Characterization of the UDP glucuronosyltransferase activity of human liver microsomes genotyped for the UGT1A1*28 polymorphism

Detection of uridine diphosphate glucuronosyltransferase 1A1 for pancreatic cancer imaging and treatment via a "turn-on" fluorescent probe

Uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1) is expressed ubiquitously in cancer cells and can metabolize exogenous substances. Studies show higher UGT1A1 levels in pancreatic cancer cells than normal cells. Therefore, we need a method to monitor the activity level of UGT1A1 in pancreatic cancer cells and in vivo. Here, we report a fluorescent probe, BCy-panc, for UGT1A1 imaging in cells and in vivo. Compared with other molecular probes, this probe is readily prepared, with high selectivity and sensitivity for the detection of UGT1A1. Our results show that BCy-panc rapidly detects UGT1A1 in pancreatic cancer. In addition, there is an urgent need for evidence to clarify the relationship between UGT1A1 and pancreatic cancer development. The present investigation found that the increase of UGT1A1 by chrysin was effective in inducing apoptosis in pancreatic cancer cells. These results indicate that the synergistic effect of chrysin and cisplatin at the cellular level is superior to that of cisplatin alone. The UGT1A1 level may be a biomarker for early diagnosis of cancer. Meanwhile, UGT1A1 plays a crucial role in pancreatic cancer, and the combination of chrysin and cisplatin may provide effective ideas for pancreatic cancer treatment.

Development of a physiologically based pharmacokinetic model to predict irinotecan disposition during inflammation

Irinotecan, a first-line chemotherapy for gastrointestinal (GI) cancers has been causing fatal toxicities like bloody diarrhea and steatohepatitis for years. Irinotecan goes through multiple-step drug metabolism after injection and one of its intermediates 7-ethyl-10-hydroxy-camptothecin (SN-38) is responsible for irinotecan side effect. However, it is unclear what is the disposition kinetics of SN-38 in the organs subjected to toxicity. No studies ever quantified the effect of each enzyme or transporter on SN-38 distribution. In current study, we established a new physiologically based pharmacokinetic (PBPK) model to predict the disposition kinetics of irinotecan. The PBPK model was calibrated with in-house mouse pharmacokinetic data and evaluated with external datasets from the literature. We separated the contribution of each parameters in irinotecan pharmacokinetics by calculating the normalized sensitivity coefficient (NSC). The model gave robust prediction of SN-38 distribution in GI tract, the site of injury. We identified that bile excretion and UDP-glucuronosyltransferases (UGT) played more important roles than fecal excretion and renal clearance in SN-38 pharmacokinetics. Our NSC showed that the impact of enzyme and transporter on irinotecan and SN-38 pharmacokinetics evolved when time continued. Additionally, we mapped out the effect of inflammation on irinotecan metabolic pathways with PBPK modelling. We discovered that inflammation significantly increased the blood and liver exposure of irinotecan and SN-38 in the mice receiving bacterial endotoxin. Inflammation suppressed UGT, microbial metabolism but increased fecal excretion. The present PBPK model can serve as an efficacious and versatile tool to quantitively assess the risk of irinotecan toxicity.

Variability in Human In Vitro Enzyme Kinetics

There are many factors which are known to cause variability in human in vitro enzyme kinetic data. Factors such as the source of enzyme and how it was prepared, the genetics and background of the donor, how the in vitro studies are designed, and how the data are analyzed contribute to variability in the resulting kinetic parameters. It is important to consider not only the factors which cause variability within an experiment, such as selection of a probe substrate, but also those that cause variability when comparing kinetic data across studies and laboratories. For example, the artificial nature of the microsomal lipid membrane and microenvironment in some recombinantly expressed enzymes, relative to those found in native tissue microsomes, has been shown to influence enzyme activity and thus can be a source of variability when comparing across the two different systems. All of these factors, and several others, are discussed in detail in the chapter below. In addition, approaches which can be used to visualize the uncertainty arising from the use of enzyme kinetic data within the context of predicting human pharmacokinetics are discussed.

Racial Disparity in Drug Disposition in the Digestive Tract

The major determinants of drug or, al bioavailability are absorption and metabolism in the digestive tract. Genetic variations can cause significant differences in transporter and enzyme protein expression and function. The racial distribution of selected efflux transporter (i.e., Pgp, BCRP, MRP2) and metabolism enzyme (i.e., UGT1A1, UGT1A8) single nucleotide polymorphisms (SNPs) that are highly expressed in the digestive tract are reviewed in this paper with emphasis on the allele frequency and the impact on drug absorption, metabolism, and in vivo drug exposure. Additionally, preclinical and clinical models used to study the impact of transporter/enzyme SNPs on protein expression and function are also reviewed. The results showed that allele frequency of the major drug efflux transporters and the major intestinal metabolic enzymes are highly different in different races, leading to different drug disposition and exposure. The conclusion is that genetic polymorphism is frequently observed in different races and the related protein expression and drug absorption/metabolism function and drug in vivo exposure can be significantly affected, resulting in variations in drug response. Basic research on race-dependent drug absorption/metabolism is expected, and FDA regulations of drug dosing adjustment based on racial disparity are suggested.

Pharmacogenomics in Pediatric Oncology: Mitigating Adverse Drug Reactions While Preserving Efficacy

Cancer is the leading cause of death in American children older than 1 year of age. Major developments in drugs such as thiopurines and optimization in clinical trial protocols for treating cancer in children have led to a remarkable improvement in survival, from approximately 30% in the 1960s to more than 80% today. Short-term and long-term adverse effects of chemotherapy still affect most survivors of childhood cancer. Pharmacogenetics plays a major role in predicting the safety of cancer chemotherapy and, in the future, its effectiveness. Treatment failure in childhood cancer-due to either serious adverse effects that limit therapy or the failure of conventional dosing to induce remission-warrants development of new strategies for treatment. Here, we summarize the current knowledge of the pharmacogenomics of cancer drug treatment in children and of statistically and clinically relevant drug-gene associations and the mechanistic understandings that underscore their therapeutic value in the treatment of childhood cancer.

Evidence-based strategies for the characterisation of human drug and chemical glucuronidation in vitro and UDP-glucuronosyltransferase reaction phenotyping

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily contribute to the elimination of drugs from almost all therapeutic classes. Awareness of the importance of glucuronidation as a drug clearance mechanism along with increased knowledge of the enzymology of drug and chemical metabolism has stimulated interest in the development and application of approaches for the characterisation of human drug glucuronidation in vitro, in particular reaction phenotyping (the fractional contribution of the individual UGT enzymes responsible for the glucuronidation of a given drug), assessment of metabolic stability, and UGT enzyme inhibition by drugs and other xenobiotics. In turn, this has permitted the implementation of in vitro - in vivo extrapolation approaches for the prediction of drug metabolic clearance, intestinal availability, and drug-drug interaction liability, all of which are of considerable importance in pre-clinical drug development. Indeed, regulatory agencies (FDA and EMA) require UGT reaction phenotyping for new chemical entities if glucuronidation accounts for ≥25% of total metabolism. In vitro studies are most commonly performed with recombinant UGT enzymes and human liver microsomes (HLM) as the enzyme sources. Despite the widespread use of in vitro approaches for the characterisation of drug and chemical glucuronidation by HLM and recombinant enzymes, evidence-based guidelines relating to experimental approaches are lacking. Here we present evidence-based strategies for the characterisation of drug and chemical glucuronidation in vitro, and for UGT reaction phenotyping. We anticipate that the strategies will inform practice, encourage development of standardised experimental procedures where feasible, and guide ongoing research in the field.

Main contribution of UGT1A1 and CYP2C9 in the metabolism of UR-1102, a novel agent for the treatment of gout

UR-1102, a novel uricosuric agent for treating gout, has been confirmed to exhibit a pharmacological effect in patients. We clarified its metabolic pathway, estimated the contribution of each metabolic enzyme, and assessed the impact of genetic polymorphisms using human in vitro materials. Glucuronide, sulfate and oxidative metabolites of UR-1102 were detected in human hepatocytes. The intrinsic clearance by glucuronidation or oxidation in human liver microsomes was comparable, but sulfation in the cytosol was much lower, indicating that the rank order of contribution was glucuronidation ≥ oxidation > sulfation. Recombinant UGT1A1 and UGT1A3 showed high glucuronidation of UR-1102. We took advantage of a difference in the inhibitory sensitivity of atazanavir to the UGT isoforms and estimated the fraction metabolised (fm) with UGT1A1 to be 70%. Studies using recombinant CYPs and CYP isoform-specific inhibitors showed that oxidation was mediated exclusively by CYP2C9. The effect of UGT1A1 and CYP2C9 inhibitors on UR-1102 metabolism in hepatocytes did not differ markedly between the wild type and variants.

Pharmacogenomics as a Tool to Limit Acute and Long-Term Adverse Effects of Chemotherapeutics: An Update in Pediatric Oncology

In the past decades, new cancer treatments have been introduced in pediatric oncology leading to improvement in clinical outcomes and survival rates. However, due to inter-individual differences, some children experience severe chemotherapy-induced toxicities or a poor clinical outcome. An explanation for the diversity in response to chemotherapy is genetic variation, leading to differences in expression and activity of metabolizing and transport enzymes as well as drug targets. Pharmacogenetic testing has emerged as a promising tool to predict and limit acute and long-term adverse effects in patients. However, in pediatric oncology, limited number of patients and a considerable diversity in study results complicate the interpretation of test results and its clinical relevance. With this review, we provide an overview of new developments over the past four years regarding relevant polymorphisms related to toxicity in pediatric oncology. The following chemotherapeutics and associated toxicities are discussed: alkylating agents, anthracyclines, asparaginase, methotrexate, platinum compounds, steroids, thiopurines, topoisomerase inhibitors, and vinca alkaloids. Our review identifies several questions regarding the role of genetic variants in chemotherapy-induced toxicities. Ambiguities in the literature stem from small population sizes, differences in (statistical) interpretation and variations in sequencing technologies as well as different clinical outcome definitions. Standardization of clinical outcome data and toxicity definitions within electronic health records combined with the increased availability of genomic sequence techniques in clinical practice will help to validate these models in upcoming years.

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.

Recent progress and challenges in screening and characterization of UGT1A1 inhibitors

Uridine-diphosphate glucuronosyltransferase 1A1 (UGT1A1) is an important conjugative enzyme in mammals that is responsible for the conjugation and detoxification of both endogenous and xenobiotic compounds. Strong inhibition of UGT1A1 may trigger adverse drug/herb-drug interactions, or result in metabolic disorders of endobiotic metabolism. Therefore, both the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA) have recommended assaying the inhibitory potential of drugs under development on the human UGT1A1 prior to approval. This review focuses on the significance, progress and challenges in discovery and characterization of UGT1A1 inhibitors. Recent advances in the development of UGT1A1 probes and their application for screening UGT1A1 inhibitors are summarized and discussed in this review for the first time. Furthermore, a long list of UGT1A1 inhibitors, including information on their inhibition potency, inhibition mode, and affinity, has been prepared and analyzed. Challenges and future directions in this field are highlighted in the final section. The information and knowledge that are presented in this review provide guidance for rational use of drugs/herbs in order to avoid the occurrence of adverse effects via UGT1A1 inhibition, as well as presenting methods for rapid screening and characterization of UGT1A1 inhibitors and for facilitating investigations on UGT1A1-ligand interactions.

6-Chloro-5-[4-(1-Hydroxycyclobutyl)Phenyl]-1H-Indole-3-Carboxylic Acid is a Highly Selective Substrate for Glucuronidation by UGT1A1, Relative to β-Estradiol

6-Chloro-5-[4-(1-hydroxycyclobutyl)phenyl]-1H-indole-3-carboxylic acid (PF-06409577) is a direct activator of the human β1-containing adenosine monophosphate-activated protein kinase (ΑMPK) isoforms. The clearance mechanism of PF-06409577 in animals and humans involves uridine diphosphoglucuronosyl transferase (UGT)-mediated glucuronidation to an acyl glucuronide metabolite of PF-06409577 [(2S,3S,4S,5R,6S)-6-((6-chloro-5-(4-(1-hydroxycyclobutyl)phenyl)-1H-indole-3-carbonyl)oxy)-3,4,5-trihydroxytetrahydro-2H-pyran-2-carboxylic acid (M1)], which retains selective activation of human β1-containing AMPK isoforms. This paper describes a detailed characterization of the human UGT isoform(s) responsible for glucuronidation of PF-06409577 to M1. Studies using a panel of 13 human recombinant UGT (hrUGT) enzymes indicated that PF-06409577 was converted to M1 in a highly selective fashion by UGT1A1, which was further verified in human liver microsomes treated with specific chemical inhibitors, and in different UGT1A1 expressers. Conversion of PF-06409577 to M1 by UGT1A1 occurred in a relatively selective fashion, compared with β-estradiol (ES), a conventional probe substrate of UGT1A1. The Michaelis-Menten constant (K M) and V max values describing the formation of M1 from PF-06409577 in hrUGT1A1 and microsomal preparations from human intestine, liver, and kidney ranged from 131 to 212 μM (K M) and 107-3834 pmol/min per milligram (V max) in the presence of 2% bovine serum albumin. Relative activity factors (RAF) were determined for UGT1A1 using PF-06409577 and ES to enable estimation of intrinsic clearance from various tissues. RAF values from PF-06409577 and ES were generally comparable with the exception of intestinal microsomes, where ES overestimated the RAF of UGT1A1 due to glucuronidation by intestinal UGT1A8 and UGT1A10. Our results suggest the potential utility of PF-06409477 as a selective probe UGT1A1 substrate for UGT reaction phenotyping and inhibition studies in preclinical discovery/development.

Association of nephrotoxicity during platinum-etoposide doublet therapy with UGT1A1 polymorphisms in small cell lung cancer patients

OBJECTIVES

Etoposide is a key agent in the treatment of small cell lung cancer (SCLC). Uridine diphosphate (UDP)-glucuronosyltransferase 1A1 (UGT1A1) is thought to be largely responsible for the glucuronidation of etoposide as well as that of irinotecan, suggesting that polymorphisms of UGT1A1 might be predictive of etoposide toxicity. We therefore examined the relation between UGT1A1 polymorphisms and toxicity profile during platinum-etoposide doublet therapy in SCLC patients.

MATERIALS AND METHODS

SCLC patients who underwent platinum-etoposide doublet therapy and molecular testing for UGT1A1 genotype were reviewed for the occurrence of adverse events during treatment.

RESULTS

A total of 41 SCLC patients received platinum-etoposide doublet therapy and were genotyped for UGT1A1*6 and UGT1A1*28 alleles. These alleles were detected in 15 (36.6%) patients, with the genotypes of *6/-, *6/*6, *28/-, *28/*28, or *6/*28 being observed in 9 (22.0%), 2 (4.9%), 2 (4.9%), 1 (2.4%), and 1 (2.4%) patients, respectively. The presence of these alleles was significantly associated with an increase in serum creatinine concentration of grade ≥2 (incidence of 66.7% for patients with the alleles versus 11.5% for those without, P <  0.001). Multivariate analysis also showed that these UGT1A1 alleles were significantly associated with therapy-induced nephrotoxicity (odds ratio of 19.30, 95% confidence interval of 2.50-149.00, P <  0.005). Although the differences did not achieve statistical significance, the incidence of other severe toxicities including febrile neutropenia was also slightly higher in patients with the UGT1A1*6 or UGT1A1*28 alleles than in those without them.

CONCLUSION

Our results reveal an association between UGT1A1 polymorphisms and toxicity of platinum-etoposide doublet therapy in SCLC patients, suggesting that close monitoring for toxicity, especially nephrotoxicity, is warranted for patients with such variant alleles receiving this treatment.

Chemical Probes for Human UDP-Glucuronosyltransferases: A Comprehensive Review

UGTs play crucial roles in the metabolism and detoxification of both endogenous and xenobiotic compounds. The key roles of UGTs in human health have garnered great interest in the design and development of specific probes for human UGTs. However, in contrast to other human enzymes, the probe substrates for human UGTs are rarely reported, owing to the highly overlapping substrate specificities of UGTs and the lack of the integrated crystal structures of UGTs. Over the past decades, many efforts are made to develop specific probe substrates for UGTs and use them in both basic research and drug discovery. This review focuses on recent progress in the development of probe substrates for UGTs and their biomedical applications. A long list of chemical probes for UGTs, including non-fluorescent and fluorescent probes along with their structural information and kinetic parameters, are prepared and analyzed. Additionally, challenges and future directions in this field are highlighted in the final section. All information and knowledge presented in this review provide practical tools/methods for measuring UGT activities in complex biological samples, which will be very helpful for rapid screening and characterization of UGT modulators, and for exploring the relevance of UGT enzymes to human diseases.

In vitro UGT1A1 inhibition by tyrosine kinase inhibitors and association with drug-induced hyperbilirubinemia

PURPOSE

Hyperbilirubinemia has been observed in patients treated with tyrosine kinase inhibitor (TKI) drugs. Therefore, it would be beneficial to understand whether there is a relationship between inhibition of uridine-5'-diphosphate glucuronosyltransferase (UGT) 1A1 activity and observed bilirubin elevations in TKI drug-treated patients. UGT1A1 is responsible for the glucuronidation of bilirubin which leads to its elimination in the bile.

METHODS

To examine this question, an in vitro glucuronidation assay was developed to determine the inhibitory effect of TKI drugs employing human liver microsomes (HLM) with varying UGT1A1 activity. Utilizing β-estradiol as the UGT1A1 probe substrate, 20 TKI drugs were evaluated at concentrations that represent clinical plasma levels. Adverse event reports were searched to generate an empirical Bayes geometric mean (EGBM) score for clinical hyperbilirubinemia with the TKI drugs.

RESULTS

Erlotinib, nilotinib, regorafenib, pazopanib, sorafenib and vemurafenib had IC₅₀ values that were lower than their clinical steady-state C_max concentrations. These TKI drugs had high incidences of hyperbilirubinemia and higher EBGM scores. The IC₅₀ values and C_max/IC₅₀ ratios correlated well with EBGM scores for hyperbilirubinemia (P < 0.005). For the TKI drugs with higher incidence of hyperbilirubinemia in Gilbert syndrome patients, who have reduced UGT1A1 activity, six of eight had smaller ratios in the low UGT1A1 activity microsomes than the wild-type microsomes for drugs, indicating greater sensitivity to the drugs in this phenotype.

CONCLUSIONS

These results suggest that in vitro UGT1A1 inhibition assays have the potential to predict clinical hyperbilirubinemia.

A Practical and High-Affinity Fluorescent Probe for Uridine Diphosphate Glucuronosyltransferase 1A1: A Good Surrogate for Bilirubin

This study aimed to develop a practical and high-affinity fluorescent probe for uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1), a key conjugative enzyme responsible for the elimination and detoxification of many potentially harmful compounds. Several substrates derived from N-butyl-4-phenyl-1,8-naphthalimide were designed and synthesized on the basis of the substrate preference of UGT1A1 and the principle of photoinduced electron transfer (PET). Following the preliminary screening, substrate 2 was found with a high specificity and high affinity toward UGT1A1, while such biotransformation brought remarkable changes in fluorescence emission. Both inhibition kinetic analyses and molecular docking simulations demonstrated that 2 could bind on UGT1A1 at the same ligand-binding site as bilirubin. Furthermore, this newly developed probe was successfully used for sensing UGT1A1 activities and the high-throughput screening of UGT1A1 modulators in complex biological samples. In conclusion, a practical and high-affinity fluorescent probe for UGT1A1 was designed and well-characterized, which could serve as a good surrogate for bilirubin to investigate UGT1A1-ligand interactions.

Reduced total serum bilirubin levels are associated with ulcerative colitis

Chronic inflammation associated with inflammatory bowel disease (IBD) results in increased oxidative stress that damages the colonic microenvironment. Low levels of serum bilirubin, an endogenous antioxidant, have been associated with increased risk for Crohn's disease (CD). Therefore, the aim of this study was to examine whether total serum bilirubin levels are associated with ulcerative colitis (UC). We identified a retrospective case-control population (n = 6,649) from a single tertiary care center, Penn State Hershey Medical Center (PSU) and a validation cohort (n = 1,996) from Virginia Commonwealth University Medical Center (VCU). Cases were age- and sex-matched to controls (PSU: CD n = 254, UC n = 187; VCU: CD n = 233, UC n = 124). Total serum bilirubin levels were obtained from de-identified medical records and segregated into quartiles. Logistic regression analysis was performed on each quartile of total serum bilirubin compared to the last quartile (highest bilirubin levels) to determine the association of total serum bilirubin with UC. Similar to CD patients, UC patients demonstrated reduced levels of total serum bilirubin compared to controls at PSU and VCU. The lowest quartile of total serum bilirubin was independently associated with UC for the PSU (OR: 1.98 [95% CI: 1.09-3.63]) and VCU cohorts (OR: 6.07 [95% CI: 3.01-12.75]). Lower levels of the antioxidant bilirubin may reduce the capability of UC patients to remove reactive oxygen species leading to an increase in intestinal injury. Therapeutics that reduce oxidative stress may be beneficial for these patients.

Pharmacogenomics in Pediatric Oncology: Review of Gene-Drug Associations for Clinical Use

During the 3rd congress of the European Society of Pharmacogenomics and Personalised Therapy (ESPT) in Budapest in 2015, a preliminary meeting was held aimed at establishing a pediatric individualized treatment in oncology and hematology committees. The main purpose was to facilitate the transfer and harmonization of pharmacogenetic testing from research into clinics, to bring together basic and translational research and to educate health professionals throughout Europe. The objective of this review was to provide the attendees of the meeting as well as the larger scientific community an insight into the compiled evidence regarding current pharmacogenomics knowledge in pediatric oncology. This preliminary evaluation will help steer the committee’s work and should give the reader an idea at which stage researchers and clinicians are, in terms of personalizing medicine for children with cancer. From the evidence presented here, future recommendations to achieve this goal will also be suggested.

Dose-finding and pharmacokinetic study to optimize the dosing of irinotecan according to the UGT1A1 genotype of patients with cancer

PURPOSE

The risk of severe neutropenia from treatment with irinotecan is related in part to UGT1A1*28, a variant that reduces the elimination of SN-38, the active metabolite of irinotecan. We aimed to identify the maximum-tolerated dose (MTD) and dose-limiting toxicity (DLT) of irinotecan in patients with advanced solid tumors stratified by the *1/*1, *1/*28, and *28/*28 genotypes.

PATIENTS AND METHODS

Sixty-eight patients received an intravenous flat dose of irinotecan every 3 weeks. Forty-six percent of the patients had the *1/*1 genotype, 41% had the *1/*28 genotype, and 13% had the *28/*28 genotype. The starting dose of irinotecan was 700 mg in patients with the *1/*1 and *1/*28 genotypes and 500 mg in patients with the *28/*28 genotype. Pharmacokinetic evaluation was performed at cycle 1.

RESULTS

In patients with the *1/*1 genotype, the MTD was 850 mg (four DLTs per 16 patients), and 1,000 mg was not tolerated (two DLTs per six patients). In patients with the *1/*28 genotype, the MTD was 700 mg (five DLTs per 22 patients), and 850 mg was not tolerated (four DLTs per six patients). In patients with the *28/*28 genotype, the MTD was 400 mg (one DLT per six patients), and 500 mg was not tolerated (three DLTs per three patients). The DLTs were mainly myelosuppression and diarrhea. Irinotecan clearance followed linear kinetics. At the MTD for each genotype, dosing by genotype resulted in similar SN-38 areas under the curve (AUCs; r(2) = 0.0003; P = .97), but the irinotecan AUC was correlated with the actual dose (r(2) = 0.39; P < .001). Four of 48 patients with disease known to be responsive to irinotecan achieved partial response.

CONCLUSION

The UGT1A1*28 genotype can be used to individualize dosing of irinotecan. Additional studies should evaluate the effect of genotype-guided dosing on efficacy in patients receiving irinotecan.

The interactions of lenalidomide with human uptake and efflux transporters and UDP-glucuronosyltransferase 1A1: lack of potential for drug-drug interactions

PURPOSE

Lenalidomide is an immunomodulatory agent used for the treatment of myelodysplastic syndromes and multiple myeloma. Renal clearance of lenalidomide is the predominant elimination route and is approximately twofold greater than the glomerular filtration rate (GFR), suggesting the potential contribution of an active secretory mechanism. In vitro studies were conducted to examine whether lenalidomide is a substrate of drug transporters, namely P-glycoprotein (P-gp), human breast cancer resistance protein (BCRP), multidrug resistance proteins (MRP1, MRP2, MRP3), organic anion transporters (OAT1, OAT3), organic cation transporters (OCT1 and OCT2), human organic cation transporter novel 1 and 2 (OCTN1 and OCTN2), multidrug and toxin extrusion (MATE1) and organic anion transporting polypeptide (OATP1B1). Lenalidomide was also evaluated as an inhibitor of P-gp, BCRP, MRP2, OCT2, OAT1, OAT3, OATP1B1, OATP1B3 and bile salt export pump (BSEP). In addition, inhibition of UDP-glucuronosyltransferase 1A1 (UGT1A1) variants by lenalidomide was also assessed.

METHOD

Cells or vesicles expressing each of the human transporters were used for uptake and inhibition studies, with appropriate probe substrates and known inhibitors.

RESULTS

Results of these studies indicate that the lenalidomide is not a substrate for the transporters examined, except that it is weak substrate of P-gp. None of the transporters studied were inhibited by lenalidomide. Lenalidomide is not an inhibitor of UGT1A1*1/*1 or its polymorphic variants UGT1A1*1/*28 and UGT1A1*28/*28.

CONCLUSIONS

Drug interactions are unlikely to occur when lenalidomide is co-administered with substrates or inhibitors of these transporters. In addition, lenalidomide is unlikely to cause interactions when co-administered with substrates of UGT1A1.

Variability in human in vitro enzyme kinetics

There are many factors which are known to cause variability in human in vitro enzyme kinetic data. Factors such as the source of enzyme and how it was prepared, the genetics and background of the donor, how the in vitro studies are designed, and how the data are analyzed contribute to variability in the resulting kinetic parameters. It is important to consider not only the factors which cause variability within an experiment, such as selection of a probe substrate, but also those that cause variability when comparing kinetic data across studies and laboratories. For example, the artificial nature of the microsomal lipid membrane and microenvironment in some recombinantly expressed enzymes, relative to those found in native tissue microsomes, has been shown to influence enzyme activity and thus can be a source of variability when comparing across the two different systems. All of these factors, and several others, are discussed in detail in the chapter below.

Substrate selectivity of human intestinal UDP-glucuronosyltransferases (UGTs): in silico and in vitro insights

The current drug development process aims to produce safe, effective drugs within a reasonable time and at a reasonable cost. Phase II metabolism (glucuronidation) can affect drug action and pharmacokinetics to a considerable extent and so its studies and prediction at initial stages of drug development are very imperative. Extensive glucuronidation is an obstacle to oral bioavailability because the first-pass glucuronidation [or premature clearance by UDP-glucuronosyltransferases (UGTs)] of orally administered agents frequently results in poor oral bioavailability and lack of efficacy. Modeling of new chemical entities/drugs for UGTs and their kinetic data can be useful in understanding the binding patterns to be used in the design of better molecules. This review concentrates on first-pass glucuronidation by intestinal UGTs, including their topology, expression profile, and pharmacogenomics. In addition, recent advances are discussed with respect to substrate selectivity at the binding pocket, structural requirements, and mechanism of enzyme actions.

Raloxifene glucuronidation in human intestine, kidney, and liver microsomes and in human liver microsomes genotyped for the UGT1A1*28 polymorphism

Raloxifene, a selective estrogen receptor modulator, exhibits quite large interindividual variability in pharmacokinetics and pharmacodynamics. In women, raloxifene is metabolized extensively by different isoforms of UDP-glucuronosyltransferase (UGT) to its glucuronides. To gain an insight into intestine, kidney, liver, and lung glucuronidation of raloxifene, human microsomes of all tested organs were used. Raloxifene-6-β-glucuronide (M1) formation followed the Michaelis-Menten kinetics in intestinal, kidney, and liver microsomes; meanwhile, raloxifene-4'-β-glucuronide (M2) formation followed the substrate inhibition kinetics. Human lung microsomes did not show any glucuronidation activity. The tissue intrinsic clearances for kidney, intestine, and liver were 3.4, 28.1, and 39.6 ml · min(-1) · kg(-1), respectively. The aim of our in vitro study was to explain the mechanism behind the observed influence of UGT1A1*28 polymorphism on raloxifene pharmacokinetics in a small-sized in vivo study (Br J Clin Pharmacol 67:437-444, 2009). Incubation of raloxifene with human liver microsomes genotyped for UGT1A1*28 showed a significantly reduced metabolic clearance toward M1 in microsomes from donors with *28 allele. On the contrary, no significant genotype influence was observed on the formation of M2 because of the high variability in estimated apparent kinetic parameters, although a clear trend toward lower glucuronidation activities was observed when UGT1A1*28 polymorphism was present. The liver intrinsic clearances of both homozygotes differed significantly, whereas the clearance of heterozygotes did not differ from the wild-type and the mutated homozygotes. In conclusion, our results show the high importance of the liver and intestine in raloxifene glucuronidation. Moreover, the significant influence of UGT1A1*28 polymorphism on metabolism of raloxifene was confirmed.

Evaluation of bisphenol A glucuronidation according to UGT1A1*28 polymorphism by a new LC-MS/MS assay

The endocrine disruptor bisphenol A (BPA) is a frequently used chemical in the manufacture of consumer products. In humans, BPA is extensively metabolized to BPA glucuronide (BPAG) by different UDP-glucuronosyltransferase (UGT) isoforms. The study has been performed with the intention to improve the accuracy of published physiologically based pharmacokinetic models and to improve regulatory risk assessments of BPA. In order to gain insight into intestine, kidney, liver, and lung glucuronidation of BPA, human microsomes of all tested organs were used. BPAG formation followed Michaelis-Menten kinetics in the intestine and kidney, but followed substrate inhibition kinetics in the liver. Human lung microsomes did not show glucuronidation activity towards BPA. While the liver intrinsic clearance was very high (857 mL min(-1)kg body weight(-1)), the tissue intrinsic clearances for the kidney and intestine were less than 1% of liver intrinsic clearance. Since BPA is a UGT1A1 substrate, we postulated that the common UGT1A1*28 polymorphism influences BPA glucuronidation, and consequently, BPA detoxification. Hepatic tissue intrinsic clearances for UGT1A1*1/*1, UGT1A1*1/*28, and UGT1A1*28/*28 microsomes were 1113, 1075, and 284 mL min(-1)kg body weight(-1), respectively. Prior to microsomal experiments, the bioproduction of BPAG and stable isotope-labeled BPAG (BPAG(d16)) was performed for the purpose of the reliable and accurate quantification of BPAG. In addition, a sensitive LC-MS/MS analytical method for the simultaneous determination of BPA and BPAG based on two stable isotope-labeled internal standards was developed and validated. In conclusion, our in vitro results show that the liver is the main site of BPA glucuronidation (K(m) 8.9 μM, V(max) 8.5 nmol min(-1) mg(-1)) and BPA metabolism may be significantly influenced by a person's genotype (K(m) 10.0-13.1 μM, V(max) 3.4-16.2 nmol min(-1) mg(-1)). This discovery may be an important fact for the currently on-going worldwide BPA risk assessments and for the improvement of physiologically based pharmacokinetic models.

Steroid catabolism in marine and freshwater fish

Steroids play important roles in regulating many physiological functions in marine and freshwater fish. Levels of active steroid in blood and tissues are determined by the balance between synthetic and catabolic processes. This review examines what is known about pathways of catabolism of steroids, primarily sex steroids, in marine and freshwater fish. Cytochrome P450 (P450) isoforms present in hepatic microsomes catalyze steroid hydroxylation to metabolites with lower or no activity at estrogen or androgen receptors. Important pathways of steroid catabolism to readily excreted metabolites are glucuronidation and sulfonation of hydroxyl groups. Estradiol, testosterone, DHEA and hydroxylated metabolites of these and other steroids readily form glucuronide and sulfate conjugates in those fish species where these pathways have been examined. Little is known, however, of the structure and function of the UDP-glucuronosyltransferase (UGT) and sulfotransferase (SULT) enzymes involved in steroid conjugation in fish. Glucuronide and sulfate conjugates of steroids may be transported into and out of cells by organic anion transporter proteins and multi-drug resistance proteins, and there is growing evidence that these proteins play important roles in steroid conjugate transport and elimination. Induction or inhibition of any of these pathways by environmental chemicals can result in alteration of the natural balance of steroid hormones and could lead to disruption of the endocrine system. Recent studies in this area are presented, with particular focus on phase II (conjugative) pathways.

Evaluation of 3,3',4'-trihydroxyflavone and 3,6,4'-trihydroxyflavone (4'-O-glucuronidation) as the in vitro functional markers for hepatic UGT1A1

Identifying uridine 5'-diphospho-(UDP)-glucuronosyltransferase (UGT)-selective probes (substrates that are primarily glucuronidated by a single isoform) is complicated by the enzymes' large overlapping substrate specificity. Here, regioselective glucuronidation of two flavonoids, 3,3',4'-trihydroxyflavone (3,3',4'-THF) and 3,6,4'-trihydroxyflavone (3,6,4'-THF), is used to probe the activity of hepatic UGT1A1. The glucuronidation kinetics of 3,3',4'-THF and 3,6,4'-THF was determined using 12 recombinant human UGT isoforms and pooled human liver microsomes (pHLM). The individual contribution of main UGT isoforms to the metabolism of the two flavonoids in pHLM was estimated using the relative activity factor approach. UGT1A1 activity correlation analyses using flavonoids-4'-O-glucuronidation vs β-estradiol-3-glucuronidation (a well-recognized marker for UGT1A1) or vs SN-38 glucuronidation were performed using a bank of HLMs (n = 12) including three UGT1A1-genotyped HLMs (i.e., UGT1A1*1*1, UGT1A1*1*28, and UGT1A1*28*28). The results showed that UGT1A1 and 1A9, followed by 1A7, were the main isoforms for glucuronidating the two flavonoids, where UGT1A1 accounted for 92 ± 7% and 91 ± 10% of 4'-O-glucuronidation of 3,3',4'-THF and 3,6,4'-THF, respectively, and UGT1A9 accounted for most of the 3-O-glucuronidation. Highly significant correlations (R(2) > 0.944, p < 0.0001) between the rates of flavonoids 4'-O-glucuronidation and that of estradiol-3-glucuronidation or SN-38 glucuronidation were observed across 12 HLMs. In conclusion, UGT1A1-mediated 4'-O-glucuronidation of 3,3',4'-THF and 3,6,4'-THF was highly correlated with the glucuronidation of estradiol (3-OH) and SN-38. This study demonstrated for the first time that regioselective glucuronidation of flavonoids can be applied to probe hepatic UGT1A1 activity in vitro.

Pharmacogenetics, pharmacogenomics, and individualized medicine

Individual variability in drug efficacy and drug safety is a major challenge in current clinical practice, drug development, and drug regulation. For more than 5 decades, studies of pharmacogenetics have provided ample examples of causal relations between genotypes and drug response to account for phenotypic variations of clinical importance in drug therapy. The convergence of pharmacogenetics and human genomics in recent years has dramatically accelerated the discovery of new genetic variations that potentially underlie variability in drug response, giving birth to pharmacogenomics. In addition to the rapid accumulation of knowledge on genome-disease and genome-drug interactions, there arises the hope of individualized medicine. Here we review recent progress in the understanding of genetic contributions to major individual variability in drug therapy with focus on genetic variations of drug target, drug metabolism, drug transport, disease susceptibility, and drug safety. Challenges to future pharmacogenomics and its translation into individualized medicine, drug development, and regulation are discussed. For example, knowledge on genetic determinants of disease pathogenesis and drug action, especially those of complex disease and drug response, is not always available. Relating the many gene variations from genomic sequencing to clinical phenotypes may not be straightforward. It is often very challenging to conduct large scale, prospective studies to establish causal associations between genetic variations and drug response or to evaluate the utility and cost-effectiveness of genomic medicine. Overcoming the obstacles holds promise for achieving the ultimate goal of effective and safe medication to targeted patients with appropriate genotypes.

Repaglinide-gemfibrozil drug interaction: inhibition of repaglinide glucuronidation as a potential additional contributing mechanism

AIM

To further explore the mechanism underlying the interaction between repaglinide and gemfibrozil, alone or in combination with itraconazole.

METHODS

Repaglinide metabolism was assessed in vitro (human liver subcellular fractions, fresh human hepatocytes, and recombinant enzymes) and the resulting incubates were analyzed, by liquid chromatography-mass spectrometry (LC-MS) and radioactivity counting, to identify and quantify the different metabolites therein. Chemical inhibitors, in addition to a trapping agent, were also employed to elucidate the importance of each metabolic pathway. Finally, a panel of human liver microsomes (genotyped for UGT1A1*28 allele status) was used to determine the importance of UGT1A1 in the direct glucuronidation of repaglinide.

RESULTS

The results of the present study demonstrate that repaglinide can undergo direct glucuronidation, a pathway that can possibly contribute to the interaction with gemfibrozil. For example, [³H]-repaglinide formed glucuronide and oxidative metabolites (M2 and M4) when incubated with primary human hepatocytes. Gemfibrozil effectively inhibited (∼78%) both glucuronide and M4 formation, but had a minor effect on M2 formation. Concomitantly, the overall turnover of repaglinide was also inhibited (∼80%), and was completely abolished when gemfibrozil was co-incubated with itraconazole. These observations are in qualitative agreement with the in vivo findings. UGT1A1 plays a significant role in the glucuronidation of repaglinide. In addition, gemfibrozil and its glucuronide inhibit repaglinide glucuronidation and the inhibition by gemfibrozil glucuronide is time-dependent.

CONCLUSIONS

Inhibition of UGT enzymes, especially UGT1A1, by gemfibrozil and its glucuronide is an additional mechanism to consider when rationalizing the interaction between repaglinide and gemfibrozil.

Correlation between bilirubin glucuronidation and estradiol-3-gluronidation in the presence of model UDP-glucuronosyltransferase 1A1 substrates/inhibitors

Inhibition of UDP-glucuronosyltransferase (UGT) 1A1-catalyzed bilirubin glucuronidation by drug compounds may potentially be of clinical concern. However, in drug discovery and development settings, bilirubin is less than an ideal in vitro probe for assessing the potential of a chemical entity to inhibit bilirubin glucuronidation. In part, this is due to the propensity of bilirubin to photodegrade and to the instability of its metabolites. To this end, the utility of estradiol-3-glucuronidation as a surrogate in vitro predictor for interactions with bilirubin was evaluated. The glucuronidation kinetics of bilirubin and estradiol were carefully characterized with recombinant UGT1A1 expressed in human embryonic kidney 293 cells. Consistent with previous reports, estradiol-3-glucuronidation displayed sigmoidal kinetics, whereas bilirubin glucuronidation exhibited typical hyperbolic kinetics. The two compounds also mutually inhibited the metabolism of the other. Sixteen UGT1A1 substrates/inhibitors were evaluated as effectors of each reaction. Fourteen compounds inhibited both bilirubin and estradiol glucuronidation. However, two compounds (ethinylestradiol and daidzein) exhibited mixed effects (concentration-dependent activation and inhibition) on estradiol-3-glucuronidation, whereas bilirubin glucuronidation was inhibited by both compounds. In addition, 7-ethyl-10-hydroxycamptothecin, a substrate of UGT1A1 (reported K(m) = 24 μM) seemed to be a weak inhibitor of bilirubin glucuronidation (IC(50) = 356.4 μM) but a partial inhibitor of estradiol-3-glucuronidation. The IC(50) values of the inhibitors against estradiol-3-glucuronidation were strongly correlated with IC(50) values against bilirubin glucuronidation, resulting in an R(2) value of 0.9604 (activator excluded) or 0.8287 (activator included). Thus, estradiol-3-glucuronidation can serve as a good surrogate for predicting inhibition of bilirubin glucuronidation with the caveat that occasionally compounds may demonstrate activation of estradiol-3-glucuronidation.

The clinical application of UGT1A1 pharmacogenetic testing: gene-environment interactions

Over the past decade, the number of pharmacogenetic tests has increased considerably, allowing for the development of our knowledge of their clinical application. The uridine diphosphate glucuronosyltransferase 1A1 gene (UGT1A1) assay is an example of a pharmacogenetic test. Numerous variants have been found in UGT1A1, the main conjugating enzyme of bilirubin and drugs such as the anticancer drug irinotecan. Recently, the US Food and Drug Administration (FDA) recommended testing for the presence of UGT1A1*28, an allele correlated with decreased transcriptional activity, to predict patients at risk of irinotecan toxicity. The administration of other drugs -- such as inhibitors of the UGT1A1 enzyme -- can clinically mimic the *28 phenotype, whereas inducers of UGT1A1 can increase the glucuronidation rate of the enzyme. The *28 polymorphism is not present in all ethnicities at a similar frequency, which suggests that it is important to study different populations to determine the clinical relevance of testing for UGT1A1*28 and to identify other clinically relevant UGT1A1 variants. Environmental factors such as lifestyle can also affect UGT1A1 activity. This review is a critical analysis of studies on drugs that can be affected by the presence of UGT1A1*28, the distribution of this polymorphism around the globe, distinct variants that may be clinically significant in African and Asian populations and how lifestyle can affect treatment outcomes that depend on UGT1A1 activity.

A humanized UGT1 mouse model expressing the UGT1A1*28 allele for assessing drug clearance by UGT1A1-dependent glucuronidation

Humanized mice that express the human UDP-glucuronosyltransferase (UGT) 1 locus have been developed in a Ugt1-null background as a model to improve predictions of human UGT1A-dependent drug clearance. Enzyme kinetic parameters (K(m) and V(max)) and pharmacokinetic properties of three probe drugs were compared using wild-type and humanized UGT1 mice that express the Gilbert's UGT1A1*28 allele [Tg(UGT1(A1*28)) Ugt1(-/-) mice]. The well characterized substrate for UGT1A1, 7-ethyl-10-hydroxy-camptothecin (SN-38), showed the greatest difference in parent drug exposure ( approximately 3-fold increase) and clearance ( approximately 3-fold decrease) in Tg(UGT1(A1*28)) Ugt1(-/-) mice after intravenous administration compared with wild-type and phenobarbital-treated animals. In contrast, the clearance of the UGT2B7 substrate (-)-17-allyl-4, 5alpha-epoxy-3, 14-dihydroxymorphinan-6-one (naloxone) was not altered in Tg(UGT1(A1*28)) Ugt1(-/-) mice. In addition, pharmacokinetic parameters with 1-(4-fluorophenyl)3(R)-[3-(4-fluorophenyl)-3(S)-hydroxypropyl]-4(S)-(4-hydroxyphenyl)-2-azetidinone (ezetimibe, Zetia; Merck & Co., Whitehouse Station, NJ), considered to be a major substrate for UGT1A1, showed small to no dependence on UGT1A1-directed glucuronidation. Enzyme kinetic parameters assessed for SN-38, ezetimibe, and naloxone using liver microsomes prepared from wild-type and Tg(UGT1(A1*28)) Ugt1(-/-) mice showed patterns consistent with the in vivo pharmacokinetic data. For SN-38 glucuronidation, V(max) decreased 5-fold in Tg(UGT1(A1*28)) Ugt1(-/-) mouse liver microsomes compared with microsomes prepared from wild-type mice, and decreased 10-fold compared with phenobarbital-treated Tg(UGT1(A1*28)) Ugt1(-/-) mice. These differences are consistent with SN-38 glucuronidation activities using HLMs isolated from individuals genotyped as UGT1A1*1 or UGT1A1*28. For ezetimibe and naloxone the differences in V(max) were minimal. Thus, Tg(UGT1(A1*28)) Ugt1(-/-) mice can serve as a pharmacokinetic model to further investigate the effects of UGT1A1 expression on drug metabolism.