in vitro and in vivo (Bidstrup et al., 2004; Kajosaari et al., 2005a). In addition to inhibiting CYP enzymes, rifampicin is an inhibitor of OATP1B1 and OATP1B3 in vitro (Vavricka et al., 2002) and also in vivo, if administered intravenously immediately before the ‘victim’ drug. Intravenous rifampicin has increased the AUC of atorvastatin ∼7-fold (Lau et al., 2007), whereas oral treatment with rifampicin over 5 days has decreased the AUC of atorvastatin by ∼80%, probably due to induction of CYP3A4 (Backman et al., 2005). Although rifampicin is a substrate of OATP1B1 (Tirona et al., 2003), the SLCO1B1 genotype has had no effect on the induction of CYP3A4 by rifampicin, as measured by the plasma concentrations of 4β-hydroxycholesterol, an endogenous marker of CYP3A4 activity (Niemi et al., 2006b). These data suggest that OATP1B3 or other uptake transporters can compensate for reduced uptake of rifampicin by OATP1B1. Rifampicin also appears to have a modest inductive effect on OATP1B1 in vitro (Jigorel et al., 2006; Sahi et al., 2006).

In healthy subjects, atorvastatin has slightly increased the AUC of repaglinide in SLCO1B1*1A/*1A participants (by ∼20%) and the C_max of repaglinide in SLCO1B1*1A/*1A (∼40%) and SLCO1B1 c.521TC (by ∼30%) participants (Kalliokoski et al., 2008e). Atorvastatin had no statistically significant effect on the blood glucose-lowering response to repaglinide, although some subjects experienced low blood glucose concentrations when repaglinide was administered together with atorvastatin. Based on the repaglinide metabolite findings of the study, atorvastatin is not a potent inhibitor of CYP3A4 or CYP2C8 in vivo and the most likely explanation for the effect of atorvastatin on repaglinide pharmacokinetics is inhibition of the OATP1B1-mediated hepatic uptake of repaglinide.

OATP1A2

OATP1A2 (previously known as OATP-A) is expressed in various tissues, including the brain, liver, kidneys and intestine (Kullak-Ublick et al., 1995; Glaeser et al., 2007), although one study found no detectable levels of SLCO1A2 mRNA in the duodenum (Meier et al., 2007). Endogenous substrates of OATP1A2 include bile acids, thyroid hormones, and steroid hormones and their conjugates (Kullak-Ublick et al., 1995, 1998; Bossuyt et al., 1996; Fujiwara et al., 2001). Moreover, OATP1A2 also transports several drugs (Table 2). Some SLCO1A2 (encoding OATP1A2) SNPs have shown decreased in vitro transport activity towards the OATP1A2 substrate estrone-3-sulfate (Lee et al., 2005), but the significance of these findings in vivo in humans is unknown. Several drugs, such as saquinavir, lovastatin, verapamil, dexamethasone and naloxone, have inhibited OATP1A2-mediated substrate uptake in vitro (Kullak-Ublick et al., 1998; Cvetkovic et al., 1999; Gao et al., 2000). Interestingly, also the flavonoids naringin, found in grapefruit juice, and hesperidin, found in orange juice, as well as these juices (at 5% soft drink strength) have inhibited OATP1A2-mediated fexofenadine uptake in vitro (Dresser et al., 2002; Bailey et al., 2007). Moreover, in studies in healthy subjects, the AUC of fexofenadine has been decreased by 25% by ingestion of naringin, and by 40–70% due to ingestion of grapefruit or orange juice (Dresser et al., 2002, 2005; Bailey et al., 2007), consistent with inhibition of OATP1A2 at the apical membrane of enterocytes.

OATP1B3

OATP1B3 (previously known as OATP8 and LST-2) was cloned based on sequence homology to the OATP1B1 (80% amino acid homology), and, similar to OATP1B1, it is mainly expressed on the sinusoidal membrane of human hepatocytes (König et al., 2000b; Abe et al., 2001). Endogenous substrates of OATP1B3 are similar to those of OATP1B1: bilirubin, bile acids, conjugated steroids, eicosanoids and thyroid hormones (König et al., 2000b; Abe et al., 2001; Cui et al., 2001; Kullak-Ublick et al., 2001), but the gastrointestinal peptide cholecystokinin is exclusively transported by OATP1B3 (Ismair et al., 2001). Moreover, the drug substrates of OATP1B3 overlap those of OATP1B1, but OATP1B3 seems to be the only hepatic OATP transporting digoxin, docetaxel and paclitaxel (Table 2). OATP1B3, in contrast to OATP1B1 and OATP2B1, has also been identified in vitro as capable of transporting amanitin, a toxin present in Amanita mushrooms (Letschert et al., 2006). The SLCO1B3 gene (encoding OATP1B3) is polymorphic (Iida et al., 2001), and some sequence variations have been associated with decreased transport activity of OATP1B3 in vitro (Letschert et al., 2004; Schwarz et al., 2006; Smith et al., 2007). In addition to inhibiting OATP1B1, cyclosporine is also an inhibitor of OATP1B3 in vitro (Ho et al., 2006b) and might thus interact with OATP1B3 substrates.

OATP2B1

OATP2B1 (previously OATP-B) is expressed at the sinusoidal membrane of hepatocytes in the liver, but also in other tissues, for example intestine and heart (Tamai et al., 2000; Kullak-Ublick et al., 2001; Kobayashi et al., 2003; Grube et al., 2006). Endogenous substrates of OATP2B1 include dehydroepiandrosterone-3-sulfate, estrone-3-sulfate and prostaglandin E2 (Tamai et al., 2000; Kullak-Ublick et al., 2001). Several drugs are substrates of OATP2B1 (Table 2). There is currently no data on the clinical relevance of SLCO2B1 polymorphism, although some SLCO2B1 sequence variations have been associated with altered transport activity of the protein in vitro (Nozawa et al., 2002; Ho et al., 2006a). Cyclosporine and gemfibrozil inhibit OATP2B1 in vitro (Ho et al., 2006b). In a study where glibenclamide was identified in vitro as a substrate for OATP2B1, grapefruit juice (at a concentration of 5%) significantly inhibited the OATP2B1-mediated uptake of estrone-3-sulfate by 80% (Satoh et al., 2005). However, grapefruit juice has had no effect on the pharmacokinetics of glibenclamide in healthy subjects (Lilja et al., 2007).

Other OATPs

OATP1C1 (previously OATP-F) is expressed in the human brain, testis and ciliary body, and shows a high affinity for thyroid hormones (Pizzagalli et al., 2002; Gao et al., 2005). OATP2A1 (hPGT) is broadly expressed in different tissues and acts as a prostaglandin transporter (Lu et al., 1996), but currently no drugs have been identified as its substrates. OATP3A1 (OATP-D) is expressed in two splice variants, and the shorter variant lacking 18 amino acids in the carboxyl terminus appears to be expressed only in the testis and brain, whereas the longer variant is ubiquitously expressed (Adachi et al., 2003; Huber et al., 2007). OATP3A1 transports estrone-3-sulfate, prostaglandin E2, thyroxine, vasopressin and benzylpenicillin (Tamai et al., 2000; Huber et al., 2007).

OATP4A1 (OATP-E) is ubiquitously expressed and it transports oestrogens, prostaglandins, thyroid hormones, taurocholate, benzylpenicillin and unaprostone (Tamai et al., 2000; Fujiwara et al., 2001; Gao et al., 2005). OATP4C1 (OATP-H) is localized at the basolateral membrane of human proximal tubule cells, and therefore it may mediate the uptake of its substrates from the blood into the kidney (Mikkaichi et al., 2004b). OATP4C1 transports thyroid hormones, digoxin, methotrexate and the antidiabetic drug sitagliptin (Mikkaichi et al., 2004b; Chu et al., 2007). OATP5A1 (OATP-J) is known only at the cDNA level (Hagenbuch and Meier, 2004), and mRNA of SLCO6A1 (OATP-I) has been detected in the testis (Lee et al., 2004).

Conclusion

OATP transporters, especially OATP1B1, 1A2, 1B3 and 2B1, may play important roles in the pharmacokinetics of several drugs. Therefore, genetic variation in OATP-encoding genes and inhibition of OATP function may have clinically significant consequences on drug therapy. The SLCO1B1 c.521T > C (p.Va.174Ala; *5 or *15 haplotype) variant allele is associated with reduced hepatic uptake and increased plasma concentrations of several OATP1B1 substrates, including repaglinide and several statins. The SLCO1B1*1B haplotype appears to be associated with enhanced hepatic uptake and reduced plasma concentrations of OATP1B1 substrates. SLCO1B1 c.521T > C SNP is strongly associated with simvastatin-induced myopathy. Inhibition of OATP1B1 is involved in the drug–drug interactions of cyclosporine, gemfibrozil and rifampicin with OATP1B1 substrates. OATP1A2 may facilitate the intestinal absorption of drugs, such as fexofenadine, and OATP1B3 and 2B1 may be important for hepatic uptake of their substrates drugs.

Glossary

Abbreviations:

AUC

area under the plasma concentration-time curve

BCRP

breast cancer resistance protein

BSEP

bile salt export pump

C_max

peak plasma concentration

CYP

cytochrome P450

HMG-CoA

3-hydroxy-3-methylglutaryl-coenzyme A

hPGT

human prostaglandin transporter

LST

liver-specific transporter

MDR1

multidrug resistance protein 1

MRP

multidrug resistance-associated protein

NTCP

sodium taurocholate co-transporting polypeptide

OAT

organic anion transporter

OATP

organic anion transporting polypeptide

OCT

organic cation transporter

SLC

solute carrier family

SLCO

solute carrier organic anion transporter family

SN-38

active metabolite of irinotecan

SNP

single nucleotide polymorphism