Functional characterization of human UDP-glucuronosyltransferase 1A9 variant, D256N, found in Japanese cancer patients

Cannabinoids and Multiple Sclerosis: A Critical Analysis of Therapeutic Potentials and Safety Concerns

Multiple sclerosis (MS) is a complicated condition in which the immune system attacks myelinated axons in the central nervous system (CNS), destroying both myelin and axons to varying degrees. Several environmental, genetic, and epigenetic factors influence the risk of developing the disease and how well it responds to treatment. Cannabinoids have recently sparked renewed interest in their therapeutic applications, with growing evidence for their role in symptom control in MS. Cannabinoids exert their roles through the endogenous cannabinoid (ECB) system, with some reports shedding light on the molecular biology of this system and lending credence to some anecdotal medical claims. The double nature of cannabinoids, which cause both positive and negative effects, comes from their actions on the same receptor. Several mechanisms have been adopted to evade this effect. However, there are still numerous limitations to using cannabinoids to treat MS patients. In this review, we will explore and discuss the molecular effect of cannabinoids on the ECB system, the various factors that affect the response to cannabinoids in the body, including the role of gene polymorphism and its relation to dosage, assessing the positive over the adverse effects of cannabinoids in MS, and finally, exploring the possible functional mechanism of cannabinoids in MS and the current and future progress of cannabinoid therapeutics.

Irinotecan toxicity during treatment of metastatic colorectal cancer: focus on pharmacogenomics and personalized medicine

Colorectal cancer, one of the most frequent types of cancer worldwide, has a high mortality rate. Irinotecan (CPT-11) has been approved for the treatment of advanced or metastatic disease either as a single agent or, more commonly, as part of combined chemotherapeutic regimens. Treatment with irinotecan is often accompanied by severe toxicity (e.g. neutropenia and diarrhea) that can result in treatment interruption or cessation, thus jeopardizing the patient's prognosis and quality of life. Irinotecan is bioactivated into its metabolite SN-38, which is subsequently detoxified by uridine diphosphate-glucuronosyl transferases (mainly UGT1A1). Further, ABC transporters (i.e. ABCB1, ABCC1-ABCC6, and ABCG2) are responsible for drug efflux into bile and urine whereas OATP transporters (SLCO1B1) enable its influx from blood into hepatocytes. Genetic polymorphisms in these enzymes/pumps may result in increased systemic SN-38 level, directly correlating with toxicity. Contemporary research is focused on the clinical implementation of genetic screenings for validated gene variations prior to treatment onset, allowing tailored individual doses or treatment regimens.

UDP-glucuronosyltransferases (UGTs) and their related metabolic cross-talk with internal homeostasis: A systematic review of UGT isoforms for precision medicine

UDP-glucuronosyltransferases (UGTs) are the primary phase II enzymes catalyzing the conjugation of glucuronic acid to the xenobiotics with polar groups for facilitating their clearance. The UGTs belong to a superfamily that consists of diverse isoforms possessing distinct but overlapping metabolic activity. The abnormality or deficiency of UGTs in vivo is highly associated with some diseases, efficacy and toxicity of drugs, and precisely therapeutic personality. Despite the great effects and fruitful results achieved, to date, the expression and functions of individual UGTs have not been well clarified, the inconsistency of UGTs is often observed in human and experimental animals, and the complex regulation factors affecting UGTs have not been systematically summarized. This article gives an overview of updated reports on UGTs involving the various regulatory factors in terms of the genetic, environmental, pathological, and physiological effects on the functioning of individual UGTs, in turn, the dysfunction of UGTs induced disease risk and endo- or xenobiotic metabolism-related toxicity. The complex cross-talk effect of UGTs with internal homeostasis is systematically summarized and discussed in detail, which would be of great importance for personalized precision medicine.

Dimerization of human uridine diphosphate glucuronosyltransferase allozymes 1A1 and 1A9 alters their quercetin glucuronidation activities

Uridine diphosphate glucuronosyltransferase 1A (UGT1A) is a major phase II drug-metabolism enzyme superfamily involved in the glucuronidation of endobiotics and xenobiotics in humans. Many polymorphisms in UGT1A genes are reported to inhibit or decrease UGT1A activity. In this study, two UGT1A1 allozymes, UGT1A1 wild-type and a splice mutant, as well as UGT1A9 wild-type and its three UGT1A9 allozymes, UGT1A9*2(C3Y), UGT1A9*3(M33T), and UGT1A9*5(D256N) were single- or double-expressed in a Bac-to-Bac expression system. Dimerization of UGT1A1 or UGT1A9 allozymes was observed via fluorescence resonance energy transfer (FRET) and co-immunoprecipitation analysis. SNPs of UGT1A altered the ability of protein-protein interaction, resulting in differential FRET efficiencies and donor-acceptor r distances. Dimerization changed the chemical regioselectivity, substrate-binding affinity, and enzymatic activity of UGT1A1 and UGT1A9 in glucuronidation of quercetin. These findings provide molecular insights into the consequences of homozygous and heterozygous UGT1A1 and UGT1A9 allozymes expression on quercetin glucuronidation.

In vitro metabolism of canagliflozin in human liver, kidney, intestine microsomes, and recombinant uridine diphosphate glucuronosyltransferases (UGT) and the effect of genetic variability of UGT enzymes on the pharmacokinetics of canagliflozin in humans

O-glucuronidation is the major metabolic elimination pathway for canagliflozin. The objective was to identify enzymes and tissues involved in the formation of 2 major glucuronidated metabolites (M7 and M5) of canagliflozin and subsequently to assess the impact of genetic variations in these uridine diphosphate glucuronosyltransferases (UGTs) on in vivo pharmacokinetics in humans. In vitro incubations with recombinant UGTs revealed involvement of UGT1A9 and UGT2B4 in the formation of M7 and M5, respectively. Although M7 and M5 were formed in liver microsomes, only M7 was formed in kidney microsomes. Participants from 7 phase 1 studies were pooled for pharmacogenomic analyses. A total of 134 participants (mean age, 41 years; men, 63%; white, 84%) were included in the analysis. In UGT1A9*3 carriers, exposure of plasma canagliflozin (Cmax,ss , 11%; AUCτ,ss , 45%) increased relative to the wild type. An increase in exposure of plasma canagliflozin (Cmax,ss , 21%; AUCt,ss , 18%) was observed in participants with UGT2B4*2 genotype compared with UGT2B4*2 noncarriers. Metabolites further delineate the role of both enzymes. The pharmacokinetic findings in participants carrying the UGT1A9*3 and UGT2B4*2 allele implicate that UGT1A9 and UGT2B4 are involved in the metabolism of canagliflozin to M7 and M5, respectively.

The effect of UGT1A and UGT2B polymorphisms on colorectal cancer risk: haplotype associations and gene–environment interactions

UDP-glucuronosyltransferases (UGTs) play an important role in the phase II metabolism of exogenous and endogenous compounds. As colorectal cancer (CRC) etiology is thought to involve the biotransformation of dietary factors, UGT polymorphisms may affect CRC risk by altering levels of exposure. Genotyping of over 1800 Caucasian subjects was completed to identify the role of genetic variation in nine UGT1A and five UGT2B genes on CRC risk. Unconditional logistic regression and haplotype analyses were conducted to identify associations with CRC risk and potential gene-environment interactions. UGT1A haplotype analysis found that the T-G haplotype in UGT1A10 exon 1 (block 2: rs17864678, rs10929251) decreased cancer risk for the colon [proximal (OR = 0.28, 95% CI = 0.11–0.69) and for the distal colon (OR = 0.32, 95% CI = 0.12–0.91)], and that the C-T-G haplotype in the 3′ region flanking the UGT1A shared exons (block 11: rs7578153, rs10203853, rs6728940) increased CRC risk in males (OR = 2.56, 95% CI = 1.10–5.95). A haplotype in UGT2B15 containing a functional variant (rs4148269, K523T) and an intronic SNP (rs6837575) was found to affect rectal cancer risk overall (OR = 2.57, 95% CI = 1.21–5.04) and in females (OR = 3.08, 95% CI = 1.08–8.74). An interaction was found between high NSAID use and the A-G-T haplotype (block 10: rs6717546, rs1500482, rs7586006) in the UGT1A shared exons that decreased CRC risk. This suggests that UGT genetic variation alters CRC risk differently by anatomical sub-site and gender and that polymorphisms in the UGT1A shared exons may have a regulatory effect on gene expression that allows for the protective effect of NSAIDs on CRC risk.

Association analysis of UGT1A genotype and haplotype with SN-38 glucuronidation in human livers

AIM

7-ethyl-10-hydroxycamptothecin (SN-38), the active metabolite of irinotecan, is mainly eliminated hepatically through glucuronidation by UGT1A1 and UGT1A9 enzymes. This study comprehensively investigates the effects of UGT1A1 and UGT1A9 genetic polymorphism on SN-38 glucuronidation activity.

MATERIALS & METHODS

Genetic polymorphisms and combinational haplotypes of UGT1A1 and UGT1A9, SN-38 glucuronidation activities, and protein levels of UGT1A1 and UGT1A9 were determined using a set of over 45 Chinese livers.

RESULTS

UGT1A1 reduced function variants UGT1A1*6, *28, *60 and *1B exhibited additive effect. The number of UGT1A1 reduced function alleles was associated with decreased SN-38G formation rates and UGT1A protein levels. UGT1A9 I399C>T and UGT1A9*1b, which were highly linked, were associated with increased SN-38 glucuronidation activity and UGT1A protein levels. However, further analysis based on UGT1A9-1A1 haplotypes confirmed that their increased effect was partly due to their close linkage with UGT1A1 reduced function alleles.

CONCLUSION

UGT1A1 genetic polymorphisms have a more important function in human liver SN-38 glucuronidation activity than UGT1A9. Original submitted 7 November 2013; Revision submitted 30 January 2014.

Predictors of irinotecan toxicity and efficacy in treatment of metastatic colorectal cancer

The colorectal cancer ranks high among the malignant tumours in incidence and mortality and irinotecan is standardly used in palliative treatment of metastatic disease in every therapeutic line. Unfortunately, the treatment with irinotecan is often associated with severe toxicities, especially neutropenia and diarrhea. The majority of the toxic manifestation is caused by the insufficient deactivation (glucuronidation) of irinotecan active metabolite SN-38 by UGT1A enzyme. The elevated SN-38 plasma concentration is responsible for the hematological and gastrointestinal toxicity that can become life-threatening. The patients carrying the mutation of the gene encoding UGT1A enzyme lack the ability of bilirubin glucuronidation, and suffer from the inherited un-conjugated hyperbilirubinemia (Gilbert syndrome, Crigler-Najjar type 1 and 2 syndrome). The mutations in other enzyme systems also play role in the etiopathogenesis of the irinotecan toxicity: CYP3A (cytochrome P-450), ABC family of transmembrane transporters (adenosine-triphosphate binding cassette). The goal of the contemporary research is to determine the predictive factors that will enable the individual adjustment of the individual drug dosage while minimising the adverse effects and maintaining the treatment benefit.

DNA damage response pathways and cell cycle checkpoints in colorectal cancer: current concepts and future perspectives for targeted treatment

Although several drugs have been designed in the last few years to target specific key pathways and functions in colorectal cancer (CRC), the backbone of CRC treatment is still made up of compounds which rely on DNA damage to accomplish their role. DNA damage response (DDR) and checkpoint pathways are intertwined signaling networks that arrest cell cycle, recognize and repair genetic mistakes which arise during DNA replication and transcription, as well as through the exposure to chemical and physical agents that interact with nucleic acids. The good but highly variable activity of DNA damaging agents in the treatment of CRC suggests that intrinsic alterations in DDR pathways and cell cycle checkpoints may contribute differentially to the way cancer cells react to DNA damage. In the present review, our aim is to depict the recent advances in understanding the molecular basis of the activity of DNA damaging agents used for the treatment of CRC. We focus on the known and potential drug targets that are part of these complex and intertwined pathways. We describe the potential role of the checkpoints in CRC, and how their pharmacological manipulation could lead to chemopotentiation or synergism with currently used drugs. Novel therapeutic agents playing a role in DDR and checkpoint inhibition are assessed. We discuss the possible rationale for combining PARP inhibition with DNA damaging agents, and we address the link between DDR and EGFR pathways in CRC.

Functional impact and prevalence of polymorphisms involved in the hepatic glucuronidation of valproic acid

Metabolism of valproic acid, a widely used drug, is only partially understood. It is mainly metabolized through glucuronidation and acts as a substrate for various UDP-glucuronosyltransferases (UGTs). UGTs metabolizing valproic acid in the liver are UGT1A3, UGT1A4, UGT1A6, UGT1A9 and UGT2B7, with UGT1A6 and UGT2B7 being the most prominent. Polymorphisms in genes expressing these enzymes may have clinical consequences, regarding dosing, blood levels of the drug and adverse reactions. Not all genes are well studied and studies, where they exist, report conflicting results. Prevalence of polymorphisms and various haplotypes is also of great importance, as it may suggest different therapeutic approaches in various populations. Presented here is a review of currently known polymorphisms, their functional impact, when known, and their prevalence in different populations, highlighting the current state of understanding and areas where there is a lack of data and suggesting new perspectives for further research.

Do pharmacokinetic polymorphisms explain treatment failure in high-risk patients with neuroblastoma?

PURPOSE

Neuroblastoma is the most common extracranial solid tumour in childhood. It accounts for 15% of all paediatric oncology deaths. In the last few decades, improvement in treatment outcome for high-risk patients has not occurred, with an overall survival rate <30-40%. Many reasons may account for such a low survival rate. The aim of this review is to evaluate whether pharmacogenetic factors can explain treatment failure in neuroblastoma.

METHODS

A literature search based on PubMed's database Medical Subject Headings (MeSH) was performed to retrieve all pertinent publications on current treatment options and new classes of drugs under investigation. One hundred and fifty-eight articles wer reviewed, and relevant data were extracted and summarised.

RESULTS AND CONCLUSIONS

Few of the large number of polymorphisms identified thus far showed an effect on pharmacokinetics that could be considered clinically relevant. Despite their clinical relevance, none of the single nucleotide polymorphisms (SNPs) investigated can explain treatment failure. These findings seem to reflect the clinical context in which anti-tumour drugs are used, i.e. in combination with multimodal therapy. In addition, many pharmacogenetic studies did not assess (differences in) drug exposure, which could contribute to explaining pharmacogenetic associations. Furthermore, it remains unclear whether the significant activity of new drugs on different neuroblastoma cell lines translates into clinical efficacy, irrespective of resistance or myelocytomatosis viral related oncogene, neuroblastoma derived (MYCN) amplification. Elucidation of the clinical role of pharmacogenetic factors in the treatment of neuroblastoma demands an integrated pharmacokinetic-pharmacodynamic approach to the analysis of treatment response data.

Phase II drug metabolizing enzymes

BACKGROUND

Phase II biotransformation reactions (also 'conjugation reactions') generally serve as a detoxifying step in drug metabolism. Phase II drug metabolising enzymes are mainly transferases. This review covers the major phase II enzymes: UDP-glucuronosyltransferases, sulfotransferases, N-acetyltransferases, glutathione S-transferases and methyltransferases (mainly thiopurine S-methyl transferase and catechol O-methyl transferase). The focus is on the presence of various forms, on tissue and cellular distribution, on the respective substrates, on genetic polymorphism and finally on the interspecies differences in these enzymes.

METHODS AND RESULTS

A literature search using the following databases PubMed, Science Direct and EBSCO for the years, 1969-2010.

CONCLUSIONS

Phase II drug metabolizing enzymes play an important role in biotransformation of endogenous compounds and xenobiotics to more easily excretable forms as well as in the metabolic inactivation of pharmacologically active compounds. Reduced metabolising capacity of Phase II enzymes can lead to toxic effects of clinically used drugs. Gene polymorphism/ lack of these enzymes may often play a role in several forms of cancer.

Uridine 5'-diphospho-glucuronosyltransferase genetic polymorphisms and response to cancer chemotherapy

Pharmacogenetics aims to elucidate how genetic variation affects the efficacy and side effects of drugs, with the ultimate goal of personalizing medicine. Clinical studies of the genetic variation in the uridine 5'-diphosphoglucuronosyltransferase gene have demonstrated how reduced-function allele variants can predict the risk of severe toxicity and help identify cancer patients who could benefit from reduced-dose schedules or alternative chemotherapy. Candidate polymorphisms have also been identified in vitro, although the functional consequences of these variants still need to be tested in the clinical setting. Future approaches in uridine 5'-diphosphoglucuronosyltransferase pharmacogenetics include genetic testing prior to drug treatment, genotype-directed dose-escalation studies, study of genetic variation at the haplotype level and genome-wide studies.

Three novel single nucleotide polymorphisms of UGT1A9 in a Thai population

The human UDP-glucuronosyltransferase, UGT1A9, catalyzes glucuronidation of various endobiotics and xenobiotics. In this study, we sequenced the promoter and exon 1 regions of the UGT1A9 gene in 93 Thai individuals and identified 7 genetic polymorphisms. The allele frequencies of all 3 novel single nucleotide polymorphisms (SNPs): 454A>G and 455A>C (N152A) and 760C>T (R254X) were 0.005. The other 4 known polymorphisms, -688A>C, -440T>C, -331C>T and -118A(T)(10)AT (UGT1A9(*)1b), were identified and found to have frequencies of 0.124, 0.978, 0.968 and 0.532, respectively.

Prediction of deleterious non-synonymous single-nucleotide polymorphisms of human uridine diphosphate glucuronosyltransferase genes

UDP glucuronosyltransferases (UGTs) are an important class of Phase II enzymes involved in the metabolism and detoxification of numerous xenobiotics including therapeutic drugs and endogenous compounds (e.g. bilirubin). To date, there are 21 human UGT genes identified, and most of them contain single-nucleotide polymorphisms (SNPs). Non-synonymous SNPs (nsSNPs) of the human UGT genes may cause absent or reduced enzyme activity and polymorphisms of UGT have been found to be closely related to altered drug clearance and/or drug response, hyperbilirubinemia, Gilbert's syndrome, and Crigler-Najjar syndrome. However, it is unlikely to study the functional impact of all identified nsSNPs in humans using laboratory approach due to its giant number. We have investigated the potential for bioinformatics approach for the prediction of phenotype based on known nsSNPs. We have identified a total of 248 nsSNPs from human UGT genes. The two algorithms tools, sorting intolerant from tolerant (SIFT) and polymorphism phenotyping (PolyPhen), were used to predict the impact of these nsSNPs on protein function. SIFT classified 35.5% of the UGT nsSNPs as "deleterious"; while PolyPhen identified 46.0% of the UGT nsSNPs as "potentially damaging" and "damaging". The results from the two algorithms were highly associated. Among 63 functionally characterized nsSNPs in the UGTs, 24 showed altered enzyme expression/activities and 45 were associated with disease susceptibility. SIFT and Polyphen had a correct prediction rate of 57.1% and 66.7%, respectively. These findings demonstrate the potential use of bioinformatics techniques to predict genotype-phenotype relationships which may constitute the basis for future functional studies.

Variability and function of family 1 uridine-5'-diphosphate glucuronosyltransferases (UGT1A)

The substrate spectrum of human UDP-glucuronosyltransferase 1A (UGT1A) proteins includes the glucuronidation of non-steroidal anti-inflammatory drugs, anticonvulsants, chemotherapeutics, steroid hormones, bile acids, and bilirubin. The unique genetic organization of the human UGT1A gene locus, and an increasing number of functionally relevant genetic variants define tissue specificity as well as a broad range of interindividual variabilities of glucuronidation. Genetic UGT1A variability has been conserved throughout the protein's evolution and shows ethnic diversity. It is the biochemical and genetic basis for clinical phenotypes such as Gilbert's syndrome and Crigler-Najjar's disease as well as for the potential for severe, unwanted drug side effects such as in irinotecan treatment. UGT1A variants influence the metabolic effects of xenobiotic exposure and therefore have been linked to cancer risk. Detailed knowledge of the organization, function, and pharmacogenetics of the human UGT1A gene locus is likely to significantly contribute to the improvement of drug safety and efficacy as well as to the provision of steps toward the goal of individualized drug therapy and disease risk prediction.

C-440T/T-331C polymorphisms in the UGT1A9 gene affect the pharmacokinetics of mycophenolic acid in kidney transplantation

INTRODUCTION

The immunosuppressive agent mycophenolic acid (MPA) is metabolized by uridine diphosphate glucuronosyltransferase 1A9 (UGT1A9) to 7-O-glucuronide (MPAG) and excreted by multidrug resistance-associated protein 2 in the bile. By contrast, the production of the acyl MPAG, a minor MPA metabolite, was ascribed to UGT2B7 and UGT1A8. Several polymorphisms in the genes encoding for UGT1A9, UGT2B7 and MRP2 proteins have been described. However, their functional role in vivo on MPA metabolism remains poorly defined.

METHODS

A total of 40 Caucasian kidney transplant patients, given induction therapies (with Campath-(1)H or the combination basiliximab/rabbit antithymocyte globulin) and on maintenance immunosuppression with cyclosporine in combination with mycophenolate mofetil (MMF) in a steroid-free regimen, were enrolled in the pharmacogenetic study. Patients had clinical and hematochemical evaluations at month 6 after transplantation, as well as complete MPA pharmacokinetic assessment. They were genotyped for SNPs in UGT1A9 C-2152T, T-1887G, C-665T, C-440T, T-331C, T-275A, T98C, for the nonsynonymous C802T SNP in UGT2B7, and for ABCC2 SNPs C-24T and G1249A. The association of these polymorphisms with MPA pharmacokinetic parameters was investigated.

RESULTS

Differences in the MPA pharmacokinetic profiles confirmed large interpatient variability of MPA exposure, with AUC(0-12) values ranging from 7.9 to 50.1 mg*h/ml. MPA AUC(0-12) was significantly associated with the presence of UGT1A9 -440/-331 genotypes (TT/CC: 61.5 +/- 2.7 mg*h/ml/g MMF; TC/CT: 45.4 +/- 14.0 mg*h/ml/g MMF; CC/TT: 40.8 +/- 10.8 mg*h/ml/g MMF; p = 0.005), whereas MPAG exposure was mainly influenced by renal function. The positive association between MPA AUC and SNPs in position -440/-331 found in kidney transplant patients confirmed previous in vitro findings showing that the abovementioned SNPs had a significant impact on UGT1A9 protein content in the liver. The presence of ABCC2 promoter C-24T and exon 10 G1249A SNPs did not cause any significant variation in MPA and MPAG pharmacokinetic parameters.

CONCLUSION

The study demonstrated a significant impact of C-440T/T-331C SNPs in the promoter region of the UGT1A9 gene on MPA pharmacokinetics in renal allograft recipients.

Lack of association between common polymorphisms in UGT1A9 and gene expression and activity

Interindividual variability in the glucuronidation of xenobiotics metabolized by UDP-glucuronosyltransferase 1A9 (UGT1A9) suggests the presence of functional UGT1A9 variants. The aim of this study was to evaluate whether the putative functionality of the UGT1A9 variants-118T(9>10) (rs3832043), I399C>T (rs2741049), -275T>A (rs6714486), and-2152C>T (rs17868320) could be confirmed in an independent study. UGT1A9 genotypes and UGT1A9 activity (i.e., flavopiridol and mycophenolic acid glucuronidation) were determined in 46 Caucasian human livers. mRNA levels were quantitated by real-time polymerase chain reaction in 35 of these livers. In addition, samples from 60 unrelated Caucasians belonging to the HapMap Project were also genotyped to confirm the allele frequencies and linkage disequilibrium (LD) pattern observed in our Caucasian livers. The allele frequencies of the-118T(9>10), I399C>T, -275T>A, and-2152C>T variants were 0.39, 0.39, 0.02, and 0.02 in the livers, respectively. The I399C>T variant was in complete LD (r(2) = 1) with-118T(9>10) (linked alleles: C and T(9), respectively). Complete LD between these two variants was also found in the HapMap samples (frequencies of-118T(9>10) and I399C>T = 0.38). I399C>T and-118T(9>10) correlated with neither UGT1A9 activities nor mRNA levels. Because of the low frequencies of the-275T>A and-2152C>T variants, an effect on phenotype could not be evaluated. Our data demonstrate that the common I399C>T and-118T(9>10) polymorphisms do not explain interindividual variation in hepatic UGT1A9 activity and mRNA expression and are in complete LD in the donor liver samples we studied.

Characterization ofN-Glucuronidation of 4-(5-Pyridin-4-yl-1H-[1,2,4]triazol-3-yl) pyridine-2-carbonitrile (FYX-051): A New Xanthine Oxidoreductase Inhibitor

In humans, orally administered 4-(5-pyridin-4-yl-1H-[1,2,4]triazol-3-yl) pyridine-2-carbonitrile (FYX-051) is excreted mainly as triazole N(1)- and N(2)-glucuronides in urine. It is important to determine the enzyme(s) that catalyze the metabolism of a new drug to estimate individual differences and/or drug-drug interactions. Therefore, the characterization and mechanism of these glucuronidations were investigated using human liver microsomes (HLMs), human intestinal microsomes (HIMs), and recombinant human UDP-glucuronosyltransferase (UGT) isoforms to determine the UGT isoform(s) responsible for FYX-051 N(1)- and N(2)-glucuronidation. FYX-051 was metabolized to its N(1)- and N(2)-glucuronide forms by HLMs, and their K(m) values were 64.1 and 72.7 microM, respectively; however, FYX-051 was scarcely metabolized to its glucuronides by HIMs. Furthermore, among the recombinant human UGT isoforms, UGT1A1, UGT1A7, and UGT1A9 catalyzed the N(1)- and N(2)-glucuronidation of FYX-051. To estimate their contribution to FYX-051 glucuronidation, inhibition analysis with pooled HLMs was performed. Mefenamic acid, a UGT1A9 inhibitor, decreased FYX-051 N(1)- and N(2)-glucuronosyltransferase activities, whereas bilirubin, a UGT1A1 inhibitor, did not affect these activities. Furthermore, in the experiment using microsomes from eight human livers, the N(1)- and N(2)-glucuronidation activity of FYX-051 was found to significantly correlate with the glucuronidation activity of propofol, a specific substrate of UGT1A9 (N(1): r(2) = 0.868, p < 0.01; N(2): r(2) = 0.775, p < 0.01). These results strongly suggested that the N(1)- and N(2)-glucuronidation of FYX-051 is catalyzed mainly by UGT1A9 in human livers.

Irinotecan pharmacokinetics/pharmacodynamics and UGT1A genetic polymorphisms in Japanese: roles of UGT1A1*6 and *28

OBJECTIVES

SN-38, an active metabolite of irinotecan, is detoxified by glucuronidation with UGT1A isoforms, 1A1, 1A7, 1A9, and 1A10. The pharmacogenetic information on UGT1A haplotypes covering all these isoforms is important for the individualized therapy of irinotecan. Associations between UGT1A haplotypes and pharmacokinetics/pharmacodynamics of irinotecan were investigated to identify pharmacogenetic markers.

METHODS

Associations between UGT1A haplotypes and the area under concentration curve ratio (SN-38 glucuronide/SN-38) or toxicities were analyzed in 177 Japanese cancer patients treated with irinotecan as a single agent or in combination chemotherapy. For association analysis, diplotypes of UGT1A gene segments [(1A1, 1A7, 1A9, 1A10), and Block C (common exons 2-5)] and combinatorial haplotypes (1A9-1A7-1A1) were used. The relationship between diplotypes and toxicities was investigated in 55 patients treated with irinotecan as a single agent.

RESULTS

Among diplotypes of UGT1A genes, patients with the haplotypes harboring UGT1A1*6 or *28 had significantly reduced area under concentration curve ratios, with the effects of UGT1A1*6 or *28 being of a similar scale. A gene dose effect on the area under concentration curve ratio was observed for the number of haplotypes containing *28 or *6 (5.55, 3.62, and 2.07 for 0, 1, and 2 haplotypes, respectively, P<0.0001). In multivariate analysis, the homozygotes and double heterozygotes of *6 and *28 (*6/*6, *28/*28 and *6/*28) were significantly associated with severe neutropenia in 53 patients who received irinotecan monotherapy.

CONCLUSIONS

The haplotypes significantly associated with reduced area under concentration curve ratios and neutropenia contained UGT1A1*6 or *28, and both of them should be genotyped before irinotecan is given to Japanese and probably other Asian patients.

Individualization of mycophenolate mofetil dose in renal transplant recipients

The immunosuppressive agent mycophenolate mofetil has been successfully used over the past 10 years to prevent acute allograft rejection after renal transplantation. It has mainly been administered as a fixed dose of mycophenolate mofetil 1000 mg b.i.d. The pharmacokinetics of mycophenolic acid, the active moiety of the prodrug mycophenolate mofetil, show large between-patient variability, and exposure to mycophenolic acid correlates with the risk for acute rejection. This suggests that already excellent clinical results can be further improved by mycophenolate mofetil dose individualization. This review discusses different arguments in favour of individualization of mycophenolate mofetil dose, as well as strategies for managing mycophenolate mofetil therapy individualization, including pharmacokinetic and pharmacodynamic monitoring and dose individualization based on pharmacogenetic information. It is expected that pharmacokinetic monitoring of mycophenolic acid will offer the most effective and feasible tool for mycophenolate mofetil dose individualization.

The impact of UGT1A8, UGT1A9, and UGT2B7 genetic polymorphisms on the pharmacokinetic profile of mycophenolic acid after a single oral dose in healthy volunteers

We studied whether polymorphisms in the UGT1A8, UGT1A9, and UGT2B7 genes, the enzymes producing the phenolic (MPAG) and acyl (AcMPAG) glucuronides of mycophenolic acid (MPA), could contribute to the interindividual variation observed in mycophenolate mofetil (MMF) pharmacokinetics (PKs). This study enrolled 17 healthy volunteers with no polymorphisms (controls) and 17 carriers of UGT1A9 -275/-2152 selected among 305 individuals genetically screened for UDP-glucuronosyltransferase (UGT) polymorphisms. Additional investigative groups included carriers of UGT1A8*2 (A173G) (n=9), UGT1A8*3 (C277Y) (n=4), and UGT1A9*3 (M33T) (n=5). Genetic analysis also included UGT2B7 to detect UGT2B7*2 (His268Tyr) and the promoter haplotype -1248A>G, -1241T>C, -1054T>C, -842G>A, -268A>G, -102T>C. Kinetics were measured in plasma and urine after a single 1.5 g oral dose of MMF, by high-performance liquid chromatography coupled with tandem mass spectrometry, over 12 h after drug intake. Compared to controls, MPA exposure was significantly lower for UGT1A9 -275/-2152 carriers, with no significant changes in MPAG. The estimates of enterohepatic (re)cycling (area under the concentration-time curve (AUC6-12 h/AUC0-12 h)) were significantly lower for MPA, MPAG, and AcMPAG in UGT1A9 -275/-2152 subjects. Compared with controls, UGT1A9*3 carriers had higher MPA and AcMPAG exposure, whereas homozygosity for the UGT1A8*2 allele and heterozygosity for UGT1A8*3 allele had no impact on MPA PKs. Compared with UGT2B7*1/*1 individuals (n=10), UGT2B7*2/*2 subjects (n=17) presented significantly higher free MPA C(max) values and elevated free and total MPA. Results indicate that after a single oral dose of MMF in healthy volunteers, specific UGT genotypes significantly alter MPA PKs and this clearly warrants additional studies with complete and detailed genetic profiling of UGT1A8, UGT1A9, and UGT2B7 genes.

Genetic linkage of UGT1A7 and UGT1A9 polymorphisms to UGT1A1*6 is associated with reduced activity for SN-38 in Japanese patients with cancer

PURPOSE

The phenotypic effects of UGT1A7 and UGT1A9 genetic polymorphisms on the in vivo pharmacokinetics of irinotecan were examined.

METHODS

Eighty-four Japanese patients with cancer who received irinotecan-based chemotherapy were enrolled. Polymorphisms present in UGT1A7 (T to G transversion at -57 and UGT1A7*2 to *9), UGT1A9 (9 or 10 repeat of T at -118 [-118(T)9 or 10] and UGT1A9*2 to *5), and UGT1A1 (UGT1A1*6, UGT1A1*27, and UGT1A1*28) were analyzed for all patients. Pharmacokinetics of irinotecan were examined in 52 patients.

RESULTS

The most frequent haplotype (haplotype I, 56.7%, 95% CI 53.1-60.4) consisted of polymorphisms related to normal catalytic or transcriptional activity [T at -57 and *1 of UGT1A7, -118(T)10 of UGT1A9, and UGT1A1*1]. The second most frequent haplotype (haplotype II, 15.0%, 95% CI 12.4-18.3) consisted of polymorphisms related to reduced catalytic or transcriptional activity [-57T > G and *3 of UGT1A7 and -118(T)9 of UGT1A9 linked to UGT1A1*6]. The AUC(SN-38)/AUC(SN-38G) ratios in three patients homozygous for haplotype II were significantly higher than those in 20 patients with I/I diplotype (P = 0.011). Neither of these patients had UGT1A1*28.

CONCLUSION

Genetic linkage of UGT1A7 and UGT1A9 polymorphisms to UGT1A1*6, related to reduced catalytic and transcriptional activities of UGTs, is associated with the decreased glucuronosyltransferase activity for SN-38 in Japanese patients with cancer.

Clinical pharmacokinetics and pharmacodynamics of mycophenolate in solid organ transplant recipients

This review aims to provide an extensive overview of the literature on the clinical pharmacokinetics of mycophenolate in solid organ transplantation and a briefer summary of current pharmacodynamic information. Strategies are suggested for further optimisation of mycophenolate therapy and areas where additional research is warranted are highlighted. Mycophenolate has gained widespread acceptance as the antimetabolite immunosuppressant of choice in organ transplant regimens. Mycophenolic acid (MPA) is the active drug moiety. Currently, two mycophenolate compounds are available, mycophenolate mofetil and enteric-coated (EC) mycophenolate sodium. MPA is a potent, selective and reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH), leading to eventual arrest of T- and B-lymphocyte proliferation. Mycophenolate mofetil and EC-mycophenolate sodium are essentially completely hydrolysed to MPA by esterases in the gut wall, blood, liver and tissue. Oral bioavailability of MPA, subsequent to mycophenolate mofetil administration, ranges from 80.7% to 94%. EC-mycophenolate sodium has an absolute bioavailability of MPA of approximately 72%. MPA binds 97-99% to serum albumin in patients with normal renal and liver function. It is metabolised in the liver, gastrointestinal tract and kidney by uridine diphosphate gluconosyltransferases (UGTs). 7-O-MPA-glucuronide (MPAG) is the major metabolite of MPA. MPAG is usually present in the plasma at 20- to 100-fold higher concentrations than MPA, but it is not pharmacologically active. At least three minor metabolites are also formed, of which an acyl-glucuronide has pharmacological potency comparable to MPA. MPAG is excreted into the urine via active tubular secretion and into the bile by multi-drug resistance protein 2 (MRP-2). MPAG is de-conjugated back to MPA by gut bacteria and then reabsorbed in the colon. Mycophenolate mofetil and EC-mycophenolate sodium display linear pharmacokinetics. Following mycophenolate mofetil administration, MPA maximum concentration usually occurs in 1-2 hours. EC-mycophenolate sodium exhibits a median lag time in absorption of MPA from 0.25 to 1.25 hours. A secondary peak in the concentration-time profile of MPA, due to enterohepatic recirculation, often appears 6-12 hours after dosing. This contributes approximately 40% to the area under the plasma concentration-time curve (AUC). The mean elimination half-life of MPA ranges from 9 to 17 hours. MPA displays large between- and within-subject pharmacokinetic variability. Dose-normalised MPA AUC can vary more than 10-fold. Total MPA concentrations should be interpreted with caution in patients with severe renal impairment, liver disease and hypoalbuminaemia. In such individuals, MPA and MPAG plasma protein binding may be altered, changing the fraction of free MPA available. Apparent oral clearance (CL/F) of total MPA appears to increase in proportion to the increased free fraction, with a reduction in total MPA AUC. However, there may be little change in the MPA free concentration. Ciclosporin inhibits biliary excretion of MPAG by MRP-2, reducing enterohepatic recirculation of MPA. Exposure to MPA when mycophenolate mofetil is given in combination with ciclosporin is approximately 30-40% lower than when given alone or with tacrolimus or sirolimus. High dosages of corticosteroids may induce expression of UGT, reducing exposure to MPA. Other co-medications can interfere with the absorption, enterohepatic recycling and metabolism of mycophenolate. Most pharmacokinetic investigations of MPA have involved mycophenolate mofetil rather than EC-mycophenolate sodium therapy. In population pharmacokinetic studies, MPA CL/F in adults ranges from 14.1 to 34.9 L/h (ciclosporin co-therapy) and from 11.9 to 25.4 L/h (tacrolimus co-therapy). Patient bodyweight, serum albumin concentration and immunosuppressant co-therapy have a significant influence on CL/F. The majority of pharmacodynamic data on MPA have been obtained in patients receiving mycophenolate mofetil therapy in the first year after kidney transplantation. Low MPA AUC is associated with increased incidence of biopsy-proven acute rejection. Gastrointestinal adverse events may be dose related. Leukopenia and anaemia have been associated with high MPA AUC, trough concentration and metabolite concentrations in some, but not all, studies. High free MPA exposure has been identified as a risk factor for leukopenia in some investigations. Targeting a total MPA AUC from 0 to 12 hours (AUC12) of 30-60 mg.hr/L is likely to minimise the risk of acute rejection and may reduce toxicity. IMPDH monitoring is in the early experimental stage. Individualisation of mycophenolate therapy should lead to improved patient outcomes. MPA AUC12 appears to be the most useful exposure measure for such individualisation. Limited sampling strategies and Bayesian forecasting are practical means of estimating MPA AUC12 without full concentration-time profiling. Target concentration intervention may be particularly useful in the first few months post-transplant and prior to major changes in anti-rejection therapy. In patients with impaired renal or hepatic function or hypoalbuminaemia, free drug measurement could be valuable in further interpretation of MPA exposure.

Prevalence of UGT1A9 and UGT2B7 nonsynonymous single nucleotide polymorphisms in West African, Papua New Guinean, and North American populations

OBJECTIVE

UDP-glucuronosyltransferases (UGTs) UGT1A9 and UGT2B7 are involved in the metabolism of antimalarial dihydroartemisinin and antiretroviral zidovudine. Our aim was to analyze the prevalence of UGT1A9 (chromosome 2) and UGT2B7 (chromosome 4) nonsynonymous single nucleotide polymorphisms (SNPs) in West African (WA), Papua New Guinean (PNG), and North American (NA) populations.

METHODS

Using a post-PCR ligation detection reaction-fluorescent microsphere assay, frequencies of UGT1A9 (8G > A, 98T > C, 766G > A) and UGT2B7 (211G > T, 802C > T, 1192G > A) SNPs were determined in WA (n = 133, 5 countries), PNG (n = 153), and NA (n = 350, 4 ethnic groups) individuals.

RESULTS

The UGT1A9 variant alleles were not common in the study populations. None of the SNPs were present in WA and PNG. Among NA, all 3 SNPs were present (1% each) in Asian-Americans, while 98T > C was present only in Caucasian-Americans (1%) and Hispanic-Americans (1%). Regarding UGT2B7 SNPs, the prevalence of 802C > T was 21% in WA, 28% in PNG, and 28-52% in NA. The SNP 211G > T was present only in Asian-Americans (9%) and Hispanic-Americans (2%), while 1192G > A was not present in any of the subjects. No significant linkage was observed at UGT1A9, UGT2B7, and between both the loci in any of the study populations.

CONCLUSIONS

Taken together, the UGT1A9-UGT2B7 polymorphism profile in WA and PNG populations is similar to African-Americans, but different from Asian-Americans. It is important to determine if these differences, along with previously reported differences in cytochrome P450 2B6 allele frequencies, are associated with altered metabolism/effectiveness of artemisinin drugs.

Multidrug Resistance Protein 2 Genetic Polymorphisms Influence Mycophenolic Acid Exposure in Renal Allograft Recipients

BACKGROUND

Mycophenolic acid (MPA) is glucuronidated by uridine diphosphate-glucuronosyltransferases (UGTs) to its pharmacologically inactive 7-O-glucuronide metabolite (MPAG). MPAG is excreted into the bile via the multidrug resistance-associated protein 2 (MRP2/ABCC2), which is essential for enterohepatic (re)circulation (EHC) of MPA(G).

METHODS

The objective of this study was to determine the relationship between single nucleotide polymorphisms (SNPs) in the MRP2 (G-1549A, G-1023A, A-1019G, C-24, G1249A, C3972T and G4544A) and UGT1A9 (C-2152T, T-275AandT98C) genes and MPA pharmacokinetics in 95 renal allograft recipients at days 7, 42, 90, and 360 after transplantation. In addition to mycophenolate mofetil, all patients received tacrolimus and corticosteroids as immunosuppression.

RESULTS

At day seven after transplantation, in the absence of the MRP2 C-24T SNP, mild liver dysfunction was associated with significantly lower MPA dose-interval exposure and higher MPA oral clearance, while liver dysfunction did not affect MPA pharmacokinetics in patients with the MRP2 C-24T variant. A similar effect is noted for the C-3972T variant, which is in linkage disequilibrium with C-24T. At later time points after transplantation the MRP2 C-24T SNP was associated with significantly higher dose-corrected MPA trough levels. Patients with the MRP2 C-24T variant had significantly more diarrhea in the first year after transplantation.

CONCLUSIONS

The MRP2 C-24T and C-3972T polymorphisms protect renal transplant recipients from a decrease in MPA exposure associated with mild liver dysfunction. Furthermore, this study suggests that the C-24T SNP is associated with a lower oral clearance of MPA in steady-state conditions.

Lack of association between UGT1A7, UGT1A9, ARP, SPINK1 and CFTR gene polymorphisms and pancreatic cancer in Italian patients

AIM: To investigate simultaneously UGT1A7, UGT1A9, ARP, SPINK and CFTR genes to verify whether genetic polymorphisms predispose to the development of pancreatic cancer (PC).

METHODS: Genomic DNA of 61 pancreatic cancer patients and 105 healthy controls (HC) were analyzed. UGT1A7 genotyping was determined by PCR-RFLP analysis. Specific PCR and sequencing were used to analyze genetic variants of UGT1A9, ARP, SPINK1 and CFTR genes.

RESULTS: Four different alleles (*1: WT; *2: N129K and R131K; *3: N129K, R131K, and W208R; and *4: W208R) in UGT1A7 and three different alleles (*1: WT; *4: Y242X; and *5: D256N) in UGT1A9 were detected. All UGT1A polymorphisms were observed at similar frequency in PC patients and HC. Seven different alleles in ARP were found in PC patients and HC at similar frequency. The SPINK1 mutations N34S and P55S occurred in five PC patients with a prevalence (4.1%) not significantly different from that observed (2.0%) in HC. The only CFTRΔF508 mutation was recognized in three PC patients with a prevalence (4.9%) similar to HC.

CONCLUSION: UGT1A7, UGT1A9, ARP, SPINK1 and CFTR gene polymorphisms are not associated with PC in Italian patients.

Pharmacogenetics of uridine diphosphoglucuronosyltransferase (UGT) 1A family members and its role in patient response to irinotecan

Glucuronidation, catalyzed by the glucuronosyltransferase (UGT) superfamily, is a major biotransformation pathway for several drugs, including irinotecan. Irinotecan is commonly used in colorectal cancer chemotherapy. Irinotecan undergoes metabolism in humans and is converted to its active metabolite SN-38, a topoisomerase I inhibitor. SN-38 is inactivated via glucuronidation catalyzed by various hepatic and extrahepatic UGT1A isozymes. Although the role of the UGT1A1 *28 genetic variant has received much attention in altered toxicity upon irinotecan treatment, other UGT1A enzymes also play an important role. This review summarizes pharmacokinetic, toxicologic, and pharmacogenetic studies carried out to date in irinotecan and SN-38 disposition.

Xenobiotic-metabolizing enzymes involved in activation and detoxification of carcinogenic polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental carcinogens and metabolized by a variety of xenobiotic-metabolizing enzymes such as cytochrome P450 (P450 or CYP), epoxide hydrolase, glutathione transferase, UDP-glucuronosyltransferase, sulfotransferase, NAD(P)H quinone oxidoreductase 1, and aldo-keto reductase. These enzymes mainly participate in the conversion of PAHs to more polar and water-soluble metabolites, and the resultant metabolites are readily excreted from the body. However, during the course of metabolism, a variety of unstable and reactive intermediates of PAHs are formed, and these metabolites attack DNA, causing cell toxicity and transformation. P450s and epoxide hydrolase convert PAHs to proximate carcinogenic metabolites, PAH-diols, and these products are further metabolized by P450s to ultimate carcinogenic metabolites, PAH diol-epoxides, or by aldo-keto reductase to reactive PAH o-quinones. PAHs are also activated by P450 and peroxidases to reactive radical cations that bind covalently to DNA. The oxygenated and reactive metabolites of PAHs are usually converted to more polar and detoxified products by phase II enzymes. Inter-individual differences exist in levels of expression and catalytic activities of a variety of enzymes that activate and/or detoxify PAHs in various organs of humans and these phenomena are thought to be critical in understanding the basis of individual differences in response to PAHs. Factors affecting such variations include induction and inhibition of enzymes by diverse chemicals and, more importantly, genetic polymorphisms of enzymes in humans.

Polycyclic aromatic hydrocarbon (PAH) metabolizing enzyme activities in human lung, and their inducibility by exposure to naphthalene, phenanthrene, pyrene, chrysene, and benzo(a)pyrene as shown in the rat lung and liver

In order to survey changes and activities in the polycyclic aromatic hydrocarbon (PAH)-metabolizing enzymes implicated in lung cancer susceptibility studies, we investigated enzyme induction by 2-5-ring-sized 'biomarker' PAHs in rat liver and lung, and the activities in five human lung specimens. Naphthalene, phenanthrene, pyrene, chrysene, and benzo[a]pyrene (BaP) were administered to rats for 3 days (25-128 mg/kg/day) and the responses compared with those of model inducers. PAH treatment increased the CYP1A-catalyzed activity of pyrene 1-hydroxylation and 7-ethoxyresorufin O-deethylation in rat liver by up to 28- and 279-fold, and in rat lung by up to 22- and 51-fold, respectively. 1-Naphthol (hUGT1A6), 1-hydroxypyrene (hUGT1A6/1A9), and entacapone (hUGT1A9) are markers of PAH-glucuronidating human uridine diphosphate-glucuronosyltransferases (UGT). These activities increased up to 6.4-fold in rat liver and up to 1.9-fold in rat lung. NADPH:quinone oxidoreductase 1 (NQO1) and glutathione S-transferase activities increased up to 5.3- and 1.6-fold (liver), and up to 4.4- and 1.4-fold (lung), respectively. CYP1A showed the best liver-to-lung relationship (R (2 )=( )0.90). The inducing efficiency by PAHs differed extensively: control <or= naphthalene < phenanthrene, pyrene << chrysene < BaP. In human lung (non-smokers), the marker activities of CYP1A1, UGT1A6/1A9, and NQO1 were lower than those in rat lung. Epoxide hydrolase activity was 1,000-fold higher than the pulmonary CYP1A1 activities. Human UGT and NQO1 displayed large variations (>60-fold), many times greater than the experimental (inducible/constitutive) variation in the rat. Kinetics of 1-hydroxypyrene glucuronidation showed two low-K (m) forms both in rat and human lung. Since the 2-4-ring PAHs (major constituents) were poor enzyme inducers, it appears that the PAH-metabolizing pathways are mainly induced by BaP-type minor constituents. Gene-environmental interactions which magnify polymorphic variability in pulmonary bioactivation/detoxification capacity probably play a key role in individual susceptibility to (or protection against) chemically induced lung cancer. Hence, human exposure to PAH mixtures with high content of BaP-type hydrocarbons confers a potentially higher health risk than PAH mixtures with low content of procarcinogens.

Uridine diphosphoglucuronosyltransferase pharmacogenetics and cancer

The uridine diphosphoglucuronosyltransferases (UGTs) belong to a superfamily of enzymes that catalyse the glucuronidation of numerous endobiotics and xenobiotics. Several human hepatic and extrahepatic UGT isozymes have been characterized with respect to their substrate specificity, tissue expression and gene structure. Genetic polymorphisms have been identified for almost all the UGT family members. A wide variety of anticancer drugs, dietary chemopreventives and carcinogens are known to be conjugated by members of both UGT1A and UGT2B subfamilies. This review examines in detail each UGT isozyme known to be associated with cancer and carcinogenesis. The cancer-related substrates for several UGTs are summarized, and the functionally relevant genetic polymorphisms of UGTs are reviewed. A number of genotype-phenotype association studies have been carried out to characterize the role of UGT pharmacogenetics in several types of cancer, and these examples are discussed here. In summary, this review focuses on the role of the human UGT genetic polymorphisms in carcinogenesis, chemoprevention and cancer risk.

Role of pharmacogenetics in irinotecan therapy

In the treatment of advanced colorectal cancer, irinotecan has become one of the most important drugs, despite its sometimes unpredictable adverse effects. To understand why some patients experience severe adverse effects (diarrhea and neutropenia), while others do not, the metabolic pathways of this drug have to be unraveled in detail. Individual variation in expression of several phase I and phase II metabolizing enzymes and ABC-transporters involved in irinotecan metabolism and excretion, at least partly explains the observed pharmacokinetic interpatient variability. Although the difference in expression-level of these proteins to a certain amount is explained by physiologic and environmental factors, the presence of specific genetic determinants also does influence their expression and function. In this review, the role of genetic polymorphisms in the main enzyme-systems (carboxylesterase, cytochrome P450 3A, and uridine diphosphate-glucuronosyltransferase) and ABC-transporters (ABCB1, ABCC2, and ABCG2) involved in irinotecan metabolism, are discussed. Since at this moment the field of pharmacogenetics and pharmacogenomics is rapidly expanding and simultaneously more rapid and cost-effective screening methods are emerging, a wealth of future data is expected to enrich our knowledge of the genetic basis of irinotecan metabolism. Eventually, this may help to truly individualize the dosing of this (and other) anti-cancer agent(s), using a personal genetic profile of the most relevant enzymes for every patient.

Haplotype structures of the UGT1A gene complex in a Japanese population

Genetic polymorphisms of UDP-glucuronosyltransferases (UGTs) are involved in individual and ethnic differences in drug metabolism. To reveal co-occurrence of the UGT1A polymorphisms, we first analyzed haplotype structures of the entire UGT1A gene complex using the polymorphisms from 196 Japanese subjects. Based on strong linkage disequilibrium between UGT1A8 and 1A10, among 1A9, 1A7, and 1A6, and between 1A3 and 1A1, the complex was divided into five blocks, Block 8/10, Block 9/6, Block 4, Block 3/1, and Block C, and the haplotypes for each block were subsequently determined/inferred. Second, using pyrosequencing or direct sequencing, additional 105 subjects were genotyped for 41 functionally tagged polymorphisms. The data from 301 subjects confirmed the robustness of block partitioning, but several linkages among the haplotypes with functional changes were found across the blocks. Thus, important haplotypes and their linkages were identified among the UGT1A gene blocks (and segments), which should be considered in pharmacogenetic studies.

Novel Single Nucleotide Polymorphism of UGT1A9 Gene in Japanese

We sequenced from 5'-franking region to intron 1 (to 337 bp downstream from exon 1) of the UDP-glucuronosyltransferase (UGT) 1A9 gene prepared from 55 Japanese cancer patients. Seven single nucleotide polymorphisms (SNPs) were found. Two of them were UGT1A9 -118(T)n (n=10) and UGT1A9*5, and four were reported SNPs in intron 1 of UGT1A9 gene (89540C>T, 89549G>A, 89616T>A and 89710A>C). A novel SNP (89587T>C) was found. The sequence is as follows: SNP, 050824FujitaK001; Gene Name, UGT1A9; Accession Number, AF297093; Length, 25 bases; 5'-CCTTCTTGAAGAT/CATGTATTTATAA-3'. Two patients were heterozygous for the mutant allele, resulting in the allele frequency of 1.82%.

Glucuronidation in therapeutic drug monitoring

BACKGROUND

Glucuronidation is a major drug-metabolizing reaction in humans. A pharmacological effect of glucuronide metabolites is frequently neglected and the value of therapeutic drug monitoring has been questioned. However, this may not always be true.

METHODS

In this review the impact of glucuronidation on therapeutic drug monitoring has been evaluated on the basis of a literature search and experience from the own laboratory.

RESULTS

The potential role of monitoring glucuronide metabolite concentrations to optimize therapeutic outcome is addressed on the basis of selected examples of drugs which are metabolized to biologically active/reactive glucuronides. Furthermore indirect effects of glucuronide metabolites on parent drug pharmacokinetics are presented. In addition, factors that may modulate the disposition of these metabolites (e.g. genetic polymorphisms, disease processes, age, and drug-drug interactions) are briefly mentioned and their relevance for the clinical situation is critically discussed.

CONCLUSION

Glucuronide metabolites can have indirect as well as direct pharmacological or toxicological effects. Although convincing evidence to support the introduction of glucuronide monitoring into clinical practice is currently missing, measurement of glucuronide concentrations may be advantageous in specific situations. If the glucuronide metabolite has an indirect effect on the pharmacokinetics of the parent compound, monitoring of the parent drug may be considered. Furthermore pharmacogenetic approaches considering uridine diphosphate (UDP) glucuronosyltransferases polymorphisms may become useful in the future to optimize therapy with drugs subject to glucuronidation.

Pharmacogenetics of irinotecan metabolism and transport: an update

The anticancer agent irinotecan (CPT-11) is converted to SN-38, which is approximately 100 to 1,000-fold more cytotoxic than the parent drug. The pharmacokinetics of irinotecan are extremely complex and have been the subject of intensive investigation in recent years. Irinotecan is subject to extensive metabolism by various polymorphic enzymes, including CES2 to form SN-38, members of the UGT1A subfamily, and CYP3A4 and CYP3A5, which form several pharmacologically inactive oxidation products. Elimination of irinotecan is also dependent on drug-transporting proteins, notably ABCB1 (P-glycoprotein), ABCC2 (cMOAT) and ABCG2 (BCRP), present on the bile canalicular membrane. The various processes mediating drug elimination, either through metabolic breakdown or excretion, likely impact substantially on interindividual variability in drug handling. This report provides an update on current strategies to individualize irinotecan chemotherapy based on each patient's genetic constitution, which may ultimately lead to more selective use of this agent.

Pharmacogenetics for individualized cancer chemotherapy

The same doses of medication cause considerable heterogeneity in efficacy and toxicity across human populations. Genetic factors are thought to represent important determinants of drug efficacy and toxicity. Pharmacogenetics focuses on the prediction of the response of tumor and normal tissue to standard therapy by genetic profiling and, thereby, to select the most appropriate medication at optimal doses for each individual patient. In the present review, we discuss the relevance of single nucleotide polymorphisms (SNP) in genes, whose gene products act upstream of the actual drug target sites, that is, drug transporters and drug metabolizing phase I and II enzymes, or downstream of them, that is, apoptosis-regulating genes and chemokines. SNPs in relevant genes, which encode for proteins that interact with anticancer drugs, were also considered, that is, enzymes of DNA biosynthesis and metabolism, DNA repair enzymes, and proteins of the mitotic spindle. A significant body of evidence supports the concept of predicting drug efficacy and toxicity by SNP genotyping. As the efficacy of cancer chemotherapy, as well as the drug-related toxicity in normal tissues is multifactorial in nature, sophisticated approaches such as genome-wide linkage analyses and integrate drug pathway profiling may improve the predictive power compared with genotyping of single genes. The implementation of pharmacogenetics into clinical routine diagnostics including genotype-based recommendations for treatment decisions and risk assessment for practitioners represents a challenge for the future.