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

CYP2C8 but not CYP3A4 is important in the pharmacokinetics of montelukast

AIM

According to product information, montelukast is extensively metabolized by CYP3A4 and CYP2C9. However, CYP2C8 was also recently found to be involved. Our aim was to study the effects of selective CYP2C8 and CYP3A4 inhibitors on the pharmacokinetics of montelukast.

METHODS

In a randomized crossover study, 11 healthy subjects ingested gemfibrozil 600 mg, itraconazole 100 mg (first dose 200 mg) or both, or placebo twice daily for 5 days, and on day 3, 10 mg montelukast. Plasma concentrations of montelukast, gemfibrozil, itraconazole and their metabolites were measured up to 72 h.

RESULTS

The CYP2C8 inhibitor gemfibrozil increased the AUC(0,∞) of montelukast 4.3-fold and its t(1/2) 2.1-fold (P < 0.001). Gemfibrozil impaired the formation of the montelukast primary metabolite M6, reduced the AUC and C(max) of the secondary (major) metabolite M4 by more than 90% (P < 0.05) and increased those of M5a and M5b (P < 0.05). The CYP3A4 inhibitor itraconazole had no significant effect on the pharmacokinetic variables of montelukast or its M6 and M4 metabolites, but markedly reduced the AUC and C(max) of M5a and M5b (P < 0.05). The effects of the gemfibrozil-itraconazole combination on the pharmacokinetics of montelukast did not differ from those of gemfibrozil alone.

CONCLUSIONS

CYP2C8 is the dominant enzyme in the biotransformation of montelukast in humans, accounting for about 80% of its metabolism. CYP3A4 only mediates the formation of the minor metabolite M5a/b, and is not important in the elimination of montelukast. Montelukast may serve as a safe and useful CYP2C8 probe drug.

Asthma outcomes: symptoms

BACKGROUND

Respiratory symptoms are commonly used to assess the impact of patient-centered interventions.

OBJECTIVE

At the request of National Institutes of Health (NIH) institutes and other federal agencies, an expert group was convened to propose which measurements of asthma symptoms should be used as a standardized measure in future clinical research studies.

METHODS

Asthma symptom instruments were classified as daily diaries (prospectively recording symptoms between research visits) or retrospective questionnaires (completed at research visits). We conducted a systematic search in PubMed and a search for articles that cited key studies describing development of instruments. We classified outcome instruments as either core (required in future studies), supplemental (used according to study aims and standardized), or emerging (requiring validation and standardization). This work was discussed at an NIH-organized workshop in March 2010 and finalized in September 2011.

RESULTS

Four instruments (3 daily diaries, 1 for adults and 2 for children; and 1 retrospective questionnaire for adults) were identified. Minimal clinically important differences have not been established for these instruments, and validation studies were only conducted in a limited number of patient populations. Validity of existing instruments may not be generalizable across racial-ethnic or other subgroups.

CONCLUSIONS

An evaluation of symptoms should be a core asthma outcome measure in clinical research. However, available instruments have limitations that preclude selection of a core instrument. The working group participants propose validation studies in diverse populations, comparisons of diaries versus retrospective questionnaires, and evaluations of symptom assessment alone versus composite scores of asthma control.

New approaches to personalized medicine for asthma: where are we?

Access to an electronic medical record is essential for personalized medicine. Currently, only 40% of US physicians have such access, but this is rapidly changing. It is expected that 100,000 Americans will have their whole genome sequenced in 2012. The cost of such sequencing is rapidly dropping, and is estimated to be $1000 by 2013. These technological advances will make interpretation of whole genome sequence data a major clinical challenge for the foreseeable future. At present, a relatively small number of genes have been identified to determine drug treatment response phenotypes for asthma. It is anticipated that this will dramatically increase over the next 10 years as personalized medicine becomes more of a reality for asthma patients.

Effect of clarithromycin and fluconazole on the pharmacokinetics of montelukast in human volunteers

OBJECTIVE

Montelukast, a leukotriene receptor antagonist, is used in the treatment of asthma. The objective of the study reported here was to determine whether multiple doses of clarithromycin or fluconazole affect the pharmacokinetics of montelukast.

METHODS

This was a four-phase cross-over study with a washout period of 2 weeks between phases. In phase 1, 12 volunteers received a single oral dose of 10 mg montelukast. In phase 2, the volunteers received a single, oral dose of 1,000 mg clarithromycin once daily for 2 days, followed by, on day 3, a single oral dose of 10 mg montelukast co-administered with clarithromycin. In phase 3, a single oral dose of 50 mg fluconazole was given once daily for 6 days, followed by, on day 7, a single oral dose of 10 mg montelukast co-administered with 50 mg fluconazole. In the last phase (phase 4), a single oral dose of 150 mg fluconazole was given once daily for 6 days, followed by, on day 7, a single oral dose of 10 mg montelukast co-administered with 150 mg fluconazole. The plasma concentration of montelukast was measured by high performance liquid chromatography for 24 h.

RESULTS

Following clarithromycin co-administration, the area under the concentration-time curve from zero to infinity ( AUC(0-∞)) of montelukast increased by 144% [90% confidence interval (CI) 2.03-2.86]. The co-administration of a single oral dose of 150 and 50 mg fluconazole decreased the montelukast AUC(0-∞) by 30.7 (90% CI 0.53-0.81) and 38.8% (90% CI 0.57-0.69), respectively.

CONCLUSIONS

Clarithromycin increased the plasma concentrations of montelukast whereas fluconazole reduced the plasma concentrations of montelukast. The mechanism of the interaction is probably due to interference of the interacting drugs with transporters mediating the uptake of montelukast.

Genetic polymorphisms of OATP transporters and their impact on intestinal absorption and hepatic disposition of drugs

There is convincing evidence that many organic anion transporting polypeptide (OATP) transporters influence the pharmacokinetics and pharmacological efficacy of their substrate drugs. Each OATP family member has a unique combination of tissue distribution, substrate specificity and mechanisms of gene expression. Among them, OATP1B1, OATP1B3 and OATP2B1 have been considered as critical molecular determinants of the pharmacokinetics of a variety of clinically important drugs. Liver-specific expression of OATP1B1 and OATP1B3 contributes to the hepatic uptake of drugs from the portal vein, and OATP2B1 may alter their intestinal absorption as well as hepatic extraction. Accordingly, changes in function and expression of these three OATPs owing to genetic polymorphisms may lead to altered pharmacological effects, including decreased drug efficacy and increased risk of adverse effects. Association of genetic polymorphisms in OATP genes with alterations in the pharmacokinetic properties of their substrate drugs has been reported; however, there still exists a degree of discordance between the reported outcomes in different clinical settings. For better understanding of the clinical relevance of genetic polymorphisms of OATP1B1, OATP1B3 and OATP2B1, the present review focuses on the association of the genotypes of these OATPs with in vitro activity changes and in vivo clinical outcomes of substrate drugs.

Functions of OATP1A and 1B transporters in vivo: insights from mouse models

Organic anion-transporting polypeptides (OATPs) are a superfamily of uptake transporters that mediate the cellular uptake of a broad range of endogenous and exogenous compounds. Of these OATP transporters, members of the 1A and 1B subfamilies have broad substrate specificities. Because they are mainly expressed in liver, kidney and small intestine, OATP1A and 1B transporters can have a major impact on the pharmacokinetics of many drugs. To study their role in physiology and drug disposition, several mouse models lacking functional expression of one or more OATPs have been generated. This review discusses recent findings for these models that have led to new insights into the impact of OATP1A and 1B transporters on pharmacokinetics and toxicokinetics, and on bilirubin detoxification and bile acid handling in normal liver physiology.

The pharmacogenetics and pharmacogenomics of asthma therapy

Despite the availability of several classes of asthma medications and their overall effectiveness, a significant portion of patients fail to respond to these therapeutic agents. Evidence suggests that genetic factors may partly mediate the heterogeneity in asthma treatment response. This review discusses important findings in asthma pharmacogenetic and pharmacogenomic studies conducted to date, examines limitations of these studies and, finally, proposes future research directions in this field. The focus will be on the three major classes of asthma medications: β-adrenergic receptor agonists, inhaled corticosteroids and leukotriene modifiers. Although many studies are limited by small sample sizes and replication of the findings is needed, several candidate genes have been identified. High-throughput technologies are also allowing for large-scale genetic investigations. Thus, the future is promising for a personalized treatment of asthma, which will improve therapeutic outcomes, minimize side effects and lead to a more cost-effective care.

Drug transporters in drug efficacy and toxicity

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

Time-effect of montelukast on protection against exercise-induced bronchoconstriction

INTRODUCTION

Montelukast has been proven to assure a protective effect against exercise-induced bronchoconstriction.

AIM

To verify exactly when montelukast begins protection in asthmatic children by evaluating different time intervals between dosing and challenge.

METHODS

In a double blind, placebo-controlled, three day doses, crossover study, patients were randomized to receive in sequence treatment with either a placebo or montelukast and assigned to one of seven groups that were tested 1, 2, 3, 4, 5, 6 and 8 h after drug administration, respectively. For each group, the exercise challenge was always performed at the same hour on the first and third days of treatment.

RESULTS

Sixty-nine asthmatic children took part in the study. On day 3, the mean FEV(1) % fall from baseline was 25.54 (95% CI = 21.63/29.46) and 14.89 (95% CI = 11.85/17.92) for the placebo and active drug (p < 0.05), respectively. On day 1, the mean fall of FEV(1) was 28.20 (95% CI = 24.46/31.94) and 19.01 (95% CI = 15.71/22.31) for the placebo and montelukast (p < 0.05), respectively. Clinical protection was achieved in 21 (30%) and 33 (48%) subjects by montelukast on the first and third days, respectively.

CONCLUSIONS

Montelukast assured protection against exercise-induced bronchoconstriction from the first through the eighth hour from the first day of treatment. However, individual susceptibility to protection was evident since some individuals were not protected at any time. We conclude that in clinical use individual responses to the drug should be carefully evaluated in the follow-up management.

SLCO2B1 and SLCO1B3 may determine time to progression for patients receiving androgen deprivation therapy for prostate cancer

PURPOSE

Androgen deprivation therapy (ADT), an important treatment for advanced prostate cancer, is highly variable in its effectiveness. We hypothesized that genetic variants of androgen transporter genes, SLCO2B1 and SLCO1B3, may determine time to progression on ADT.

PATIENTS AND METHODS

A cohort of 538 patients with prostate cancer treated with ADT was genotyped for SLCO2B1 and SLCO1B3 single nucleotide polymorphisms (SNP). The biologic function of a SLCO2B1 coding SNP in transporting androgen was examined through biochemical assays.

RESULTS

Three SNPs in SLCO2B1 were associated with time to progression (TTP) on ADT (P < .05). The differences in median TTP for each of these polymorphisms were about 10 months. The SLCO2B1 genotype, which allows more efficient import of androgen, enhances cell growth and is associated with a shorter TTP on ADT. Patients carrying both SLCO2B1 and SLCO1B3 genotypes, which import androgens more efficiently, exhibited a median 2-year shorter TTP on ADT, demonstrating a gene-gene interaction (P(interaction) = .041).

CONCLUSION

Genetic variants of SLCO2B1 and SLCO1B3 may function as pharmacogenomic determinants of resistance to ADT in prostate cancer.

Influence of dietary substances on intestinal drug metabolism and transport

Successful delivery of promising new chemical entities via the oral route is rife with challenges, some of which cannot be explained or foreseen during drug development. Further complicating an already multifaceted problem is the obvious, yet often overlooked, effect of dietary substances on drug disposition and response. Some dietary substances, particularly fruit juices, have been shown to inhibit biochemical processes in the intestine, leading to altered pharmacokinetic (PK), and potentially pharmacodynamic (PD), outcomes. Inhibition of intestinal CYP3Amediated metabolism is the major mechanism by which fruit juices, including grapefruit juice, enhances systemic exposure to new and already marketed drugs. Inhibition of intestinal non-CYP3A enzymes and apically-located transport proteins represent recently identified mechanisms that can alter PK and PD. Several fruit juices have been shown to inhibit these processes in vitro, but some interactions have not translated to the clinic. The lack of in vitroin vivo concordance is due largely to a lack of rigorous methods to elucidate causative ingredients prior to clinical testing. Identification of specific components and underlying mechanisms is challenging, as dietary substances frequently contain multiple, often unknown, bioactive ingredients that vary in composition and bioactivity. A translational research approach, combining expertise from clinical pharmacologists and natural products chemists, is needed to develop robust models describing PK/PD relationships between a given dietary substance and drug of interest. Validation of these models through well-designed clinical trials would facilitate development of common practice guidelines for managing drug-dietary substance interactions appropriately.

Asthma in sickle cell disease: implications for treatment

Objective. To review issues related to asthma in sickle cell disease and management strategies. Data Source. A systematic review of pertinent original research publications, reviews, and editorials was undertaken using MEDLlNE, the Cochrane Library databases, and CINAHL from 1947 to November 2010. Search terms were [asthma] and [sickle cell disease]. Additional publications considered relevant to the sickle cell disease population of patients were identified; search terms included [sickle cell disease] combined with [acetaminophen], [pain medications], [vitamin D], [beta agonists], [exhaled nitric oxide], and [corticosteroids]. Results. The reported prevalence of asthma in children with sickle cell disease varies from 2% to approximately 50%. Having asthma increases the risk for developing acute chest syndrome , death, or painful episodes compared to having sickle cell disease without asthma. Asthma and sickle cell may be linked by impaired nitric oxide regulation, excessive production of leukotrienes, insufficient levels of Vitamin D, and exposure to acetaminophen in early life. Treatment of sickle cell patients includes using commonly prescribed asthma medications; specific considerations are suggested to ensure safety in the sickle cell population. Conclusion. Prospective controlled trials of drug treatment for asthma in patients who have both sickle cell disease and asthma are urgently needed.

Genetic variations of bile salt transporters as predisposing factors for drug-induced cholestasis, intrahepatic cholestasis of pregnancy and therapeutic response of viral hepatitis

INTRODUCTION

Drug-induced cholestasis, intrahepatic cholestasis of pregnancy and viral hepatitis are acquired forms of liver disease. Cholestasis is a pathophysiologic state with impaired bile formation and subsequent accumulation of bile salts in hepatocytes. The bile salt export pump (BSEP) (ABCB11) is the key export system for bile salts from hepatocytes.

AREAS COVERED

This article provides an introduction into the physiology of bile formation followed by a summary of the current knowledge on the key bile salt transporters, namely, the sodium-taurocholate co-transporting polypeptide NTCP, the organic anion transporting polypeptides (OATPs), BSEP and the multi-drug resistance protein 3. The pathophysiologic consequences of altered functions of these transporters, with an emphasis on molecular and genetic aspects, are then discussed.

EXPERT OPINION

Knowledge of the role of hepatocellullar transporters, especially BSEP, in acquired cholestasis is continuously increasing. A common variant of BSEP (p.V444A) is now a well-established susceptibility factor for acquired cholestasis and recent evidence suggests that the same variant also influences the therapeutic response and disease progression of viral hepatitis C. Studies in large independent cohorts are now needed to confirm the relevance of p.V444A. Genome-wide association studies should lead to the identification of additional genetic factors underlying cholestatic liver disease.

Expression of SLCO transport genes in castration-resistant prostate cancer and impact of genetic variation in SLCO1B3 and SLCO2B1 on prostate cancer outcomes

BACKGROUND

Metastases from men with castration-resistant prostate cancer (CRPC) harbor increased tumoral androgens versus untreated prostate cancers. This may reflect steroid uptake by OATP (organic anion transporting polypeptide)/SLCO transporters. We evaluated SLCO gene expression in CRPC metastases and determined whether prostate cancer outcomes are associated with single nucleotide polymorphisms (SNP) in SLCO2B1 and SLCO1B3, transporters previously shown to mediate androgen uptake.

METHODS

Transcripts encoding eleven SLCO genes were analyzed in untreated prostate cancer and in metastatic CRPC tumors obtained by rapid autopsy. SNPs in SLCO2B1 and SLCO1B3 were genotyped in a population-based cohort of 1,309 Caucasian prostate cancer patients. Median survival follow-up was 7.0 years (0.77-16.4). The risk of prostate cancer recurrence/progression and prostate cancer-specific mortality (PCSM) was estimated with Cox proportional hazards analysis.

RESULTS

Six SLCO genes were highly expressed in CRPC metastases versus untreated prostate cancer, including SLCO1B3 (3.6-fold; P = 0.0517) and SLCO2B1 (5.5-fold; P = 0.0034). Carriers of the variant alleles SLCO2B1 SNP rs12422149 (HR: 1.99; 95% CI: 1.11-3.55) or SLCO1B3 SNP rs4149117 (HR: 1.76; 95% CI: 1.00-3.08) had an increased risk of PCSM.

CONCLUSIONS

CRPC metastases show increased expression of SLCO genes versus primary prostate cancer. Genetic variants of SLCO1B3 and SLCO2B1 are associated with PCSM. Expression and genetic variation of SLCO genes which alter androgen uptake may be important in prostate cancer outcomes.

IMPACT

OATP/SLCO genes may be potential biomarkers for assessing risk of PCSM. Expression and genetic variation in these genes may allow stratification of patients to more aggressive hormonal therapy or earlier incorporation of nonhormonal-based treatment strategies.

Frequencies of single-nucleotide polymorphisms of SLCO1A2, SLCO1B3 and SLCO2B1 genes in a Finnish population

Organic anion transporting polypeptides 1A2, 1B3 and 2B1 (OATP1A2, OATP1B3 and OATP2B1) are expressed in tissues important for pharmacokinetics, and mediate the cellular influx of various endogenous and exogenous compounds, including drugs. The aim of the study was to investigate the frequencies of single-nucleotide polymorphisms (SNP) of SLCO1A2, SLCO1B3 and SLCO2B1 in a Finnish population. The distribution of nine non-synonymous SLCO1A2, SLCO1B3 and SLCO2B1 SNPs was determined in 552 healthy Finnish Caucasian participants by using allelic discrimination with TaqMan 5'nuclease assays. The SLCO1A2 c.38T>C (p.Ile13Thr) and c.516C>T (p.Glu172Asp) SNPs were found with variant allele frequencies of 12.9% (95% confidence interval: 11.0-15.0) and 7.2% (5.8-8.8). The variant allele frequencies of SLCO1B3 c.334T>G (p.Ser112Ala), c.699G>A (p.Met233Ile) and c.767G>C (p.Gly256Ala) were 77.0% (74.4-79.4), 76.9% (74.3-79.3) and 12.8% (10.9-14.9), respectively. None of the participants carried the SLCO1B3 c.1309G>A (p.Gly437Ser) SNP. The SLCO2B1 c.601G>A (p.Val201Met), c.935G>A (p.Arg312Gln) and c.1457C>T (p.Ser486Phe) variant allele frequencies were 2.1% (1.4-3.1), 13.6% (11.7-15.7) and 2.8% (2.0-4.0), respectively. The SLCO1B3 c.334T>G and c.699G>A SNPs were in a nearly complete linkage disequilibrium (r² = 0.99, D' = 1.00), all other SNP pairs showed only a weak correlation. In conclusion, non-synonymous sequence variations of SLCO1A2, SLCO1B3 and SLCO2B1 occur at high frequencies in the Finnish population.

Uptake transporters of the human OATP family: molecular characteristics, substrates, their role in drug-drug interactions, and functional consequences of polymorphisms

Organic anion transporting polypeptides (OATPs, gene family: SLC21/SLCO) mediate the uptake of a broad range of substrates including several widely prescribed drugs into cells. Drug substrates for members of the human OATP family include HMG-CoA-reductase inhibitors (statins), antibiotics, anticancer agents, and cardiac glycosides. OATPs are expressed in a variety of different tissues including brain, intestine, liver, and kidney, suggesting that these uptake transporters are important for drug absorption, distribution, and excretion. Because of their wide tissue distribution and broad substrate spectrum, altered transport kinetics, for example, due to drug-drug interactions or due to the functional consequences of genetic variations (polymorphisms), can contribute to the interindividual variability of drug effects. Therefore, the molecular characteristics of human OATP family members, the role of human OATPs in drug-drug interactions, and the in vitro analysis of the functional consequences of genetic variations in SLCO genes encoding OATP proteins are the focus of this chapter.

Complex systems in pulmonary medicine: a systems biology approach to lung disease

The lung is a highly complex organ that can only be understood by integrating the many aspects of its structure and function into a comprehensive view. Such a view is provided by a systems biology approach, whereby the many layers of complexity, from the molecular genetic, to the cellular, to the tissue, to the whole organ, and finally to the whole body, are synthesized into a working model of understanding. The systems biology approach therefore relies on the expertise of many disciplines, including genomics, proteomics, metabolomics, physiomics, and, ultimately, clinical medicine. The overall structure and functioning of the lung cannot be predicted from studying any one of these systems in isolation, and so this approach highlights the importance of emergence as the fundamental feature of systems biology. In this paper, we will provide an overview of a systems biology approach to lung disease by briefly reviewing the advances made at many of these levels, with special emphasis on recent work done in the realm of pulmonary physiology and the analysis of clinical phenotypes.

Effect of citrus juice and SLCO2B1 genotype on the pharmacokinetics of montelukast

Previously the authors found that a common polymorphism, rs12422149 (SLCO2B1{NM_007256.2}:c.935G>A), in the gene coding for OATP2B1, was associated with absorption of and response to montelukast in humans. In vitro studies showed that citrus juice could reduce the permeability of montelukast consistent with known inhibition of organic anion-transporting polypeptides. To study the clinical significance of c.935G>A, the authors conducted a single-dose, pharmacokinetic study of montelukast co-ingested with citrus juice. On average, co-ingestion with either orange juice or 4× concentrated grapefruit juice had a minimal effect on the area under the plasma concentration-time curve from time zero extrapolated to infinite time (AUC(0→∞)) of montelukast relative to co-ingestion with Gatorade control (n = 24). However when the data were stratified by genotype at c.935 (G/G n = 21, A/G n = 5), a significant reduction in AUC(0→∞) was detected with orange juice in G/G homozygotes (AUC(0→∞), G/G, Gatorade = 2560 ± 900 ng·h·mL(-1) vs AUC(0→∞), G/G, orange juice = 2010 ± 650 ng·h·mL(-1), P = .032). Significantly, A/G heterozygotes showed reduced AUC(0→∞) relative to G/G homozygotes, independent of treatment (AUC(0→∞), G/G, combined treatments = 2310 ± 820 ng·h·mL(-1) vs AUC(0→∞), A/G, combined treatments = 1460 ± 340 ng·h·mL(-1), P = 2.0 × 10(-5)) replicating previous observations.

Recent advances in antileukotriene therapy

PURPOSE OF REVIEW

Despite profound effects of leukotrienes in experimental models, clinical responses to antileukotriene drugs are highly heterogeneous. This review discusses recent advances concerning the molecular mechanisms of antileukotrienes as well as their efficacy in various clinical scenarios and patient groups.

RECENT FINDINGS

Appreciation of the role of leukotriene E4 and the existence of its distinct receptors may explain the limited efficacy of current leukotriene receptor antagonists. Pharmacogenetic studies highlight the influence of several leukotriene pathway genes on clinical responsiveness. Benefits of addition of antileukotrienes to inhaled corticosteroids in chronic adult asthmatics have been shown, but their role in acute asthma is unclear. Evidence suggests they are not a first-line treatment for allergic rhinitis or urticaria, but may provide useful additional therapy. In children antileukotrienes provide symptomatic benefit in preschool wheezers, but have no clear role in bronchiolitis or acute asthma. Adherence to montelukast appears superior to inhaled corticosteroids. Use in sleep-disordered breathing and eosinophilic gastroenteropathies warrants further investigation. Despite recent concerns thorough analysis of existing data suggests antileukotrienes are well tolerated drugs. The possible link with Churg-Strauss syndrome requires further investigation.

SUMMARY

The leukotriene pathway remains an attractive target in asthma and allergic disease, particularly in light of renewed appreciation of the role of leukotriene E4. Clarification of the clinical role of antileukotrienes is needed.

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

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

An update on the role of leukotrienes in asthma

PURPOSE OF REVIEW

Leukotrienes are lipid mediators involved in the pathogenesis of asthma. There is significant new information about the actions of leukotrienes in asthma and the evolving role of antileukotriene therapies. We review recent findings on regulation of leukotriene synthesis, biological function of leukotrienes in disease models, and use of leukotriene modifiers in clinical practice.

RECENT FINDINGS

Our understanding of the regulation of leukotriene synthesis at a molecular level has greatly advanced. Recent evidence indicates that genetic variation in the leukotriene synthetic pathway affects the clinical response to leukotriene modifiers. The participation of leukotriene B4 in the allergic sensitization process in animal models suggests a larger role for leukotriene B4 in asthma. Preclinical and in-vitro models suggest that the cysteinyl leukotrienes are important in airway remodeling. Leukotrienes are key mediators of exercise-induced bronchoconstriction with recent studies demonstrating that leukotriene modifiers reduce the severity of exercise-induced bronchoconstriction during short-term and long-term use.

SUMMARY

Leukotrienes are clearly involved in airway inflammation and certain clinical features of asthma. Evolving evidence indicates that leukotriene B4 has an important role in the development of asthma and that cysteinyl leukotrienes are key mediators of the airway remodeling process.

The role of pharmacogenomics in improving the management of asthma

There is a large amount of interindividual variability in both therapeutic and adverse responses to asthma therapies. Genetic variability can account for 50% to 60% of this variability. Pharmacogenomics holds out the promise of allowing clinicians to prospectively choose therapies that have the greatest likelihood to be effective for individual patients and to avoid those that might have a high likelihood of producing adverse effects. In this article we review the principles of pharmacogenomic investigation. We explore the data developed from the early pharmacogenomic studies with the most common asthma therapies. Furthermore, we explore the potential use of pharmacogenomics, as well as caveats in interpreting such information.

Heterogeneity of response to asthma controller therapy: clinical implications

PURPOSE OF REVIEW

Patients with asthma exhibit heterogeneous responses to all classes of asthma medication. This review examines clinical characteristics, biomarkers of inflammation, and genetic polymorphisms associated with treatment responsiveness in studies of adults and children with asthma, with an emphasis on inhaled corticosteroids and leukotriene modifiers.

RECENT FINDINGS

Recent clinical studies extended previous observations of associations between biomarkers of allergic inflammation and responsiveness to inhaled corticosteroids, and between cigarette smoke exposure and responsiveness to montelukast. Pharmacogenetic studies revealed associations between treatment response and genetic variations in CRHR1, as well as a number of genes encoding proteins involved in the absorption, production, and action of the cysteinyl leukotrienes. Very few studies have attempted to test the ability of these phenotypic and genotypic associations to predict treatment responsiveness.

SUMMARY

Additional prospective studies of sufficient size, quality, and ethnic diversity are needed to determine how best to incorporate information about genetic variations, clinical characteristics, and biomarkers into decisions about asthma therapy for individual patients.

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

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

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

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

Concentration-dependent noncysteinyl leukotriene type 1 receptor-mediated inhibitory activity of leukotriene receptor antagonists

The use of cysteinyl leukotriene receptor antagonists (LTRAs) for asthma therapy has been associated with a significant degree of interpatient variability in response to treatment. Some of that variability may be attributable to noncysteinyl leukotriene type 1 receptor (CysLT(1))-mediated inhibitory mechanisms that have been demonstrated for this group of drugs. We used a model of CysLT(1) signaling in human monocytes to characterize CysLT(1)-dependent and -independent anti-inflammatory activity of two chemically different, clinically relevant LTRAs (montelukast and zafirlukast). Using receptor-desensitization experiments in monocytes and CysLT(1)-transfected HEK293 cells and IL-10- and CysLT(1) small interfering RNA-induced downregulation of CysLT(1) expression, we showed that reported CysLT(1) agonists leukotriene D(4) and UDP signal through calcium mobilization, acting on separate receptors, and that both pathways were inhibited by montelukast and zafirlukast. However, 3-log greater concentrations of LTRAs were required for the inhibition of UDP-induced signaling. In monocytes, UDP, but not leukotriene D(4), induced IL-8 production that was significantly inhibited by both drugs at micromolar concentrations. At low micromolar concentrations, both LTRAs also inhibited calcium ionophore-induced leukotriene (leukotriene B(4) and leukotriene C(4)) production, indicating 5-lipoxygenase inhibitory activities. We report herein that montelukast and zafirlukast, acting in a concentration-dependent manner, can inhibit non-CysLT(1)-mediated proinflammatory reactions, suggesting activities potentially relevant for interpatient variability in response to treatment. Higher doses of currently known LTRAs or new compounds derived from this class of drugs may represent a new strategy for finding more efficient therapy for bronchial asthma.

Drug interactions

Drugs for allergy are often taken in combination with other drugs, either to treat allergy or other conditions. In common with many pharmaceuticals, most such drugs are subject to metabolism by P450 enzymes and to transmembrane transport. This gives rise to considerable potential for drug-drug interactions, to which must be added consideration of drug-diet interactions. The potential for metabolism-based drug interactions is increasingly being taken into account during drug development, using a variety of in silico and in vitro approaches. Prediction of transporter-based interactions is not as advanced. The clinical importance of a drug interaction will depend upon a number of factors, and it is important to address concerns quantitatively, taking into account the therapeutic index of the compound.

Drug metabolism and pharmacokinetics

In this article, aspects of absorption, distribution, metabolism, and excretion have been described bearing in mind the pathogenesis of allergic diseases and their possible therapeutic opportunities. The importance of the routes of administration of the different therapeutic groups has been emphasized. The classical aspects of drug metabolism and disposition related to oral administration have been reviewed, but special emphasis has been given to intranasal, cutaneous, transdermal, and ocular administration as well as to the absorption and the subsequent bioavailability of drugs. Drug-metabolizing enzymes and transporters present in extrahepatic tissues, such as nasal mucosa and the respiratory tract, have been particularly discussed. As marketed antiallergic drugs include both racemates and enantiomers, aspects of stereoselective absorption, distribution, metabolism, and excretion have been discussed. Finally, a new and promising methodology, microdosing, has been presented, although it has not yet been applied to drugs used in the treatment of allergic diseases.

Effect of SLCO1B1 polymorphism on the plasma concentrations of bile acids and bile acid synthesis marker in humans

BACKGROUND AND OBJECTIVE

Organic anion transporting polypeptide 1B1 (OATP1B1, encoded by SLCO1B1) is a sinusoidal influx transporter of human hepatocytes. Our aim was to characterize the role of OATP1B1 in the hepatic uptake of bile acids in vivo.

METHODS

Fasting blood samples were drawn from 24 healthy volunteers with SLCO1B1 c.388AA-c.521TT (*1A/*1A) genotype, eight with c.388GG-c.521TT (*1B/*1B) genotype, 24 with c.521TC genotype, and nine with c.521CC genotype. Plasma concentrations of 15 endogenous bile acids, their synthesis marker, and cholesterol were determined by liquid chromatography-tandem mass spectrometry.

RESULTS

The concentrations of ursodeoxycholic acid, glycoursodeoxycholic acid, chenodeoxycholic acid, and glycochenodeoxycholic acid were approximately 50-240% higher in individuals with the SLCO1B1 c.521CC, c.521TC, or c.388AA-c.521TT genotype than in those with the c.388GG-c.521TT genotype (P<0.05), with the largest differences seen between the c.521CC and c.388GG-c.521TT individuals. The concentration of tauroursodeoxycholic acid was approximately 120% higher in individuals with the c.521TC genotype and that of taurochenodeoxycholic acid 110% higher in individuals with the c.521CC or c.521TC genotype than in those with the c.388GG-c.521TT genotype (P<0.05). The cholic acid concentration was approximately 30% higher in individuals with the c.521CC or c.388AA-c.521TT genotype than in those with the c.388GG-c.521TT genotype (P<0.05), but its conjugates remained unaffected by the genotype. The bile acid synthesis marker 7alpha-hydroxy-4-cholesten-3-one/cholesterol concentration ratio was 62 or 45% higher in the c.388AA-c.521TT participants than in the c.388GG-c.521TT or c.521TC participants, respectively (P<0.05).

CONCLUSION

SLCO1B1 polymorphism considerably affects the disposition of several endogenous bile acids and bile acid synthesis marker, indicating that OATP1B1 plays an important role in the hepatic uptake of bile acids in vivo in humans.

Genetics and pharmacogenetics of the leukotriene pathway

Leukotrienes are now established contributors to the inflammatory process in asthma, and leukotriene modifiers are mainstays in the therapy of asthma. This review focuses on published association studies implicating the role of leukotriene pathway genes in asthma pathogenesis and treatment response, specifically focusing on those genetic variants associated with asthma affection status, the development of aspirin-exacerbated respiratory disease, and pharmacogenetic response. Although published studies have been limited by small sample sizes and a lack of independent replication, multiple loci within multiple leukotriene pathway genes have now been associated in more than 1 study related to asthma or asthma treatment response. Those specific variants include 2 variants in the 5-lipoxygenase gene (ALOX5) that are both associated with response to 5-lipoxygenase inhibition and to leukotriene receptor antagonists, variants in genes encoding the 2 established cysteinyl leukotriene receptor antagonists (CYSLTR1 and CYSLTR2) that are both associated with asthma susceptibility in at least 2 independent populations, and a leukotriene C(4) synthase promoter polymorphism (LTC4s) that has been associated with asthma affection status and asthma-exacerbated respiratory disease. Despite these successes, genetic investigations into this pathway remain in their formative stages. Future studies aimed at providing a broader scope of investigation through increased sample sizes and through genome-wide approaches are needed.

The effect of probenecid and MK-571 on the feto-maternal transfer of saquinavir in dually perfused human term placenta

Human placenta, particularly the blood-placenta barrier, with various transporters has crucial role to protect the fetus and, on the other hand, to facilitate movement of compounds towards the fetal circulation. This study aimed to characterize the role of basal transporters of the syncytiotrophoblast, which appear to be yet less studied, in the fetal-to-maternal transfer of saquinavir by use of dually perfused human placentas. A dual perfusion of human placenta was performed to study effect of MK-571 and probenecid, inhibitors of the MRP1 and OATP transporters, expressed in the basal trophoblast membrane, on the transfer of saquinavir. The fetal-to-maternal placental transfer of saquinavir in the control group as measured by TPT(AUC)% (absolute fraction of the dose crossing placenta) was 14.0%, which is 73% less than the transfer of the freely diffusible antipyrine. The two inhibitors, MK-571 and probenecid caused a non-significant (P = 0.34 for ANOVA) reduction of 43% and 24%, respectively, in the mean amount of saquinavir transferred from the fetal to the maternal side. MK-571 also somewhat (by 31%) reduced the TPT(AUC)% of antipyrine, but this finding did not reach statistical significance (P = 0.25). Neither of the employed inhibitors had an effect on the placental transfer index of saquinavir transfer (P = 0.77). The present results indicated lack of significant effect by MK-571 and probenecid on the fetal-to-maternal transfer of saquinavir and suggest that MRP1 and, possibly, OATP2B1 do not play a significant role in the fetal-to-maternal transfer of saquinavir.