Influence of uridine diphosphate (UDP)-glucuronosyltransferases and ABCC2 genetic polymorphisms on the pharmacokinetics of mycophenolic acid and its metabolites in Chinese renal transplant recipients

Identification of UDP-glucuronosyltransferase (UGT) isoforms involved in the metabolism of Chlorophenols (CPs)

Chlorophenols (CPs) are a group of pollutants that pose a great threat to the environment, they are widely used in industrial and agricultural wastes, pesticides, herbicides, textiles, pharmaceuticals and plastics. Among CPs, pentachlorophenol was listed as one of the persistent organic pollutants (POPs) by the Stockholm convention. This study aims to identify the UDP-glucosyltransferase (UGT) isoforms involved in the metabolic elimination of CPs. CPs' mono-glucuronide was detected in the human liver microsomes (HLMs) incubation mixture with co-factor uridine-diphosphate glucuronic acid (UDPGA). HLMs-catalyzed glucuronidation metabolism reaction equations followed Michaelis-Menten or substrate inhibition type. Recombinant enzymes and chemical reagents inhibition experiments were utilized to phenotype the main UGT isoforms involved in the glucuronidation of CPs. UGT1A6 might be the major enzyme in the glucuronidation of mono-chlorophenol isomer. UGT1A1, UGT1A6, UGT1A9, UGT2B4 and UGT2B7 were the most important five UGT isoforms for metabolizing the di-chlorophenol and tri-chlorophenol isomers. UGT1A1 and UGT1A3 were the most important UGT isoforms in the catalysis of tetra-chlorophenol and pentachlorophenol isomers. Species differences were investigated using rat liver microsomes (RLMs), pig liver microsomes (PLMs), dog liver microsomes (DLMs), and monkey liver microsomes (MyLMs). All these results were helpful for elucidating the metabolic elimination and toxicity of CPs.

Influence of genetic polymorphisms on pharmacokinetics and treatment response of mycophenolic acid: a scoping review

This scoping review explores the impact of genetic polymorphisms on the pharmacokinetics and treatment responses of mycophenolic acid (MPA), an immunosuppressant. The study includes 83 articles from 1226 original studies, focusing on transplantation (n = 80) and autoimmune disorders (n = 3). Genetic variants in uridine 5'-diphospho-glucuronosyltransferase (UGT1A9, UGT1A8 and UGT2B7) and transmembrane transporters (ABCC2, SLCO1B1, SLCO1B3 and ABCB1) significantly affected MPA's pharmacokinetics and susceptibility to its adverse effect. Whereas variants in several genes including UGT1A9, UGT2B7, IMPDH1 and IMPDH2 have been associated with a higher risk of transplant rejection. However, there is a lack of studies on MPA's impact on autoimmune disorders and limited research on the Asian population. The findings underscore the need for further research on MPA's impact across different populations and diseases, particularly among other Asian ethnic groups, to advance personalized medicine in MPA therapy.

Influence of Calcineurin Inhibitors and Genetic Polymorphism of Transporters on Enterohepatic Circulation and Exposure of Mycophenolic Acid in Chinese Adult Renal Allograft Recipients

There is significant enterohepatic circulation (EHC) during the disposition of mycophenolic acid (MPA). The aim of this study was to elucidate factors influencing the EHC of MPA in Chinese adult renal allograft recipients. After 2 weeks of therapy with mycophenolate mofetil or enteric-coated mycophenolate sodium, blood samples were collected from 125 patients at 0 to 12 hours post-administration and MPA concentrations were determined. The influence of calcineurin inhibitors (CNIs) and genetic polymorphisms on MPA exposure and EHC was studied. The Shapley additive explanations method was used to estimate the impact of various factors on the area under the plasma drug concentration-time curve (AUC_0-12h ) for MPA. An extreme gradient boosting (XGboost) machine learning-based model was established to predict AUC_0-12h . Results showed that the dose-normalized AUC_6-12h (dn-AUC_6-12h ) of MPA was significantly lower in patients co-administered with cyclosporine (CsA) than in patients co-administered with tacrolimus (TAC) (P < .05). For patients co-administered with TAC, patients with ABCC2 C-24T CC or SLCO1B1 T521C TT genotypes had significantly higher values of dn-AUC_6-12h (P < .05). Patients with SLCO1B3 334T/699G alleles had significantly lower dn-AUC_6-12h values than homozygotes (P < .05). By introducing body weight, age, and EHC-related factors, including co-administered CNIs and genetic polymorphism of drug transporters, as covariates in the XGboost machine learning model, the prediction performance of AUC_0-12h for MPA in Chinese adult renal allograft recipients can be improved.

Association of ABCC2 Haplotypes to Mycophenolic Acid Pharmacokinetics in Stable Kidney Transplant Recipients

Mycophenolic acid exhibits significant interpatient pharmacokinetic variability attributed to factors including race, sex, concurrent medications, and enterohepatic circulation of the mycophenolic acid glucuronide metabolite to mycophenolic acid. This conversion by enterohepatic circulation is mediated by the multidrug resistance-associated protein 2, encoded by ABCC2. This study investigated ABCC2 haplotype associations with mycophenolic acid pharmacokinetics in 147 stable kidney transplant recipients receiving mycophenolic acid in combination with calcineurin inhibitors. The role of the ABCC2 genotypes -24C>T (rs717620), 1249C>T (rs2273697), and 3972C>T (rs3740066) were evaluated in prospective, cross-sectional pharmacokinetic studies of stable recipients receiving mycophenolic acid and either tacrolimus or cyclosporine. Haplotype phenotypic associations with mycophenolic acid pharmacokinetic parameters were computed using THESIAS (v. 3.1). Four ABCC2 haplotypes with estimated frequencies greater than 10% were identified (H1:CGC [wild type], H9:CGT, H2:CAC, H12:TGT). There were no differences in haplotype frequencies by either race or sex. There were significant associations of pharmacokinetic parameters with ABCC2 haplotypes for mycophenolic acid clearance (L/h), mycophenolic acid AUC0-12h (mg·h/L), and the ratio of mycophenolic acid glucuronide to mycophenolic acid AUC0-12h . The wild-type haplotype ABCC2 CGC had greater mycophenolic acid AUC0-12h (P = .017), slower clearance (P = .013), and lower mycophenolic acid glucuronide to mycophenolic acid AUC0-12h ratio (P = .047) compared with the reduced function ABCC2 haplotype CGT. These differences were most pronounced among patients receiving tacrolimus cotreatment. No phenotypic associations were found with the cyclosporine-mycophenolic acid regimen. Variation in ABCC2 haplotypes contributes to subtherapeutic mycophenolic acid exposure and influences interpatient variability in pharmacokinetic phenotypes based on concurrent calcineurin inhibitor treatment.

Effect of UGT polymorphisms on pharmacokinetics and adverse reactions of mycophenolic acid in kidney transplant patients

Mycophenolic acid (MPA) is a common immunosuppressive drug for kidney transplantation patients, and is characterized by a narrow therapeutic index and significant individual variability. UGTs are the main enzymes responsible for the metabolism of MPA. Although, many studies have focused on the relationship between UGT polymorphisms and pharmacokinetics and adverse reactions of MPA, the conclusion are controversial. We reviewed the relevant literature and summarized the significant influences of UGT polymorphisms, such as UGT1A8 (rs1042597, rs17863762), UGT1A9 (rs72551330, rs6714486, rs17868320, rs2741045, rs2741045) and UGT2B7 (rs7438135, rs7439366, rs7662029), on the pharmacokinetics of MPA and its metabolites and adverse reactions. The review provides a reference for guiding the individualized administration of MPA and reducing adverse reactions to MPA.

Influence of SLCO1B1 521T>C, UGT2B7 802C>T and IMPDH1 -106G>A Genetic Polymorphisms on Mycophenolic Acid Levels and Adverse Reactions in Chinese Autoimmune Disease Patients

Introduction

Mycophenolate mofetil (MMF), a new type of immunosuppressant, has emerged as a frontline agent for treating autoimmune diseases. Mycophenolic acid (MPA) is an active metabolite of MMF. MPA exposure varies greatly among individuals, which may lead to adverse drug reactions such as gastrointestinal side effects, infection, and leukopenia. Genetic factors play an important role in the variation of MPA levels and its side effects. Although many published studies have focused on MMF use in patients after organ transplant, studies that examine the use of MMF in patients with autoimmune diseases are still lacking.

Methods

This study will not only explore the genetic factors affecting MPA levels and adverse reactions but also investigate the relationships between UGT1A9 −118(dT)9/10, UGT1A9 - 1818T>C, UGT2B7 802C>T, SLCO1B1 521T>C, SLCO1B3 334T>G, IMPDH1 −106G>A and MPA trough concentration (MPA C₀), along with adverse reactions among Chinese patients with autoimmune diseases. A total of 120 patients with autoimmune diseases were recruited. The MPA trough concentration was detected using the enzyme multiplied immunoassay technique (EMIT). Genotyping was performed using a real-time polymerase chain reaction (PCR) system and validated allelic discrimination assays. Clinical data were collected for the determination of side effects.

Results

SLCO1B1 521T>C demonstrated a significant association with MPA C₀/d (p=0.003), in which patients with the CC type showed a higher MPA C₀/d than patients with the TT type (p=0.001) or the CT type (p=0.000). No significant differences were found in MPA C₀/d among the other SNPs. IMPDH1 −106G>A was found to be significantly related to infections (p=0.006). Subgroup analysis revealed that UGT2B7 802C>T was significantly related to Pneumocystis carinii pneumonia infection (p=0.036), while SLCO1B1 521T>C was associated with anemia (p=0.029).

Conclusion

For Chinese autoimmune disease patients, SLCO1B1 521T>C was correlated with MPA C₀/d and anemia. IMPDH1 −106G>A was significantly related to infections. UGT2B7 802C>T was significantly related to Pneumocystis carinii pneumonia infection.

The Impact of Genetic Polymorphisms on the Pharmacokinetics and Pharmacodynamics of Mycophenolic Acid: Systematic Review and Meta-analysis

BACKGROUND

Mycophenolic acid (MPA) is among the most commonly prescribed medications for immunosuppression following organ transplantation. Highly variable MPA exposure and drug response are observed among individuals receiving the same dosage of the drug. Identification of candidate genes whose polymorphisms could be used to predict MPA exposure and clinical outcome is of clinical value.

OBJECTIVES

This study aimed to determine the impact of genetic polymorphisms on the pharmacokinetics and pharmacodynamics of MPA in humans by means of a systematic review and meta-analysis.

METHODS

A systematic search was conducted on PubMed, EMBASE, Web of Sciences, Scopus, and the Cochrane Library databases. A meta-analysis was conducted to determine any associations between genetic polymorphisms and pharmacokinetic or pharmacodynamic parameters of MPA. Pooled-effect estimates were calculated by means of the random-effects model.

RESULTS

A total of 37 studies involving 3844 individuals were included in the meta-analysis. Heterozygous carriers of the UGT1A9 -275T>A polymorphism were observed to have a significantly lower MPA exposure than wild-type individuals. Four single nucleotide polymorphisms (SNPs), namely UGT1A9 -2152C>T, UGT1A8 518C>G, UGT2B7 211G>T, and SLCO1B1 521T>C, were also significantly associated with altered MPA pharmacokinetics. However, none of the investigated SNPs, including SNPs in the IMPDH gene, were found to be associated with the clinical efficacy of MPA. The only SNP that was associated with adverse outcomes was SLCO1B3 344T>G.

CONCLUSIONS

The present systematic review and meta-analysis identified six SNPs that were significantly associated with pharmacokinetic variability or adverse effects of MPA. Our findings represent the basis for future research and clinical implications with regard to the role of pharmacogenetics in MPA pharmacokinetics and drug response.

Personalized Therapy for Mycophenolate: Consensus Report by the International Association of Therapeutic Drug Monitoring and Clinical Toxicology

ABSTRACT

When mycophenolic acid (MPA) was originally marketed for immunosuppressive therapy, fixed doses were recommended by the manufacturer. Awareness of the potential for a more personalized dosing has led to development of methods to estimate MPA area under the curve based on the measurement of drug concentrations in only a few samples. This approach is feasible in the clinical routine and has proven successful in terms of correlation with outcome. However, the search for superior correlates has continued, and numerous studies in search of biomarkers that could better predict the perfect dosage for the individual patient have been published. As it was considered timely for an updated and comprehensive presentation of consensus on the status for personalized treatment with MPA, this report was prepared following an initiative from members of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology (IATDMCT). Topics included are the criteria for analytics, methods to estimate exposure including pharmacometrics, the potential influence of pharmacogenetics, development of biomarkers, and the practical aspects of implementation of target concentration intervention. For selected topics with sufficient evidence, such as the application of limited sampling strategies for MPA area under the curve, graded recommendations on target ranges are presented. To provide a comprehensive review, this report also includes updates on the status of potential biomarkers including those which may be promising but with a low level of evidence. In view of the fact that there are very few new immunosuppressive drugs under development for the transplant field, it is likely that MPA will continue to be prescribed on a large scale in the upcoming years. Discontinuation of therapy due to adverse effects is relatively common, increasing the risk for late rejections, which may contribute to graft loss. Therefore, the continued search for innovative methods to better personalize MPA dosage is warranted.

ABCB1, ABCG2, ABCC1, ABCC2, and ABCC3 drug transporter polymorphisms and their impact on drug bioavailability: what is our current understanding?

INTRODUCTION

Interindividual differences in drug response are a frequent clinical challenge partly due to variation in pharmacokinetics. ATP-binding cassette (ABC) transporters are crucial determinants of drug disposition. They are subject of gene regulation and drug-interaction; however, it is still under debate to which extend genetic variants in these transporters contribute to interindividual variability of a wide range of drugs.

AREAS COVERED

This review discusses the current literature on the impact of genetic variants in ABCB1, ABCG2 as well as ABCC1, ABCC2, and ABCC3 on pharmacokinetics and drug response. The aim was to evaluate if results from recent studies would increase the evidence for potential clinically relevant pharmacogenetic effects.

EXPERT OPINION

Although enormous efforts have been made to investigate effects of ABC transporter genotypes on drug pharmacokinetics and response, the majority of studies showed only weak if any associations. Despite few unique results, studies mostly failed to confirm earlier findings or still remained inconsistent. The impact of genetic variants on drug bioavailability is only minor and other factors regulating the transporter expression and function seem to be more critical. In our opinion, the findings on the so far investigated genetic variants in ABC efflux transporters are not suitable as predictive biomarkers.

Pharmacogenetics and pharmacokinetics modeling of unexpected and extremely severe toxicities after sorafenib intake

A 53-year-old woman with papillary thyroid cancer treated with 800 mg sorafenib therapy rapidly experienced grade 3 toxicities. Dosing was reduced in a step-wise manner with several treatment discontinuations down to 200 mg every 2 days but severe toxicities continued. Plasma drug monitoring showed high exposure, even at low dose. Dosing was then further reduced at 200 mg every 3 days and tolerance was finally acceptable (i.e., grade 1 toxicity) with stable disease upon RECIST imaging. Pharmacogenetic investigations showed polymorphisms affecting both UGT1A9 (UGT1A9-rs3832043) and nuclear receptor PXR (NR1I2-rs3814055, NR1I2-rs2472677 and NR1I2-rs10934498), possibly resulting in downregulation of liver metabolizing enzymes of sorafenib (i.e., CYP and UGT). Patient's clearance (0.48 l/h) estimated by Bayesian approach was consistently lower than usually described. This is the first time that, in addition to mutations affecting UGT1A9, genetic polymorphisms of NR1I2 have possibly been associated with both plasma overexposure and severe toxicities upon sorafenib intake.

Human variability in isoform-specific UDP-glucuronosyltransferases: markers of acute and chronic exposure, polymorphisms and uncertainty factors

UDP-glucuronosyltransferases (UGTs) are involved in phase II conjugation reactions of xenobiotics and differences in their isoform activities result in interindividual kinetic differences of UGT probe substrates. Here, extensive literature searches were performed to identify probe substrates (14) for various UGT isoforms (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B7 and UGT2B15) and frequencies of human polymorphisms. Chemical-specific pharmacokinetic data were collected in a database to quantify interindividual differences in markers of acute (Cmax) and chronic (area under the curve, clearance) exposure. Using this database, UGT-related uncertainty factors were derived and compared to the default factor (i.e. 3.16) allowing for interindividual differences in kinetics. Overall, results show that pharmacokinetic data are predominantly available for Caucasian populations and scarce for other populations of different geographical ancestry. Furthermore, the relationships between UGT polymorphisms and pharmacokinetic parameters are rarely addressed in the included studies. The data show that UGT-related uncertainty factors were mostly below the default toxicokinetic uncertainty factor of 3.16, with the exception of five probe substrates (1-OH-midazolam, ezetimibe, raltegravir, SN38 and trifluoperazine), with three of these substrates being metabolised by the polymorphic isoform 1A1. Data gaps and future work to integrate UGT-related variability distributions with in vitro data to develop quantitative in vitro-in vivo extrapolations in chemical risk assessment are discussed.

Polymorphic Expression of UGT1A9 is Associated with Variable Acetaminophen Glucuronidation in Neonates: A Population Pharmacokinetic and Pharmacogenetic Study

INTRODUCTION

Acetaminophen (paracetamol, APAP) is widely used as an analgesic and antipyretic drug in children and neonates. A number of enzymes contribute to the metabolism of acetaminophen, and genetic factors might be important to explain variability in acetaminophen metabolism among individuals.

METHODS

The current investigation utilized a previously published parent-metabolite population pharmacokinetic model describing acetaminophen glucuronidation, sulfation, and oxidation to examine the potential role of genetic variability on the relevant metabolic pathways. Neonates were administered 30-min intravenous infusions of acetaminophen 15 mg/kg every 12 h (< 28 weeks' gestational age [GA]) or every 8 h (≥ 28 weeks GA) for 48 h. A total of 18 sequence variations (SVs) in UDP-glucuronosyltransferase (UGT), sulfotransferase (SULT), and cytochrome P450 (CYP) genes from 33 neonates (aged 1-26 days) were examined in a stepwise manner for an effect on the metabolic formation clearance of acetaminophen by glucuronidation (UGT), sulfation (SULT), and oxidation (CYP). The stepwise covariate modeling procedure was performed using NONMEM^® version 7.3.

RESULTS

Incorporation of genotype as a covariate for one SV located in the UGT1A9 gene promoter region (rs3832043, - 118 > insT, T₉ > T₁₀) significantly improved model fit (likelihood ratio test, p < 0.001) and reduced between-subject variability in glucuronide formation clearance. Individuals with the UGT1A9 T₁₀ polymorphism, indicating insertion of an additional thymidine nucleotide, had a 42% reduction in clearance to APAP-glucuronide as compared to their wild-type counterparts.

CONCLUSION

This study shows a pharmacogenetic effect of an SV in the UGT1A9 promoter region on the metabolism of acetaminophen in neonates.

Effects of Comedication and Genetic Factors on the Population Pharmacokinetics of Lamotrigine: A Prospective Analysis in Chinese Patients With Epilepsy

Lamotrigine (LTG) is a second-generation anti-epileptic drug widely used for focal and generalized seizures in adults and children, and as a first-line medication in pregnant women and women of childbearing age. However, LTG pharmacokinetics shows high inter-individual variability, thus potentially leading to therapeutic failure or side effects in patients. This prospective study aimed to establish a population pharmacokinetics model for LTG in Chinese patients with epilepsy and to investigate the effects of genetic variants in uridine diphosphate glucuronosyltransferase (UGT) 1A4, UGT2B7, MDR1, ABCG2, ABCC2, and SLC22A1, as well as non-genetic factors, on LTG pharmacokinetics. The study population consisted of 89 patients with epilepsy, with 419 concentrations of LTG. A nonlinear mixed effects model was implemented in NONMEM software. A one-compartment model with first-order input and first-order elimination was found to adequately characterize LTG concentration. The population estimates of the apparent volume of distribution (V/F) and apparent clearance (CL/F) were 12.7 L and 1.12 L/h, respectively. The use of valproic acid decreased CL/F by 38.5%, whereas the co-administration of rifampicin caused an increase in CL/F of 64.7%. The CL/F decreased by 52.5% in SLC22A1-1222AA carriers. Patients with the ABCG2-34AA genotype had a 42.0% decrease in V/F, whereas patients with the MDR1-2677TT and C3435TT genotypes had a 136% increase in V/F. No obvious genetic effect of UGT enzymes was found relative to the concentrations of LTG in Chinese patients. Recommended dose regimens for patients with different gene polymorphisms and comedications were estimated on the basis of Monte Carlo simulations and the established model. These findings should be valuable for developing individualized dosage regimens in adult and adolescent Chinese patients 13–65 years of age.

Impact of UGT2B7 and ABCC2 genetic polymorphisms on mycophenolic acid metabolism in Chinese renal transplant recipients

AIM

To evaluate genetic variants affecting mycophenolic acid (MPA) metabolism in Chinese renal transplant recipients.

METHODS

Total 11 SNPs of UGT1A9, UGT1A8, UGT2B7, ABCC2, ABCG2 and SLCO1B3 were genotyped in 408 Chinese renal transplant recipients. Associations between SNPs and MPA concentration/dose ratio (C₀/D) were analyzed using different genetic models. Multivariate linear regression was used to analyze associations between log (C₀/D) and clinical factors. Results: After adjustment by clinical factors, UGT2B7 rs7662029 was associated with log (C₀/D) using a dominant (p = 0.041) and an additive (p = 0.038) model, ABCC2 rs717620 was associated with log (C₀/D) using a recessive model (p = 0.019). Using additive model, SNP-SNP interactions were identified (p = 0.002) between ABCC2 rs717620 and UGT1A9 rs2741049, with interactions (p = 0.002) between ABCC2 rs717620 and UGT1A8 rs1042597. Age, albumin and serum creatinine were associated with log (C₀/D).

CONCLUSION

rs7662029 and rs717620 may affect MPA pharmacokinetics. SNP-SNP interactions and clinical factors may have significant effects on MPA metabolism.

Pharmacokinetics and pharmacogenomics of mycophenolic acid and its clinical correlations in maintenance immunosuppression for lupus nephritis

Background

There is little data on mycophenolic acid (MPA) pharmacokinetics and pharmacogenomics and optimal MPA exposure in lupus nephritis (LN) patients during long-term maintenance.

Methods

We measured blood MPA levels at 1, 2, 4, 8, 10 and 12-h post-dose (i.e. C1, C2, C4, C8, C10 and C12) in 88 stable LN patients receiving maintenance prednisolone and mycophenolate mofetil, repeated every 6 months. The relationship between MPA exposure and single nucleotide polymorphisms (SNPs) of adenosine triphosphate–binding cassette subfamily C member 2 (ABCC2; rs2273697, rs3740066, rs717620 and rs17222723), organic anion-transporting polypeptides (OATPs; rs7311358 and rs4149117) and uridine diphosphate glucuronosyltransferase (UGT; rs17863762, rs6714486, rs17868320 and rs72551330) was also investigated.

Results

C1, C2 and C12 were 8.3 ± 6.6 , 7.2 ± 5.2 and 2.0 ± 1.4 mg/L and all correlated with the 12-h area under the curve (AUC0–12; r = 0.51, 0.85 and 0.73; P = 0.02, &lt;0.001 and &lt;0.001, respectively). C12 inversely correlated with hemoglobin, immunoglobulins and leukocyte levels (P &lt; 0.05 for all). Five renal flares, 11 episodes of infection and 10 episodes of anemia (hemoglobin &lt;10 g/dL) occurred over 96 weeks, with a corresponding C12 of 1.3 ± 0.5, 4.3 ± 2.6 and 2.9 ± 1.5 mg/L, respectively (versus 2.4 ± 1.2, 1.8 ± 1.2 and 1.7 ± 1.1 mg/L in patients without these complications; P = 0.041, &lt;0.001 and 0.004). SNP rs2273697 A/G in the ABCC2 gene was associated with lower MPA exposure compared with G/G (1075.9 ± 239.9 versus 1891.5 ± 918.9 mgh/L per g/kg; P = 0.003). SNPs of OATP and UGT were unrelated to MPA level.

Conclusion

MPA C12 correlates with the AUC0–12 and is related to renal flare, infection and anemia. SNP rs2273697 A/G is associated with lower MPA exposure.

The Pharmacogenetics of Immune-Modulating Therapy

Immunosuppressive drugs are a prerequisite in organ transplantation to prevent rejection and are also widely used in inflammatory diseases such as inflammatory bowel disease (IBD) or also in some hematologic malignancies-depending on the mode of action. For thiopurine analogs the polymorphic thiopurine S-methyltransferase (TPMT) was early detected to be associated with thiopurine-induced leukopenia; recent studies identified also NUDT15 to be related to this severe side effect. For drugs like methotrexate and mycophenolate mofetil a number of ADME genes like UDP-glucuronosyltransferases (UGTs) and ABC efflux transporters were investigated, however, with partly contradicting results. For calcineurin inhibitors like cyclosporine and in particular tacrolimus however, cytochrome P450 3A4 and 3A5 variants were found to significantly affect the pharmacokinetics. Genetic variants in genes encoding relevant pharmacodynamic proteins, however, lacked compelling evidence to affect the clinical outcome. This chapter reviews the current evidence on the association of pharmacogenetic traits to dose finding and clinical outcome of small-molecule immunosuppressants. Moreover this chapter critically summarizes suitability to apply pharmacogenetics in clinical practice in order to optimize immunosuppressant therapy.

ABCC2 polymorphisms and survival in the Princess Margaret cohort study and the NCIC clinical trials group BR.24 trial of platinum-treated advanced stage non-small cell lung cancer patients

BACKGROUND

The drug transporter ABCC2 is upregulated in non-small cell lung cancer (NSCLC) and implicated in platinum resistance. We evaluated the association between germline polymorphisms in the ABCC2 gene and survival outcomes of platinum-treated advanced NSCLC patients.

MATERIAL AND METHODS

Ten candidate and tagging germline polymorphisms in the ABCC2 gene were genotyped in a discovery cohort of 170 platinum-treated stage IV NSCLC patients from the Princess Margaret Cancer Centre. Associations with overall survival were assessed using multivariate Cox proportional hazard models adjusted for prognostic variables. To validate our results, we analyzed the association of the two top polymorphisms in the ABCC2 gene on survival outcomes of 219 stage IIIB-IV NSCLC patients enrolled on the NCIC Clinical Trials Group BR.24 clinical trial.

RESULTS

Only one polymorphism was validated across both cohorts for an association with overall survival: the A allele of the ABCC2 polymorphism, rs8187710 (4544G>A), was associated with adverse overall survival (adjusted hazard ratio [aHR] 2.22; 95% CI: 1.2-4.0; p=0.009) among our stage IV NSCLC patients. A significant association with overall survival (aHR 1.73; 95% CI: 1.0-2.9; p=0.036) was observed for the same ABCC2 polymorphism in the BR.24 validation cohort. No other ABCC2 polymorphisms were associated with outcome.

CONCLUSION

The ABCC2 polymorphism, rs8187710 (4544G>A), is associated with overall survival in platinum-treated advanced NSCLC patients. Additional studies are needed to evaluate the predictive versus prognostic nature of this relationship, and to explore the functional effect of this polymorphism on the pharmacokinetics of platinum drugs.

An Expanded Analysis of Pharmacogenetics Determinants of Efavirenz Response that Includes 3'-UTR Single Nucleotide Polymorphisms among Black South African HIV/AIDS Patients

Introduction: Efavirenz (EFV) is a non-nucleoside reverse transcriptase inhibitor prescribed as part of first-line highly active antiretroviral therapy (HAART) in South Africa. Despite administration of fixed doses of EFV, inter-individual variability in plasma concentrations has been reported. Poor treatment outcomes such as development of adverse drug reactions or treatment failure have been linked to EFV plasma concentrations outside the therapeutic range (1–4 μg/mL) in some studies. The drug metabolizing enzyme (DME), CYP2B6, is primarily responsible for EFV metabolism with minor contributions by CYP1A2, CYP2A6, CYP3A4, CYP3A5, and UGT2B7. DME coding genes are also regulated by microRNAs through targeting the 3′-untranslated region. Expanded analysis of 30 single nucleotide polymorphisms (SNPs), including those in the 3′-UTR, was performed to identify pharmacogenetics determinants of EFV plasma concentrations in addition to CYP2B6 c.516G>T and c.983T>C SNPs.

Methods: SNPs in CYP1A2, CYP2B6, UGT2B7, and NR1I2 (PXR) were selected for genotyping among 222 Bantu-speaking South African HIV-infected patients receiving EFV-containing HAART. This study is a continuation of earlier pharmacogenetics studies emphasizing the role of genetic variation in the 3′-UTR of genes which products are either pharmacokinetic or pharmacodynamic targets of EFV.

Results: Despite evaluating thirty SNPs, CYP2B6 c.516G>T and c.983T>C SNPs remain the most prominent predictors of EFV plasma concentration.

Conclusion: We have shown that CYP2B6 c.516G>T and c.983T>C SNPs are the most important predictors of EFV plasma concentration after taking into account all other SNPs, including genetic variation in the 3′-UTR, and variables affecting EFV metabolism.

Impact of Single Nucleotide Polymorphisms (SNPs) on Immunosuppressive Therapy in Lung Transplantation

Lung transplant patients present important variability in immunosuppressant blood concentrations during the first months after transplantation. Pharmacogenetics could explain part of this interindividual variability. We evaluated SNPs in genes that have previously shown correlations in other kinds of solid organ transplantation, namely ABCB1 and CYP3A5 genes with tacrolimus (Tac) and ABCC2, UGT1A9 and SLCO1B1 genes with mycophenolic acid (MPA), during the first six months after lung transplantation (51 patients). The genotype was correlated to the trough blood drug concentrations corrected for dose and body weight (C0/Dc). The ABCB1 variant in rs1045642 was associated with significantly higher Tac concentration, at six months post-transplantation (CT vs. CC). In the MPA analysis, CT patients in ABCC2 rs3740066 presented significantly lower blood concentrations than CC or TT, three months after transplantation. Other tendencies, confirming previously expected results, were found associated with the rest of studied SNPs. An interesting trend was recorded for the incidence of acute rejection according to NOD2/CARD15 rs2066844 (CT: 27.9%; CC: 12.5%). Relevant SNPs related to Tac and MPA in other solid organ transplants also seem to be related to the efficacy and safety of treatment in the complex setting of lung transplantation.

Associations of UDP-glucuronosyltransferases polymorphisms with mycophenolate mofetil pharmacokinetics in Chinese renal transplant patients

AIM

To evaluate the effects of UDP-glucuronosyltransferases (UGTs) polymorphisms on the pharmacokinetics of the immunosuppressant mycophenolate mofetil (MMF) in Chinese renal transplant recipients.

METHODS

A total of 127 renal transplant patients receiving MMF were genotyped for polymorphisms in UGT1A9 -1818T>C, I399C>T, -118T9/10, -440C>T, -331T>C, UGT2B7 IVS1+985A>G, 211G>T, -900A>G, UGT1A8 518C>G and UGT1A7 622T>C. The plasma concentrations of the MMF active moiety mycophenolic acid (MPA) and main metabolite 7-O-MPA-glucuronide (MPAG) were analyzed using HPLC. Univariate and multivariate analyses were used to assess the effects of UGT-related gene polymorphisms on MPA pharmacokinetics.

RESULTS

The dose-adjusted MPA AUC0-12 h of the patients with the UGT2B7 IVS1+985AG genotype was 48% higher than that of the patients with the IVS1+985AA genotype, which could explain 11.2% of the inter-individual variation in MPA pharmacokinetics. The dose-adjusted MPAG AUC0-12 h of the patients with the UGT1A7 622CC and UGT1A9 -440CT/-331TC genotypes, respectively, was significantly higher than that of the patients with 622T homozygotes and -440C/-331T homozygotes. Furthermore, the genotypes UGT1A9 -1818T>C and UGT1A8 518C>G were associated with a low dose-adjusted MPAG AUC0-12 h.

CONCLUSION

The UGT2B7 11+985A>G genotype is associated with the pharmacokinetics of MPA in Chinese renal transplant patients, which demonstrates the usefulness of this SNP for individualizing MMF dosing.

Polymorphisms of the drug transporters ABCB1, ABCG2, ABCC2 and ABCC3 and their impact on drug bioavailability and clinical relevance

INTRODUCTION

Human ATP-binding cassette (ABC) transporters act as translocators of numerous substrates across extracellular and intracellular membranes, thereby contributing to bioavailability and consequently therapy response. Genetic polymorphisms are considered as critical determinants of expression level or activity and subsequently response to selected drugs.

AREAS COVERED

Here the influence of polymorphisms of the prominent ABC transporters P-glycoprotein (MDR1, ABCB1), breast cancer resistance protein (BCRP, ABCG2) and the multidrug resistance-associated protein (MRP) 2 (ABCC2) as well as MRP3 (ABCC3) on the pharmacokinetic of drugs and associated consequences on therapy response and clinical outcome is discussed.

EXPERT OPINION

ABC transporter genetic variants were assumed to affect interindividual differences in pharmacokinetics and subsequently clinical response. However, decades of medical research have not yielded in distinct and unconfined reproducible outcomes. Despite some unique results, the majority were inconsistent and dependent on the analyzed cohort or study design. Therefore, variability of bioavailability and drug response may be attributed only by a small amount to polymorphisms in transporter genes, whereas transcriptional regulation or post-transcriptional modification seems to be more critical. In our opinion, currently identified genetic variants of ABC efflux transporters can give some hints on the role of transporters at interfaces but are less suitable as biomarkers to predict therapeutic outcome.

Renal glucuronidation and multidrug resistance protein 2-/ multidrug resistance protein 4-mediated efflux of mycophenolic acid: interaction with cyclosporine and tacrolimus

Mycophenolic acid (MPA) is an immunosuppressant used in transplant rejection, often in combination with cyclosporine (CsA) and tacrolimus (Tac). The drug is cleared predominantly via the kidneys, and 95% of the administered dose appears in urine as 7-hydroxy mycophenolic acid glucuronide (MPAG). The current study was designed to unravel the renal excretory pathway of MPA and MPAG, and their potential drug-drug interactions. The role of multidrug resistance protein (MRP) 2 and MRP4 in MPA disposition was studied using human embryonic kidney 293 (HEK293) cells overexpressing the human transporters, and in isolated, perfused kidneys of Mrp2-deficient rats and Mrp4-deficient mice. Using these models, we identified MPA as substrate of MRP2 and MRP4, whereas its MPAG appeared to be a substrate of MRP2 only. CsA inhibited MPAG transport via MRP2 for 50% at 8 μM (P < 0.05), whereas Tac had no effect. This was confirmed by cell survival assays, showing a 10-fold increase in MPA cytotoxicity (50% reduction in cell survival changed from 12.2 ± 0.3 μM to 1.33 ± 0.01 μM by MPA + CsA; P < 0.001) and in perfused kidneys, showing a 50% reduction in MPAG excretion (P < 0.05). The latter effect was observed in Mrp2-deficient animals as well, supporting the importance of Mrp2 in MPAG excretion. CsA, but not Tac, inhibited MPA glucuronidation by rat kidney homogenate and human uridine 5'-diphospho-glucuronosyltransferase-glucuronosyltransferase 1A9 (P < 0.05 and P < 0.01, respectively). We conclude that MPA is a substrate of both MRP2 and MRP4, but MRP2 is the main transporter involved in renal MPAG excretion. In conclusion, CsA, but not Tac, influences MPA clearance by inhibiting renal MPA glucuronidation and MRP2-mediated MPAG secretion.

Genes and beans: pharmacogenomics of renal transplant

Advances in the management of patients after solid organ transplantation have led to dramatic decreases in rates of acute rejection, but long-term graft and patient survival have remained unchanged. Individualized therapy after transplant will ideally provide adequate immunosuppression while limiting the adverse effects of drug therapy that significantly impact graft survival. Therapeutic drug monitoring represents the best approximation of individualized drug therapy in transplant at this time; however, obtaining pharmacogenomic data in transplant patients has the potential to enhance our current practice. Polymorphisms of target genes that impact pharmacokinetics have been identified for most immunosuppressants, including tacrolimus, cyclosporine, mycophenolate, azathioprine and sirolimus. In the future, pre-emptive assessment of a patient's genetic profile may inform drug selection and provide information on specific doses that will improve efficacy and limit toxicity.

Association of ABCC2 -24C>T polymorphism with high-dose methotrexate plasma concentrations and toxicities in childhood acute lymphoblastic leukemia

Methotrexate (MTX) is a key agent for the treatment of childhood acute lymphoblastic leukemia (ALL). Increased MTX plasma concentrations are associated with a higher risk of adverse drug effects. ATP-binding cassette subfamily C member 2 (ABCC2) is important for excretion of MTX and its toxic metabolite. The ABCC2 −24C>T polymorphism (rs717620) reportedly contributes to variability of MTX kinetics. In the present study, we assessed the association between the ABCC2 −24C>T polymorphism and methotrexate (MTX) toxicities in childhood ALL patients treated with high-dose MTX. A total of 112 Han Chinese ALL patients were treated with high-dose MTX according to the ALL-Berlin-Frankfurt-Muenster 2000 protocol. Our results showed that presence of the −24T allele in ABCC2 gene led to significantly higher MTX plasma concentrations at 48 hours after the start of infusion, which would strengthen over repeated MTX infusion. The −24T allele in ABCC2 gene was significantly associated with higher risks of high-grade hematologic (leucopenia, anemia, and thrombocytopenia) and non-hematologic (gastrointestinal and mucosal damage/oral mucositis) MTX toxicities. This study provides the first evidence that the −24T allele in ABCC2 gene is associated with the severity of MTX toxicities, which add fresh insights into clinical application of high-dose MTX and individualization of MTX treatment.

Relevance of UDP-glucuronosyltransferase polymorphisms for drug dosing: A quantitative systematic review

UDP-glucuronosyltransferases (UGT) catalyze the biotransformation of many endobiotics and xenobiotics, and are coded by polymorphic genes. However, knowledge about the effects of these polymorphisms is rarely used for the individualization of drug therapy. Here, we present a quantitative systematic review of clinical studies on the impact of UGT variants on drug metabolism to clarify the potential for genotype-adjusted therapy recommendations. Data on UGT polymorphisms and dose-related pharmacokinetic parameters in man were retrieved by a systematic search in public databases. Mean estimates of pharmacokinetic parameters were extracted for each group of carriers of UGT variants to assess their effect size. Pooled estimates and relative confidence bounds were computed with a random-effects meta-analytic approach whenever multiple studies on the same variant, ethnic group, and substrate were available. Information was retrieved on 30 polymorphic metabolic pathways involving 10 UGT enzymes. For irinotecan and mycophenolic acid a wealth of data was available for assessing the impact of genetic polymorphisms on pharmacokinetics under different dosages, between ethnicities, under comedication, and under toxicity. Evidence for effects of potential clinical relevance exists for 19 drugs, but the data are not sufficient to assess effect size with the precision required to issue dose recommendations. In conclusion, compared to other drug metabolizing enzymes much less systematic research has been conducted on the polymorphisms of UGT enzymes. However, there is evidence of the existence of large monogenetic functional polymorphisms affecting pharmacokinetics and suggesting a potential use of UGT polymorphisms for the individualization of drug therapy.

Population pharmacokinetics of unbound mycophenolic acid in adult allogeneic haematopoietic cell transplantation: effect of pharmacogenetic factors

AIM

To evaluate pharmacogenetic factors as contributors to the variability of unbound mycophenolic acid (MPA) exposure in adult allogeneic haematopoietic cell transplantation (alloHCT) recipients.

METHODS

A population-based pharmacokinetic (PK) model of unbound MPA was developed using non-linear mixed-effects modelling (nonmem). Previously collected intensive unbound MPA PK data from 132 adult alloHCT recipients after oral and intravenous dosing of the prodrug mycophenolate mofetil (MMF) were used. In addition to clinical covariates, genetic polymorphisms in UGT1A8, UGT1A9, UGT2B7 and MRP2 were evaluated for their impact on unbound MPA PK.

RESULTS

Unbound MPA concentration-time data were well described by a two compartment model with first order absorption and linear elimination. For the typical patient (52 years of age, creatinine clearance 86 ml min(-1)), the median estimated values [coefficient of variation, %, (CV)] of systemic clearance, intercompartmental clearance, central and peripheral volumes of MPA were 1610 l h(-1) (37.4%), 541 l h(-1) (75.6%), 1230 l (37.5%), and 6140 l (120%), respectively. After oral dosing, bioavailability was low (0.56) and highly variable (CV 46%). No genetic polymorphisms tested significantly explained the variability among individuals. Creatinine clearance was a small but significant predictor of unbound MPA CL. No other clinical covariates impacted unbound MPA PK.

CONCLUSIONS

In adult alloHCT recipients, variability in unbound MPA AUC was large and remained largely unexplained even with the inclusion of pharmacogenetic information. Targeting unbound MPA AUC in a patient will require therapeutic drug monitoring.

The influence of UGT polymorphisms as biomarkers in solid organ transplantation

In solid organ transplant patients, it is important to maintain a fine balance between preventing rejection and reducing adverse effects. Several immunosuppressive agents such tacrolimus, cyclosporine, sirolimus and everolimus require therapeutic drug monitoring. The study of germline variation of the genome has opened novel opportunities to individualize therapy. Among the currently available immunosuppressive agents, cyclosporine, tacrolimus and mycophenolic acid are in vitro substrates of the UGT1A and 2B families of glucuronidation enzymes. Mycophenolic acid, either given as mycophenolate mofetil or mycophenolate sodium, is the most frequently used antiproliferative immunosuppressant. Mycophenolic acid is a prodrug which is rapidly de-esterified in the gut wall, blood, liver and tissue to the active moiety, mycophenolic acid (MPA). MPA undergoes significant hepatic metabolism to several metabolites. The 7-hydroxyglucuronide MPA is the major metabolite and is inactive. This paper reviews the current status of the genetic associations between germline UGT variants and the pharmacokinetics and pharmacodynamics of mycophenolic acid. Our conclusive assessment of the studies conducted so far is that these germline markers are not ready to be used in the clinic to individualize mycophenolic acid dosing and improve outcome. Novel approaches are required to identify new genetic determinants of outcomes in transplantation.

The association of the UGT1A8, SLCO1B3 and ABCC2/ABCG2 genetic polymorphisms with the pharmacokinetics of mycophenolic acid and its phenolic glucuronide metabolite in Chinese individuals

BACKGROUND

This study aimed to evaluate the effect of UGT1A8*2, SLCO1B3 T334G, ABCC2 C-24T and ABCG2 C421A polymorphisms on the pharmacokinetics (PKs) of mycophenolic acid (MPA) and its phenolic glucuronide (MPAG) in healthy Chinese volunteers and in stable renal transplant patients.

METHODS

The data were extracted from comparative bioavailability studies conducted in 42 healthy individuals and 37 renal transplant patients. A complete PK profile was obtained over 48 h for healthy volunteers and over 12h for the transplant patients. The MPA/MPAG plasma concentrations were measured by HPLC. The genotypes were determined using either the Taqman probe technique or direct sequencing. A multivariate analysis was used to assess the effect of the genotypes (UGT1A8*2, SLCO1B3 T334G, ABCC2 C-24T and ABCG2 C421A) and other covariates (age, weight, height, calculated creatinine clearance, serum albumin, haemoglobin and drug comedication) on the AUC(4-12) and AUC(0-12) for MPA and MPAG in the healthy volunteers and patients.

RESULTS

In the healthy volunteers, the dose-adjusted geometric means (GM) of the MPA AUC(4-12) in individuals with the SLCO1B3 334T allele were 30.4% lower than those values in the 334G homozygote carriers (P<0.05); in the transplant patients, the steroid dose was associated with a negative effect on the AUC of MPAG (P<0.03) and weight was associated with a negative effect on the AUC for MPA in the healthy volunteers and patients (P<0.03). No other significant effect of genotype or of the other studied variables on AUC(4-12) or AUC(0-12) of MPA/MPAG was found in the healthy volunteers or patients.

CONCLUSIONS

The PKs of MPA is affected by the SLCO1B3 polymorphism in healthy Chinese individuals. The absence of an effect of SLCO1B3 polymorphisms in transplant patients may be due to the co-administration of cyclosporine (CsA). Concomitant steroid dose and weight are two important covariates of the AUC of MPA and MPAG, which should be taken into account in clinical use. Further confirmatory in vivo studies are needed.

ATP-binding cassette transporters as pharmacogenetic biomarkers for kidney transplantation

Immunosuppressive drugs used in organ transplantation are highly effective in preventing acute rejection. However, the clinical use of these drugs is complicated by the fact that they display highly variable pharmacokinetics and pharmacodynamics between individual patients. The influence of genetic variation on the interindividual variability in immunosuppressive drug disposition, efficacy, and toxicity has been explored in recent years. The polymorphically-expressed ATP-binding cassette (ABC) transporter proteins, in particular ABCB1 and ABCC2, have been investigated extensively because they play an important role in the absorption, distribution and elimination of many immunosuppressive drugs in use today. From these studies it can be concluded that polymorphisms in ABCB1 and ABCC2 have no consistent effect on immunosuppressant pharmacokinetics and toxicity although polymorphisms in ABCB1 appear to be related to the risk of developing calcineurin inhibitor-related nephrotoxicity. However, the latter needs to be replicated before an individual's ABCB1 genotype can become a useful marker that is applied in clinical practice. Future studies evaluating the influence of ABC transporter gene polymorphisms should explore the relationship with intracellular rather than systemic drug concentrations further in well-designed clinical studies. Until then, single-nucleotide polymorphisms in ABC transporter genes are not suitable to act as biomarkers for solid organ transplantation.

The influence of pharmacogenetics and cofactors on clinical outcomes in kidney transplantation

INTRODUCTION

Immunosuppressive drugs have a narrow therapeutic range and large inter-individual response variability. This has prompted pharmacogenetic studies, mostly with regard to their dose-concentration relationships, but also about proteins involved in their pharmacodynamics. Some polymorphisms of genes involved in their disposition pathways were shown to affect their dose-concentration relationships. The impact of pharmacogenetics on tissue distribution and the resulting clinical effects have less often been studied. More importantly, a few single nucleotide polymorphisms seem to have a significant impact on the incidence of acute rejection or the adverse effects of immunosuppressants. Environmental factors often interact with such genotype-phenotype relationships.

AREAS COVERED

This article reviews the impact of genetic polymorphisms of the metabolic enzymes, membrane transporters and target proteins of mycophenolic acid, calcineurin inhibitors and mammalian target of rapamycin inhibitors on clinical outcomes in kidney transplantation.

EXPERT OPINION

The current level of evidence is not yet high enough to recommend pharmacogenetic personalization of immunosuppressive regimens in transplant recipients. The prevention of cellular toxicity associated with local metabolism or transport, which cannot be addressed by routine monitoring, is worth investigating further.

Pharmacogenetic impact of UDP-glucuronosyltransferase metabolic pathway and multidrug resistance-associated protein 2 transport pathway on mycophenolic acid in thoracic transplant recipients: an exploratory study

STUDY OBJECTIVE

To assess the contribution of polymorphisms in the uridine diphosphate glucuronosyltransferase gene (UGT) and the multidrug resistance-associated protein 2 gene (ABCC2) to mycophenolic acid (MPA) pharmacokinetics and clinical outcomes in thoracic transplant recipients.

DESIGN

Open-label, cross-sectional study.

SETTING

Transplant clinic in Vancouver, British Columbia, Canada.

PATIENTS

Sixty-eight thoracic (36 lung, 32 heart) transplant recipients who were receiving steady-state oral mycophenolate mofetil.

MEASUREMENTS AND MAIN RESULTS

Eleven blood samples were obtained from each patient over a 12-hour dosing period. Plasma concentrations of MPA (active metabolite of mycophenolate mofetil), the MPA metabolites 7-Omycophenolic acid glucuronide (MPAG) and acyl glucuronide (AcMPAG), and free MPA were measured, and dose-normalized conventional pharmacokinetic parameters were determined by noncompartmental methods. Genetic polymorphisms in UGT and ABCC2 were determined by sequencing, and their contributions to pharmacokinetic variability were investigated by using multivariate analysis. For both the lung and heart transplant groups, the UGT2B7 variant 802T (Tyr(268) or UGT2B7*2, rs7439366) and the UGT2B7 variant -138A modified AcMPAG exposure (2.5-3.7-fold and 9.3-12.3-fold higher AcMPAG area under the concentration-time curve [AUC] and AcMPAG:MPA ratio, respectively). In an exploratory analysis, occurrences of rejection, infection, anemia, and leukopenia were associated with an AcMPAG AUC greater than 50 μg·hour/ml and an AcMPAG:MPA ratio greater than 2.

CONCLUSION

UGT2B7 is a promising gene candidate that may influence MPA pharmacokinetics clinically; however, larger clinical pharmacogenetic studies in thoracic transplant subpopulations are warranted to corroborate the role of AcMPAG and UGT2B7 variants in optimizing mycophenolate mofetil therapy.

Effects of uridine diphosphate glucuronosyltransferase 2B7 and 1A7 pharmacogenomics and patient clinical parameters on steady-state mycophenolic acid pharmacokinetics in glomerulonephritis

PURPOSE

The role of pharmacogenomics, clinical and demographic parameters in pharmacokinetic predictions was evaluated in patients receiving mycophenolic acid (MPA).

METHODS

A cohort study design of patients with glomerulonephritis secondary to lupus nephritis and anti-neutrophil cytoplasmic antibody (ANCA) vasculitis was employed. Forty-six patients with lupus nephritis and ANCA vasculitis who were receiving MPA were recruited from the nephrology clinic. The study assessed the relative single and combined roles of genomic, clinical, and demographic characteristics on pharmacokinetic parameters using general linear models. The study focused on polymorphisms in UGT1A7, UGT2B7, and ABCB1/MDR1; all of which have limited data available concerning MPA pharmacokinetics. All patients had pharmacokinetic assessments for MPA and glucuronide metabolites (MPAG, AcMPAG). Genotyping was performed for known variants of UGTs (UGT1A9, UGT1A7, UGT2B7), and multidrug resistance protein (ABCB1/MDR1), involved in MPA disposition. Analyses included univariate and multivariate linear modeling.

RESULTS

In univariate analyses, UGT2B7 heterozygosity (coefficient 0.3508; R (2)=0.0873) and UGT1A7 heterozygosity (coefficient 0.3778; R (2)=0.0966) predicted increased apparent oral clearance of MPA. UGT1A7 heterozygosity (coefficient -0.4647; R (2) 0.0897) predicted lower MPA trough concentrations. In multivariate assessments, higher urinary protein excretion, lower serum creatinine, and increased weight predicted greater apparent oral clearance of MPA (p < 0.0001). White race and higher serum creatinine predicted higher MPA trough concentrations (p < 0.0001). Higher exposure to MPA was predicted by decreased urinary protein excretion and increased serum creatinine.

CONCLUSIONS

Clinical and demographic parameters were 2-4 times more important in MPA disposition than genotypes and explained 30-40% of the pharmacokinetic parameters.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Pharmacogenetic influences on mycophenolate therapy

Mycophenolic acid (MPA) is a cornerstone immunosuppressant therapy in solid organ transplantation. MPA is metabolized by uridine diphosphate glucuronosyltransferase to inactive 7-O-MPA-glucuronide (MPAG). At least three minor metabolites are also formed, including a pharmacologically active acyl-glucuronide. MPA and MPAG are subject to enterohepatic recirculation. Biliary excretion of MPA/MPAG involves several transporters, including organic anion transporting polypeptides and multidrug resistant protein-2 (MRP-2). MPA metabolites are also excreted via the kidney, at least in part by MRP-2. MPA exerts its immunosuppressive effect through the inhibition of inosine-5-monophosphate dehydrogenase. Several SNPs have been identified in the genes encoding for uridine diphosphate glucuronosyltransferase, organic anion transporting polypeptides, MRP-2 and inosine-5-monophosphate dehydrogenase. This article provides an extensive overview of the known effects of these SNPs on the pharmacokinetics and pharmacodynamics of MPA.

Limited sampling strategies for mycophenolic acid in solid organ transplantation: a systematic review

BACKGROUND

Mycophenolic acid (MPA) is the active metabolite of mycophenolate mofetil, a widely used immunosuppressant. Numerous studies have developed limited sampling strategies (LSSs) to predict MPA AUC in solid organ transplant recipients.

OBJECTIVES

To systematically review and assess quality of literature pertaining to MPA LSSs, evaluate clinical implications and provide suggestions for future research.

METHODS

Literature searches of MEDLINE (1966 - May 2009) and EMBASE (1980 - May 2009) for English articles in solid organ transplantation, along with manual review of article references were conducted. Included articles were categorized according to criteria adapted from levels of evidence of the US Preventative Services Task Force.

RESULTS

Of a total of 29 studies identified, 20 were in kidney, 4 in heart, 4 in liver and 1 in lung transplantation and 7 were in pediatrics. A total of 14 studies were deemed to be Level I evidence studies, 3 were Level II-1, 1 was Level II-2 and 11 were Level III.

CONCLUSIONS

Although various LSSs that are well correlated to MPA AUC while being relatively unbiased and precise to predict MPA AUC have been developed, further research is needed to determine validity of these LSSs in a variety of patient populations and to determine if these LSSs improve patient outcomes.

Pharmacogenomics: a new paradigm to personalize treatments in nephrology patients

Although notable progress has been made in the therapeutic management of patients with chronic kidney disease in both conservative and renal replacement treatments (dialysis and transplantation), the occurrence of medication-related problems (lack of efficacy, adverse drug reactions) still represents a key clinical issue. Recent evidence suggests that adverse drug reactions are major causes of death and hospital admission in Europe and the United States. The reasons for these conditions are represented by environmental/non-genetic and genetic factors responsible for the great inter-patient variability in drugs metabolism, disposition and therapeutic targets. Over the years several genetic settings have been linked, using pharmacogenetic approaches, to the effects and toxicity of many agents used in clinical nephrology. However, these strategies, analysing single gene or candidate pathways, do not represent the gold standard, being the overall pharmacological effects of medications and not typically monogenic traits. Therefore, to identify multi-genetic influence on drug response, researchers and clinicians from different fields of medicine and pharmacology have started to perform pharmacogenomic studies employing innovative whole genomic high-throughput technologies. However, to date, only few pharmacogenomics reports have been published in nephrology underlying the need to enhance the number of projects and to increase the research budget for this important research field. In the future we would expect that, applying the knowledge about an individual's inherited response to drugs, nephrologists will be able to prescribe medications based on each person's genetic make-up, to monitor carefully the efficacy/toxicity of a given drug and to modify the dosage or number of medications to obtain predefined clinical outcomes.