Metabolism of Repaglinide by CYP2C8 and CYP3A4 in vitro: Effect of Fibrates and Rifampicin

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.

Efficacy and safety of mitiglinide in diabetic patients on maintenance hemodialysis

Mitiglinide is a rapid- and short-acting insulinotropic sulfonylurea receptor ligand and features rapid hypoglycemic action. To date, no prospective study has evaluated the use of mitiglinide in diabetic patients receiving hemodialysis (HD). In this study we evaluated the efficacy and safety of mitiglinide in diabetic patients on HD. Following an 8-week baseline period, we enrolled a study population of poorly controlled diabetic HD patients who had mean hemoglobin (Hb)A(1c) levels greater than 6.5% at baseline and who were not receiving insulin injection therapy. Patients were administered mitiglinide, 15 mg for those who were younger than 70 years and 7.5 mg for those who were 70 years and older, daily with each meal for the first 8 weeks. Subsequently, the doses were titrated by dose-doubling to a maximum of 30 mg/day if no adverse effects appeared. The efficacy was determined by monitoring glycemic control (plasma glucose, HbA(1c), and glycated albumin levels). Safety and tolerance were determined by monitoring clinical and laboratory parameters during the 24-week study period. The average final dose of mitiglinide was 20.0 +/- 8.6 mg daily. Mitiglinide was effective in reducing not only HbA(1c) and glycated albumin but also fasting plasma glucose levels from baseline from week 4 after the start of treatment. The agent was also effective in reducing triglyceride levels. No serious adverse effects such as hypoglycemia or liver impairment were observed in any patient. However, we could not completely rule out the possibility of a hypoglycemic episode, including silent hypoglycemia due to autonomic neuropathy, and therefore further clinical studies are required. It is necessary to adjust the dose of mitiglinide according to the status of glycemic control or hypoglycemic symptoms of individual patients. Although mitiglinide was effective as a treatment for diabetic patients on HD therapy, it should be initiated at a lower dose in the HD population, compared with the general population of diabetic patients. Mitiglinide can be safely used for diabetic patients on HD, if careful attention is paid to hypoglycemia.

Cytochrome P450 2C8 pharmacogenetics: a review of clinical studies

Cytochrome P450 (CYP) 2C8 is responsible for the oxidative metabolism of many clinically available drugs from a diverse number of drug classes (e.g., thiazolidinediones, meglitinides, NSAIDs, antimalarials and chemotherapeutic taxanes). The CYP2C8 enzyme is encoded by the CYP2C8 gene, and several common nonsynonymous polymorphisms (e.g., CYP2C8*2 and CYP2C8*3) exist in this gene. The CYP2C8*2 and *3 alleles have been associated in vitro with decreased metabolism of paclitaxel and arachidonic acid. Recently, the influence of CYP2C8 polymorphisms on substrate disposition in humans has been investigated in a number of clinical pharmacogenetic studies. Contrary to in vitro data, clinical data suggest that the CYP2C8*3 allele is associated with increased metabolism of the CYP2C8 substrates, rosiglitazone, pioglitazone and repaglinide. However, the CYP2C8*3 allele has not been associated with paclitaxel pharmacokinetics in most clinical studies. Furthermore, clinical data regarding the impact of the CYP2C8*3 allele on the disposition of NSAIDs are conflicting and no definitive conclusions can be made at this time. The purpose of this review is to highlight these clinical studies that have investigated the association between CYP2C8 polymorphisms and CYP2C8 substrate pharmacokinetics and/or pharmacodynamics in humans. In this review, CYP2C8 clinical pharmacogenetic data are provided by drug class, followed by a discussion of the future of CYP2C8 clinical pharmacogenetic research.

Impact of OATP transporters on pharmacokinetics

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.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Multicentre, Double-blind, Randomized Study of Mitiglinide Compared with Nateglinide in Type 2 Diabetes Mellitus Patients in China

This study investigated the non-inferiority of mitiglinide 10–20 mg versus nateglinide 120 mg, given orally three times daily for 20 weeks, in 291 Chinese type 2 diabetes mellitus (DM) patients in whom adequate blood glucose control had not been achieved by diet and exercise. Mitiglinide was started at 10 mg three times daily for 12 weeks and thereafter could be increased to 20 mg three times daily, depending on symptoms. In the mitiglinide and nateglinide groups the change in glycosylated haemoglobin (HbA1c) from baseline to week 12 was −0.53 ± 1.08% and −0.58 ± 1.04%, respectively, and further decreased by −0.77 ± 1.06% and −0.71 ± 1.12%, respectively, at week 20 compared with baseline. The intergroup difference (95% confidence interval) of HbA1c was 0.05% (−0.19, 0.30) at week 12 and 0.06% (−0.31, 0.19) at week 20. Similar changes in HbA1c were seen in patients whose baseline value was < 8.0% as well as in those aged ≥ 60 years. Comparable reductions in other blood glucose variables were noted in the two groups. Adverse events in the mitiglinide and nateglinide groups were noted in 54.5% and 57.9% of patients, respectively. In conclusion, mitiglinide 10–20 mg three times daily provided similar blood glucose control to nateglinide 120 mg three times daily in type 2 DM patients, including the elderly and patients with mild diabetes mellitus.

Effects of the SLCO1B1*1B haplotype on the pharmacokinetics and pharmacodynamics of repaglinide and nateglinide

OBJECTIVE

Organic anion transporting polypeptide 1B1 (OATP1B1), encoded by SLCO1B1, is an influx transporter expressed on the sinusoidal membrane of human hepatocytes. The aim of this study was to investigate whether the SLCO1B1*1B haplotype affects the pharmacokinetics and pharmacodynamics of repaglinide and nateglinide.

METHODS

Eight healthy volunteers with the SLCO1B1*1B/*1B genotype and 16 with the SLCO1B1*1A/*1A genotype ingested a single 0.5-mg dose of repaglinide and, after a washout period of 1 week, a single 60-mg dose of nateglinide. Plasma repaglinide and nateglinide and blood glucose concentrations were measured for 7 h.

RESULTS

The AUC(0-infinity) and Cmax of repaglinide were 32% (P=0.007) and 24% lower (P=0.056) in the individuals with the SLCO1B1*1B/*1B genotype than in those with the SLCO1B1*1A/*1A genotype. The mean blood glucose concentration from 0 to 7 h after repaglinide intake was 10% higher in the SLCO1B1*1B/*1B participants than in the SLCO1B1*1A/*1A participants (P=0.007). The Cmax of nateglinide occurred earlier in the SLCO1B1*1B/*1B participants than in the SLCO1B1*1A/*1A participants (P=0.004), but no differences were seen in the other pharmacokinetic variables of nateglinide.

CONCLUSION

The SLCO1B1*1B/*1B genotype is associated with reduced plasma concentrations of repaglinide, consistent with an enhanced hepatic uptake by OATP1B1, but has limited effects on the pharmacokinetics of nateglinide.

The effect of SLCO1B1 polymorphism on repaglinide pharmacokinetics persists over a wide dose range

AIMS

To establish whether the effect of SLCO1B1[encoding organic anion transporting polypeptide 1B1 (OATP1B1)] c.521T-->C (p.Val174Ala) polymorphism on the pharmacokinetics of repaglinide is dose-dependent.

METHODS

Twelve healthy volunteers with the SLCO1B1 c.521TT genotype (controls) and eight with the c.521CC genotype ingested a single 0.25-, 0.5-, 1- or 2-mg dose of repaglinide in a dose-escalation study with a wash-out period of > or =1 week.

RESULTS

The mean area under the plasma concentration-time curve from time 0 to infinity (AUC(0-infinity)) of 0.25, 0.5, 1 or 2 mg repaglinide was 82% (95% confidence interval 47, 125), 72% (24, 138), 56% (24, 95) or 108% (59, 171) (P < or = 0.001) larger in participants with the SLCO1B1 c.521CC genotype than in those with the c.521TT genotype, respectively. Repaglinide peak plasma concentration and AUC(0-infinity) increased linearly along with repaglinide dose in both genotype groups (r > 0.88, P < 0.001). There was a tendency towards lower blood glucose concentrations after repaglinide administration in the participants with the c.521CC genotype than in those with the c.521TT genotype.

CONCLUSIONS

The effect of SLCO1B1 c.521T-->C polymorphism on the pharmacokinetics of repaglinide persists throughout the clinically relevant dose range.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Interaction of oral antidiabetic drugs with hepatic uptake transporters: focus on organic anion transporting polypeptides and organic cation transporter 1

OBJECTIVE

The uptake of drugs into hepatocytes is a key determinant for hepatic metabolism, intrahepatic action, their subsequent systemic plasma concentrations, and extrahepatic actions. In vitro and in vivo studies indicate that many drugs used for treatment of cardiovascular diseases (e.g., oral antidiabetic drugs, statins) are taken up into hepatocytes by distinct organic anion transporters (organic anion transporting polypeptides [OATPs]; gene symbol SLCO/SLC21) or organic cation transporters (OCTs; gene symbol SLC22). Because most patients with type 2 diabetes receive more than one drug and inhibition of drug transporters has been recognized as a new mechanism underlying drug-drug interactions, we tested the hypothesis of whether oral antidiabetic drugs can inhibit the transport mediated by hepatic uptake transporters.

RESEARCH DESIGN AND METHODS

Using stably transfected cell systems recombinantly expressing the uptake transporters OATP1B1, OATP1B3, OATP2B1, or OCT1, we analyzed whether the antidiabetic drugs repaglinide, rosiglitazone, or metformin influence the transport of substrates and drugs (for OATPs, sulfobromophthalein [BSP] and pravastatin; for OCT1, 1-methyl-4-phenylpyridinium [MPP(+)] and metformin).

RESULTS

Metformin did not inhibit the uptake of OATP and OCT1 substrates. However, OATP-mediated BSP and pravastatin uptake and OCT1-mediated MPP(+) and metformin uptake were significantly inhibited by repaglinide (half-maximal inhibitory concentration [IC(50)] 1.6-5.6 micromol/l) and rosiglitazone (IC(50) 5.2-30.4 micromol/l).

CONCLUSIONS

These in vitro results demonstrate that alterations of uptake transporter function by oral antidiabetic drugs have to be considered as potential mechanisms underlying drug-drug interactions.

Prediction of the metabolic interaction of nateglinide with other drugs based on in vitro studies

Nateglinide is an antidiabetic agent metabolized by CYP2C9 and CYP3A4; hence inhibitors of these CYP isozymes may interact with nateglinide. There are, however, only limited in vitro data on how to predict drug-drug interactions in vivo. We examined the effects of 18 drugs that may be prescribed together with nateglinide (metformin, buformin, aspirin, gemfibrozil, simvastatin, pioglitazone, rosiglitazone, carbamazepine, clarithromycin, gliclazide, clofibrate, fluconazole, bezafibrate, phenylbutazone, nifedipine, famotidine, ibuprofen and miconazole) on the conversion of nateglinide to its major metabolite (N-[trans-4-(1-hydroxy-1-methylethyl)-cyclohexanecarbonyl]-D-phenylalanine) using human liver microsomes. Eight compounds showed a<50% inhibitory effect and we estimated the K(i) values for the remaining 10 compounds. Except for fluconazole and miconazole, 1+I(in, max, u)/K(i) calculated from the K(i) values, was approximately 1 and thus the possibility of a drug-drug interaction was considered low. The value for fluconazole suggested the risk of interaction and agreed with the results of clinical studies in which the AUC of nateglinide increased by 48% when it was co-administered with fluconazole. The present study showed that nateglinide metabolism would hardly be affected by the drugs used in this study, except for miconazole and fluconazole that are potent inhibitors of multiple isoforms of CYPs.

Different effects of SLCO1B1 polymorphism on the pharmacokinetics and pharmacodynamics of repaglinide and nateglinide

Thirty-two healthy volunteers with different SLCO1B1 genotypes ingested a 0.5-mg dose of repaglinide and 60-mg dose of nateglinide with a washout period of 1 week. Participants with SLCO1B1 c.521CC genotype (n = 4) had a 59% (P = 0.001) or 72% (P < 0.001) greater mean area under the plasma repaglinide concentration-time curve (AUC(0-infinity)) than participants with c.521TC (n = 12) or c.521TT (n = 16) genotypes. The AUC(0-infinity) of repaglinide metabolites M2 and M4 were 112% (P = 0.004) and 81% (P = 0.002) larger in participants with c.521CC genotype than in those with c.521TT genotype, but no differences existed in the pharmacokinetics of M1. Maximum decrease in blood glucose concentration correlated with repaglinide AUC(0-infinity) (r = 0.412, P = 0.019). SLCO1B1 polymorphism had no significant effect on the pharmacokinetics or pharmacodynamics of nateglinide or its M7 metabolite. Thus, in contrast to repaglinide, the disposition of nateglinide is unaffected by the SLCO1B1 c.521T>C polymorphism.

Characterization of novel CYP2C8 haplotypes and their contribution to paclitaxel and repaglinide metabolism

Cytochrome P450 2C8 (CYP2C8) plays a major role in the metabolism of therapeutically important drugs which exhibit large interindividual differences in their pharmacokinetics. In order to evaluate any genetic influence on this variation, a CYP2C8 phenotype-genotype evaluation was carried out in Caucasians. Two novel CYP2C8 haplotypes, named B and C with frequencies of 24 and 22% in Caucasians, respectively, were identified and caused a significantly increased and reduced paclitaxel 6alpha-hydroxylation, respectively, as evident from analyses of 49 human liver samples. In healthy white subjects, CYP2C8*3 and the two novel haplotypes significantly influenced repaglinide pharmacokinetics in SLCO1B1c.521T/C heterozygous individuals: haplotype B was associated with reduced and haplotype C with increased repaglinide AUC (0-infinity). Functional studies suggested -271C>A (CYP2C8*1B) as a causative SNP in haplotype B. In conclusion, two novel common CYP2C8 haplotypes were identified and significantly associated with altered rate of CYP2C8-dependent drug metabolism in vitro and in vivo.

Multiple Inhibition Mechanisms and Prediction of Drug–Drug Interactions: Status of Metabolism and Transporter Models as Exemplified by Gemfibrozil–Drug Interactions

PURPOSE

To assess the consequences of multiple inhibitors and differential inhibition mechanisms on the prediction of 12 gemfibrozil drug-drug interactions (DDIs). In addition, qualitative zoning of transporter-related gemfibrozil and cyclosporine DDIs was investigated.

METHODS

The effect of gemfibrozil and its acyl-glucuronide on different enzymes was incorporated into a metabolic prediction model. The impact of CYP2C8 time-dependent inhibition by gemfibrozil acyl-glucuronide was assessed using repaglinide, cerivastatin, loperamide, rosiglitazone and pioglitazone DDIs. Gemfibrozil and cyclosporine inhibition data obtained in human embryonic kidney cells expressing OATP1B1 and hepatic input concentration ([I]in) were used for qualitative zoning of 14 transporter-mediated DDIs.

RESULTS

Incorporation of time-dependent inhibition by gemfibrozil glucuronide showed no significant improvement in the prediction, as CYP2C8 contributed <65% to the overall elimination of the victim drugs investigated. Qualitative zoning of OATP1B1 DDIs resulted in no false negative predictions; yet the magnitude of observed interactions was significantly over-predicted.

CONCLUSIONS

Time-dependent inhibition by gemfibrozil glucuronide is only important for victim drugs eliminated predominantly (>80%) via CYP2C8. Qualitative zoning of OATP1B1 inhibitors based on [I]in/K (i) is valid in drug screening to avoid false negatives. Refinement of the transporter model by incorporating the fraction of drug transported by a particular transporter is recommended.

Bosentan is a substrate of human OATP1B1 and OATP1B3: inhibition of hepatic uptake as the common mechanism of its interactions with cyclosporin A, rifampicin, and sildenafil

The elimination process of the endothelin receptor antagonist bosentan (Tracleer) in humans is entirely dependent on metabolism mediated by two cytochrome P450 (P450) enzymes, i.e., CYP3A4 and CYP2C9. Most interactions with concomitantly administered drugs can be rationalized in terms of inhibition of these P450 enzymes. The increased bosentan concentrations observed in the presence of cyclosporin A, rifampicin, or sildenafil, however, are incompatible with this paradigm and prompted the search for alternative mechanisms governing these interactions. In the present article, we identify bosentan and its active plasma metabolite, Ro 48-5033 (4-(2-hydroxy-1,1-dimethyl-ethyl)-N-[6-(2-hydroxy-ethoxy)-5-(2-methoxy-phenoxy)-[2,2']bipyrimidinyl-4-yl]-benzenesulfonamide), as substrates of the human organic anion transporting polypeptides (OATP) OATP1B1 and OATP1B3. Bosentan uptake into Chinese hamster ovary cells expressing these OATP transporters was efficiently inhibited by cyclosporin A and rifampicin with IC(50) values significantly below their effective plasma concentrations in humans. The phosphodiesterase-5 inhibitor sildenafil was also shown to interfere with OATP-mediated transport, however, at concentrations above those achieved in therapeutic use. Therefore, inhibition of bosentan hepatic uptake may represent an alternative/complementary mechanism to rationalize some of the pharmacokinetic interactions seen in therapeutic use. A similar picture has been drawn for drugs like pitavastatin and fexofenadine, drugs that are mainly excreted in unchanged form. Bosentan elimination, in contrast, is entirely dependent on metabolism. Therefore, the described interactions with rifampicin, cyclosporin A, and, to a lesser extent, sildenafil represent evidence that inhibition of hepatic uptake may become the rate-limiting step in the overall elimination process even for drugs whose elimination is entirely dependent on metabolism.

Drug-drug and food-drug pharmacokinetic interactions with new insulinotropic agents repaglinide and nateglinide

This review describes the current knowledge on drug-drug and food-drug interactions with repaglinide and nateglinide. These two meglitinide derivatives, commonly called glinides, have been developed for improving insulin secretion of patients with type 2 diabetes mellitus. They are increasingly used either in monotherapy or in combination with other oral antihyperglycaemic agents for the treatment of type 2 diabetes. Compared with sulfonylureas, glinides have been shown to (i) provide a better control of postprandial hyperglycaemia, (ii) overcome some adverse effects, such as hypoglycaemia, and (iii) have a more favourable safety profile, especially in patients with renal failure. The meal-related timing of administration of glinides and the potential influence of food and meal composition on their bioavailability may be important. In addition, some food components (e.g. grapefruit juice) may cause pharmacokinetic interactions. Because glinides are metabolised via cytochrome P450 (CYP) 3A4 isoenzyme, they are indeed exposed to pharmacokinetic interactions. In addition to CYP3A4, repaglinide is metabolised via CYP2C8, while nateglinide metabolism also involves CYP2C9. Furthermore, both compounds and their metabolites may undergo specialised transport/uptake in the intestine, another source of pharmacokinetic interactions. Clinically relevant drug-drug interactions are those that occur when glinides are administered together with other glucose-lowering agents or compounds widely coadministered to diabetic patients (e.g. lipid-lowering agents), with drugs that are known to induce (risk of lower glinide plasma levels and thus of deterioration of glucose control) or inhibit (risk of higher glinide plasma levels leading to hypoglycaemia) CYP isoenzymes concerned in their metabolism, or with drugs that have a narrow efficacy : toxicity ratio. Pharmacokinetic interactions reported in the literature appear to be more frequent and more important with repaglinide than with nateglinide. Rifampicin (rifampin) reduced repaglinide area under the plasma concentration-time curve (AUC) by 32-85% while it reduced nateglinide AUC by almost 25%. Reported increases in AUCs with coadministration of drugs inhibiting CYP isoenzymes never exceeded 80% for repaglinide (except with ciclosporin and with gemfibrozil) and 50% for nateglinide. Ciclosporin more than doubled repaglinide AUC (+144%), a finding that should raise caution when using these two drugs in combination. The most impressive pharmacokinetic interaction was reported with combined administration of gemfibrozil (a strong CYP2C8 inhibitor) and repaglinide (8-fold increase in repaglinide AUC). Although no studies have been performed in patients with type 2 diabetes, the latter combination should be avoided in clinical practice.

Carboxyl-glucuronidation of mitiglinide by human UDP-glucuronosyltransferases

Mitiglinide (MGN) is a new potassium channel antagonist for the treatment of type 2 diabetes mellitus. In the present study, a potential metabolic pathway of MGN, via carboxyl-linked glucuronic acid conjugation, was found. MGN carboxyl-glucuronide was isolated from a reaction mixture consisting of MGN and human liver microsomes fortified with UDP-glucuronic acid (UDPGA) and identified by a hydrolysis reaction with beta-glucuronidase and HPLC-MS/MS. Kinetic analysis indicated that MGN from four species had the highest affinity for the rabbit liver microsomal enzyme (K(m)=0.202 mM) and the lowest affinity for the dog liver microsomal enzyme (K(m)=1.164 mM). The metabolic activity (V(max)/K(m)) of MGN to the carboxyl-glucuronidation was in the following order: rabbit>dog>rat>human. With the assessment of MGN glucuronide formation across a panel of recombinant UDP-glucuronosyltransferase (UGT) isoforms (UGT1A3, UGT1A4, UGT1A6, UGT1A9, and UGT2B7), only UGT1A3 and UGT2B7 exhibited high MGN glucuronosyltransferase activity. The K(m) values of MGN glucuronidation in recombinant UGT1A3 and UGT2B7 microsomes were close to those in human liver microsomes. The formation of MGN glucuronidation by human liver microsomes was effectively inhibited by quercetin (substrate for UGT1A3) and diclofenac (substrate for UGT2B7), respectively. The MGN glucuronidation activities in 15 human liver microsomes were significantly correlated with quercetin (r(2)=0.806) and diclofenac glucuronidation activities (r(2)=0.704), respectively. These results demonstrate that UGT1A3 and UGT2B7 are catalytic enzymes in MGN carboxyl-glucuronidation in human liver.

Inhibitory and Inductive Effects of Rifampin on the Pharmacokinetics of Bosentan in Healthy Subjects

This study was conducted to investigate the effect of rifampin on the pharmacokinetics of bosentan. Healthy male subjects received bosentan 125 mg b.i.d. for 6.5 days in the presence or absence of rifampin 600 mg once a day. In vitro experiments were performed to investigate the effect of rifampin on the uptake of bosentan into Chinese hamster ovary cells expressing the human organic anion-transporting polypeptide (OATP)1B1, -1B3, and -2B1. Following the first concomitant administration, there was a fivefold increase in bosentan trough concentrations. At steady state, concomitant rifampin significantly decreased exposure to bosentan by 58%. Rifampin potently inhibited the uptake of bosentan into cells expressing human OATP1B1 and -1B3. Rifampin decreased the exposure to bosentan consistent with its known cytochrome P450 enzyme-inductive properties. The initial increase in bosentan concentrations can be explained by an inhibitory effect of rifampin on hepatic drug transporters.

effect of OATP1B transporter inhibition on the pharmacokinetics of atorvastatin in healthy volunteers

The inhibition of hepatic uptake transporters, such as OATP1B1, on the pharmacokinetics of atorvastatin is unknown. Here, we investigate the effect of a model hepatic transporter inhibitor, rifampin, on the kinetics of atorvastatin and its metabolites in humans. The inhibitory effect of a single rifampin dose on atorvastatin kinetics was studied in 11 healthy volunteers in a randomized, crossover study. Each subject received two 40-mg doses of atorvastatin, one on study day 1 and one on study day 8, separated by 1 week. One intravenous 30-min infusion of 600 mg rifampin was administered to each subject on either study day 1 or study day 8. Plasma concentrations of atorvastatin and metabolites were above the limits of quantitation for up to 24 h after dosing. Rifampin significantly increased the total area under the plasma concentration-time curve (AUC) of atorvastatin acid by 6.8+/-2.4-fold and that of 2-hydroxy-atorvastatin acid and 4-hydroxy-atorvastatin acid by 6.8+/-2.5- and 3.9+/-2.4-fold, respectively. The AUC values of the lactone forms of atorvastatin, 2-hydroxy-atorvastatin and 4-hydroxy-atorvastatin, were also significantly increased, but to a lower extent. An intravenous dose of rifampin substantially increased the plasma concentrations of atorvastatin and its acid and lactone metabolites. The data confirm that OATP1B transporters represent the major hepatic uptake systems for atorvastatin and its active metabolites. Inhibition of hepatic uptake may have consequences for efficacy and toxicity of drugs like atorvastatin that are mainly eliminated by the hepatobiliary system.

Pioglitazone is metabolised by CYP2C8 and CYP3A4 in vitro: potential for interactions with CYP2C8 inhibitors

Our objective was to identify the cytochrome P450 (CYP) enzymes that metabolise pioglitazone and to examine the effects of the CYP2C8 inhibitors montelukast, zafirlukast, trimethoprim and gemfibrozil on pioglitazone metabolism in vitro. The effect of different CYP isoform inhibitors on the elimination of a clinically relevant concentration of pioglitazone (1 microM) and the formation of the main primary metabolite M-IV were studied using pooled human liver microsomes. The metabolism of pioglitazone by CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4 and CYP3A5 was investigated using human recombinant CYP isoforms. In particular, the inhibitors of CYP2C8, but also those of CYP3A4, markedly inhibited the elimination of pioglitazone and the formation of M-IV by HLM. Inhibitors selective to other CYP isoforms had a minor effect only. Of the recombinant isoforms, CYP2C8 (20 pmol/ml) metabolised pioglitazone markedly (56% in 60 min.), and also CYP3A4 had a significant effect (37% in 60 min.). Montelukast, zafirlukast, trimethoprim and gemfibrozil inhibited pioglitazone elimination in HLM with IC50 values of 0.51 microM, 1.0 microM, 99 microM and 98 microM, respectively, and the formation of the metabolite M-IV with IC50 values of 0.18 microM, 0.78 microM, 71 microM and 59 microM, respectively. In conclusion, pioglitazone is metabolised mainly by CYP2C8 and to a lesser extent by CYP3A4 in vitro. CYP2C9 is not significantly involved in the elimination of pioglitazone. The effect of different CYP2C8 inhibitors on pioglitazone pharmacokinetics needs to be evaluated also in vivo because, irrespective of their in vitro CYP2C8 inhibitory potency, their pharmacokinetic properties may affect the extent of interaction.

Effect of rifampicin on the pharmacokinetics of pioglitazone

AIMS

The effect of enzyme induction on the pharmacokinetics of pioglitazone, a thiazolidinedione antidiabetic drug that is metabolized primarily by CYP2C8, is not known. Rifampicin is a potent inducer of several CYP enzymes and our objective was to study its effects on the pharmacokinetics of pioglitazone in humans.

METHODS

In a randomized, two-phase crossover study, ten healthy subjects ingested either 600 mg rifampicin or placebo once daily for 6 days. On the last day, they received a single oral dose of 30 mg pioglitazone. The plasma concentrations and cumulative excretion of pioglitazone and its active metabolites M-IV and M-III into urine were measured up to 48 h.

RESULTS

Rifampicin decreased the mean total area under the plasma concentration-time curve (AUC(0-infinity)) of pioglitazone by 54% (range 20-66%; P = 0.0007; 95% confidence interval -78 to -30%) and shortened its dominant elimination half-life (t(1/2)) from 4.9 to 2.3 h (P = 0.0002). No significant effect on peak concentration (C(max)) or time to peak (t(max)) was observed. Rifampicin increased the apparent formation rate of M-IV and shortened its t(max) (P < 0.01). It also decreased the AUC(0-infinity) of M-IV (by 34%; P = 0.0055) and M-III (by 39%; P = 0.0026), shortened their t1/2 (M-IV by 50%; P = 0.0008, and M-III by 55%; P = 0.0016) and increased the AUC(0-infinity) ratios of M-IV and M-III to pioglitazone by 44% (P = 0.0011) and 32% (P = 0.0027), respectively. Rifampicin increased the M-IV/pioglitazone and M-III/pioglitazone ratios in urine by 98% (P = 0.0015) and 95% (P = 0.0024). A previously unrecognized metabolite M-XI, tentatively identified as a dihydroxy metabolite, was detected in urine during both phases, and rifampicin increased the ratio of M-XI to pioglitazone by 240% (P = 0.0020).

CONCLUSIONS

Rifampicin caused a substantial decrease in the plasma concentration of pioglitazone, probably by induction of CYP2C8. Concomitant use of rifampicin with pioglitazone may decrease the efficacy of the latter drug.

Montelukast and zafirlukast do not affect the pharmacokinetics of the CYP2C8 substrate pioglitazone

OBJECTIVE

Pioglitazone, a thiazolidinedione antidiabetic drug, is metabolised mainly by the cytochrome P450 (CYP) 2C8 enzyme. The leukotriene receptor antagonists montelukast and zafirlukast have potently inhibited CYP2C8 activity and the metabolism of pioglitazone in vitro. Our objective was to determine whether montelukast and zafirlukast increase the plasma concentrations of pioglitazone in humans.

METHODS

In a randomised, double-blind crossover study with three phases and a washout period of 3 weeks, 12 healthy volunteers took either 10 mg montelukast once daily and placebo once daily, or 20 mg zafirlukast twice daily, or placebo twice daily, for 6 days. On day 3, they received a single oral dose of 15 mg pioglitazone. The plasma concentrations of pioglitazone and its metabolites M-IV, M-III, M-V and M-XI were measured for 96 h.

RESULTS

The total area under the plasma concentration-time curve of pioglitazone during the montelukast and zafirlukast phases was 101% (range 71-143%) and 103% (range 78-146%), respectively, of that during the placebo phase. Also, the peak plasma concentration and elimination half-life of pioglitazone remained unaffected by montelukast and zafirlukast. There were no statistically significant differences in the pharmacokinetics of any of the metabolites of pioglitazone between the phases.

CONCLUSIONS

Montelukast and zafirlukast do not increase the plasma concentrations of pioglitazone, indicating that their inhibitory effect on CYP2C8 is negligible in vivo, despite their strong inhibitory effect on CYP2C8 in vitro. The results highlight the importance of in vivo interaction studies and of the incorporation of relevant pharmacokinetic properties of drugs, including plasma protein binding data, to in vitro-in vivo interaction predictions.

Itraconazole, gemfibrozil and their combination markedly raise the plasma concentrations of loperamide

OBJECTIVE

Loperamide is biotransformed in vitro by the cytochromes P450 (CYP) 2C8 and 3A4 and is a substrate of the P-glycoprotein efflux transporter. Our aim was to investigate the effects of itraconazole, an inhibitor of CYP3A4 and P-glycoprotein, and gemfibrozil, an inhibitor of CYP2C8, on the pharmacokinetics of loperamide.

METHODS

In a randomized crossover study with 4 phases, 12 healthy volunteers took 100 mg itraconazole (first dose 200 mg), 600 mg gemfibrozil, both itraconazole and gemfibrozil, or placebo, twice daily for 5 days. On day 3, they ingested a single 4-mg dose of loperamide. Loperamide and N-desmethylloperamide concentrations in plasma were measured for up to 72 h and in urine for up to 48 h. Possible central nervous system effects of loperamide were assessed by the Digit Symbol Substitution Test and by subjective drowsiness.

RESULTS

Itraconazole raised the peak plasma loperamide concentration (Cmax) 2.9-fold (range, 1.2-5.0; P < 0.001) and the total area under the plasma loperamide concentration-time curve (AUC(0-infinity)) 3.8-fold (1.4-6.6; P < 0.001) and prolonged the elimination half-life (t(1/2)) of loperamide from 11.9 to 18.7 h (P < 0.001). Gemfibrozil raised the Cmax of loperamide 1.6-fold (0.9-3.2; P < 0.05) and its AUC(0-infinity) 2.2-fold (1.0-3.7; P < 0.05) and prolonged its t(1/2) to 16.7 h (P < 0.01). The combination of itraconazole and gemfibrozil raised the Cmax of loperamide 4.2-fold (1.5-8.7; P < 0.001) and its AUC(0-infinity) 12.6-fold (4.3-21.8; P < 0.001) and prolonged the t(1/2) of loperamide to 36.9 h (P < 0.001). The amount of loperamide excreted into urine within 48 h was increased 3.0-fold, 1.4-fold and 5.3-fold by itraconazole, gemfibrozil and their combination, respectively (P < 0.05). Itraconazole, gemfibrozil and their combination reduced the plasma AUC(0-72) ratio of N-desmethylloperamide to loperamide by 65%, 46% and 88%, respectively (P < 0.001). No significant differences were seen in the Digit Symbol Substitution Test or subjective drowsiness between the phases.

CONCLUSION

Itraconazole, gemfibrozil and their combination markedly raise the plasma concentrations of loperamide. Although not seen in the psychomotor tests used, an increased risk of adverse effects should be considered during concomitant use of loperamide with itraconazole, gemfibrozil and especially their combination.

Pharmacokinetics of atorvastatin and its hydroxy metabolites in rats and the effects of concomitant rifampicin single doses: relevance of first-pass effect from hepatic uptake transporters, and intestinal and hepatic metabolism

Pharmacokinetic coadministration experiments with atorvastatin (ATV) and rifampicin (RIF) in rats were performed to investigate the potential involvement of hepatic uptake transporters, Oatps (organic anion-transporting polypeptides), during hepatic drug elimination, as an in vivo extension of our recently published cellular and isolated perfused liver studies. ATV was administered orally (10 mg/kg) and intravenously (2 mg/kg) to rats in the absence and presence of a single intravenous dose of RIF (20 mg/kg), and pharmacokinetic parameters were compared between control and RIF-treatment groups. RIF markedly increased the plasma concentrations of ATV and its metabolites when ATV was administered orally. The area under the plasma concentration-time curve (AUC(0-infinity)) for ATV also increased significantly after intravenous dosing of ATV with RIF, but the extent was much less than that observed for oral ATV dosing. Significant increases in plasma levels were observed for both metabolites as well. The 7-fold higher AUC ratio of metabolites to parent drug following oral versus intravenous ATV dosing suggests that ATV undergoes extensive gut metabolism. Both hepatic and intestinal metabolism contribute to the low oral bioavailability of ATV in rats. In the presence of RIF, the liver metabolic extraction was significantly reduced, most likely because of RIF's inhibition on Oatp-mediated uptake, which leads to reduced hepatic amounts of parent drug for subsequent metabolism. Gut extraction was also significantly reduced, but we were unable to elucidate the mechanism of this effect because intravenous RIF caused gut changes in availability. These studies reinforce our hypothesis that hepatic uptake is a major contributor to the elimination of ATV and its metabolites in vivo.

Pharmacogenomics of human OATP transporters

Organic anion transporting polypeptides (OATPs) mediate the uptake of a broad range of compounds into cells. Substrates for members of the OATP family include bile salts, hormones, and steroid conjugates as well as drugs like the HMG-CoA-reductase inhibitors (statins), cardiac glycosides, anticancer agents like methotrexate, and antibiotics like rifampicin. OATPs are expressed in a variety of different tissues, including intestine, liver, kidney, and brain, suggesting that they play a critical role in drug absorption, distribution, and excretion. The identification and functional characterisation of naturally occurring variations in genes encoding human OATP (SLCO) family members is in the focus of transporter research. As a result of their broad substrate spectrum and their wide tissue distribution, altered transport characteristics or protein localisation can contribute significantly to interindividual variations of drug effects. The analysis of the consequences of genetic variations in genes encoding transport proteins may, therefore, contribute to a better understanding of interindividual differences in drug effects and to individualise treatment regimens with drugs that are substrates for human OATP proteins. In this review, we summarise the current knowledge on genetic variations in transporter genes encoding human OATP family members and their functional consequences analysed by in vitro and in vivo studies.

Pioglitazone, an in vitro inhibitor of CYP2C8 and CYP3A4, does not increase the plasma concentrations of the CYP2C8 and CYP3A4 substrate repaglinide

OBJECTIVE

Pioglitazone, a thiazolidinedione antidiabetic, inhibits cytochrome P450 (CYP) 2C8 and CYP3A4 enzymes in vitro. Repaglinide, a meglitinide analogue antidiabetic, is metabolised by CYP2C8 and CYP3A4. In patients with type 2 diabetes, the pioglitazone-repaglinide combination has acted synergistically on glycaemic parameters. Our aim was to determine whether pioglitazone increases the plasma concentrations of repaglinide.

METHODS

In a randomized, 2-phase cross-over study, 12 healthy volunteers received 30 mg pioglitazone or placebo once daily for 5 days. On day 5, they ingested a single 0.25 mg dose of repaglinide 1 h after the last pretreatment dose. Plasma repaglinide and pioglitazone, and blood glucose concentrations were measured for 12 h.

RESULTS

During the pioglitazone phase, the mean peak plasma repaglinide concentration (C(max)) and the total area under the concentration-time curve [AUC(0-infinity)] of repaglinide were 100% (range 53-157%, P=0.99) and 90% (range 63-120%, P=0.22), respectively, of those during the placebo phase. Also the half-life of repaglinide was unaffected, but the median peak time of repaglinide was shortened from 40 min to 20 min by pioglitazone (P=0.014). The short-term pioglitazone administration did not modify the blood glucose-lowering effect of a single dose of repaglinide.

CONCLUSIONS

Pioglitazone does not increase the plasma concentrations of repaglinide, indicating that the inhibitory effect of pioglitazone on CYP2C8 and CYP3A4 is very weak in vivo, probably due to its extensive plasma protein binding. The synergistic effect of repaglinide and pioglitazone on the glycaemic parameters, seen in patients with type 2 diabetes during their long-term use, is unlikely to be caused by inhibition of repaglinide metabolism by pioglitazone.

GLUCURONIDATION CONVERTS GEMFIBROZIL TO A POTENT, METABOLISM-DEPENDENT INHIBITOR OF CYP2C8: IMPLICATIONS FOR DRUG-DRUG INTERACTIONS

Gemfibrozil more potently inhibits CYP2C9 than CYP2C8 in vitro, and yet the opposite inhibitory potency is observed in the clinic. To investigate this apparent paradox, we evaluated both gemfibrozil and its major metabolite, an acyl-glucuronide (gemfibrozil 1-O-beta-glucuronide) as direct-acting and metabolism-dependent inhibitors of the major drug-metabolizing cytochrome P450 enzymes (CYP1A2, 2B6, 2C8, 2C9, 2C19, 2D6, and 3A4) in human liver microsomes. Gemfibrozil most potently inhibited CYP2C9 (IC50 of 30 microM), whereas gemfibrozil glucuronide most potently inhibited CYP2C8 (IC50 of 24 microM). Unexpectedly, gemfibrozil glucuronide, but not gemfibrozil, was found to be a metabolism-dependent inhibitor of CYP2C8 only. The IC50 for inhibition of CYP2C8 by gemfibrozil glucuronide decreased from 24 microM to 1.8 microM after a 30-min incubation with human liver microsomes and NADPH. Inactivation of CYP2C8 by gemfibrozil glucuronide required NADPH, and proceeded with a K(I) (inhibitor concentration that supports half the maximal rate of enzyme inactivation) of 20 to 52 microM and a k(inact) (maximal rate of inactivation) of 0.21 min(-1). Potent inhibition of CYP2C8 was also achieved by first incubating gemfibrozil with alamethicin-activated human liver microsomes and UDP-glucuronic acid (to form gemfibrozil glucuronide), followed by a second incubation with NADPH. Liquid chromatography-tandem mass spectrometry analysis established that human liver microsomes and recombinant CYP2C8 both convert gemfibrozil glucuronide to a hydroxylated metabolite, with oxidative metabolism occurring on the dimethylphenoxy moiety (the group furthest from the glucuronide moiety). The results described have important implications for the mechanism of the clinical interaction reported between gemfibrozil and CYP2C8 substrates such as cerivastatin, repaglinide, rosiglitazone, and pioglitazone.