Glucuronidation of dihydrotestosterone and trans-androsterone by recombinant UDP-glucuronosyltransferase (UGT) 1A4: evidence for multiple UGT1A4 aglycone binding sites

Kinetic Characterization of Estradiol Glucuronidation by Liver Microsomes and Expressed UGT Enzymes: The Effects of Organic Solvents

BACKGROUND AND OBJECTIVE

In vitro glucuronidation of 17β-estradiol (estradiol) is often performed to assess the role of uridine 5'-diphospho-glucuronosyltransferase 1A1 (UGT1A1) in xenobiotic/drug metabolism. The objective of this study was to determine the effects of four commonly used organic solvents [i.e., dimethyl sulfoxide (DMSO), methanol, ethanol, and acetonitrile] on the glucuronidation kinetics of estradiol, which can be glucuronidated at C3 and C17 positions.

METHODS

The impacts of organic solvents on estradiol glucuronidation were determined by using expressed UGT enzymes and liver microsomes from both human and animals.

RESULTS

In human liver microsomes (HLM), methanol, ethanol, and acetonitrile significantly altered estradiol glucuronidation kinetics with increased Vmax (up to 2.6-fold) and CLmax (up to 2.8-fold) values. Altered estradiol glucuronidation in HLM was deduced to be attributed to the enhanced metabolic activities of UGT1A1 and UGT2B7, whose activities differ at the two glucuronidation positions. The effects of organic solvents on estradiol glucuronidation were glucuronidation position-, isozyme-, and solvent-specific. Furthermore, both ethanol and acetonitrile have a greater tendency to modify the glucuronidation activity of estradiol in animal liver microsomes.

CONCLUSION

Organic solvents such as methanol, ethanol, and acetonitrile showed great potential in adjusting the glucuronidation of estradiol. DMSO is the most suitable solvent due to its minimal influence on estradiol glucuronidation. Researchers should be cautious in selecting appropriate solvents to get accurate results when assessing the metabolism of a new chemical entity.

Ciprofol is primarily glucuronidated by UGT1A9 and predicted not to cause drug-drug interactions with typical substrates of CYP1A2, CYP2B6, and CYP2C19

Ciprofol is a novel intravenous anesthetic agent. Its major glucuronide metabolite, M4, is found in plasma and urine. However, the specific isoforms of UDP-glucuronosyltransferases (UGTs) that metabolize ciprofol to M4 remain unknown. This study systematically characterized UGTs that contribute to the formation of M4 using human liver microsomes (HLM), human intestinal microsomes (HIM), and human recombinant UGTs. The inhibitory potential of ciprofol and M4 against major human UGTs and cytochrome P450 enzymes (P450s) was also explored. In vitro-in vivo extrapolation (IVIVE) and physiologically-based pharmacokinetic (PBPK) simulations were performed to predict potential in vivo drug-drug interactions (DDIs) caused by ciprofol. Glucuronidation of ciprofol followed Michaelis-Menten kinetics in both HLM and HIM with apparent Km values of 345 and 412 μM, Vmax values of 2214 and 444 nmol min-1·mg protein-1, respectively. The in vitro intrinsic clearances (CLint = Vmax/Km) for ciprofol glucuronidation by HLM and HIM were 6.4 and 1.1 μL min-1·mg protein-1, respectively. Human recombinant UGT studies revealed that UGT1A9 is the predominant isoform mediating M4 formation, followed by UGT1A7, with UGT1A8 playing a minor role. Ciprofol competitively inhibited CYP1A2 (Ki = 12 μM) and CYP2B6 (Ki = 4.7 μM), and noncompetitively inhibited CYP2C19 (Ki = 29 μM). No time-dependent inhibition by ciprofol was noted for CYP1A2, CYP2B6, or CYP2C19. In contrast, M4 showed limited or no inhibitory effects against selected P450s. Neither ciprofol nor M4 inhibited UGTs significantly. Initial IVIVE suggested potential ciprofol-mediated inhibition of CYP1A2, CYP2B6, and CYP2C19 inhibition in vivo. However, PBPK simulations showed no significant effect on phenacetin, bupropion, and S-mephenytoin exposure or peak plasma concentration. Our findings are pertinent for future DDI studies of ciprofol as either a perpetrator or victim drug.

Inhibition of human UDP-glucuronosyltransferase enzyme by ripretinib: Implications for drug-drug interactions

Ripretinib, a tyrosine kinase inhibitor (TKI), is the first FDA approved fourth-line therapy for adults with advanced gastrointestinal stromal tumor (GIST). Studies have shown that several TKIs for treating GIST were potent inhibitors of human UDP- glucosyltransferase (UGTs) enzymes. However, whether ripretinib affects the activity of UGTs remains unclear. The aim of this study was to investigate the effects of ripretinib on major UGT isoforms, as well as to evaluate its potential drug-drug interactions (DDIs) risk caused by the inhibition of UGTs activities. The inhibitory effects and inhibition modes of ripretinib on UGTs were systematically evaluated using high-performance liquid chromatography (HPLC) and enzyme kinetic studies, respectively. Our data showed that ripretinib exhibited potent inhibition against UGT1A1, UGT1A3, UGT1A4, UGT1A7 and UGT1A8. Enzyme kinetic studies indicated that ripretinib was not only a competitive inhibitor of UGT1A1, UGT1A4 and UGT1A7, but also a noncompetitive inhibitor of UGT1A3, as well as a mixed inhibitor of UGT1A8. The prediction results of in vitro-in vivo extrapolation (IVIVE) demonstrated that ripretinib might bring the potential risk of DDIs when combined with substrates of UGT1A1, UGT1A3, UGT1A4, UGT1A7 or UGT1A8. Therefore, special attention should be paid when ripretinib is used in conjunction with other drugs metabolized by UGTs to avoid risk of DDIs in clinic.

Cytochrome P450 metabolism studies of [6]-gingerol, [8]-gingerol, and [10]-gingerol by liver microsomes of humans and different species combined with expressed CYP enzymes

Gingerols, mainly [6]-gingerol (6G), [8]-gingerol (8G), and [10]-gingerol (10G), are the functional and specific pungent phytochemicals in ginger. However, poor oral bioavailability limits their applications owing to extensive metabolism. The present study aims to characterize the cytochrome P450 (CYP) metabolic characteristics of 6G, 8G, and 10G by using pooled human liver microsomes (HLM), different animal liver microsomes, and the expressed CYP enzymes. It is shown that NADPH-dependent oxidation and hydrogenation metabolisms of gingerols are the main metabolic types in HLM. With the increase of the carbon chain, the polarity of gingerols decreases and the formation of hydrogenated metabolites is more efficient (CL_int: 1.41 μL min^-1 mg^-1 for 6G, 7.79 μL min^-1 mg^-1 for 8G and 14.11 μL min^-1 mg^-1 for 10G), indicating that the phase I metabolism of gingerols by HLM varied with the chemical structure of the substrate. The phase I metabolism of gingerols revealed considerable species variations, and compared to HLM, novel metabolites such as (3S,5S)-gingerdiols and demethylated metabolites are generated in some animal liver microsomes. The primary enzymes involved in the oxidized and demethylated metabolism of these gingerols are CYP1A2 and CYP2C19, but their affinities for gingerols are not the same. CYP2D6 and CYP2B6 contributed significantly to the formation of (3R,5S)-[8]-gingerdiol and (3R,5S)-[10]-gingerdiol, respectively; however, the enzyme responsible for the production of (3R,5S)-[6]-gingerediol is yet to be identified. Some metabolites in microsomes cannot be detected by the 12 investigated CYP enzymes, which may be related to the combined effects of multiple enzymes in microsomes, the different affinity of mixed liver microsomes and CYP enzymes, gene polymorphisms, etc. Overall, this work provides a deeper knowledge of the influence of CYP metabolism on the gingerols, as well as the mode of action and the possibility for drug-herbal interactions.

Diverse effects of α-/β-estradiol on catalytic activities of human UDP-glucuronosyltransferases (UGT)

β-estradiol (β-E2) and α-estradiol (α-E2) act as an endo- and an exon-estrogen in humans, respectively. There is a structural variation in C17-OH configuration of the two estrogens. UDP-glucuronosyltransferases (UGT) are responsible for termination of activities of a variety of endogenous hormones, clinical drugs, and environmental toxicants. The current study was conducted to investigate the effects of the two estrogens towards catalytic activities of UGTs. It was found that β-E2 could decrease activities of UGT1A9, - 2B4 and - 2B7, with K_i values of a few micro-molars. β-E2 could additionally accelerate the activity of UGT2B17 via promoting enzyme-substrate binding and increasing the turn over number. Comparatively, α-E2 displayed much stronger inhibitory potentials towards UGT2B7 and - 2B4, but showed little influence to UGT1A9 and - 2B17. The K_i values for inhibition of UGT2B7 in glucuronidation of different substrates by α-E2 were in a nanomolar range that is only about 1/100-1/50 of β-E2. UGT2B7 structural model was fatherly constructed to explore the mechanism underlying dramatically different inhibition selectivity of the two estrogens. Compared to β-E2, α-E2 formed more hydrophobic and hydrogen-bonded interactions with the residues in the active pocket. It is concluded that the configuration of E2-17-OH determines the inhibitory potentials towards UGTs. The results are useful in better understanding ligand selectivity of UGTs, as well as in further development of α-E2 in health protection.

Non-invasive skin sampling detects systemically administered drugs in humans

Clinical testing typically relies on invasive blood draws and biopsies. Alternative methods of sample collection are continually being developed to improve patient experience; swabbing the skin is one of the least invasive sampling methods possible. To show that skin swabs in combination with untargeted mass spectrometry (metabolomics) can be used for non-invasive monitoring of an oral drug, we report the kinetics and metabolism of diphenhydramine in healthy volunteers (n = 10) over the course of 24 hours in blood and three regions of the skin. Diphenhydramine and its metabolites were observed on the skin after peak plasma levels, varying by compound and skin location, and is an illustrative example of how systemically administered molecules can be detected on the skin surface. The observation of diphenhydramine directly from the skin supports the hypothesis that both parent drug and metabolites can be qualitatively measured from a simple non-invasive swab of the skin surface. The mechanism of the drug and metabolites pathway to the skin’s surface remains unknown.

Comparison of Sweated and Non-Sweated Ethanol Extracts of Salvia miltiorrhiza Bge. (Danshen) Effects on Human and rat Hepatic UDP-Glucuronosyltransferase and Preclinic Herb-Drug Interaction Potential Evaluation

BACKGROUND

The ethanol of Danshen (DEE) preparation has been widely used to treat cardiac-cerebral disease and cancer. Sweating is one of the primary processing methods of Danshen, which greatly influenced its quality and pharmacological properties. Sweated and non-sweated DEE preparation combining with various synthetic drugs, adding up the possibility of herbal-drug interactions.

OBJECTIVE

This study explored the effects of sweated and non-sweated DEE on human and rat hepatic UGT enzymes expression and activity and proposed a potential mechanism.

METHODS

The expression of two processed DEE on rat UGT1A, UGT2B and nuclear receptors including pregnane X receptor (PXR), constitutive androstane receptor (CAR), and peroxisome proliferator-activated receptor α (PPARα) were investigated after intragastric administration in rats by Western blot. Enzyme activity of DEE and its active ingredients (Tanshinone I, Cryptotanshinone, and Tanshinone I) on UGT isoenzymes was evaluated by quantifying probe substrate metabolism and metabolite formation in vitro using Ultra Performance Liquid Chromatography.

RESULTS

The two processed DEE (5.40 g/kg) improved UGT1A (P<0.01) and UGT2B (P<0.05) protein expression, and the non-sweated DEE (2.70 g/kg) upregulated UGT2B expression protein (P<0.05), compared with the CMCNa group. On day 28, UGT1A protein expression was increased (P<0.05) both in two processed DEE groups, meanwhile the non-sweated DEE significantly enhanced UGT2B protein expression (P<0.05) on day 21, compared with the CMCNa group. The process underlying this mechanism involved with the activation of nuclear receptors CAR, PXR, and PPARα; In vitro, sweated DEE (0-80 μg/mL) significantly inhibited the activity of human UGT1A7 (P<0.05) and rat UGT1A1, 1A8, and 1A9 (P<0.05). Non-sweated DEE (0-80 μg/mL) dramatically suppressed the activity of human UGT1A1, 1A3, 1A6, 1A7, 2B4, and 2B15, and rat UGT1A1, 1A3, 1A7, and 1A9 (P<0.05); Tanshinone I (0-1 μM) inhibited the activity of human UGT1A3, 1A6, and 1A7 (P<0.01) and rat UGT1A3, 1A6, 1A7, and 1A8 (P<0.05). Cryptotanshinone (0-1 μM) remarkably inhibited the activity of human UGT1A3 and 1A7 (P<0.05) and rat UGT1A7, 1A8, and 1A9 (P<0.05). Nonetheless, Tanshinone IIA (0-2 μM) is not a potent UGT inhibitor both in humans and rats; Additionally, there existed significant differences between two processed DEE in expression of PXR, and the activity of human UGT1A1, 1A3, 1A6, and 2B15 and rat UGT1A3 and 2B15 (P<0.05).

CONCLUSION

The effects of two processed DEE on hepatic UGT enzyme expression and activity were different. Accordingly, the combined usage of related UGTs substrates with DEE and its monomer components preparations may call for caution, depending on the drug's exposure-response relationship and dose adjustment. Besides, it is vital to pay attention to the distinction between sweated and non-sweated Danshen in clinic, which exerted an important influence on its pharmacological activity.

Fundamentals of Enzyme Kinetics: Michaelis-Menten and Non-Michaelis-Type (Atypical) Enzyme Kinetics

This chapter will provide a general introduction to the kinetics of enzyme-catalyzed reactions, including a general discussion of catalysts, reaction rates, and binding constants. This section will be followed by a discussion of various types of enzyme kinetics observed in drug metabolism reactions. A large number of enzymatic reactions can be adequately described by Michaelis-Menten kinetics. The Michaelis-Menten equation represents a rectangular hyperbola, with a y-asymptote at the V_max value. However, in other cases, more complex kinetic models are required to explain the observed data. Atypical kinetic profiles are believed to arise from the simultaneous binding of multiple molecules within the active site of the enzyme (Tracy and Hummel, Drug Metab Rev 36:231-242, 2004). Several cytochromes P450 (CYPs) have large active sites that enable binding of multiple molecules (Yano et al., J Biol Chem 279:38091-38094, 2004; Wester et al., J Biol Chem 279:35630-35637, 2004). Thus, atypical kinetics are not uncommon in in vitro drug metabolism studies.

Atypical kinetics of cytochrome P450 2J2: Epoxidation of arachidonic acid and reversible inhibition by xenobiotic inhibitors

Extrahepatic CYP2J2 metabolism of arachidonic acid (AA) to bioactive regioisomeric epoxyeicosatrienoic acids (EETs) is implicated in both physiological and pathological conditions. Here, we aimed to characterize atypical substrate inhibition kinetics of this endogenous metabolic pathway and its reversible inhibition by xenobiotic inhibitors when AA is used as the physiologically-relevant substrate vis-à-vis conventional probe substrate astemizole (AST). As compared to typical Michaelis-Menten kinetics observed for AST, complete substrate inhibition was observed for CYP2J2 metabolism of AA to 14,15-EET whereby velocity of the reaction declined significantly at concentrations of AA above 20-30 µM with an estimated substrate inhibition constant (K_s) of 31 µM. In silico sequential docking of two AA substrates to orthosteric (OBS) and adjacent secondary binding sites (SBS) within a 3-dimensional homology model of CYP2J2 revealed favorable and comparable binding poses of glide-scores -3.1 and -3.8 respectively. Molecular dynamics (MD) simulations ascertained CYP2J2 conformational stability with dual AA substrate binding as time-dependent root mean squared deviation (RMSD) of protein Cα atoms and ligand heavy atoms stabilized to a plateau in all but one trajectory (n=6). The distance between heme-iron and ω6 (C14, C15) double bond of AA in OBS also increased from 7.5 ± 1.4 Å to 8.5 ± 1.8 Å when CYP2J2 was simulated with only AA in OBS versus the presence of AA in both OBS and SBS (p<0.001), supporting the observed in vitro substrate inhibition phenomenon. Poor correlation was observed between inhibitory constants (K_i) determined for a panel of nine competitive and mixed mode xenobiotic inhibitors against CYP2J2 metabolism of AA as compared to AST, whereby 4 out of 9 drugs had a greater than 5-fold difference between K_i values. Nonlinear Eadie-Hofstee plots illustrated that complete substrate inhibition of CYP2J2 by AA was not attenuated even at high concentrations of xenobiotic inhibitors which further corroborates that CYP2J2 may accommodate three or more ligands simultaneously. In light of the atypical kinetics, our results highlight the importance of using physiologically-relevant substrates in in vitro enzymatic inhibition assays for the characterization of xenobiotic-endobiotic interactions which is applicable to other complex endogenous metabolic pathways beyond CYP2J2 metabolism of AA to EETs. The accurate determination of K_i would further facilitate the association of xenobiotic-endobiotic interactions to observed therapeutic or toxic outcomes.

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bisubstrate reaction that requires the aglycone and the cofactor, UDP-GlcUA. Accumulating evidence suggests that the bisubstrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modeling of glucuronidation reactions in vitro, UDP-GlcUA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for during experimental design and data interpretation. While the assessment of drug-drug interactions resulting from UGT inhibition has been challenging in the past, the increasing availability of UGT enzyme-selective substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of drug-drug interaction potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often underpredicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation. Physiologically based pharmacokinetic (PBPK) modeling has also shown to be of value for predicting PK of drugs eliminated by glucuronidation.

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.

Whole-genome sequencing of wild Siberian musk deer (Moschus moschiferus) provides insights into its genetic features

Background

Siberian musk deer, one of the seven species, is distributed in coniferous forests of Asia. Worldwide, the population size of Siberian musk deer is threatened by severe illegal poaching for commercially valuable musk and meat, habitat losses, and forest fire. At present, this species is categorized as Vulnerable on the IUCN Red List. However, the genetic information of Siberian musk deer is largely unexplored.

Results

Here, we produced 3.10 Gb draft assembly of wild Siberian musk deer with a contig N50 of 29,145 bp and a scaffold N50 of 7,955,248 bp. We annotated 19,363 protein-coding genes and estimated 44.44% of the genome to be repetitive. Our phylogenetic analysis reveals that wild Siberian musk deer is closer to Bovidae than to Cervidae. Comparative analyses showed that the genetic features of Siberian musk deer adapted in cold and high-altitude environments. We sequenced two additional genomes of Siberian musk deer constructed demographic history indicated that changes in effective population size corresponded with recent glacial epochs. Finally, we identified several candidate genes that may play a role in the musk secretion based on transcriptome analysis.

Conclusions

Here, we present a high-quality draft genome of wild Siberian musk deer, which will provide a valuable genetic resource for further investigations of this economically important musk deer.

FoxO3a as a Positive Prognostic Marker and a Therapeutic Target in Tamoxifen-Resistant Breast Cancer

Background: Resistance to endocrine treatments is a major clinical challenge in the management of estrogen receptor positive breast cancers. Although multiple mechanisms leading to endocrine resistance have been proposed, the poor outcome of this subgroup of patients demands additional studies. Methods: FoxO3a involvement in the acquisition and reversion of tamoxifen resistance was assessed in vitro in three parental ER+ breast cancer cells, MCF-7, T47D and ZR-75-1, in the deriving Tamoxifen resistant models (TamR) and in Tet-inducible TamR/FoxO3a stable cell lines, by growth curves, PLA, siRNA, RT-PCR, Western blot, Immunofluorescence, Transmission Electron Microscopy, TUNEL, cell cycle, proteomics analyses and animal models. FoxO3a clinical relevance was validated in silico by Kaplan–Meier survival curves. Results: Here, we show that tamoxifen resistant breast cancer cells (TamR) express low FoxO3a levels. The hyperactive growth factors signaling, characterizing these cells, leads to FoxO3a hyper-phosphorylation and subsequent proteasomal degradation. FoxO3a re-expression by using TamR tetracycline inducible cells or by treating TamR with the anticonvulsant lamotrigine (LTG), restored the sensitivity to the antiestrogen and strongly reduced tumor mass in TamR-derived mouse xenografts. Proteomics data unveiled novel potential mediators of FoxO3a anti-proliferative and pro-apoptotic activity, while the Kaplan–Meier analysis showed that FoxO3a is predictive of a positive response to tamoxifen therapy in Luminal A breast cancer patients. Conclusions: Altogether, our data indicate that FoxO3a is a key target to be exploited in endocrine-resistant tumors. In this context, LTG, being able to induce FoxO3a, might represent a valid candidate in combination therapy to prevent resistance to tamoxifen in patients at risk.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Metabolic profiling of corylin in vivo and in vitro

Corylin, an phenolic compound from Psoralea corylifolia, has been reported with various pharmacological properties but has poor bioavailability due to massive metabolism. In this study, twelve metabolites of corylin mainly involving in oxidation, hydration, glucuronidation and sulfation were detected in mice. Furthermore, the oxidation and hydration of corylin (M4) in human liver microsomes (HLM) and human intestine microsomes (HIM) were both efficient with high CL_int (intrinsic clearance) values of 24.29 and 42.85 μL/min/mg, respectively. CYP1A1, 1B1 and 2C19 contributed most for M4 with the CL_int values of 26.63, 33.09 and 132.41 μL/min/mg, respectively. Besides, M4 was strongly correlated with phenacetin-N-deacetylation (r = 0.885, p = 0.0001) and tolbutamide-4-oxidation (r = 0.727, p = 0.001) in twelve individual HLMs, respectively. In addition, corylin was efficiently glucuronidated (M7) in HLM (125.33 μL/min/mg) and in HIM (108.74 μL/min/mg). UGT1A1 contributed the most for M7 with the CL_int value of 122.32 μL/min/mg. Meanwhile, M7 was significantly correlated with β-estradiol-3-O-glucuronidation (r = 0.742, p = 0.006) in twelve individual HLMs. Moreover, the metabolism of corylin showed marked species differences. Taken together, corylin was subjected to massive first-pass metabolism in liver and intestine, while CYP1A1, 1B1, 2C19 and UGT1A1 were the main contributors. Finally, the proposed metabolic pathway of corylin involed CYP and UGT isoforms were summarized, which could help to understand the metabolic fate of corylin in vivo.

Glucuronidation of [6]-shogaol, [8]-shogaol and [10]-shogaol by human tissues and expressed UGT enzymes: identification of UGT2B7 as the major contributor

Shogaols, mainly [6]-shogaol (6S), [8]-shogaol (8S) and [10]-shogaol (10S), the predominant and characteristic pungent phytochemicals in ginger, are responsible for most of its beneficial effects. However, poor oral bioavailability owing to extensive glucuronidation limits their application. The present study aimed to characterize the glucuronidation pathways of 6S, 8S and 10S by using pooled human liver microsomes (HLM), human intestine microsomes (HIM) and recombinant human UDP-glucosyltransferases (UGTs). The rates of glucuronidation were determined by incubating shogaols with uridine diphosphate glucuronic acid-supplemented microsomes. Kinetic parameters were derived by appropriate model fitting. Reaction phenotyping assays, activity correlation analyses and relative activity factors were performed to identify the main UGT isoforms. As a result, one mono-4′-O-glucuronide was detected after incubating each shogaol with HLM and HIM. Enzymes kinetic analysis demonstrated that glucuronidation of shogaols consistently displayed the substrate inhibition profile, and the liver showed higher metabolic activity for shogaols (CL_int = 1.37–2.87 mL min⁻¹ mg⁻¹) than the intestine (CL_int = 0.67–0.85 mL min⁻¹ mg⁻¹). Besides, reaction phenotyping assays revealed that UGT2B7 displayed the highest catalytic ability (CL_int = 0.47–1.17 mL min⁻¹ mg⁻¹) among all tested UGTs. In addition, glucuronidation of shogaols was strongly correlated with AZT glucuronidation (r = 0.886, 0.803 and 0.871 for glucuronidation of 6S, 8S and 10S, respectively; p < 0.01) in a bank of individual HLMs (n = 9). Furthermore, UGT2B7 contributed to 40.8%, 34.2% and 36.0% for the glucuronidation of 6S, 8S and 10S in HLM, respectively. Taken altogether, shogaols were efficiently metabolized through the glucuronidation pathway, and UGT2B7 was the main contributor to their glucuronidation.

The glucuronidation pathways of shogaols ([6]-shogaol, [8]-shogaol and [10]-shogaol) were characterized in human tissues and recombinant human UDP-glucosyltransferases, and UGT2B7 was identified as the main contributor to their glucuronidation.

In vitro inhibition of UGT1A3, UGT1A4 by ursolic acid and oleanolic acid and drug-drug interaction risk prediction

1. Ursolic acid (UA) and oleanolic acid (OA) may have important activity relevant to health and disease prevention. Thus, we studied the activity of UA and OA on UDP-glucuronosyltransferases (UGTs) and used trifluoperazine as a probe substrate to test UGT1A4 activity. Recombinant UGT-catalyzed 4-methylumbelliferone (4-MU) glucuronidation was used as a probe reaction for other UGT isoforms. 2. UA and OA inhibited UGT1A3 and UGT1A4 activity but did not inhibit other tested UGT isoforms. 3. UA-mediated inhibition of UGT1A3 catalyzed 4-MU-β-d-glucuronidation was via competitive inhibition (IC50 0.391 ± 0.013 μM; Ki 0.185 ± 0.015 μM). UA also competitively inhibited UGT1A4-mediated trifluoperazine-N-glucuronidation (IC50 2.651 ± 0.201 μM; Ki 1.334 ± 0.146 μM). 4. OA offered mixed inhibition of UGT1A3-mediated 4-MU-β-d-glucuronidation (IC50 0.336 ± 0.013 μM; Ki 0.176 ± 0.007 μM) and competitively inhibited UGT1A4-mediated trifluoperazine-N-glucuronidation (IC50 5.468 ± 0.697 μM; Ki 6.298 ± 0.891 μM). 5. Co-administering OA or UA with drugs or products that are substrates of UGT1A3 or UGT1A4 may produce drug-mediated side effects.

Identifying and applying a highly selective probe to simultaneously determine the O-glucuronidation activity of human UGT1A3 and UGT1A4

Glucuronidation mediated by uridine 5′-diphospho (UDP)-glucuronosyltransferase is an important detoxification pathway. However, identifying a selective probe of UDP- glucuronosyltransferase is complicated because of the significant overlapping substrate specificity displayed by the enzyme. In this paper, desacetylcinobufagin (DACB) 3-O- and 16-O-glucuronidation were found to be isoform-specific probe reactions for UGT1A4 and UGT1A3, respectively. DACB was well characterized as a probe for simultaneously determining the catalytic activities of O-glucuronidation mediated by UGT1A3 and UGT1A4 from various enzyme sources, through a sensitive analysis method.

Regioselective glucuronidation of gingerols by human liver microsomes and expressed UDP-glucuronosyltransferase enzymes: reaction kinetics and activity correlation analyses for UGT1A9 and UGT2B7

OBJECTIVES

To determine the reaction kinetics for regioselective glucuronidation of gingerols (i.e. 6-, 8- and 10-gingerol) by human liver microsomes and expressed UDP-glucuronosyltransferase (UGT) enzymes, and to identify the main UGT enzymes involved in regioselective glucuronidation of gingerols.

METHODS

The rates of glucuronidation were determined by incubating the gingerols with uridine diphosphoglucuronic acid-supplemented microsomes. Kinetic parameters were derived by fitting an appropriate model to the data. Activity correlation analyses were performed to identify the main UGT enzymes contributing to hepatic metabolism of gingerols.

KEY FINDINGS

Glucuronidation at the 4'-OH group was much more favoured than that at 5-OH. The degree of position preference was compound-dependent; the catalytic efficiency ratios of 4'-O- to 5-O-glucuronidation were 9.1, 19.7 and 2.9 for 6-, 8- and 10-gingerol, respectively. UGT1A8 (an intestinal enzyme), UGT1A9 and UGT2B7 were the enzymes showing the highest activity towards gingerols. Formation of 5-O-glucuronide was mainly catalysed by UGT1A9. UGT2B7 was the only enzyme that generated glucuronides at both 4'-OH and 5-OH sites, although a strong position preference was observed with 4'-OH (≥80.2%). Further, activity correlation analyses indicated that UGT2B7 and UGT1A9 were primarily responsible for 4'-O-glucuronidation and 5-O-glucuronidation of gingerols in the liver, respectively.

CONCLUSIONS

Gingerols were metabolized by multiple hepatic and gastrointestinal UGT enzymes. Also, UGT1A9 and 2B7 were the main contributors to regioselective glucuronidation of gingerols in the liver.

Posttranscriptional regulation of uridine diphosphate glucuronosyltransferases

INTRODUCTION

The uridine diphosphate (UDP)-glucuronosyltransferase (UGT) superfamily of enzymes (EC 2.4.1.17) conjugates glucuronic acid to an aglycone substrate to make them more polar and readily excreted. In general, this reaction terminates the activities of chemicals, drugs and toxins, although occasionally a more active or toxic species is produced.

AREAS COVERED

In addition to their well-known transcriptional responsiveness, UGTs are also regulated by posttranscriptional mechanisms. Here, the authors review these mechanisms, including latency, modulation of co-substrate accessibility and binding, dimerization and oligomerization, protein-protein interactions, allosteric inhibition and activation, posttranslational structural and functional modifications and developmental switching for UGTs.

EXPERT OPINION

Posttranscriptional regulation of UGTs has traditionally received less attention than nuclear regulation, in part because mechanisms involving ribosomes and endoplasmic reticula are challenging to investigate. Most promising of the posttranscriptional mechanisms reviewed are likely to be effects on co-substrate (UDP-glucuronic acid) transport and availability and structure-function changes to UGT proteins through, for example, glycosylation and phosphorylation. Although classical biochemistry continues to illuminate many aspects of UGT function, advances in proteomics and structural biology are beginning to assist in the determination of posttranscriptional regulation mechanisms for UGTs.

The UGT1 locus is a determinant of prostate cancer recurrence after prostatectomy

The prognostic significance of common deletions in uridine diphospho-glucuronosyltransferase 2B (UGT2B) genes encoding sex steroid metabolic enzymes has been recently recognized in localized prostate cancer (PCa) after radical prostatectomy (RP). However, the role of germline variations at the UGT1 locus, encoding half of all human UGTs and primarily involved in estrogen metabolism, remains unexplored. We investigated whether variants of UGT1 are potential prognostic markers. We studied 526 Caucasian men who underwent RP for clinically localized PCa. Genotypes of patients for 34 haplotype-tagged single-nucleotide polymorphisms (htSNPs) and 11 additional SNPs across the UGT1 locus previously reported to mark common variants including functional polymorphisms were determined. The risk of biochemical recurrence (BCR) was estimated using adjusted Cox proportional hazards regression and Kaplan-Meier analysis. We further investigated whether variants are associated with plasma hormone levels by mass spectrometry. In multivariable models, seven htSNPs were found to be significantly associated with BCR. A greater risk was revealed for four UGT1 intronic variants with hazard ratios (HRs) of 1.59-1.88 (P<0.002) for htSNPs in UGT1A10, UGT1A9, and UGT1A6. Conversely, decreased BCR was associated with three htSNPs in introns of UGT1A10 and UGT1A9 (HR=0.56-058; P≤0.01). An unfavorable UGT1 haplotype comprising all risk alleles, with a frequency of 14%, had a HR of 1.68 (95% CI=1.13-2.50; P=0.011). Significant alteration in circulating androsterone levels was associated with this haplotype, consistent with changes in hormonal exposure. This study provides the first evidence, to our knowledge, that germline polymorphisms of UGT1 are potential predictors of recurrence of PCa after prostatectomy.

Metabolism of androstenone, 17β-estradiol and dihydrotestosterone in primary cultured pig hepatocytes and the role of 3β-hydroxysteroid dehydrogenase in this process

Steroids metabolism plays an important role in mammals and contributes to quality of pig meat. The main objective of this study was to identify metabolites of androstenone, 17β-estradiol and dihydrotestosterone using primary cultured pig hepatocytes as a model system. The role of 3β-hydroxysteroid dehydrogenase (3βHSD) in regulation of steroid metabolism was also validated using trilostane, a specific 3βHSD inhibitor. Steroid glucuronide conjugated metabolites were detected by liquid chromatography time of flight mass spectrometry (LC-TOF-MS). 3βHSD enzyme was essential for metabolism of androstenone to 5α-androst-16-en-3β-ol, which then formed androstenone glucuronide conjugate. Metabolism of 17β-estradiol was accompanied by formation of glucuronide-conjugated estrone and glucuronide-conjugated estradiol. The ratio of the two metabolites was around 5∶1. 3βHSD enzyme was not involved in 17β-estradiol metabolism. 5α-Dihydrotestosterone-17β-glucuronide was identified as a dihydrotestosterone metabolite, and this metabolism was related to 3βHSD enzyme activity as demonstrated by inhibition study.

Diethylstilbestrol can effectively accelerate estradiol-17-O-glucuronidation, while potently inhibiting estradiol-3-O-glucuronidation

This in vitro study investigates the effects of diethylstilbestrol (DES), a widely used toxic synthetic estrogen, on estradiol-3- and 17-O- (E2-3/17-O) glucuronidation, via culturing human liver microsomes (HLMs) or recombinant UDP-glucuronosyltransferases (UGTs) with DES and E2. DES can potently inhibit E2-3-O-glucuronidation in HLM, a probe reaction for UGT1A1. Kinetic assays indicate that the inhibition follows a competitive inhibition mechanism, with the Ki value of 2.1±0.3μM, which is less than the possible in vivo level. In contrast to the inhibition on E2-3-O-glucuronidation, the acceleration is observed on E2-17-O-glucuronidation in HLM, in which cholestatic E2-17-O-glucuronide is generated. In the presence of DES (0-6.25μM), Km values for E2-17-O-glucuronidation are located in the range of 7.2-7.4μM, while Vmax values range from 0.38 to 1.54nmol/min/mg. The mechanism behind the activation in HLM is further demonstrated by the fact that DES can efficiently elevate the activity of UGT1A4 in catalyzing E2-17-O-glucuronidation. The presence of DES (2μM) can elevate Vmax from 0.016 to 0.81nmol/min/mg, while lifting Km in a much lesser extent from 4.4 to 11μM. Activation of E2-17-O-glucuronidation is well described by a two binding site model, with KA, α, and β values of 0.077±0.18μM, 3.3±1.1 and 104±56, respectively. However, diverse effects of DES towards E2-3/17-O-glucuronidation are not observed in liver microsomes from several common experimental animals. In summary, this study issues new potential toxic mechanisms for DES: potently inhibiting the activity of UGT1A1 and powerfully accelerating the formation of cholestatic E2-17-O-glucuronide by UGT1A4.

Warfarin is an effective modifier of multiple UDP-glucuronosyltransferase enzymes: evaluation of its potential to alter the pharmacokinetics of zidovudine

In this study, we aimed to determine the modulatory effects of warfarin (an extensively used anticoagulant drug) and its metabolites on UDP-glucuronosyltransferase (UGT) activity and to assess the potential of warfarin to alter the pharmacokinetics of zidovudine (AZT). The effects of warfarin and its metabolites on glucuronidation were determined using human and rat liver microsomes (HLM and RLM) as well as expressed UGTs. The mechanisms of warfarin-UGT interactions were explored through kinetic characterization and modeling. Pharmacokinetic studies with rats were performed to evaluate the potential of warfarin to alter the pharmacokinetics of AZT. We found that warfarin was an effective modifier of a panel of UGT enzymes. The effects of warfarin on glucuronidation were inhibitory for UGT1A1, 2B7, and 2B17, but activating for UGT1A3. Mixed effects were observed for UGT1A7 and 1A9. Consistent with its inhibitory effects on UGT2B7 activity, warfarin inhibited AZT glucuronidation in HLM (Ki = 74.9-96.3 μM) and RLM (Ki = 190-230 μM). Inhibition of AZT glucuronidation by UGT2B7, HLM, and RLM was also observed with several hydroxylated metabolites of warfarin. Moreover, the systemic exposure (AUC) of AZT in rats was increased by a 1.5- to 2.1-fold upon warfarin coadministration. The elevated AUC was associated with suppressed glucuronidation that was probably attained through a combined action of warfarin and its hydroxylated metabolites. In conclusion, the activities of multiple UGT enzymes can be modulated by warfarin and the nature of modulation was isoform dependent. Also, pharmacokinetic interactions of zidovudine with warfarin were highly possible through inhibition of UGT metabolism.

Transcriptional regulation of human UDP-glucuronosyltransferase genes

Glucuronidation is an important metabolic pathway for many small endogenous and exogenous lipophilic compounds, including bilirubin, steroid hormones, bile acids, carcinogens and therapeutic drugs. Glucuronidation is primarily catalyzed by the UDP-glucuronosyltransferase (UGT) 1A and two subfamilies, including nine functional UGT1A enzymes (1A1, 1A3-1A10) and 10 functional UGT2 enzymes (2A1, 2A2, 2A3, 2B4, 2B7, 2B10, 2B11, 2B15, 2B17 and 2B28). Most UGTs are expressed in the liver and this expression relates to the major role of hepatic glucuronidation in systemic clearance of toxic lipophilic compounds. Hepatic glucuronidation activity protects the body from chemical insults and governs the therapeutic efficacy of drugs that are inactivated by UGTs. UGT mRNAs have also been detected in over 20 extrahepatic tissues with a unique complement of UGT mRNAs seen in almost every tissue. This extrahepatic glucuronidation activity helps to maintain homeostasis and hence regulates biological activity of endogenous molecules that are primarily inactivated by UGTs. Deciphering the molecular mechanisms underlying tissue-specific UGT expression has been the subject of a large number of studies over the last two decades. These studies have shown that the constitutive and inducible expression of UGTs is primarily regulated by tissue-specific and ligand-activated transcription factors (TFs) via their binding to cis-regulatory elements (CREs) in UGT promoters and enhancers. This review first briefly summarizes published UGT gene transcriptional studies and the experimental models and tools utilized in these studies, and then describes in detail the TFs and their respective CREs that have been identified in the promoters and/or enhancers of individual UGT genes.

Enzyme kinetics of uridine diphosphate glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bi-substrate reaction that requires the aglycone and a cofactor, UDPGA. Accumulating evidence suggests that the bi-substrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modelling of glucuronidation reactions in vitro, UDPGA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for in experimental design and data interpretation. Assessing drug-drug interactions (DDIs) involving UGT inhibition remains challenging. However, the increasing availability of UGT enzyme-specific substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of DDI potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often under-predicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation (IVIVE).

A potential role for human UDP-glucuronosyltransferase 1A4 promoter single nucleotide polymorphisms in the pharmacogenomics of tamoxifen and its derivatives

Tamoxifen (Tam) is a selective estrogen receptor modulator used to inhibit breast tumor growth. Tam can be directly N-glucuronidated via the tertiary amine group or O-glucuronidated after cytochrome P450-mediated hydroxylation. In this study, the glucuronidation of Tam and its hydroxylated and/or chlorinated derivatives [4-hydroxytamoxifen (4OHTam), toremifene (Tor), and 4-hydroxytoremifene (4OHTor)] was examined using recombinant human UDP-glucuronosyltransferases (UGTs) from the 1A subfamily and human hepatic microsomes. Recombinant UGT1A4 catalyzed the formation of N-glucuronides of Tam and its derivatives and was the most active UGT enzyme toward these compounds. Therefore, it was hypothesized that single nucleotide polymorphisms (SNPs) in the promoter region of UGT1A4 have the ability to significantly decrease the glucuronidation rates of Tam metabolites in the human liver. In vitro activity of 64 genotyped human liver microsomes was used to determine the association between the UGT1A4 promoter and coding region SNPs and the glucuronidation rates of Tam, 4OHTam, Tor, and 4OHTor. Significant decreases in enzymatic activity were observed in microsomes for individuals heterozygous for -163G/A and -217T/G. These alterations in glucuronidation may lead to prolonged circulating half-lives and may potentially modify the effectiveness of these drugs in the treatment of breast cancer.

Different enzyme kinetic models

As described in Chapter 2 , a large number of enzymatic reactions can be adequately described by Michaelis-Menten kinetics. The Michaelis-Menten equation represents a rectangular hyperbola, with a y-asymptote at the V max value. In many cases, more complex kinetic models are required to explain the observed data. Atypical kinetic profiles are believed to arise from the simultaneous binding of multiple molecules within the active site of the enzyme (Tracy and Hummel, Drug Metab Rev 36:231-242, 2004). Several cytochromes P450 have large active sites that enable binding of multiple molecules (Wester et al. J Biol Chem 279:35630-35637, 2004; Yano et al. J Biol Chem 279:38091-38094, 2004). Thus, atypical kinetics are not uncommon in in vitro drug metabolism studies. This chapter covers enzyme kinetic reactions in which a single enzyme has multiple binding sites for substrates and/or inhibitors as well as reactions catalyzed by multiple enzymes.

Potential role of UGT1A4 promoter SNPs in anastrozole pharmacogenomics

Anastrozole belongs to the nonsteroidal triazole-derivative group of aromatase inhibitors. Recently, clinical trials demonstrated improved antitumoral efficacy and a favorable toxicity with third-generation aromatase inhibitors, compared with tamoxifen. Anastrozole is predominantly metabolized by phase I oxidation with the potential for further phase II glucuronidation. It also, however, is subject to direct N-glucuronidation by UDP-glucuronosyltransferase 1A4 (UGT1A4). Anastrozole pharmacokinetics vary widely among patients, but pharmacogenomic studies of patients treated with anastrozole are sparse. In this study, we examined individual variability in the glucuronidation of anastrozole and its association with UGT1A4 promoter and coding region polymorphisms. In vitro assays using liver microsomal preparations from individual subjects (n = 96) demonstrated 235-fold variability in anastrozole glucuronidation. Anastrozole glucuronidation was correlated (r = 0.99; P < 0.0001) with lamotrigine glucuronidation (a diagnostic substrate for UGT1A4) and with UGT1A4 mRNA expression levels in human liver microsomes (r = 0.99; P < 0.0001). Recombinant UGT1A4 catalyzed anastrozole glucuronidation, which was inhibited by hecogenin (IC50 = 15 µM), a UGT1A4 specific inhibitor. The promoter region of UGT1A4 is polymorphic, and compared with those homozygous for the common allele, lower enzymatic activity was observed in microsomes from individuals heterozygous for -163G<A, -219T<G, and -217C<T (P = 0.009, P = 0.014, and P = 0.009, respectively). These results indicate that variability in glucuronidation could contribute to response to anastrozole in the treatment of breast cancer.

Albumin stimulates the activity of the human UDP-glucuronosyltransferases 1A7, 1A8, 1A10, 2A1 and 2B15, but the effects are enzyme and substrate dependent

Human UDP-glucuronosyltransferases (UGTs) are important enzymes in metabolic elimination of endo- and xenobiotics. It was recently shown that addition of fatty acid free bovine serum albumin (BSA) significantly enhances in vitro activities of UGTs, a limiting factor in in vitro–in vivo extrapolation. Nevertheless, since only few human UGT enzymes were tested for this phenomenon, we have now performed detailed enzyme kinetic analysis on the BSA effects in six previously untested UGTs, using 2–4 suitable substrates for each enzyme. We also examined some of the previously tested UGTs, but using additional substrates and a lower BSA concentration, only 0.1%. The latter concentration allows the use of important but more lipophilic substrates, such as estradiol and 17-epiestradiol. In five newly tested UGTs, 1A7, 1A8, 1A10, 2A1, and 2B15, the addition of BSA enhanced, to a different degree, the in vitro activity by either decreasing reaction’s K_m, increasing its V_max, or both. In contrast, the activities of UGT2B17, another previously untested enzyme, were almost unaffected. The results of the assays with the previously tested UGTs, 1A1, 1A6, 2B4, and 2B7, were similar to the published BSA only as far as the BSA effects on the reactions’ K_m are concerned. In the cases of V_max values, however, our results differ significantly from the previously published ones, at least with some of the substrates. Hence, the magnitude of the BSA effects appears to be substrate dependent, especially with respect to V_max increases. Additionally, the BSA effects may be UGT subfamily dependent since K_m decreases were observed in members of subfamilies 1A, 2A and 2B, whereas large V_max increases were only found in several UGT1A members. The results shed new light on the complexity of the BSA effects on the activity and enzyme kinetics of the human UGTs.

A new strategy to rapidly evaluate kinetics of glucuronide efflux by breast cancer resistance protein (BCRP/ABCG2)

PURPOSE

The efflux transporter breast cancer resistance protein (BCRP/ABCG2) plays an important role in excretion of anionic drugs and metabolites including glucuronides in humans.

METHODS

In this article, our recently published cell model (i.e., HeLa cells over-expressing UGT1A9 (HeLa1A9)) is used to determine the kinetic parameters of BCRP-mediated transport of glucuronides.

RESULTS

After incubation of the aglycone with the cells, a steady-state (i.e., zero-order or near zero-order) excretion of its glucuronide is rapidly achieved and then maintained. Kinetic profiling with different (intracellular) glucuronide concentrations and their corresponding excretion rates is enabled by varying the concentration of the aglycone, which allows for the determination of kinetic parameters responsible for BCRP-mediated efflux of glucuronides. This approach was validated theoretically using a cellular pharmacokinetic model incorporating various enzymatic and transporter-mediated kinetic processes. It was also validated experimentally in that kinetic parameters of efflux of glucuronides of 6-hydroxyflavone and 4-methylumberiferone in the HeLa1A9 cell model were shown to be consistent with those derived with BCRP-overexpressing membrane vesicles.

CONCLUSION

This study provides a new strategy for rapidly evaluating the kinetics of glucuronide efflux by BCRP.

Functional analysis of UGT1A4P24T and UGT1A4L48V variant enzymes

Aim: To investigate the effects of two nonsynonymous SNPs, UGT1A4*2 (rs#: 6755571, 70C>A, P24T) and UGT1A4*3 (rs#: 2011425, 142T>G, L48V), on the function of UGT1A4 against dihydrotestosterone (DHT), transandrosterone (t-AND), lamotrigine (LTG) and tamoxifen (TAM). Materials & methods: Detailed kinetic experiments were conducted with recombinant UGT1A4wild-type, UGT1A4P24T and UGT1A4L48V, which were overexpressed in HEK293 cell lines. The kinetic profiles and kinetic parameters (Km, Vmax and CLint) obtained with either UGT1A4P24T or UGT1A4L48V were compared with those obtained with the wild-type enzyme. The interaction of TAM on UG1A4-catalyzed DHT glucuronidation was also investigated with the three UGT1A4 polymorphic enzymes. Results: UGT1A4L48V had higher enzyme efficiency (CLint) compared with wild-type UGT1A4 on DHT glucuronidation; UGT1A4P24T and UGT1A4L48V had lower CLint than wild-type UGT1A4 for t-AND and LTG glucuronidation. The TAM CLint with UGT1A4P24T and UGT1A4L48V glucuronidation and the UGT1A4P24T-catalyzed DHT glucuronidation were, on the other hand, similar to those of the wild-type enzyme. With all three enzymes, TAM activated UGT1A4-catalyzed DHT glucuronidation in a concentration-dependent fashion. Conclusion: Decreased CLint of UGT1A4P24T and UGT1A4L48V on LTG glucuronidation may lead to interindividual variations in LTG metabolism in vivo. However, it is less likely that these polymorphisms would have impact on DHT and t-AND metabolism in vivo because these compounds are glucuronidated by multiple enzymes.

Original submitted 31 May 2011; Revision submitted 19 July 2011

Substrate inhibition kinetics in drug metabolism reactions

Inhibition of enzyme activity at high substrate concentrations, so-called "substrate inhibition," is commonly observed and has been recognized in drug metabolism reactions since the last decade. Although the importance of such "atypical" kinetics in vivo remains poorly understood, a substrate with substrate inhibition kinetics has been shown to unconventionally alter the metabolism of other substrates. In recent years, it is becoming increasingly evident that the mechanisms for substrate inhibition are highly complex, which are possibly contributed by multiple (at least two) binding sites within the enzyme protein, the formation of a ternary dead-end enzyme complex, and/or the ligand-induced changes in enzyme conformation. This review primarily discusses the mechanisms for substrate inhibition displayed by the important drug-metabolizing enzymes, such as cytochrome p450s, UDP-glucuronyltransferases, and sulfotransferases. Kinetic modeling of substrate inhibition in the absence or presence of a modifier is another central issue in this review because of its importance in the determination of kinetic parameters and in vitro/in vivo predictions.