Absorption of montelukast is transporter mediated: a common variant of OATP2B1 is associated with reduced plasma concentrations and poor response

The effects of dietary and herbal phytochemicals on drug transporters

Membrane transporter proteins (the ABC transporters and SLC transporters) play pivotal roles in drug absorption and disposition, and thus determine their efficacy and safety. Accumulating evidence suggests that the expression and activity of these transporters may be modulated by various phytochemicals (PCs) found in diets rich in plants and herbs. PC absorption and disposition are also subject to the function of membrane transporter and drug metabolizing enzymes. PC-drug interactions may involve multiple major drug transporters (and metabolizing enzymes) in the body, leading to alterations in the pharmacokinetics of substrate drugs, and thus their efficacy and toxicity. This review summarizes the reported in vitro and in vivo interactions between common dietary PCs and the major drug transporters. The oral absorption, distribution into pharmacological sanctuaries and excretion of substrate drugs and PCs are considered, along with their possible interactions with the ABC and SLC transporters which influence these processes.

Recent advance in the pharmacogenomics of human Solute Carrier Transporters (SLCs) in drug disposition

Drug pharmacokinetics is influenced by the function of metabolising enzymes and influx/efflux transporters. Genetic variability of these genes is known to impact on clinical therapies. Solute Carrier Transporters (SLCs) are the primary influx transporters responsible for the cellular uptake of drug molecules, which consequently, impact on drug efficacy and toxicity. The Organic Anion Transporting Polypeptides (OATPs), Organic Anion Transporters (OATs) and Organic Cation Transporters (OCTs/OCTNs) are the most important SLCs involved in drug disposition. The information regarding the influence of SLC polymorphisms on drug pharmacokinetics is limited and remains a hot topic of pharmaceutical research. This review summarises the recent advance in the pharmacogenomics of SLCs with an emphasis on human OATPs, OATs and OCTs/OCTNs. Our current appreciation of the degree of variability in these transporters may contribute to better understanding the inter-patient variation of therapies and thus, guide the optimisation of clinical treatments.

Food and Drug Interactions

Natural foods and vegetal supplements have recently become increasingly popular for their roles in medicine and as staple foods. This has, however, led to the increased risk of interaction between prescribed drugs and the bioactive ingredients contained in these foods. These interactions range from pharmacokinetic interactions (absorption, distribution, metabolism, and excretion influencing blood levels of drugs) to pharmacodynamic interactions (drug effects). In a quantitative respect, these interactions occur mainly during metabolism. In addition to the systemic metabolism that occurs mainly in the liver, recent studies have focused on the metabolism in the gastrointestinal tract endothelium before absorption. Inhibition of metabolism causes an increase in the blood levels of drugs and could have adverse reactions. The food-drug interactions causing increased blood levels of drugs may have beneficial or detrimental therapeutic effects depending on the intensity and predictability of these interactions. It is therefore important to understand the potential interactions between foods and drugs should and the specific outcomes of such interactions.

Pharmacogenomics of inhaled corticosteroids and leukotriene modifiers: a systematic review

BACKGROUND

Pharmacogenetics studies of anti-inflammatory medication of asthma have expanded rapidly in recent decades, but the clinical value of their findings remains limited.

OBJECTIVE

To perform a systematic review of pharmacogenomics and pharmacogenetics of inhaled corticosteroids (ICS) and leukotriene modifiers (LTMs) in patients with asthma.

METHODS

Articles published between 1999 and June 2015 were searched using PubMed and EMBASE. Pharmacogenomics/genetics studies of patients with asthma using ICS or LTMs were included if ≥1 of the following outcomes were studied: lung function, exacerbation rates or asthma symptoms. The studies of Single Nucleotide Polymorphisms (SNPs) that had been replicated at least once were assessed in more detail.

RESULTS

In total, 59 publications were included in the systematic review: 26 addressed LTMs (including two genomewide Genome-Wide association studies [GWAS]) and 33 addressed ICS (including four GWAS). None of the GWAS reported similar results. Furthermore, none of the SNPs assessed in candidate gene studies were identified in a GWAS. No consistent reports were found for candidate gene studies of LTMs. In candidate gene studies of ICS, the most consistent results were found for rs28364072 in FCER2. This SNP was associated with all three outcomes of poor response, and the largest effect was reported with the risk of exacerbations (hazard ratio, 3.95; 95% CI, 1.64-9.51).

CONCLUSION AND CLINICAL RELEVANCE

There is a lack of replication of genetic variants associated with poor ICS or LTM response. The most consistent results were found for the FCER2 gene [encoding for a low-affinity IgE receptor (CD23)] and poor ICS response. Larger studies with well-phenotyped patients are needed to assess the clinical applicability of ICS and LTM pharmacogenomics/genetics.

The role of organic anion transporting polypeptides in drug absorption, distribution, excretion and drug-drug interactions

INTRODUCTION

The in vivo fate and effectiveness of a drug depends highly on its absorption, distribution, metabolism, excretion and toxicity (ADME-Tox). Organic anion transporting polypeptides (OATPs) are membrane proteins involved in the cellular uptake of various organic compounds, including clinically used drugs. Since OATPs are significant players in drug absorption and distribution, modulation of OATP function via pharmacotherapy with OATP substrates/inhibitors, or modulation of their expression, affects drug pharmacokinetics. Given their cancer-specific expression, OATPs may also be considered anticancer drug targets. Areas covered: We describe the human OATP family, discussing clinically relevant consequences of altered OATP function. We offer a critical analysis of published data on the role of OATPs in ADME and in drug-drug interactions, especially focusing on OATP1A2, 1B1, 1B3 and 2B1. Expert opinion: Four members of the OATP family, 1A2, 1B1, 1B3 and 2B1, have been characterized in detail. As biochemical and pharmacological knowledge on the other OATPs is lacking, it seems timely to direct research efforts towards developing the experimental framework needed to investigate the transport mechanism and substrate specificity of the poorly described OATPs. In addition, elucidating the role of OATPs in tumor development and therapy response are critical avenues for further research.

In Vitro Metabolism of Montelukast by Cytochrome P450s and UDP-Glucuronosyltransferases

Montelukast has been recommended as a selective in vitro and in vivo probe of cytochrome P450 (P450) CYP2C8 activity, but its selectivity toward this enzyme remains unclear. We performed detailed characterization of montelukast metabolism in vitro using human liver microsomes (HLMs), expressed P450s, and uridine 5'-diphospho-glucuronosyltransferases (UGTs). Kinetic and inhibition experiments performed at therapeutically relevant concentrations reveal that CYP2C8 and CYP2C9 are the principal enzymes responsible for montelukast 36-hydroxylation to 1,2-diol. CYP3A4 was the main catalyst of montelukast sulfoxidation and stereoselective 21-hydroxylation, and multiple P450s participated in montelukast 25-hydroxylation. We confirmed direct glucuronidation of montelukast to an acyl-glucuronide. We also identified a novel peak that appears consistent with an ether-glucuronide. Kinetic analysis in HLMs and experiments in expressed UGTs indicate that both metabolites were exclusively formed by UGT1A3. Comparison of in vitro intrinsic clearance in HLMs suggest that direct glucuronidation may play a greater role in the overall metabolism of montelukast than does P450-mediated oxidation, but the in vivo contribution of UGT1A3 needs further testing. In conclusion, our in vitro findings provide new insight toward montelukast metabolism. The utility of montelukast as a probe of CYP2C8 activity may be compromised owing to involvement of multiple P450s and UGT1A3 in its metabolism.

Montelukast, current indications and prospective future applications

INTRODUCTION

Montelukast is recommended for the treatment of asthma, exercise -induced bronchospasm and allergic rhinitis. Several trials demonstrated potential therapeutic effects in other respiratory conditions, and different animal-model-based studies explored potential pharmacological actions in non-respiratory conditions.

AREAS COVERED

Clinical investigations on the pharmacotherapeutic effects of montelukast, in addition to in-vivo studies on animal models of non-respiratory diseases. The data discussed in this review were mainly obtained from clinical randomized trials, real-life studies, and studies based on animal models as approve of concept. As a condition, all of the discussed articles were published in journals cited by Pubmed. Expert commentary: The current clinical data are in favor of montelukast use in the management of chronic asthma as an add-on or alternative therapy to the inhaled corticosteroids. Further clinical trials are required to confirm the effectiveness and feasibility of montelukast for the treatment of conditions other than the current clinical indications.

[Research advances in gene polymorphisms in biological pathways of drugs for asthma]

The studies on gene polymorphisms in biological pathways of the drugs for the treatment of asthma refer to the studies in which pharmacogenetic methods, such as genome-wide association studies, candidate gene studies, genome sequencing, admixture mapping analysis, and linkage disequilibrium, are used to identify, determine, and repeatedly validate the effect of one or more single nucleotide polymorphisms on the efficacy of drugs. This can provide therapeutic strategies with optimal benefits, least side effects, and lowest costs to patients with asthma, and thus realize individualized medicine. The common drugs for asthma are β2 receptor agonists, glucocorticoids, and leukotriene modifiers. This article reviews the research achievements in polymorphisms in biological pathways of the common drugs for asthma, hoping to provide guidance for pharmacogenetic studies on asthma in future and realize individualized medicine for patients with asthma soon.

Genome-wide association study of leukotriene modifier response in asthma

Heterogeneous therapeutic responses to leukotriene modifiers (LTMs) are likely due to variation in patient genetics. Although prior candidate gene studies implicated multiple pharmacogenetic loci, to date, no genome-wide association study (GWAS) of LTM response was reported. In this study, DNA and phenotypic information from two placebo-controlled trials (total N=526) of zileuton response were interrogated. Using a gene-environment (G × E) GWAS model, we evaluated 12-week change in forced expiratory volume in 1 second (ΔFEV1) following LTM treatment. The top 50 single-nucleotide polymorphism associations were replicated in an independent zileuton treatment cohort, and two additional cohorts of montelukast response. In a combined analysis (discovery+replication), rs12436663 in MRPP3 achieved genome-wide significance (P=6.28 × 10(-08)); homozygous rs12436663 carriers showed a significant reduction in mean ΔFEV1 following zileuton treatment. In addition, rs517020 in GLT1D1 was associated with worsening responses to both montelukast and zileuton (combined P=1.25 × 10(-07)). These findings implicate previously unreported loci in determining therapeutic responsiveness to LTMs.

Insights into solute carriers: physiological functions and implications in disease and pharmacokinetics

SLCs transport many endogenous and exogenous compounds including drugs; SLCs dysfunction has implications in pharmacokinetics, drug toxicity or lack of efficacy.

Montelukast Disposition: No Indication of Transporter-Mediated Uptake in OATP2B1 and OATP1B1 Expressing HEK293 Cells

Clinical studies with montelukast show variability in effect and polymorphic OATP2B1-dependent absorption has previously been implicated as a possible cause. This claim has been challenged with conflicting data and here we used OATP2B1-transfected HEK293 cells to clarify the mechanisms involved. For montelukast, no significant difference in cell uptake between HEK-OATP2B1 and empty vector cell lines was observed at pH 6.5 or pH 7.4, and no concentration-dependent uptake was detected. Montelukast is a carboxylic acid, a relatively potent inhibitor of OATP1B1, OATP1B3, and OATP2B1, and has previously been postulated to be actively transported into human hepatocytes. Using OATP1B1-transfected HEK293 cells and primary human hepatocytes in the presence of OATP inhibitors we demonstrate for the first time that active OATP-dependent transport is unlikely to play a significant role in the human disposition of montelukast.

Comparison of genetic variations of the SLCO1B1, SLCO1B3, and SLCO2B1 genes among five ethnic groups

Organic anion-transporting polypeptide (OATP; gene symbol, SLCO) transporters are generally involved in the uptake of multiple drugs and their metabolites at most epithelial barriers. The pattern of single-nucleotide polymorphisms (SNPs) in these transporters may be determinants of interindividual variability in drug disposition and response. The objective of this study was to define the distribution of SNPs of three SLCO genes, SLCO1B1, SLCO1B3, and SLCO2B1, in a Korean population and other ethnic groups. The study was screened using the Illumina GoldenGate assay for genomic DNA from 450 interethnic subjects, including 11 pharmacogenetic core variants and 76 HapMap tagging SNPs. The genotype distribution of the Korean population was similar to East Asian populations, but significantly different from African American and European American cohorts. These interethnic differences will be useful information for prospective studies, including genetic association and pharmacogenetic studies of drug metabolism by SLCO families.

Asthma Pharmacogenomics: 2015 Update

There is evidence that genetic factors are implicated in the observed differences in therapeutic responses to the common classes of asthma therapy such as β2-agonists, corticosteroids, and leukotriene modifiers. Pharmacogenomics explores the roles of genetic variation in drug response and continues to be a field of great interest in asthma therapy. Prior studies have focused on candidate genes and recently emphasized genome-wide association analyses. Newer integrative omics and system-level approaches have recently revealed novel understanding of drug response pathways. However, the current known genetic loci only account for a fraction of variability in drug response and ongoing research is needed. While the field of asthma pharmacogenomics is not yet fully translatable to clinical practice, ongoing research should hopefully achieve this goal in the near future buttressed by the recent precision medicine efforts in the USA and worldwide.

Genome-Wide Association Study Identifies Novel Pharmacogenomic Loci For Therapeutic Response to Montelukast in Asthma

Background

Genome-wide association study (GWAS) is a powerful tool to identify novel pharmacogenetic single nucleotide polymorphisms (SNPs). Leukotriene receptor antagonists (LTRAs) are a major class of asthma medications, and genetic factors contribute to variable responses to these drugs. We used GWAS to identify novel SNPs associated with the response to the LTRA, montelukast, in asthmatics.

Methods

Using genome-wide genotype and phenotypic data available from American Lung Association - Asthma Clinical Research Center (ALA-ACRC) cohorts, we evaluated 8-week change in FEV₁ related to montelukast administration in a discovery population of 133 asthmatics. The top 200 SNPs from the discovery GWAS were then tested in 184 additional samples from two independent cohorts.

Results

Twenty-eight SNP associations from the discovery GWAS were replicated. Of these, rs6475448 achieved genome-wide significance (combined P = 1.97 x 10^-09), and subjects from all four studies who were homozygous for rs6475448 showed increased ΔFEV₁ from baseline in response to montelukast.

Conclusions

Through GWAS, we identified a novel pharmacogenomic locus related to improved montelukast response in asthmatics.

Personalized medicine in children with asthma

Personalized medicine for children with asthma aims to provide a tailored management of asthma, which leads to faster and better asthma control, has less adverse events and may be cost saving. Several patient characteristics, lung function parameters and biomarkers have been shown useful in predicting treatment response or predicting successful reduction of asthma medication. As treatment response to the main asthma therapies is partly genetically determined, pharmacogenetics may open the way for personalized medicine in children with asthma. However, the number of genes identified for the various asthma drug response phenotypes remains small and randomized controlled trials are lacking. Biomarkers in exhaled breath or breath condensate remain promising but did not find their way from bench to bedside yet, except for the fraction of exhaled nitric oxide. E-health will most likely find its way to clinical practice and most interventions are at least non-inferior to usual care. More studies are needed on which interventions will benefit most individual children.

Asthma pharmacogenetics and the development of genetic profiles for personalized medicine

Human genetics research will be critical to the development of genetic profiles for personalized or precision medicine in asthma. Genetic profiles will consist of gene variants that predict individual disease susceptibility and risk for progression, predict which pharmacologic therapies will result in a maximal therapeutic benefit, and predict whether a therapy will result in an adverse response and should be avoided in a given individual. Pharmacogenetic studies of the glucocorticoid, leukotriene, and β₂-adrenergic receptor pathways have focused on candidate genes within these pathways and, in addition to a small number of genome-wide association studies, have identified genetic loci associated with therapeutic responsiveness. This review summarizes these pharmacogenetic discoveries and the future of genetic profiles for personalized medicine in asthma. The benefit of a personalized, tailored approach to health care delivery is needed in the development of expensive biologic drugs directed at a specific biologic pathway. Prior pharmacogenetic discoveries, in combination with additional variants identified in future studies, will form the basis for future genetic profiles for personalized tailored approaches to maximize therapeutic benefit for an individual asthmatic while minimizing the risk for adverse events.

Minireview: SLCO and ABC transporters: a role for steroid transport in prostate cancer progression

Androgens play a critical role in the development and progression of prostate cancer (PCa), and androgen deprivation therapy via surgical or medical castration is front-line therapy for patients with advanced PCa. However, intratumoral testosterone levels are elevated in metastases from patients with castration-resistant disease, and residual intratumoral androgens have been implicated in mediating ligand-dependent mechanisms of androgen receptor activation. The source of residual tissue androgens present despite castration has not been fully elucidated, but proposed mechanisms include uptake and conversion of adrenal androgens, such as dehdroepiandrosterone to testosterone and dihydrotestosterone, or de novo androgen synthesis from cholesterol or progesterone precursors. In this minireview, we discuss the emerging evidence that suggests a role for specific transporters in mediating transport of steroids into or out of prostate cells, thereby influencing intratumoral androgen levels and PCa development and progression. We focus on the solute carrier and ATP binding cassette gene families, which have the most published data for a role in PCa-related steroid transport, and review the potential impact of genetic variation on steroid transport activity and PCa outcomes. Continued assessment of transport activity in PCa models and human tumor tissue is needed to better delineate the different roles these transporters play in physiologic and neoplastic settings, and in order to determine whether targeting the uptake of steroid substrates by specific transporters may be a clinically feasible therapeutic strategy.

The intestinal absorption of folates

The properties of intestinal folate absorption were documented decades ago. However, it was only recently that the proton-coupled folate transporter (PCFT) was identified and its critical role in folate transport across the apical brush-border membrane of the proximal small intestine established by the loss-of-function mutations identified in the PCFT gene in subjects with hereditary folate malabsorption and, more recently, by the Pcft-null mouse. This article reviews the current understanding of the properties of PCFT-mediated transport and how they differ from those of the reduced folate carrier. Other processes that contribute to the transport of folates across the enterocyte, along with the contribution of the enterohepatic circulation, are considered. Important unresolved issues are addressed, including the mechanism of intestinal folate absorption in the absence of PCFT and regulation of PCFT gene expression. The impact of a variety of ions, organic molecules, and drugs on PCFT-mediated folate transport is described.

Design and anticipated outcomes of the eMERGE-PGx project: a multicenter pilot for preemptive pharmacogenomics in electronic health record systems

We describe here the design and initial implementation of the eMERGE-PGx project. eMERGE-PGx, a partnership of the eMERGE and PGRN consortia, has three objectives : 1) Deploy PGRNseq, a next-generation sequencing platform assessing sequence variation in 84 proposed pharmacogenes, in nearly 9,000 patients likely to be prescribed drugs of interest in a 1–3 year timeframe across several clinical sites; 2) Integrate well-established clinically-validated pharmacogenetic genotypes into the electronic health record with associated clinical decision support and assess process and clinical outcomes of implementation; and 3) Develop a repository of pharmacogenetic variants of unknown significance linked to a repository of EHR-based clinical phenotype data for ongoing pharmacogenomics discovery. We describe site-specific project implementation and anticipated products, including genetic variant and phenotype data repositories, novel variant association studies, clinical decision support modules, clinical and process outcomes, approaches to manage incidental findings, and patient and clinician education methods.

Towards structural systems pharmacology to study complex diseases and personalized medicine

Genome-Wide Association Studies (GWAS), whole genome sequencing, and high-throughput omics techniques have generated vast amounts of genotypic and molecular phenotypic data. However, these data have not yet been fully explored to improve the effectiveness and efficiency of drug discovery, which continues along a one-drug-one-target-one-disease paradigm. As a partial consequence, both the cost to launch a new drug and the attrition rate are increasing. Systems pharmacology and pharmacogenomics are emerging to exploit the available data and potentially reverse this trend, but, as we argue here, more is needed. To understand the impact of genetic, epigenetic, and environmental factors on drug action, we must study the structural energetics and dynamics of molecular interactions in the context of the whole human genome and interactome. Such an approach requires an integrative modeling framework for drug action that leverages advances in data-driven statistical modeling and mechanism-based multiscale modeling and transforms heterogeneous data from GWAS, high-throughput sequencing, structural genomics, functional genomics, and chemical genomics into unified knowledge. This is not a small task, but, as reviewed here, progress is being made towards the final goal of personalized medicines for the treatment of complex diseases.

Genetic predictors associated with improvement of asthma symptoms in response to inhaled corticosteroids

BACKGROUND

To date, genome-wide association studies (GWASs) of inhaled corticosteroid (ICS) response in asthmatic patients have focused primarily on lung function and exacerbations.

OBJECTIVE

We hypothesized that GWAS analysis could identify novel genetic markers predicting a symptomatic response to ICSs.

METHODS

We analyzed differences in asthma symptoms in response to ICSs in 124 white children from the Childhood Asthma Management Program (CAMP) trial using scores from diary cards. Of the 440,862 single nucleotide polymorphisms (SNPs) analyzed, the top 100 ranked SNPs were pursued for replication initially in subjects from the pediatric Childhood Asthma Research and Education trials (77 white children) and then in subjects from the adult Asthma Clinical Research Network (110 white adults) and Leukotriene Modifier or Corticosteroid or Corticosteroid-Salmeterol trials (110 white adults).

RESULTS

The lowest P value for GWAS analysis in the CAMP trial was 8.94 × 10(-8) (rs2388639). Of the 60 SNPs available in the Childhood Asthma Research and Education Network trials, rs1558726 (combined P = 1.02 × 10(-5)), rs2388639 (combined P = 8.56 × 10(-9)), and rs10044254 (combined P = 9.16 × 10(-8)) independently replicated. However, these 3 SNPs were not additionally replicated in the adult asthmatic patients of the remaining trials. rs10044254 lies in the intronic region of F-box and leucine-rich repeat protein 7 (FBXL7) and is associated with decreased expression in immortalized B cells derived from CAMP participants.

CONCLUSIONS

We have identified a novel SNP, rs10044254, associated with both decreased expression of FBXL7 and improved symptomatic response to ICSs in 2 independent pediatric cohorts. Our results suggest that there might be a specific genetic mechanism regulating symptomatic response to ICSs in children that does not carry over to adults.

Case study 4. Predicting the drug interaction potential for inhibition of CYP2C8 by montelukast

Predicting Drug-Drug Interactions (DDIs) from in vitro data is made difficult by not knowing concentrations of substrate and inhibitor at the target site. For in vivo targets, this is understandable, since intracellular concentrations can differ from extracellular concentrations. More vexing is that the concentration of the drug at the target for some in vitro assays can also be unknown. This uncertainty has resulted in standard in vitro practices that cannot accurately predict human pharmacokinetics. This case study highlights the impact of drug distribution, both in vitro and in vivo, with the example of the drug interaction potential of montelukast.

Organic anion-transporting polypeptides: a novel approach for cancer therapy

Organic anion-transporting polypeptides (OATPs) encoded by the SLCO genes constitute an important transporter superfamily that mediates transmembrane transport of various clinical drugs and endogenous nutrients. Eleven human OATPs with different transport functions are expressed in various tissues. Bile acids, steroid hormone conjugates, prostaglandins, testosterone and thyroid hormones that promote cell proliferation are typical substrates of OATPs. Many important clinical drugs have been identified as substrates of OATP1B1, OATP1B3, OATP2B1 and OATP1A2. Liver-specific OATP1B1 and OATP1B3 as well as testis-specific OATP6A1 are expressed in malignancies and can act as biomarkers for many tumours. Various studies have shown the associations of genetic polymorphisms in OATP genes with the uptake pharmacokinetics of their substrates. Because of their abundant expression in tumours and their high transport activity for many cancer drugs, OATPs should be considered as important therapeutic targets in anti-cancer drug design.

Effects of polymorphisms of the SLCO2B1 transporter gene on the pharmacokinetics of montelukast in humans

Montelukast, a leukotriene receptor antagonist, is a substrate of organic anion transporting OATP2B1 encoded by the SLCO2B1. We evaluated the effects of six non-synonymous (c.1175C>T, c.1457C>T, c.43C>T, c.935G>A, c.601G>A, and c.644A>T) polymorphisms and one promoter (g.-282G>A) polymorphism on the pharmacokinetics of montelukast. A single dose of 10 mg montelukast was administered in 24 healthy subjects. Its levels were measured up to 24 hours and a pharmacokinetic analysis was performed based on the SLCO2B1 polymorphisms. We did not encounter subjects with c.1175C>T, c.43C>T, or c.644A>T polymorphisms. The remaining SLCO2B1 polymorphisms did not affect plasma levels of montelukast, and pharmacokinetic parameters of montelukast did not differ among genotype groups. Oral clearance results were as follows: (1) 3.3 L/h for c.935GG, 3.0 L/h for c.935GA, and 3.5 L/h for c.935AA; (2) 3.4 L/h for c.1457CC, 2.9 L/h for c.1457CT, and 3.2 L/h for c.1457TT; (3) 3.2 L/h for c.601GG, 3.4 L/h for c.601GA, and 3.4 L/h for c.601AA; (4) 3.2 L/h for g.-282GG, 3.4 L/h for g.-282GA, and 3.2 L/h for g.-282AA. The findings suggest that SLCO2B1 polymorphisms do not affect the pharmacokinetics of montelukast and that SLCO2B1 polymorphisms appear to be a minor determinant of inter-individual variability of montelukast.

Role of biomarkers in understanding and treating children with asthma: towards personalized care

Asthma is one of the most common chronic diseases affecting children. Despite publicized expert panels on asthma management and the availability of high-potency inhaled corticosteroids, asthma continues to pose an enormous burden on quality of life for children. Research into the genetic and molecular origins of asthma are starting to show how distinct disease entities exist within the syndrome of "asthma". Biomarkers can be used to diagnose underlying molecular mechanisms that can predict the natural course of disease or likely response to drug treatment. The progress of personalized medicine in the care of children with asthma is still in its infancy. We are not yet able to apply stratified asthma treatments based on molecular phenotypes, although that time may be fast approaching. This review discusses some of the recent advances in asthma genetics and the use of current biomarkers that can help guide improved treatment. For example, the fraction of expired nitric oxide and serum Immunoglobulin E (IgE) (including allergen-specific IgE), when evaluated in the context of recurrent asthma symptoms, are general predictors of allergic airway inflammation. Biomarker assays for secondhand tobacco smoke exposure and cysteinyl leukotrienes are both promising areas of study that can help personalize management, not just for pharmacologic management, but also education and prevention efforts.

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

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

SLCO2B1 c.935G>A single nucleotide polymorphism has no effect on the pharmacokinetics of montelukast and aliskiren

OBJECTIVE

A nonsynonymous single nucleotide polymorphism (SNP) in the SLCO2B1 gene encoding organic anion transporting polypeptide 2B1 (OATP2B1), c.935G>A (p.R312Q; rs12422149), has been associated with reduced plasma concentrations of montelukast in patients with asthma. Our aim was to examine the possible effects of the SLCO2B1 c.935G>A SNP on the single-dose pharmacokinetics of the suggested OATP2B1 substrates montelukast and aliskiren.

METHODS

Sixteen healthy volunteers with the SLCO2B1 c.935GG genotype, 12 with the c.935GA genotype, and five with the c.935AA genotype ingested a single 10 mg dose of montelukast or a 150 mg dose of aliskiren, with a washout period of 1 week. Plasma montelukast concentrations were measured up to 24 h. Plasma and urine aliskiren concentrations were measured up to 72 and 12 h, respectively, and plasma renin activity up to 24 h after aliskiren intake.

RESULTS

The SLCO2B1 genotypes had no significant effect on the pharmacokinetics of montelukast or aliskiren. The geometric mean ratios with 90% confidence intervals of montelukast area under the plasma concentration-time curve from 0 h to infinity (AUC(0-∞)) in participants with the c.935GA or the c.935AA genotype to those with the c.935GG genotype were 1.02 (0.87, 1.21) or 0.88 (0.71, 1.10), respectively (P=0.557). The geometric mean ratios (90% confidence interval) of aliskiren AUC(0-∞) in participants with the c.935GA or the c.935AA genotype to those with the c.935GG genotype were 0.98 (0.74, 1.30) or 1.24 (0.85, 1.80), respectively (P=0.576).

CONCLUSION

These data do not support the suggested functional significance of the SLCO2B1 c.935G>A SNP on OATP2B1 activity in vivo.

Integrative systems biology approaches in asthma pharmacogenomics

In order to improve therapeutic outcomes, there is a tremendous need to identify patients who are likely to respond to a given asthma treatment. Pharmacogenomic studies have explained a portion of the variability in drug response and provided an increasing list of candidate genes and SNPs. However, as phenotypic variation arises from a network of complex interactions among genetic and environmental factors, rather than individual genes or SNPs, a multidisciplinary, systems-level approach is required in order to understand the inter-relationships among these factors. Systems biology, which seeks to capture interactions between genetic factors and other variables, offers a promising approach to improved therapeutic outcomes in asthma. This aritcle will review and update progress in the pharmacogenomics of asthma and then discuss the application of systems biology approaches to asthma pharmacogenomics.

SLCO2B1 genetic polymorphisms in a Korean population: pyrosequencing analyses and comprehensive comparison with other populations

SLCO2B1, also known as OATP2B1 (Organic Anion Transporter) or OATP-B or SLC21A9, is an organic anion uptake transporter that is encoded by the SLCO2B1 gene. In this study we assessed the frequencies of SLCO2B1 polymorphisms in a Korean population using newly developed pyrosequencing methods and compared their frequencies with those in other ethnic groups. We developed pyrosequencing methods to identify the following six SLCO2B1 non-synonymous polymorphisms: c.1175C > T (rs1621378), c.1457C > T (rs2306168), c.43C > T (rs56837383), c.935G > A (rs12422149), c.601G > A (rs35199625) and c.644A > T (rs72559740). The allele frequencies of these polymorphisms were analyzed in 227 Korean subjects. The allele frequencies of SLCO2B1 polymorphisms in the population tested were as follows: 0.0 for c.1175C > T, c.43C > T and c.644A > T; 0.2687 for c.1457C > T; 0.4273 for c.935G > A; and 0.0727 for c. 601G > A. Even though the allele frequencies of the c.1175C > T and c.1457C > T polymorphisms were comparable to those in Japanese subjects, the frequencies in this Korean population differed from those in other ethnic groups. The developed pyrosequencing methods are rapid and reliable for detecting non-synonymous SLCO2B1 polymorphisms. Large ethnic differences in the frequency of SLCO2B1 genetic polymorphisms were noted among ethnic groups. The SLCO2B1 polymorphisms at c.1175C > T, c.43C > T and c.644A > T were not found in the Korean population while c.1457C > T, c.935G > A and c.601G > A exhibited mostly higher frequencies in Koreans compared with Finnish, Caucasian and African-American populations.

Interaction of silymarin flavonolignans with organic anion-transporting polypeptides

Organic anion-transporting polypeptides (OATPs) are multispecific transporters mediating the uptake of endogenous compounds and xenobiotics in tissues that are important for drug absorption and elimination, including the intestine and liver. Silymarin is a popular herbal supplement often used by patients with chronic liver disease; higher oral doses than those customarily used (140 mg three times/day) are being evaluated clinically. The present study examined the effect of silymarin flavonolignans on OATP1B1-, OATP1B3-, and OATP2B1-mediated transport in cell lines stably expressing these transporters and in human hepatocytes. In overexpressing cell lines, OATP1B1- and OATP1B3-mediated estradiol-17β-glucuronide uptake and OATP2B1-mediated estrone-3-sulfate uptake were inhibited by most of the silymarin flavonolignans investigated. OATP1B1-, OATP1B3-, and OATP2B1-mediated substrate transport was inhibited efficiently by silymarin (IC₅₀ values of 1.3, 2.2 and 0.3 µM, respectively), silybin A (IC₅₀ values of 9.7, 2.7 and 4.5 µM, respectively), silybin B (IC₅₀ values of 8.5, 5.0 and 0.8 µM, respectively), and silychristin (IC₅₀ values of 9.0, 36.4, and 3.6 µM, respectively). Furthermore, silymarin, silybin A, and silybin B (100 µM) significantly inhibited OATP-mediated estradiol-17β-glucuronide and rosuvastatin uptake into human hepatocytes. Calculation of the maximal unbound portal vein concentrations/IC₅₀ values indicated a low risk for silymarin-drug interactions in hepatic uptake with a customary silymarin dose. The extent of silymarin-drug interactions depends on OATP isoform specificity and concentrations of flavonolignans at the site of drug transport. Higher than customary doses of silymarin, or formulations with improved bioavailability, may increase the risk of flavonolignan interactions with OATP substrates in patients.

Targeted drug delivery to treat pain and cerebral hypoxia

Limited drug penetration is an obstacle that is often encountered in treatment of central nervous system (CNS) diseases including pain and cerebral hypoxia. Over the past several years, biochemical characteristics of the brain (i.e., tight junction protein complexes at brain barrier sites, expression of influx and efflux transporters) have been shown to be directly involved in determining CNS permeation of therapeutic agents; however, the vast majority of these studies have focused on understanding those mechanisms that prevent drugs from entering the CNS. Recently, this paradigm has shifted toward identifying and characterizing brain targets that facilitate CNS drug delivery. Such targets include the organic anion-transporting polypeptides (OATPs in humans; Oatps in rodents), a family of sodium-independent transporters that are endogenously expressed in the brain and are involved in drug uptake. OATP/Oatp substrates include drugs that are efficacious in treatment of pain and/or cerebral hypoxia (i.e., opioid analgesic peptides, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors). This clearly suggests that OATP/Oatp isoforms are viable transporter targets that can be exploited for optimization of drug delivery to the brain and, therefore, improved treatment of CNS diseases. This review summarizes recent knowledge in this area and emphasizes the potential that therapeutic targeting of OATP/Oatp isoforms may have in facilitating CNS drug delivery and distribution. Additionally, information presented in this review will point to novel strategies that can be used for treatment of pain and cerebral hypoxia.

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

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

Fruit juices as perpetrators of drug interactions: the role of organic anion-transporting polypeptides

Grapefruit juice is widely recognized to cause important drug interactions via inhibition of CYP3A4, and a wider variety of fruit juices have been shown to inhibit influx transporters in enterocytes known as organic anion-transporting polypeptides (OATPs). Fruit juice coadministration significantly reduces the oral bioavailability of numerous important medicines relying on this anion transporter pathway for absorption. This article reviews the current literature on interactions between clinically used OATP substrates and fruit juice consumption.

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

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

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

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

Mechanisms underlying food-drug interactions: inhibition of intestinal metabolism and transport

Food-drug interaction studies are critical to evaluate appropriate dosing, timing, and formulation of new drug candidates. These interactions often reflect prandial-associated changes in the extent and/or rate of systemic drug exposure. Physiologic and physicochemical mechanisms underlying food effects on drug disposition are well-characterized. However, biochemical mechanisms involving drug metabolizing enzymes and transport proteins remain underexplored. Several plant-derived beverages have been shown to modulate enzymes and transporters in the intestine, leading to altered pharmacokinetic (PK) and potentially negative pharmacodynamic (PD) outcomes. Commonly consumed fruit juices, teas, and alcoholic drinks contain phytochemicals that inhibit intestinal cytochrome P450 and phase II conjugation enzymes, as well as uptake and efflux transport proteins. Whereas myriad phytochemicals have been shown to inhibit these processes in vitro, translation to the clinic has been deemed insignificant or undetermined. An overlooked prerequisite for elucidating food effects on drug PK is thorough knowledge of causative bioactive ingredients. Substantial variability in bioactive ingredient composition and activity of a given dietary substance poses a challenge in conducting robust food-drug interaction studies. This confounding factor can be addressed by identifying and characterizing specific components, which could be used as marker compounds to improve clinical trial design and quantitatively predict food effects. Interpretation and integration of data from in vitro, in vivo, and in silico studies require collaborative expertise from multiple disciplines, from botany to clinical pharmacology (i.e., plant to patient). Development of more systematic methods and guidelines is needed to address the general lack of information on examining drug-dietary substance interactions prospectively.

Genetic basis for personalized medicine in asthma

There is heterogeneity in patient responses to current asthma medications. Significant progress has been made identifying genetic polymorphisms that influence the efficacy and potential for adverse effects to asthma drugs, including; β(2)-adrenergic receptor agonists, corticosteroids and leukotriene modifiers. Pharmacogenetics holds great promise to maximise clinical outcomes and minimize adverse effects. Asthma is heterogeneous with respect to clinical presentation and inflammatory mechanisms underlying the disease, which is likely to contribute to variable results in clinical trials targeting specific inflammatory mediators. Genome-wide association studies have begun to identify genes underlying asthma (e.g., IL1RL1), which represent future therapeutic targets. In this article, we review and update the pharmacogenetics of current asthma therapies and discuss the genetics underlying selected Phase II and future targets.

Pharmacogenetics of drug transporters in the enterohepatic circulation

This article summarizes the impact of the pharmacogenetics of drug transporters expressed in the enterohepatic circulation on the pharmacokinetics and pharmacodynamics of drugs. The role of pharmacogenetics in the function of drug transporter proteins in vitro is now well established and evidence is rapidly accumulating from in vivo pharmacokinetic studies, which suggests that genetic variants of drug transporter proteins can translate into clinically relevant phenotypes. However, a large amount of conflicting information on the clinical relevance of drug transporter proteins has so far precluded the emergence of a clear picture regarding the role of drug transporter pharmacogenetics in medical practice. This is very well exemplified by the case of P-glycoprotein (MDR1, ABCB1). The challenge is now to develop pharmacogenetic models with sufficient predictive power to allow for translation into drug therapy. This will require a combination of pharmacogenetics of drug transporters, drug metabolism and pharmacodynamics of the respective drugs.

Systems pharmacology: network analysis to identify multiscale mechanisms of drug action

Systems approaches have long been used in pharmacology to understand drug action at the organ and organismal levels. The application of computational and experimental systems biology approaches to pharmacology allows us to expand the definition of systems pharmacology to include network analyses at multiple scales of biological organization and to explain both therapeutic and adverse effects of drugs. Systems pharmacology analyses rely on experimental "omics" technologies that are capable of measuring changes in large numbers of variables, often at a genome-wide level, to build networks for analyzing drug action. A major use of omics technologies is to relate the genomic status of an individual to the therapeutic efficacy of a drug of interest. Combining pathway and network analyses, pharmacokinetic and pharmacodynamic models, and a knowledge of polymorphisms in the genome will enable the development of predictive models of therapeutic efficacy. Network analyses based on publicly available databases such as the U.S. Food and Drug Administration's Adverse Event Reporting System allow us to develop an initial understanding of the context within which molecular-level drug-target interactions can lead to distal effectors in a process that results in adverse phenotypes at the organ and organismal levels. The current state of systems pharmacology allows us to formulate a set of questions that could drive future research in the field. The long-term goal of such research is to develop polypharmacology for complex diseases and predict therapeutic efficacy and adverse event risk for individuals prior to commencement of therapy.

Oral drug delivery utilizing intestinal OATP transporters

Transporters play important roles in tissue distribution and urinary- and biliary-excretion of drugs and transporter molecules involved in those processes have been elucidated well. Furthermore, an involvement of efflux transporters such as P-glycoproteins, multidrug resistance associated protein 2, and breast cancer resistance protein as the intestinal absorption barrier and/or intestinal luminal secretion mechanisms has been demonstrated. However, although there are many suggestions for the contribution of uptake/influx transporters in intestinal absorption of drugs, information on the transporter molecules responsible for the intestinal absorptive process is limited. Among them, most studied absorptive drug transporter is peptide transporter PEPT1. However, utilization of PEPT1 for oral delivery of drugs may not be high due to the chemical structural requirement of PEPT1 limited to peptide-mimetics. Recently, organic anion transporting polypeptide (OATP) family such as OATP1A2 and OATP2B1 has been suggested to mediate intestinal absorption of several drugs. Since OATPs exhibit species difference in expressed tissues and functional properties between human and animals, human studies are essential to clarify the intestinal absorption mechanisms of drugs via OATPs. Recent pharmacogenomic studies demonstrated that OATP2B1 is involved in the drug absorption in human. In addition, information of drug-juice interaction in the intestine also uncovered the contribution of OATP1A2 and OATP2B1 in drug absorption. Since OATP1A2 and OATP2B1 exhibit broader substrate selectivity compared with PEPT1, their potential to be applied for oral delivery should be high. In this review, current understanding of characteristics and contribution as the absorptive transporters of OATPs in small intestine in human is described. Now, it is getting clearer that OATPs have significant roles in intestinal absorption of drugs, therefore, there are higher possibility to utilize OATPs as the tools for oral delivery.