CHARACTERIZATION OF AFLOQUALONE N-GLUCURONIDATION: SPECIES DIFFERENCES AND IDENTIFICATION OF HUMAN UDP-GLUCURONOSYLTRANSFERASE ISOFORM(S)

In Vitro Ciclopirox Glucuronidation in Liver Microsomes from Humans and Various Experimental Animals

BACKGROUND AND OBJECTIVE

Ciclopirox is a widely used antifungal drug, redisposition of which has drawn increasing attentions due to multiple promising activities. The drug undergoes extensive glucuronidation, which acts as a major obstacle in the ongoing novel application and still remains poorly understood. The current study aims to phenotype ciclopirox glucuronidation pathway and as well to decipher the related species differences.

METHODS

Ciclopirox glucuronidation was investigated in liver microsomes from humans (HLM) and various experimental animals. Assays with recombinant uridine diphosphate glucuronosyltransferases (UGTs), enzyme kinetic analyses and selective inhibitors were used to determine the role of individual UGTs in ciclopirox glucuronidation.

RESULTS

HLM is highly active in ciclopirox glucuronidation with Michaelis-Menten constant (Km), maximum velocity (Vmax), and intrinsic clearance (CLint) values of 139 μM, 7.89 nmol/min/mg, and 56 μL/min/mg, respectively. UGT1A9 displays by far the highest activity, whereas several other isoforms (UGT1A6, UGT1A7, and UGT1A8) catalyze formation of traced glucuronides. Further kinetic analysis demonstrates that UGT1A9 has a closed Km value (167 μM) to HLM. UGT1A9 selective inhibitor (magnolol) can potently inhibit ciclopirox glucuronidation in HLM with the IC50 value of 0.12 μM. The reaction displays remarkable differences across liver microsomes from mice, rats, cynomolgus monkey, minipig, and beagle dog, with the CLint values in the range of 26-369 μL/min/mg. In addition, ciclopirox glucuronidation activities of experimental animals' liver microsomes were less sensitive to magnolol than that of HLM.

CONCLUSIONS

Ciclopirox glucuronidation displays remarkable species differences with UGT1A9 as a dominant contributor in humans. It is suggested that the pharmacological or toxicological effects of ciclopirox may be UGT1A9 and species dependent.

Identification of a Discrete Diglucuronide of GDC-0810 in Human Plasma after Oral Administration

GDC-0810 is a small molecule therapeutic agent having potential to treat breast cancer. In plasma of the first-in-human study, metabolite M2, accounting for 20.7% of total drug-related materials, was identified as a discrete diglucuronide that was absent in rats. Acyl glucuronide M6 and N-glucuronide M4 were also identified as prominent metabolites in human plasma. Several in vitro studies were conducted in incubations of [14C]GDC-0810, synthetic M6 and M4 with liver microsomes, intestinal microsomes, and hepatocytes of different species as well as recombinant UDP-glucuronosyltransferase (UGT) enzymes to further understand the formation of M2. The results suggested that 1) M2 was more efficiently formed from M6 than from M4, and 2) acyl glucuronidation was mainly catalyzed by UGT1A8/7/1 that is highly expressed in the intestines whereas N-glucuronidation was mainly catalyzed by UGT1A4 that is expressed in the human liver. This complicated mechanism presented challenges in predicting M2 formation using human in vitro systems. The absence of M2 and M4 in rats can be explained by low to no expression of UGT1A4 in rodents. M2 could be the first discrete diglucuronide that was formed from both acyl- and N-glucuronidation on a molecule identified in human plasma. SIGNIFICANCE STATEMENT: A discrete diglucuronidation metabolite of GDC-0810, a breast cancer drug candidate, was characterized as a unique circulating metabolite in humans that was not observed in rats or little formed in human in vitro system.

Drug-oxidizing and conjugating non-cytochrome P450 (non-P450) enzymes in cynomolgus monkeys and common marmosets as preclinical models for humans

Many drug oxidations and conjugations are mediated by a variety of cytochromes P450 (P450) and non-P450 enzymes in humans and non-human primates. These non-P450 enzymes include aldehyde oxidases (AOX), carboxylesterases (CES), flavin-containing monooxygenases (FMO), glutathione S-transferases (GST), arylamine N-acetyltransferases (NAT),sulfotransferases (SULT), and uridine 5'-diphospho-glucuronosyltransferases (UGT) and their substrates include both endobiotics and xenobiotics. Cynomolgus macaques (Macaca fascicularis, an Old-World monkey) are widely used in preclinical studies because of their genetic and physiological similarities to humans. However, many reports have indicated the usefulness of common marmosets (Callithrix jacchus, a New World monkey) as an alternative non-human primate model. Although knowledge of the drug-metabolizing properties of non-P450 enzymes in non-human primates is relatively limited, new research has started to provide an insight into the molecular characteristics of these enzymes in cynomolgus macaques and common marmosets. This mini-review provides collective information on the isoforms of non-P450 enzymes AOX, CES, FMO, GST, NAT, SULT, and UGT and their enzymatic profiles in cynomolgus macaques and common marmosets. In general, these non-P450 cynomolgus macaque and marmoset enzymes have high sequence identities and similar substrate recognitions to their human counterparts. However, these enzymes also exhibit some limited differences in function between species, just as P450 enzymes do, possibly due to small structural differences in amino acid residues. The findings summarized here provide a foundation for understanding the molecular mechanisms of polymorphic non-P450 enzymes and should contribute to the successful application of non-human primates as model animals for humans.

Comprehensive insights into the formation of metabolites of the ghrelin mimetics capromorelin, macimorelin and tabimorelin as potential markers for doping control purposes

Analytical methods to determine the potential misuse of the ghrelin mimetics capromorelin (CP-424,391), macimorelin (macrilen, EP-01572) and tabimorelin (NN703) in sports were developed. Therefore, different extraction strategies, i.e. solid-phase extraction, protein precipitation, as well as a "dilute-and-inject" approach, from urine and EDTA-plasma were assessed and comprehensive in vitro/in vivo experiments were conducted, enabling the identification of reliable target analytes by means of high resolution mass spectrometry. The drugs' biotransformation led to the preliminary identification of 51 metabolites of capromorelin, 12 metabolites of macimorelin and 13 metabolites of tabimorelin. Seven major metabolites detected in rat urine samples collected post-administration of 0.5-1.0 mg of a single oral dose underwent in-depth characterization, facilitating their implementation into future confirmatory test methods. In particular, two macimorelin metabolites exhibiting considerable abundances in post-administration rat urine samples were detected, which might contribute to an improved sensitivity, specificity, and detection window in case of human sports drug testing programs. Further, the intact drugs were implemented into World Anti-Doping Agency-compliant initial testing (limits of detection 0.02-0.60 ng/ml) and confirmation procedures (limits of identification 0.18-0.89 ng/ml) for human urine and blood matrices. The obtained results allow extension of the test spectrum of doping agents in multitarget screening assays for growth hormone-releasing factors from human urine.

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.

Characterizing the in vitro species differences in N-glucuronidation of a potent pan-PIM inhibitor GNE-924 containing a 3,5-substituted 6-azaindazole

1. Glucuronidation of amines has been shown to exhibit large species differences, where the activity is typically more pronounced in human than in many preclinical species such as rat, mouse, dog and monkey. The purpose of this work was to characterize the in vitro glucuronidation of GNE-924, a potent pan-PIM inhibitor, to form M1 using liver microsomes (LM) and intestinal microsomes (IM). 2. M1 formation kinetics varied highly across species and between liver and intestinal microsomes. In LM incubations, rat exhibited the highest rate of M1 formation (CL_int,app) at 140 ± 10 µL/min/mg protein, which was approximately 30-fold higher than human. In IM incubations, mouse exhibited the highest CL_int,app at 484 ± 40 µL/min/mg protein, which was >1000-fold higher than human. In addition, CL_int,app in LM was markedly higher than IM in human and monkey. In contrast, CL_int,app in IM was markedly higher than LM in dog and mouse. 3. Reaction phenotyping indicated that UGT1A1, UGT1A3, UGT1A9, UGT2B4 and the intestine-specific UGT1A10 contributed to the formation of M1. 4. This is one of the first reports showing that N-glucuronidation activity is significantly greater in multiple preclinical species than in humans, and suggests that extensive intestinal N-glucuronidation may limit the oral exposure of GNE-924.

N-glucuronidation of drugs and other xenobiotics by human and animal UDP-glucuronosyltransferases

Metabolic disposition of drugs and other xenobiotics includes glucuronidation reactions that are catalyzed by the uridine diphosphate glucuronosyltransferases (UGTs). The most common glucuronidation reactions are O- and N-glucuronidation and in this review, we discuss both, while the emphasis is on N-glucuronidation. Interspecies difference in glucuronidation is another central issue in this review due to its importance in drug development. Accordingly, the available data on glucuronidation in different animals comes mainly from the species that are used in preclinical studies to assess the safety of drugs under development. Both O- and N-glucuronidation reactions are chemically diverse. Different O-glucuronidation reactions are described and discussed, and many drugs that undergo such reactions are indicated. The compounds that undergo N-glucuronidation include primary aromatic amines, hydroxylamines, amides, tertiary aliphatic amines, and aromatic N-heterocycles. The interspecies variability in N-glucuronidation is particularly high, above all when it comes to aliphatic tertiary amines and aromatic N-heterocycles. The N-glucuronidation rates in humans are typically much higher than in animals, largely due to the activity of two enzymes, the extensively studied UGT1A4, and the more recently identified as a main player in N-glucuronidation, UGT2B10. We discuss both enzymes and review the findings that revealed the role of UGT2B10 in N-glucuronidation.

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.

Metabolites in safety testing (MIST): considerations of mechanisms of toxicity with dose, abundance, and duration of treatment

In previous papers, we have offered a strategic framework regarding metabolites of drugs in humans and the need to assess these in laboratory animal species (also termed Metabolites in Safety Testing or MIST; Smith and Obach, Chem. Res. Toxicol. (2006) 19, 1570-1579). Three main tenets of this framework were founded in (i) comparisons of absolute exposures (as circulating concentrations or total body burden), (ii) the nature of the toxicity mechanism (i.e., reversible interaction at specific targets versus covalent binding to multiple macromolecules), and (iii) the biological matrix in which the metabolite was observed (circulatory vs excretory). In the present review, this framework is expanded to include a fourth tenet: considerations for the duration of exposure. Basic concepts of pharmacology are utilized to rationalize the relationship between exposure (to parent drug or metabolite) and various effects ranging from desired therapeutic effects through to severe toxicities. Practical considerations of human ADME (absorption-distribution-metabolism-excretion) data, to determine which metabolites should be further evaluated for safety, are discussed. An analysis of recently published human ADME studies shows that the number of drug metabolites considered to be important for MIST can be excessively high if a simple percentage-of-parent-drug criterion is used without consideration of the aforementioned four tenets. Concern over unique human metabolites has diminished over the years as experience has shown that metabolites of drugs in humans will almost always be observed in laboratory animals, although the proportions may vary. Even if a metabolite represents a high proportion of the dose in humans and a low proportion in animals, absolute abundances in animals frequently exceed that in humans because the doses used in animal toxicology studies are much greater than therapeutic doses in humans. The review also updates the enzymatic basis for the differences between species and how these relate to MIST considerations.

Identification and characterization of two chloramphenicol glucuronides from thein vitroglucuronidation of chloramphenicol in human liver microsomes

This study reports the development of a specific and sensitive liquid chromatography coupled with tandem mass spectrometry detection (LC-MS/MS) assay for the quantification of the in vitro O-glucuronidation of chloramphenicol (CP), the determination of the kinetic parameters for the O-glucuronidation of CP in pooled human liver microsomes (HLM), the biosynthesis of the CP glucuronides (CPGlu), and identification of the structures of CPGlu by (1)H-nuclear magnetic resonance (NMR) and MS. Two glucuronyl derived metabolites of CP were obtained from the incubation of alamethicin-activated HLM with CP and uridine 5'-diphosphoglucuronic acid (UDPGA) in pH 7.4 TRIS buffer. Their identification and structural confirmation were achieved by beta-glucuronidase hydrolysis, in the presence and absence of UDPGA, and by (1)H-NMR and LC-MS/MS. These two metabolites were biosynthesized, isolated, and purified using high-performance liquid chromatography (HPLC). Their structures were further identified as the 1-O-CPGlu (the minor glucuronide formed at the secondary alcohol of CP) and 3-O-CPGlu (the major glucuronide formed at the primary alcohol of CP) by LC-MS/MS and two-dimensional NMR. The enzymatic kinetic parameters K(m) and V(max) in HLM for the 3-O-CPGlu were determined to be 650 microM and 0.26 nmoles min(-1) mg(-1), respectively, and for the 1-O-CPGlu to be 301 microM and 0.014 nmoles min(-1) mg(-1), respectively. This study also provides a sensitive and specific method for the measurement of in vitro CP-UDP-glucuronosyltransferase (UGT) activity.

Absorption, Metabolism, and Excretion of [14C]Imidafenacin, a New Compound for Treatment of Overactive Bladder, After Oral Administration to Healthy Male Subjects

The absorption, metabolism, and excretion of imidafenacin [KRP-197/ONO-8025, 4-(2-methyl-1H-imidazol-1-yl)-2,2-diphenylbutanamide], a new antimuscarinic drug developed for treatment of overactive bladder, were assessed in six healthy male subjects after a single oral administration of 0.25 mg of [(14)C]imidafenacin (approximately 46 microCi). The highest radioactivity in the plasma was observed at 1.5 h after administration. The apparent terminal elimination half-life of the total radioactivity was 72 h. Approximately 65.6 and 29.4% of the administered radioactivity were recovered in the urine and feces, respectively, within 192 h after administration. The metabolite profiling by high-performance liquid chromatography-radiodetector and liquid chromatography/tandem mass spectrometry demonstrated that the main component of radioactivity was unchanged imidafenacin in the 2-h plasma. The N-glucuronide conjugate (M-9) was found as the major metabolite and the oxidized form of the 2-methylimidazole moiety (M-2) and the ring-cleavage form (M-4) were detected as the minor metabolites in the 2-h plasma, but M-4 was found to be the main component in the 12-h plasma. Unchanged imidafenacin, M-9, M-2, and other oxidized metabolites were excreted in the urine, but the unchanged imidafenacin and M-9 were not found in the feces. Two unique metabolites were found in the urine and feces, which were identified as the interchangeable cis- and trans-isomers of 4,5-dihydrodiol forms of the 2-methylimidazole moiety. These findings indicate that imidafenacin is rapidly and well absorbed (at least 65% of dose recovered in urine) after oral administration, circulates in human plasma as the unchanged form, its glucuronide, and other metabolites, and is then excreted in urine and feces as the oxidized metabolites of 2-methylimidazole moiety.

Carboxyl-glucuronidation of mitiglinide by human UDP-glucuronosyltransferases

Mitiglinide (MGN) is a new potassium channel antagonist for the treatment of type 2 diabetes mellitus. In the present study, a potential metabolic pathway of MGN, via carboxyl-linked glucuronic acid conjugation, was found. MGN carboxyl-glucuronide was isolated from a reaction mixture consisting of MGN and human liver microsomes fortified with UDP-glucuronic acid (UDPGA) and identified by a hydrolysis reaction with beta-glucuronidase and HPLC-MS/MS. Kinetic analysis indicated that MGN from four species had the highest affinity for the rabbit liver microsomal enzyme (K(m)=0.202 mM) and the lowest affinity for the dog liver microsomal enzyme (K(m)=1.164 mM). The metabolic activity (V(max)/K(m)) of MGN to the carboxyl-glucuronidation was in the following order: rabbit>dog>rat>human. With the assessment of MGN glucuronide formation across a panel of recombinant UDP-glucuronosyltransferase (UGT) isoforms (UGT1A3, UGT1A4, UGT1A6, UGT1A9, and UGT2B7), only UGT1A3 and UGT2B7 exhibited high MGN glucuronosyltransferase activity. The K(m) values of MGN glucuronidation in recombinant UGT1A3 and UGT2B7 microsomes were close to those in human liver microsomes. The formation of MGN glucuronidation by human liver microsomes was effectively inhibited by quercetin (substrate for UGT1A3) and diclofenac (substrate for UGT2B7), respectively. The MGN glucuronidation activities in 15 human liver microsomes were significantly correlated with quercetin (r(2)=0.806) and diclofenac glucuronidation activities (r(2)=0.704), respectively. These results demonstrate that UGT1A3 and UGT2B7 are catalytic enzymes in MGN carboxyl-glucuronidation in human liver.

N-glucuronidation of perfluorooctanesulfonamide by human, rat, dog, and monkey liver microsomes and by expressed rat and human UDP-glucuronosyltransferases

N-Alkylperfluorooctanesulfonamides have been used in a range of industrial and commercial applications. Perfluorooctanesulfonamide (FOSA) is a major metabolite of N-alkylperfluorooctanesulfonamides and has a long half-life in animals and in the environment and is biotransformed to FOSA N-glucuronide. The objective of this study was to identify and characterize the human and experimental animal liver UDP-glucuronosyltransferases (UGTs) that catalyze the N-glucuronidation of FOSA. The results showed that pooled human liver and rat liver microsomes had high N-glucuronidation activities. Expressed rat UGT1.1, UGT2B1, and UGT2B12 in HK293 cells catalyzed the N-glucuronidation of FOSA but at rates that were lower than those observed in rat liver microsomes. Of the 10 expressed human UGTs (1A1, 1A3, 1A4, 1A6, 1A9, 2B4, 2B7, 2B15, and 2B17) studied, only hUGT2B4 and hUGT2B7 catalyzed the N-glucuronidation of FOSA. The kinetics of N-glucuronidation of FOSA by rat liver microsomes and by hUGT2B4/7 was consistent with a single-enzyme Michaelis-Menten model, whereas human liver microsomes showed sigmoidal kinetics. These data show that rat liver UGT1.1, UGT2B1, and UGT2B12 catalyze the N-glucuronidation of FOSA, albeit at low rates, and that hUGT2B4 and hUGT2B7 catalyze the N-glucuronidation of FOSA.

N-GLUCURONIDATION OF THE PLATELET-DERIVED GROWTH FACTOR RECEPTOR TYROSINE KINASE INHIBITOR 6,7-(DIMETHOXY-2,4-DIHYDROINDENO[1,2-C]PYRAZOL-3-YL)-(3-FLUORO-PHENYL)-AMINE BY HUMAN UDP-GLUCURONOSYLTRANSFERASES

The potential cancer therapeutic agent, 6,7-(dimethoxy-2, 4-dihydroindeno[1,2-c]pyrazol-3-yl)-(3-fluoro-phenyl)-amine (JNJ-10198409), formed three N-glucuronides that were positively identified by liquid chromatography-tandem mass spectrometry and NMR as N-amine-glucuronide (Glu-A), 1-N-pyrazole-glucuronide (Glu-B), and 2-N-pyrazole-glucuronide (Glu-C). All three N-glucuronides were detected in rat liver microsomes, whereas only Glu-A and -B were found in monkey and human liver microsomes. In contrast to common glucuronides, Glu-B was completely resistant to beta-glucuronidase. Kinetic analyses revealed that glucuronidation of JNJ-10198409 in human liver microsomes exhibited atypical kinetics that may be described by a two-site binding model. For the high affinity binding, K(m) values were 1.2 and 5.0 microM, and V(max) values were 2002 and 2,403 nmol min(-1) mg(-1) for Glu-A and Glu-B, respectively. Kinetic constants of low affinity binding were not determined due to low solubility of the drug. Among the human UDP-glucuronosyltransferases (UGTs) tested, UGT1A9, 1A8, 1A7, and 1A4 were the most active isozymes to produce Glu-A; for the formation of Glu-B, UGT1A9 was the most active enzyme, followed by UGT1A3, 1A7, and 1A4. Glucuronidation of JNJ-10198409 by those UGT1A enzymes followed classic Michaelis-Menten kinetics. In contrast, no glucuronides were formed by all UGT2B isozymes tested, including UGT2B4, 2B7, 2B15, and 2B17. Collectively, these results suggested that glucuronidation of JNJ-10198409 in human liver microsomes is catalyzed by multiple UGT1A enzymes. Since UGT1A enzymes are widely expressed in various tissues, it is anticipated that both hepatic and extrahepatic glucuronidation will likely contribute to the elimination of the drug in humans. Additionally, conjugation at the nitrogens of the pyrazole ring represents a new structural moiety for UGT1A-mediated reactions.

Identification of Human UDP-Glucuronosyltransferase Isoform(s) Responsible for the Glucuronidation of 2-(4-Chlorophenyl)-5-(2-Furyl)-4-Oxazoleacetic Acid (TA-1801A)

We characterized the hepatic and intestinal UDP-glucuronosyltransferase (UGT) isoform(s) responsible for the glucuronidation of 2-(4-chlorophenyl)-5-(2-furyl)-4-oxazoleacetic acid (TA-1801A) in humans through several in vitro mechanistic studies. Assessment of a panel of recombinant UGT isoforms revealed the TA-1801A glucuronosyltransferase activity of UGT1A1, UGT1A3, UGT1A7, UGT1A9, and UGT2B7. Kinetic analyses of the TA-1801A glucuronidation by recombinant UGT1A1, UGT1A3, UGT1A9, and UGT2B7 showed that the K(m) value for UGT2B7 was apparently consistent with those in human liver and jejunum microsomes. The TA-1801A glucuronosyltransferase activity in human liver microsomes was inhibited by bilirubin (typical substrate for UGT1A1), propofol (typical substrate for UGT1A9), diclofenac (substrate for UGT1A9 and UGT2B7), and genistein (substrate for UGT1A1, UGT1A3, and UGT1A9). The inhibition by bilirubin, propofol, and diclofenac of the TA-1801A glucuronidation was less pronounced in jejunum microsomes than liver microsomes, suggesting that the contribution of UGT1A1, UGT1A9, and UGT2B7 to the TA-1801A glucuronidation is smaller in the intestine than the liver. In contrast, genistein strongly inhibited the TA-1801A glucuronosyltransferase activity in both human liver and jejunum microsomes. These results suggest that the glucuronidation of TA-1801A is mainly catalyzed by UGT1A1, UGT1A9, and UGT2B7 in the liver, and by UGT1A1, UGT1A3, and UGT2B7 in the intestine in humans.

Glucuronidation of 2-(4-Chlorophenyl)-5-(2-Furyl)-4-Oxazoleacetic Acid (TA-1801A) in Humans: Species Differences in Liver and Intestinal Microsomes

The metabolism of ethyl 2-(4-chlorophenyl)-5-(2-furyl)-4-oxazoleacetate (TA-1801), a potent hypolipidemic agent, was studied in humans after oral administration and compared with that found in rats, rabbits, and dogs previously. Hydrolysis of the ethyl ester to produce metabolite M1 (TA-1801 active form; TA-1801A) is the first metabolic step and the subsequent biotransformation includes the glucuronidation to form the metabolite M4 and the oxidation to form the metabolites M2 and M3. The metabolism of TA-1801 in humans was qualitatively similar to that in the experimental animals studied, although species differences were seen in the amount of metabolites. M4, the glucuronide of TA-1801A was the most abundant metabolite in human urine (24.3% of the dose). In vitro studies using human liver and jejunum microsomes indicated that the TA-1801A glucuronosyltransferase activity in human jejunum microsomes was 2-fold higher than that in liver microsomes. With regard to the interspecies differences in the TA-1801A glucuronosyltransferase activities, the intrinsic clearance for the TA-1801A glucuronidation in liver microsomes was in the following order: rabbit>monkey>human=rat=dog. In jejunum microsomes, the intrinsic clearance for the TA-1801A glucuronidation was in the following order: human>monkey>rabbit>rat=dog. These results suggest that the species differences in the intestinal TA-1801A glucuronidation contribute to the species differences in the excretion rate of TA-1801A glucuronide into the urine.

Regioselective glucuronidation of denopamine: marked species differences and identification of human udp-glucuronosyltransferase isoform

Denopamine is one of the oral beta(1)-adrenoceptor-selective partial agonists. Denopamine glucuronide is the most abundant metabolite in human, rat, and dog urine when administered orally. Species differences in denopamine glucuronidation were investigated with liver microsomes obtained from humans and experimental animals. In rat and rabbit, only the phenolic glucuronide was detected, whereas in dog and monkey, not only the phenolic glucuronide but also the alcoholic glucuronide was found. In contrast, in humans, the alcoholic glucuronide was detected exclusively. The kinetics of denopamine glucuronidation in human liver microsomes showed a typical Michaelis-Menten plot. The K(m) and V(max) values accounted for 2.87 +/- 0.17 mM and 7.29 +/- 0.23 nmol/min/mg protein, respectively. With the assessment of denopamine glucuronide formation across a panel of recombinant UDP-glucuronosyltransferase (UGT) isoforms (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9, UGT1A10, UGT2B4, UGT2B7, UGT2B15, and UGT2B17), only UGT2B7 exhibited high denopamine glucuronosyltransferase activity. The K(m) value of denopamine glucuronidation in recombinant UGT2B7 microsomes was close to those in human liver and jejunum microsomes. The formation of denopamine glucuronidation by human liver, jejunum, and recombinant UGT2B7 microsomes was effectively inhibited by diclofenac, a known substrate for UGT2B7. The denopamine glucuronidation activities in seven human liver microsomes were significantly correlated with diclofenac glucuronidation activities (r(2) = 0.685, p < 0.05). These results demonstrate that the denopamine glucuronidation in human liver and intestine is mainly catalyzed by UGT2B7 and that glucuronidation of the alcoholic hydroxyl group, but not the phenolic hydroxyl group, occurs regioselectively in humans.