Impact of genetic variation in OCT1 on drugs pharmacokinetics and response

Single nucleotide polymorphisms in OCT1 affect transporter function (Figure 1.4). There-fore, they may cause inter-individual variations in drug response and may influence pharmacotherapy. Any of the polymorphisms R61C, C88R, G401S, G465R, or a deletion of methinone420 (M420del) causes partial or complete loss of OCT1 function. In Caucasians the frequency of these variants have been reported to be on average 9.6 %, 0.6 %, 2.8 %, 1.8 %, and 15.4 %, respectively (Kerb et al., 2002; Tzvetkov et al., 2012).

Individuals can be divided to carriers of zero, one, or two loss-of function alleles in OCT1.

In Caucasians 42 % of the individuals carry one and further 9 % two loss-of-function OCT1 alleles (Shu et al., 2007; Tzvetkov et al., 2012). Data is rapidly accumulating that this genetically-determined loss of OCT1 activity may affect the pharmacokinetics, efficacy, and toxicity of drugs that are metabolized or act in the liver. Affected are cationic and weak basic like the opioids morphine and tramadol, the antiemetics tropisetron and ondansetron, and the very commonly administrated oral antidiabetic metformin.

Figure 1.4 Localization and function of OCT1 in hepatocytes. (A) OCT1 is mainly expressed in the sinusoidal membrane of human hepatocytes. Next to OCT1, the following uptake transporters are expressed in the sinusoidal membrane: the organic anion transporter 2 (OAT2;SLC22A7) and 7 (OAT7; SLC22A9), three members of organic anion transport polypeptides (OATP1B1 (SLCO1B1), OATP1B3 (SLCO1B3), OATP2B1 (SLCO2B1), and the uptake transporter sodium/taurocholate co-transporting peptide (NTCP;

SLC10A1). Transporters in the basolateral membrane are the efflux transporters multidrug resistance protein MRP3 (ABCC3), MRP4 (ABCC4), and MRP6 (ABCC6). Transporters in the canalicular membrane are the efflux pumps multidrug resistance 1 transporter (MDR1; ABCB1), the bile-salt export pump (BSEP;

ABCB11), the breast cancer resitance protein (BCRP; ABCG2), MRP2 (ABCC2), and the multidrug and toxin extrusion protein 1 (MATE1; SLC47A1), the heteromeric organic solute transporter (OSTα-OSTβ;

SLC51a/Slc51b) is located in the apical membrane. (B) OCT1 as a mediator of drugs as the first step of metabolism and excretion. For drugs that rely on OCT1 uptake, e.g. morphine, tropisetron, and O-desmethyltramadol, as the first step of metabolism and excretion, loss of OCT1 function lead to higher plasma levels of these drugs. This results in higher efficacy, but also increases the risk of adverse effects.

Drugs like metformin rely on OCT1 uptake in order to reach their side of action. Loss of OCT1 function decreases metformin efficacy.

Morphine is a natural opioid analgesic, which directly acts on the central nervous system (Mayer and Price, 1976). Codeine is the pro-drug of morphine and needs to be metabolized to morphine by CYP2D6. Morphine and codeine are used for treatment of moderate to serve pain e.g. during surgery or cancer therapy (Hanks et al., 2001). Polymorphisms in the CYP6D6 gene were associated with increased risk of morphine adverse effects (Gasche et al., 2004; Kirchheiner et al., 2007; Crews et al., 2012). But polymorphisms in the CYP2D6 gene could only partially explain the high morphine plasma concentrations after treatment with codeine (Lotsch et al., 2009; Sistonen et al., 2012). Morphine is a hydrophilic, weak base that is positively charged at physiological pH (pKa = 7.4). Morphine is a substrate of OCT1 whereas the uptake of the pro-drug codeine into hepatocytes is not OCT1 dependent (Tzvetkov et al., 2013). Codeine is highly membrane permeable and enters hepatocytes by passive diffusion. Moreover, codeine was shown to inhibit OCT1 transport activity (Tzvetkov et al., 2013). Recently, the increase in morphine plasma concentration was also related to higher sensibility to adverse effects in healthy volunteers (Tzvetkov et al., 2013).

Therefore, the knowledge of individual OCT1 genotypes may help to adjust drug medication also in regard of adverse effects. Similar effects were also observed in patients.

Children with loss of function alleles in OCT1 showed increased morphine plasma concentrations after morphine administration. Moreover, it was shown that these patients had reduced levels of morphine 3-glucoronid, the main metabolite of morphine generated in hepatocytes (Fukuda et al., 2013).

Tramadol is a synthetic opioid analgesic used in the treatment of moderate pain. It is a prodrug that needs to be metabolized to its active metabolite O-desmethyltramadol, which binds with high affinity to the µ-opioid receptor (Sevcik et al., 1993). The reaction is catalyzed by CYP2D6 in hepatocytes (Paar et al., 1997). Polymorphisms in the CYP2D6 gene were shown to affect tramadol pharmacokinetics. But these polymorphisms could just partially explain inter-individual differences in subgroups defined as individuals with high, intermediate or low CYP2D6 activity (Stamer et al., 2003; Kirchheiner et al., 2008).

Whereas tramadol can cross plasma membranes by carrier independent passive diffusion, the uptake of its hydrophilic metabolite O-desmethyltramadol into hepatocytes depends on OCT1 (Tzvetkov et al., 2011). After re-uptake of O-desmethyltramadol into hepatocytes it is bio-inactivated by glucuronidation by UGT2B7 (Lehtonen et al., 2010). Individuals carrying two loss-of-function OCT1 alleles showed higher plasma levels of O-desmethyl-tramadol (Tzvetkov et al., 2011). In line with the increased plasma concentrations O-desmethyl-tramadol

induced miosis, which was used as an indicator of opioid effects, was stronger and prolonged in these individuals. Hence, the pharmacokinetic of tramadol is not only influenced by polymorphisms in the CYP2D6 gene but also by polymorphisms in the OCT1 gene as OCT1 is assumed as the rate limiting step of O-desmethyltramadol bioin-activation (Tzvetkov et al., 2011).

Tropisetron is a serotonin antagonist of the 5-hydroxytryptamine 3 receptor (5-HT3

receptor) (Simpson et al., 2000). It is used as an antiemetic for the treatment of chemotherapy induced and postoperative nausea and vomiting (Simpson et al., 2000). The 5-HT3 receptor mediates nausea and vomiting in the peripheral and central nervous system.

Although tropisetron was shown to be effective in treatment of nausea, some patients still do not respond sufficiently to tropisetorn therapy (Bruntsch et al., 1993; Adams et al., 1995; Mystakidou et al., 1998). Genetic polymorphisms in the metabolizing enzyme CYP2D6 are well known to strongly contribute, but could not completely explain the observed strong inter-individual differences in response to tropisetron (Kees et al., 2001;

Kaiser et al., 2002; Kim et al., 2003; Pickering et al., 2012). More recently also genetically-determined loss of OCT1 activity was associated with a variation in the pharmacokinetics and activity of tropisetron and ondansetron (Tzvetkov et al., 2012). In a study of Tzvetkov et al. 253 patients were genotyped for five common amino acid substitutions that are associated with reduced activity of OCT1: R61C, C88R, G401S, M420del or G465R. Of these patients 12 % carried two loss-of-function OCT1 alleles, further 38 % carried one. In patients carrying two loss-of-function alleles the plasma concentration of tropisetron after 3 and 6 hours of administration was higher than in patients with only one variant allele or two active OCT1 alleles. Moreover, these patients showed greater therapeutic effect of tropisetron treatment as vomiting was less observed than in patients carrying two active OCT1 alleles (Tzvetkov et al., 2012).

Next to opioids and antiemetics, metformin was also shown to be a substrate of OCT1 (Shu et al., 2007). Metformin is a hydrophilic cation with a biguanide structure, which is widely used for the treatment of diabetes type 2 (Tahrani et al., 2011). Metformin leads to decreased absorption of glucose in the intestine and reduces gluconeogenesis (Ikeda et al., 2000; Kim et al., 2008; Foretz et al., 2010). The exact molecular mechanism underlying metformin action remains partially understood (An and He, 2016). It is assumed that it acts via activation of AMP-activated protein kinase (AMPK), which inhibits enzymes

responsible for gluconeogenesis and leads to an increased uptake of glucose into muscle and liver cells (Sarabia et al., 1992; Abbud et al., 2000; Zhou et al., 2001; Zhang et al., 2009). Metformin is a substrate of OCT1 and genetic polymorphisms in OCT1 were shown to affect metformin uptake in vitro (Shu et al., 2007). When performing an oral glucose tolerance test in healthy volunteers that were carriers of loss-of-function OCT1 alleles, the plasma glucose level was significantly higher after 180 min of metformin administration than in carriers of the OCT1 wild type reference (Shu et al., 2007; Chen et al., 2010). In contrast, the variant P341L was not shown to influence metformin pharmacokinetics in healthy volunteers (Yoon et al., 2013). Metformin is primarily eliminated by renal excretion (Pentikainen et al., 1979). Although in much lower levels as in the liver, the human OCT1 protein was detected in apical membrane of proximal and distal renal tubules (Tzvetkov et al., 2009). It was shown in a study of healthy male Caucasians that metformin clearance significantly increased in carriers of loss of function OCT1 alleles and account for 9.1 % of inter-individual variation in metformin clearance (Tzvetkov et al., 2009).