Characterization of rabbit UDP-glucuronosyltransferase UGT1A7: tertiary amine glucuronidation is catalyzed by UGT1A7 and UGT1A4

The Use of Long-term Hepatocyte Cultures for Detecting Induction of Drug Metabolising Enzymes: The Current Status

In this report, metabolically competent in vitro systems have been reviewed, in the context of drug metabolising enzyme induction. Based on the experience of the scientists involved, a thorough survey of the literature on metabolically competent long-term culture models was performed. Following this, a prevalidation proposal for the use of the collagen gel sandwich hepatocyte culture system for drug metabolising enzyme induction was designed, focusing on the induction of the cytochrome P450 enzymes as the principal enzymes of interest. The ultimate goal of this prevalidation proposal is to provide industry and academia with a metabolically competent in vitro alternative for long-term studies. In an initial phase, the prevalidation study will be limited to the investigation of induction. However, proposals for other long-term applications of these systems should be forwarded to the European Centre for the Validation of Alternative Methods for consideration. The prevalidation proposal deals with several issues, including: a) species; b) practical prevalidation methodology; c) enzyme inducers; and d) advantages of working with independent expert laboratories. Since it is preferable to include other alternative tests for drug metabolising enzyme induction, when such tests arise, it is recommended that they meet the same level of development as for the collagen gel sandwich long-term hepatocyte system. Those tests which do so should begin the prevalidation and validation process.

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.

Looking back through the MIST: a perspective of evolving strategies and key focus areas for metabolite safety analysis

The publication of the US FDA MIST guidance document in 2008 reignited the debate around the most appropriate strategies to underwrite metabolite safety for novel compounds. Whilst some organizations have suggested that the guidelines necessitate a paradigm shift to more thorough metabolite analysis during early development, an evaluation of historical practices shows that the principles of the guidelines have always largely underpinned metabolism studies within the pharmaceutical industry. Therefore, it is argued that existing practices, when coupled to appropriate emerging analytical tools and a case-by-case consideration of the relevance of the generated metabolism data in terms of structure, physicochemisty, abundance and activity, represent a fit-for-purpose approach to metabolite-safety assessments.

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.

Quaternary ammonium-linked glucuronidation of trans-4-hydroxytamoxifen, an active metabolite of tamoxifen, by human liver microsomes and UDP-glucuronosyltransferase 1A4

Tamoxifen (TAM), a nonsteroidal antiestrogen, is the most widely used drug for chemotherapy of hormone-dependent breast cancer in women. Trans-4-hydroxy-TAM (trans-4-HO-TAM), one of the TAM metabolites in humans, has been considered to be an active metabolite of TAM because of its higher affinity toward estrogen receptors (ERs) than the parent drug and other side-chain metabolites. In the present study, we found a new potential metabolic pathway of trans-4-HO-TAM and its geometrical isomer, cis-4-HO-TAM, via N-linked glucuronic acid conjugation for excretion in humans. N+-Glucuronides of 4-HO-TAM isomers were isolated along with O-glucuronides from a reaction mixture consisting of trans- or cis-4-HO-TAM and human liver microsomes fortified with UDP-glucuronic acid and identified with their respective synthetic specimens by high performance liquid chromatography-electrospray ionization time-of-flight mass spectrometry. Although N- and O-glucuronidating activities of human liver microsomes toward trans-4-HO-TAM were nearly comparable, O-glucuronidation was predominant for cis-4-HO-TAM conjugation. Only UGT1A4 catalyzed the N-linked glucuronidation of 4-HO-TAM among recombinant human UGT isoforms (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9, UGT1A10, UGT2B4, UGT2B7, UGT2B15, and UGT2B17) expressed in insect cells. In contrast, all UGT isoforms, except for UGT1A3 and UGT1A4, catalyzed O-glucuronidation of 4-HO-TAM. Although O-glucuronidation of 4-HO-TAM greatly decreased binding affinity for human ERs, 4-HO-TAM N+-glucuronide still had binding affinity similar to 4-HO-TAM itself, suggesting that N+-glucuronide might contribute to the biological activity of TAM in vivo.

Quaternary ammonium-linked glucuronidation of tamoxifen by human liver microsomes and UDP-glucuronosyltransferase 1A4

Tamoxifen (TAM), a nonsteroidal antiestrogen, is the most widely used drug for chemotherapy of hormone-dependent breast cancer in women. In the present study, we found a new potential metabolic pathway of TAM via N-linked glucuronic acid conjugation for excretion in humans. TAM N(+)-glucuronide was isolated from a reaction mixture consisting of TAM and human liver microsomes fortified with UDP-glucuronic acid (UDPGA) and identified with a synthetic specimen by high-performance liquid chromatography-electrospray ionization-mass spectrometry. However, no TAM-glucuronidating activity was detected in microsomes from rat, mouse, monkey, dog, and guinea pig livers. A strong correlation (r(2) =0.92 ) was observed between N-glucuronidating activities toward TAM and trifluoperazine, a probe substrate for human UDP-glucuronosyltransferase (UGT) 1A4, in human liver microsomes from eight donors (five females, three males). However, no correlation ( (r(2) =0.02 )) was observed in the activities between 7-hydroxy-4-(trifluoromethyl)coumarin and TAM. Only UGT1A4 catalyzed the N-linked glucuronidation of TAM among recombinant UGTs (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B4, UGT2B7, UGT2B15, and UGT2B17) expressed in insect cells. Apparent K(m) values for TAM N-glucuronidation by human liver microsomes and recombinant UGT1A4 were 35.8 and 32.4 microM, respectively. These results strongly suggested that UGT1A4 could play a role in metabolism and excretion of TAM without Phase I metabolism in human liver. TAM N(+)-glucuronide still had binding affinity similar to TAM itself for human estrogen receptors, ERalpha and ERbeta, suggesting that TAM N(+)-glucuronide might contribute to the biological activity of TAM in vivo.

Quaternary ammonium-linked glucuronidation of 1-substituted imidazoles by liver microsomes: interspecies differences and structure-metabolism relationships

N-Glucuronidation at an aromatic tertiary amine of 5-membered polyaza ring systems was investigated for a model series of eight 1-substituted imidazoles in liver microsomes from five species. The major objectives were to investigate substrate specificities of the series in human microsomes and interspecies variation for the prototype molecule, 1-phenylimidazole. The formed quaternary ammonium-linked metabolites were characterized by positive ion electrospray mass spectrometry. The incubation conditions for the N-glucuronidation of 1-substituted imidazoles were optimized; where for membrane disrupting agents, alamethicin was more effective than the detergents examined. The need to optimize alamethicin concentration was indicated by 4-fold interspecies variation in optimal concentration and by a change in effect from removal of glucuronidation latency to inhibition on increasing concentration. For the four species with quantifiable N-glucuronidation of 1-phenylimidazole, there were 8- and 18-fold variations in the determined apparent K(m) (range, 0.63 to 4.8 mM) and V(max) (range, 0.08 to 1.4 nmol/min/mg of protein) values, respectively. The apparent clearance values (V(max)/K(m)) were in the following order: human congruent with guinea pig congruent with rabbit > rat congruent with dog (no metabolite detected). Monophasic kinetics were observed for the N-glucuronidation of seven substrates by human liver microsomes, which suggests that one enzyme is involved in each metabolic catalysis. No N-glucuronidation was observed for the substrate containing the para-phenyl substituent with the largest electron withdrawing effect, 1-(4-nitrophenyl)imidazole. Linear correlation analyses between apparent microsomal kinetics and substrate physicochemical parameters revealed significant correlations between K(m) and lipophilicity (pi(para) or log P values) and between V(max)/K(m) and both electronic properties (sigma(para) value) and pKa.

Small intestinal UDP-glucuronosyltransferase sheUGT1A07: partial purification and cDNA cloning from sheep small intestine

A phenol UDPglucuronosyltransferase (UGT) was partially purified, and the cDNA encoding the isoform was cloned and sequenced from sheep small intestine. The purified preparation containing a one major band (57 kDa) and one minor band (50 kDa) revealed high activities toward xenobiotics such as 1-naphthol (1-NA), 4-nitrophenol, and 4-methylumbelliferone. The preparation, however, had only little activity toward 4-hydroxybiphenyl and no activity toward bilirubin, suggesting that the preparation contains UGT1 isoforms. The NH2-terminal amino acid sequence of the major band was determined to be Gly-Lys-Leu-Leu-Val-Val-Pro-Met-Asp-Gly-Ser. A full-length UGT cDNA was obtained by reverse transcription-polymerase chain reaction with the degenerated 5'-primer from the NH2-terminal amino acid sequence of the purified major one and rapid amplification of cDNA ends from sheep small intestine. The cloned cDNA named sheUGT1A07 by amino acid similarity has a NH2-terminus sequence identical to that of the purified major one. Another phenol UGT cDNA named sheUGT1A6 was also cloned from sheep liver. sheUGT1A6 was expressed mainly in the liver, whereas sheUGT1A07 mRNA was expressed almost only in the alimentary organs, suggesting that sheUGT1A6 plays a role as a general drug metabolizing UGT isoform in the liver and sheUGT1A07 plays important role in the xenobiotics glucuronidation in the sheep small intestine.

Transcriptional activation of the UDP-glucuronosyltransferase 1A7 gene in rat liver by aryl hydrocarbon receptor ligands and oltipraz

UDP-glucuronosyltransferase UGT1A7 catalyzes the glucuronidation of benzo(a)pyrene metabolites and other bulky aromatic compounds. Both UGT1A7 mRNA and an associated enzyme activity (benzo(a)pyrene7, 8-dihydrodioltransferase activity) are markedly increased in livers of rats treated with beta-naphthoflavone or 4-methyl-5-pyrazinyl-3H-1,2-dithiole-3-thione (oltipraz). Nuclear runoff assays show that the effects of both inducers are primarily due to transcriptional activation. A 27-kilobase region that included the UGT1A7/UGT1A6 promoter regions was cloned. Primer extension and RNase protection studies indicated >/=30 transcription start sites in five clusters between bases -85 and -40 respective to the translation start codon. There was no recognizable TATA box, but the promoter region is TA-rich. Sequence analysis revealed potential binding sites for CCAAT enhancer-binding protein, activator protein 1, and hepatic nuclear factors 1, 3, and 4, but no xenobiotic response elements or antioxidant response elements, implicated in the regulation of other genes by beta-naphthoflavone or oltipraz, were found. A UGT1A7 gene reporter plasmid directed strong constitutive expression in transient transfection assays using primary rat hepatocytes. Treatment with 3-methylcholanthrene or oltipraz had no effect compared with similarly treated pGL3-Basic-transfected cells. These results suggest that the regulatory elements controlling xenobiotic inducibility of UGT1A7 transcription are located either 5' or 3' of bases -1600 to +54. One possibility is that the polycyclic aromatic-mediated regulation of UGT1A7 occurs via the xenobiotic response element flanking the UGT1A6 locus 7 kilobase pairs downstream.