Mechanistic studies of uridine diphosphate glucuronosyltransferase

Intestinal Glucuronidation, Prior to Hepatic Glucuronidation, Plays an Important Role in the Low Circulating Levels of Calycosin

Calycosin is a dietary flavonoid with favorable activities, which seems to be inconsistent with its low circulating levels in vivo. To address this issue, we developed a strategy to understand calycosin distribution by integrating qualitative and quantitative analyses of calycosin and its metabolites in portal vein plasma, the liver, and systemic plasma after oral administration to rats. Consequently, 21 metabolites were characterized in total, including the first report of a reductive biotransformation and 14 new metabolites. Compared with the low levels of calycosin, calycosin glucuronides were predominant in circulation, and both the hepatic and intestinal regions contributed to the high exposure of these calycosin glucuronides. However, intestinal glucuronidation, prior to hepatic glucuronidation, plays a key role in the low circulating levels of calycosin.

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.

UDP-glucose Dehydrogenase: The First-step Oxidation Is an NAD+-dependent Bimolecular Nucleophilic Substitution Reaction (SN2)

UDP-glucose dehydrogenase (UGDH) catalyzes the conversion of UDP-glucose to UDP-glucuronic acid by NAD⁺-dependent two-fold oxidation. Despite extensive investigation into the catalytic mechanism of UGDH, the previously proposed mechanisms regarding the first-step oxidation are somewhat controversial and inconsistent with some biochemical evidence, which instead supports a mechanism involving an NAD⁺-dependent bimolecular nucleophilic substitution (S_N2) reaction. To verify this speculation, the essential Cys residue of Streptococcus zooepidemicus UGDH (SzUGDH) was changed to an Ala residue, and the resulting Cys260Ala mutant and SzUGDH were then co-expressed in vivo via a single-crossover homologous recombination method. Contrary to the previously proposed mechanisms, which predict the formation of the capsular polysaccharide hyaluronan, the resulting strain instead produced an amide derivative of hyaluronan, as validated via proteinase K digestion, ninhydrin reaction, FT-IR and NMR. This result is compatible with the NAD⁺-dependent S_N2 mechanism.

Glucuronidation: driving factors and their impact on glucuronide disposition

Glucuronidation is a well-recognized phase II metabolic pathway for a variety of chemicals including drugs and endogenous substances. Although it is usually the secondary metabolic pathway for a compound preceded by phase I hydroxylation, glucuronidation alone could serve as the dominant metabolic pathway for many compounds, including some with high aqueous solubility. Glucuronidation involves the metabolism of parent compound by UDP-glucuronosyltransferases (UGTs) into hydrophilic and negatively charged glucuronides that cannot exit the cell without the aid of efflux transporters. Therefore, elimination of parent compound via glucuronidation in a metabolic active cell is controlled by two driving forces: the formation of glucuronides by UGT enzymes and the (polarized) excretion of these glucuronides by efflux transporters located on the cell surfaces in various drug disposition organs. Contrary to the common assumption that the glucuronides reaching the systemic circulation were destined for urinary excretion, recent evidences suggest that hepatocytes are capable of highly efficient biliary clearance of the gut-generated glucuronides. Furthermore, the biliary- and enteric-eliminated glucuronides participate into recycling schemes involving intestinal microbes, which often prolong their local and systemic exposure, albeit at low systemic concentrations. Taken together, these recent research advances indicate that although UGT determines the rate and extent of glucuronide generation, the efflux and uptake transporters determine the distribution of these glucuronides into blood and then to various organs for elimination. Recycling schemes impact the apparent plasma half-life of parent compounds and their glucuronides that reach intestinal lumen, in addition to prolonging their gut and colon exposure.

C-8 Mannich base derivatives of baicalein display improved glucuronidation stability: exploring the mechanism by experimentation and theoretical calculations

The glucuronidation of 7-OH is blocked by the intramolecular hydrogen bond between 7-OH and C-8 Mannich base substituent in BA-a.

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).

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

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

UDP-glucuronic acid binds first and the aglycone substrate binds second to form a ternary complex in UGT1A9-catalyzed reactions, in both the presence and absence of bovine serum albumin

The presence of bovine serum albumin (BSA) largely modulates the enzyme kinetics parameters of the human UDP-glucuronosyltransferase (UGT) 1A9, increasing both the apparent aglycone substrate affinity of the enzyme and its limiting reaction velocity (Drug Metab Dispos 39:2117-2129, 2011). For a better understanding of the BSA effects and an examination of whether its presence changes the catalytic mechanism, we have studied the enzyme kinetics of 4-methylumbelliferone glucuronidation by UGT1A9 in the presence and absence of 0.1% BSA, using bisubstrate enzyme kinetic experiments, in both the forward and reverse directions, as well as product and dead-end inhibition. The combined results strongly suggest that the reaction mechanism of UGT1A9, and presumably other human UGTs as well, involves the formation of a compulsory-order ternary-complex, with UDP-α-d-glucuronic acid (UDPGA) as the first binding substrate. Based on the enzyme kinetic parameters measured for the forward and reverse reactions, the equilibrium constant of the overall reaction was calculated (Keq = 574) and the relative magnitudes of the reaction rate constants were elucidated. The inclusion of BSA in the bisubstrate kinetic experiments quantitatively changed the apparent enzyme kinetic parameters, presumably by removing internal inhibitors that bind to the binary enzyme-UDPGA (E-UDPGA) complex, as well as to the ternary E-UDPGA-aglycone complex. Nevertheless, the underlying compulsory-order ternary-complex mechanism with UDPGA binding first is the same in both the absence and presence of BSA. The results offer a novel understanding of UGT enzyme kinetic mechanism and BSA effects.

Mangiferin glucuronidation: important hepatic modulation of antioxidant activity

Mangiferin displays an extensive spectrum of pharmacological properties, including antioxidant activity. Its phase II metabolism in the presence of Aroclor 1254-induced and un-induced microsomal and cytosolic fractions from rat liver and the antioxidant potency of the glucuronidated conjugates were investigated. Mangiferin was not a substrate for the cytosolic sulphotransferases. Glucuronidation led to the formation of two monoglucuronidated metabolites of mangiferin and a monoglucuronidated metabolite of homomangiferin (a minor constituent of the mangiferin standard). Deconjugation utilising glucuronidase resulted in the disappearance of the metabolites, with the concomitant formation of the two parent compounds. Considering steric hinderance caused by the C-2 glucosyl moiety and the relative acidity of the xanthone OH groups, the 6-OH of mangiferin and, to a lesser degree the 7-OH, are likely to be the primary glucuronidation targets. The ferric iron reducing ability of the glucuronidated reaction mixture was reduced, while the free radical scavenging abilities of mangiferin, utilising on-line post-column HPLC-DAD-DPPH· and HPLC-DAD-ABTS·+ assays, were eliminated, providing further evidence that the catechol arrangement at C-6 and C-7 was the preferred site of conjugation. This paper provides the first evidence that the glucuronidated metabolites of mangiferin resulted in a loss in free radical scavenging and ferric iron reducing ability.

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.

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.

Metabolism, oral bioavailability and pharmacokinetics of chemopreventive kaempferol in rats

The purpose of this study was to compare the hepatic and small intestinal metabolism, and to examine bioavailability and gastro-intestinal first-pass effects, of kaempferol in rats. Liver and small intestinal microsomes fortified with either NADPH or UDPGA were incubated with varying concentrations of kaempferol for up to 120 min. Based on the values of the kinetic constants (K(m) and V(max)), the propensity for UDPGA-dependent conjugation compared with NADPH-dependent oxidative metabolism was higher for both hepatic and small intestinal microsomes. Male Sprague-Dawley rats were administered kaempferol intravenously (i.v.) (10, 25 mg/kg) or orally (100, 250 mg/kg). Gastro-intestinal first-pass effects were observed by collecting portal blood after oral administration of 100 mg/kg kaempferol. Pharmacokinetic parameters were obtained by non-compartmental analysis using WinNonlin. After i.v. administration, the plasma concentration-time profiles for 10 and 25 mg/kg were consistent with high clearance (approximately 3 L/hr/kg) and large volumes of distribution (8-12 L/hr/kg). The disposition was characterized by a terminal half-life value of 3-4 h. After oral administration the plasma concentration-time profiles demonstrated fairly rapid absorption (t(max) approximately 1-2 h). The area under the curve (AUC) values after i.v. and oral doses increased approximately proportional to the dose. The bioavailability (F) was poor at approximately 2%. Analysis of portal plasma after oral administration revealed low to moderate absorption. Taken together, the low F of kaempferol is attributed in part to extensive first-pass metabolism by glucuronidation and other metabolic pathways in the gut and in the liver.

Discovery of 4-(benzylaminomethylene)isoquinoline-1,3-(2H,4H)-diones and 4-[(pyridylmethyl)aminomethylene]isoquinoline-1,3-(2H,4H)-diones as potent and selective inhibitors of the cyclin-dependent kinase 4

The series of 4-(benzylaminomethylene)isoquinoline-1,3-(2H,4H)-dione and 4-[(pyridylmethyl)aminomethylene]isoquinoline-1,3-(2H,4H)-dione derivatives reported here represents a novel class of potential antitumor agents, which potently and selectively inhibit CDK4 over CDK2 and CDK1. In the benzylamino headpiece, a 3-OH substituent is required on the phenyl ring for CDK4 inhibitory activity, which is further enhanced when an iodo, aryl, heteroaryl, t-butyl, or cyclopentyl substituent is introduced at the C-6 position of the isoquinoline-1,3-dione core. To circumvent the metabolic liability associated with the phenolic OH group on the 4-substituted 3-OH phenyl headpiece, we take two approaches: first, introduce a nitrogen o- or p- to the 3-OH group in the phenyl ring; second, replace the phenyl headpiece with N-substituted 2-pyridones. We present here the synthesis, SAR data, metabolic stability data, and a CDK4 mimic model that explains the binding, potency, and selectivity of our CDK4 selective inhibitors.

Intestinal first-pass glucuronidation activities of selected dihydroxyflavones

Flavonoids have low bioavailabilities due to extensive intestinal first-pass metabolisms, especially glucuronidation. The present study aimed to evaluate the intestinal glucuronidation of dihydroxyflavones and provide more information on their structure-activity relationships. Seven dihydroxyflavones, namely 3,7-, 5,7-, 6,7-, 7,8-, 2',7-, 3',7-, and 4',7-dihydroxyflavone and a monohydroxyflavone, 7-hydroxyflavone, were investigated by incubating each hydroxyflavone at various concentrations with either human jejunum microsome or rat intestinal microsome. Two mono-glucuronides were identified for each dihydroxyflavone. For human jejunum microsome, most of the studied dihydroxyflavones demonstrated greater glucuronidation activities than that of 7-hydroxyflavone except for 3,7-dihydroxyflavone and 4',7-dihydroxyflavone. 3',7-dihydroxyflavone had the greatest intrinsic clearance which was at least seven times greater than that of all other dihydroxyflavones. In addition, species difference in glucuronidation activity was observed with human jejunum microsome higher than rat intestinal microsome for all hydroxyflavones except for 3,7-dihydroxyflavone. The results further demonstrated that the hydroxyl group positions do affect the intestinal glucuronidation activity of hydroxyflavones. Increasing the number of hydroxyl groups on A- or B-ring (except for 4'-OH) would enhance the glucuronidation activity of flavones, whereas adding a 3-OH on C-ring might not. Furthermore, existence of hydroxyl group at 3' position may enhance the glucuronidation activity of flavonoids.

Design and synthesis of a functionally selective D3 agonist and its in vivo delivery via the intranasal route

This paper reports the synthesis and biological activity of a novel series of aryl-morpholine dopamine receptor agonists. Several compounds show high levels of functional selectivity for the D3 over the D2 dopamine receptor. Compound 26 has >1000-fold functional selectivity and has been successfully progressed in vivo using an intranasal delivery route.

Purification and characterization of a novel glucosyltransferase specific to 27β-hydroxy steroidal lactones from Withania somnifera and its role in stress responses

Sterol glycosyltransferases catalyze the synthesis of diverse glycosterols in plants. Withania somnifera is a medically important plant, known for a variety of pharmacologically important withanolides and their glycosides. In this study, a novel 27beta-hydroxy glucosyltransferase was purified to near homogeneity from cytosolic fraction of W. somnifera leaves and studied for its biochemical and kinetic properties. The purified enzyme showed activity with UDP-glucose but not with UDP-galactose as sugar donor. It exhibited broad sterol specificity by glucosylating a variety of sterols/withanolides with beta-OH group at C-17, C-21 and C-27 positions. It transferred glucose to the alkanol at C-25 position of the lactone ring, provided an alpha-OH was present at C-17 in the sterol skeleton. A comparable enzyme has not been reported earlier from plants. The enzyme is distinct from the previously purified W. somnifera 3beta-hydroxy specific sterol glucosyltransferase and does not glucosylate the sterols at C-3 position; though it also follows an ordered sequential bisubstrate reaction mechanism, in which UDP-glucose and sterol are the first and second binding substrates. The enzyme activity with withanolides suggests its role in secondary metabolism in W. somnifera. Results on peptide mass fingerprinting showed its resemblance with glycuronosyltransferase like protein. The enzyme activity in the leaves of W. somnifera was enhanced following the application of salicylic acid. In contrast, it decreased rapidly on exposure of the plants to heat shock, suggesting functional role of the enzyme in biotic and abiotic stresses.

Prediction of metabolic clearance of bisphenol A (4,4 '-dihydroxy-2,2-diphenylpropane) using cryopreserved human hepatocytes

This study investigated the kinetics of glucuronidation of bisphenol A (BPA; 4,4'-dihydroxy-2,2-diphenylpropane) in cryopreserved human hepatocytes (HCs). Incubation conditions were developed using Sprague-Dawley rat HCs. For determination of the kinetic constants of BPA glucuronidation rates with human HCs, viable HCs (0.125 x 10(6)) were incubated with [(14)C]BPA (1.3-52 microM) for 10 min. The glucuronidation reaction demonstrated Michaelis-Menten kinetics and yielded a mean K(m) for males and females of 9 +/- 3 and 8 +/- 2 microM, respectively. The V(max) values of these reactions were 438 +/- 129 pmol/min/10(6) for male HCs and 480 +/- 208 pmol/min/10(6) for female HCs. The scaled intrinsic clearance (CL(int)) for male human HCs was 149 +/- 67 ml/min/kg (range 53-246) and for female HCs was 165 +/- 89 ml/min/kg (range 73-336). Overall, there are no apparent gender differences in the glucuronidation of BPA. These CL(int) values were then extrapolated to estimate total hepatic metabolic clearance (CL(met)) using a nonrestrictive well stirred model. The estimated CL(met) value for both male and female HCs was 6 ml/min/kg, which represents 30% of hepatic blood flow. Thus, in vivo clearance seems to depend highly on plasma protein binding. These in vitro results correlate well with in vivo studies in humans, which report extensive glucuronidation of BPA.

Purification and physico-kinetic characterization of 3beta-hydroxy specific sterol glucosyltransferase from Withania somnifera (L) and its stress response

Sterol glycosyltransferases catalyze the synthesis of diverse glycosteroids in plants, leading to a change in their participation in cellular metabolism. Withania somnifera is a medically important plant, known for a variety of pharmacologically important withanolides and their glycosides. In this study, a cytosolic sterol glucosyltransferase was purified 3406 fold to near homogeneity from W. somnifera leaves and studied for its biochemical and kinetic properties. The purified enzyme was active with UDP-glucose but not with UDP-galactose as sugar donor. It exhibited broad sterol specificity by glucosylating a variety of sterols and phytosterols with 3beta-OH group. It showed a low level of activity with flavonoids and isoflavonoids. The enzyme gave maximum K(cat)/K(m) value (0.957) for 24-methylenecholesterol that resembles aglycone structure of pharmacologically important sitoindosides VII and VIII from W. somnifera. The enzyme follows ordered sequential bisubstrate mechanism of reaction, in which UDP-glucose and sterol are the first and second binding substrates. This is the first detailed kinetic study on purified plant cytosolic sterol glucosyltransferases. Results on peptide mass fingerprinting and substrate specificity suggested that the enzyme belongs to the family of secondary metabolite glucosylating glucosyltransferases. The enzyme activity exhibited a rapid in vivo response to high temperature and salicylic acid treatment of plants, suggesting its physiological role in abiotic and biotic stress.

Position preference on glucuronidation of mono-hydroxylflavones in human intestine

Extensive intestinal glucuronidation has been previously reported in both human and animals after oral administration of naturally occurred flavonoids. The present study aims to investigate the relationship between human intestinal glucuronidation activity and the position of hydroxyl substitution on flavonoids. Seven commercially available mono-hydroxyflavones (HF), namely 3-, 5-, 6-, 7-, 2'-, 3'- and 4'-mono-hydroxyflavones, were chosen as model compounds. Glucuronidation activity of the selected seven HFs was investigated by incubating each HF at various concentrations with human jejunum S9 at 37 degrees C for 10 min. The generated glucuronides were identified by HPLC/MS and quantified by HPLC/UV. Metabolic kinetics parameters including Km and Vmax of each HF were determined. The results demonstrated that the glucuronidation activity of 6- and 3'-mono-hydroxyflavones was much greater than that of 3-, 4'-, 7- and 2'-HF with 5-HF to be the lowest. The findings imply that nucleophilicity and stereo-conformation of OH substituents are crucial for the intestinal glucuronidation of flavonoids.

KINETIC CHARACTERIZATION OF THE 1A SUBFAMILY OF RECOMBINANT HUMAN UDP-GLUCURONOSYLTRANSFERASES

The initial glucuronidation rates were determined for eight recombinant human UDP-glucuronosyltransferases (UGTs) of the 1A subfamily, and the bisubstrate kinetics and inhibition patterns were analyzed. At low substrate concentrations, the reactions followed general ternary complex kinetics, whereas at higher concentrations of both substrates, the reactions were mostly characterized by ternary complex kinetics with substrate inhibition. The glucuronidation of entacapone by UGT1A9 was inhibited by 1-naphthol in a competitive fashion, with respect to entacapone, and an uncompetitive fashion, with respect to UDP-glucuronic acid (UDPGA). Its inhibition by UDP, on the other hand, was noncompetitive with respect to entacapone and competitive with respect to UDPGA. These inhibition patterns are compatible with a compulsory ordered bi bi mechanism in which UDPGA is the first-binding substrate. Despite the identical primary structure of the C-terminal halves of the UGT1A isoforms, there were marked differences in the respective K(m) values for UDPGA, ranging from 52 microM for UGT1A6 to 1256 microM for UGT1A8. Relative specificity constants were calculated for the eight UGT1A isoforms with 1-hydroxypyrene, 4-nitrophenol, scopoletin, 4-methylumbelliferone, and entacapone as aglycone substrates. The results demonstrated that seven of the UGT1A isoforms are capable of conjugating phenolic substrates with similar highest k(cat) values, and UGT1A4 has a lower relative turnover rate. The highest specificity constants were obtained for 1-hydroxypyrene, even with UGT1A6, which has been regarded as a specific isoform for small planar phenols. A k(cat) value of 1.9 s(-1) was calculated for the glucuronidation of scopoletin by purified UGT1A9.

Discovery of potent and orally bioavailable N,N′-diarylurea antagonists for the CXCR2 chemokine receptor

A series of 3-substituted N,N'-diarylureas was prepared and the structure-activity relationship relative to CXCR2 receptor affinity as well as their pharmacokinetic properties were examined. In vitro microsomal metabolism studies indicated that the lower clearance rates of the 3-sulfonamido-substituted compounds were most likely due to the suppression of glucuronidation.

Towards integrated ADME prediction: past, present and future directions for modelling metabolism by UDP-glucuronosyltransferases

Undesirable absorption, distribution, metabolism, excretion (ADME) properties are the cause of many drug development failures and this has led to the need to identify such problems earlier in the development process. This review highlights computational (in silico) approaches that have been used to identify the characteristics of ligands influencing molecular recognition and/or metabolism by the drug-metabolising enzyme UDP-gucuronosyltransferase (UGT). Current studies applying pharmacophore elucidation, 2D-quantitative structure metabolism relationships (2D-QSMR), 3D-quantitative structure metabolism relationships (3D-QSMR), and non-linear pattern recognition techniques such as artificial neural networks and support vector machines for modelling metabolism by UGT are reported. An assessment of the utility of in silico approaches for the qualitative and quantitative prediction of drug glucuronidation parameters highlights the benefit of using multiple pharmacophores and also non-linear techniques for classification. Some of the challenges facing the development of generalisable models for predicting metabolism by UGT, including the need for screening of more diverse structures, are also outlined.

PREDICTINGHUMANDRUGGLUCURONIDATIONPARAMETERS: Application of In Vitro and In Silico Modeling Approaches

Cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT), which both exist as enzyme "superfamilies," are together responsible for the metabolism of most hepatically cleared drugs. There is currently intense interest in the development of techniques that permit identification of the CYP and UGT isoform(s) involved in the metabolism of a newly discovered drug, and hence prediction of factors likely to alter elimination in vivo. In addition, the quantitative scaling of kinetic parameters for a metabolic pathway assumes importance for identifying newly discovered drugs with undesirable in vivo pharmacokinetic properties. Although qualitative and quantitative in vitro-in vivo correlation based on data generated using human liver tissue or recombinant enzymes have been applied successfully to many drugs eliminated by CYP, these strategies have proved less definitive for glucuronidated compounds. Computational (in silico) modeling techniques that potentially provide a facile and economic alternative to the in vitro methods are now emerging. This review assesses the utility of in vitro and in silico approaches for the qualitative and quantitative prediction of drug glucuronidation parameters and the challenges facing the development of generalizable models.

In silico insights: Chemical and structural characteristics associated with uridine diphosphate-glucuronosyltransferase substrate selectivity

1. Undesirable absorption, distribution, metabolism, excretion properties are the cause of many drug development failures and this has led to the need to identify such problems earlier in the development process. This work highlights computational (in silico) approaches used to identify characteristics influencing the metabolism of uridine diphosphate (UDP)-glucuronosyltransferase (UGT) substrates. Uridine diphosphate-glucuronosyltransferase facilitates conjugation between glucuronic acid and a nucleophilic site within a substrate and is one of the major drug-metabolizing enzymes. 2. An understanding of the relevant structural and chemical characteristics of the ligand and the enzyme active site will lead to greater utilization of metabolically relevant structural information in drug design. However, an X-ray crystal structure of UGT is not yet available, little has been reported about important structurally or catalytically relevant amino acids and only recently has the reported substrate profile of UGT isoforms reached an interpretable level. 3. A database of all the known substrates and non-substrates for each human UGT isoform was assembled and a range of modelling approaches assessed. Currently, pharmacophore models developed using Catalyst (Accelrys, San Diego, CA, USA) indicate that substrates of the UGT1A family share two key hydrophobic regions 3 and 6-7 A from the site of glucuronidation in a well-defined spatial geometry. Furthermore, two-dimensional quantitative structure-activity relationship models show significant reliance on substrate lipophilicity and a range of other descriptors that are known to capture information relevant to ligand-protein interactions. 4. In conclusion, substrate-based modelling of UGT appears both useful and feasible, with significant potential for determining aspects of chemical structure associated with metabolism and to quantify the nature of the relationship for UGT substrates. The development of a novel, user-defined 'glucuronidation feature' for alignment was crucial to the development of pharmacophore-based UGT models.

Pharmacophore and quantitative structure activity relationship modelling of UDP-glucuronosyltransferase 1A1 (UGT1A1) substrates

UDP-glucuronosyltransferase 1A1 (UGT1A1) is a polymorphic enzyme responsible for the glucuronidation of structurally diverse drugs, non-drug xenobiotics and endogenous compounds (e.g. bilirubin). Thus, definition of UGT1A1 substrate and inhibitor selectivities and binding affinities assumes importance for the identification of compounds whose elimination may be impaired in subjects with variant genotypes, and for the prediction of potentially inhibitory interactions involving xenobiotics and endogenous compounds metabolized by UGT1A1. We report the generation of two- and three-dimensional (2D and 3D) quantitative structure activity relationships (QSAR) and pharmacophore models for 23 known UGT1A1 substrates with diverse structure and binding affinity. Initially, a simple procedure was developed to determine apparent inhibition constants (Ki,app) for these compounds. Eighteen substrates were subsequently used to construct models and the remaining five to validate the predictive ability of the models. Three different models were constructed: (i) three feature pharmacophore model able to predict the Ki,app on the basis of the degree to which a substrate can fit to the arrangement of 3D features (r2 = 0.87, Ki,app for all five test substrates predicted within log unit); (ii) 3D-QSAR using a 'common features' pharmacophore to align the substrates (r2 = 0.71, Ki,app for four out of five test substrates predicted within one log unit); (iii) 2D-QSAR constructed with six chemical descriptors (r2 = 0.92, Ki,app of all five test substrates predicted within one log unit). The common features pharmacophore demonstrated the importance of two hydrophobic domains separated from the glucuronidation site by 4 A and 7 A, respectively. These models, which represent the first generalized predictive models for a UGT isoform, complement each other and are an important first step towards computer based (in silico) models of UGT1A1 for high throughput prediction of metabolism.

Quantitative structure activity relationships for the glucuronidation of simple phenols by expressed human UGT1A6 and UGT1A9

UGT1A6 and UGT1A9 have both been demonstrated to rapidly glucuronidate simple phenolic compounds. A series of simple phenols were selected and screened with both isoforms and then used as model substrates for the generation of V(max) and K(m) values. UGT1A6 showed a more restricted acceptance of phenolic substrates compared with UGT1A9. However, the affinity of UGT1A6 for these compounds exhibited higher K(m) values than UGT1A9, although rates of turnover were similar. Molecular surface-weighted holistic invariant molecular descriptors were generated for each substrate and used to produce the first quantitative structure activity relationship models generated for expressed human UGTs. Models relating log of the K(m) value to the generated descriptors correlated well with the experimental data r(2) value of 0.996 for UGT1A6 and r(2) value of 0.83 for UGT1A9. Cross validation by a leave-one-out method also showed good predictive capability within the subset with a q(2) value of 0.98 for UGT1A6 and q(2) value of 0.73 for UGT1A9. Empirically, UGT1A6 V(max) decreased as the 4-substituent increased in size, and a trend was observed when UGT1A6 V(max) was plotted against molecular volume. The larger UGT1A6 substrates were typified by low activity and lower K(m) values than their smaller counterparts. Extrapolating from this, it was demonstrated that phenols with large 4-substituents, which were not UGT1A6 substrates, could inhibit 4-ethylphenol glucuronidation. The K(m) values for UGT1A9 showed a similar relationship to UGT1A6 but with much lower K(m) values and greater variability in range of this value.

A strategy for the solution-phase parallel synthesis of N-(pyrrolidinylmethyl)hydroxamic acids

Both five- and six-membered iminocyclitols have proven to be useful transition-state analogue inhibitors of glycosidases. They also mimic the transition-state sugar moiety of the nucleoside phosphate sugar in glycosyltransferase-catalyzed reactions. Described here is the development of a general strategy toward the parallel synthesis of a five-membered iminocyclitol linked to a hydroxamic acid group designed to mimic the transition state of GDP-fucose complexed with Mn(II) in fucosyltransferase reactions. The iminocyclitol 8 containing a protected hydroxylamine unit was prepared from D-mannitol. The hydroxamic acid moiety was introduced via the reaction of 8 with various acid chlorides. The strategy is generally applicable to the construction of libraries for identification of glycosyltransferase inhibitors.

Structural and functional studies of UDP-glucuronosyltransferases

UDP-Glucuronosyltransferases (UGTs) are glycoproteins localized in the endoplasmic reticulum (ER) which catalyze the conjugation of a broad variety of lipophilic aglycon substrates with glucuronic acid using UDP-glucuronic acid (UDP-GIcUA) as the sugar donor. Glucuronidation is a major factor in the elimination of lipophilic compounds from the body. In this review, current information on the substrate specificities of UGT1A and 2B family isoforms is discussed. Recent findings with regard to UGT structure and topology are presented, including a dynamic topological model of UGTs in the ER. Evidence from experiments on UGT interactions with inhibitors directed at specific amino acids, photoaffinity labeling, and analysis of amino acid alignments suggest that UDP-GIcUA interacts with residues in both the N- and C-terminal domains, whereas aglycon binding sites are localized in the N-terminal domain. The amino acids identified so far as crucial for substrate binding and catalysis are arginine, lysine, histidine, proline, and residues containing carboxylic acid. Site-directed mutagenesis experiments are critical for unambiguous identification of the active-site architecture.

Synthesis of the acceptor analog alphaFuc(1-->2)alphaGal-O(CH2)7 CH3: a probe for the kinetic mechanism of recombinant human blood group B glycosyltransferase

We report the chemical synthesis of alphaFuc(1-->2)alphaGal-O(CH2)7CH3 (1) an analog of the natural blood group (O)H disaccharide alphaFuc(1-->2)betaGal-OR. Compound 1 was a good substrate for recombinant blood group B glycosyltransferase (GTB) and was used as a precursor for the enzymatic synthesis of the blood group B analog (alphaGal(-->3)alphaFuc(1-->2)]alphaGal-O(CH2)7CH3+ ++ (2). To probe the mechanism of the GTB reaction, kinetic evaluations were carried out employing compound 1 or the natural acceptor disaccharide alphaFuc(1-->2)betaGal-O(CH2)7CH3 (3) with UDP-Gal and UDP-GalNAc donors. Comparisons of the kinetic constants for alternative donor and acceptor pairs suggest that the GTB mechanism is Theorell-Chance where donor binding precedes acceptor binding. GTB operates with retention of configuration at the anomeric center of the donor. Retaining reactions are thought to occur via a double-displacement mechanism with formation of a glycosyl-enzyme intermediate consistent with the proposed Theorell-Chance mechanism.

Purification and kinetic analysis of a baculovirus ecdysteroid UDP-glucosyltransferase

The baculovirus ecdysteroid UDP-glucosyltransferase (EGT) disrupts the hormonal balance of the insect host by catalysing the conjugation of ecdysteroids, the moulting hormones, with the sugar moiety from UDP-glucose or UDP-galactose. In this study, Autographa californica nucleopolyhedrovirus EGT has been overproduced and purified, and its kinetic properties determined. The enzyme was purified 1100-fold to near-homogeneity using only two major steps, ion-exchange and gel-filtration chromatography. EGT activity was eluted from the gel-filtration column as a single peak corresponding to a 260+/-50 kDa protein, suggesting that the enzyme is an oligomer of three to five subunits, as the subunit molecular mass is approximately 56 kDa. Kinetic analysis showed that EGT has broadly similar specificities for UDP-galactose and UDP-glucose (kcat/Km=1790.8 and 902.1 respectively) when ecdysone is used as the other substrate. On the other hand, it shows marked differences in specificity for the various ecdysteroids tested. Ecdysone seems to be the optimal substrate (kcat/Km=7101.1), whereas 3-dehydroecdysone, an ecdysone precursor in Lepidoptera, is seven times less favourable (kcat/Km=1085.7). Notably, 20-hydroxyecdysone, the active form of the hormone, is conjugated very poorly (kcat/Km=31.6). Analysis of the data revealed that the enzyme mechanism involves the formation of an ecdysteroid-UDP-sugar-enzyme ternary complex. This work represents the most detailed biochemical characterization of an EGT to date.

Glucuronidation of entacapone, nitecapone, tolcapone, and some other nitrocatechols by rat liver microsomes

PURPOSE

Nitrocatechol COMT inhibitors are a new class of bioactive compounds, for which glucuronidation is the most important metabolic pathway. The objective was to characterize the enzyme kinetics of nitrocatechol glucuronidation to improve the understanding and predicting of the pharmacokinetic behavior of this class of compounds.

METHODS

The glucuronidation kinetics of seven nitrocatechols and 4-nitrophenol, the reference substrate for phenol UDP-glucuronosyltransferase activity, was measured in liver microsomes from creosote-treated rats and determined by non-linear fitting of the experimental data to the Michaelis-Menten equation. A new method that combined densitometric and radioactivity measurement of the glucuronides separated by HPTLC was developed for the quantification.

RESULTS

Apparent K(m) values for the nitrocatechols varied greatly depending on substitution pattern being comparable with 4-nitrophenol (0.11 mM) only in the case of 4-nitrocatechol (0.19 mM). Simple nitrocatechols showed two-fold Vmax values compared with 4-nitrophenol (68.6 nmol min-1 mg-1), while all disubstituted catechols exhibited much lower glucuronidation rate. Vmax/K(m) values were about 10 times higher for monosubstituted catechols compared to disubstituted ones. The kinetic parameters for COMT inhibitors were in the following order: K(m) nitecapone > > entacapone > tolcapone; Vmax nitecapone > entacapone > tolcapone; Vmax/K(m) tolcapone > nitecapone > entacapone.

CONCLUSIONS

Nitrocatechols can in principle be good substrates of UGTs. However, substituents may have a remarkable effect on the enzyme kinetic parameters. The different behaviour of nitecapone compared to the other COMT inhibitors may be due to its hydrophilic 5-substituent. The longer elimination half-life of tolcapone in vivo compared to entacapone could not be explained by glucuronidation kinetics in vitro.

Designing safer chemicals: predicting the rates of metabolism of halogenated alkanes

A computational model is presented that can be used as a tool in the design of safer chemicals. This model predicts the rate of hydrogen-atom abstraction by cytochrome P450 enzymes. Excellent correlations between biotransformation rates and the calculated activation energies (delta Hact) of the cytochrome P450-mediated hydrogen-atom abstractions were obtained for the in vitro biotransformation of six halogenated alkanes (1-fluoro-1,1,2,2-tetrachloroethane, 1,1-difluoro-1,2,2-trichloroethane, 1,1,1-trifluro-2,2-dichloroethane, 1,1,1,2-tetrafluoro-2-chloroethane, 1,1,1,2,2,-pentafluoroethane, and 2-bromo-2-chloro-1,1,1-trifluoroethane) with both rat and human enzyme preparations: In(rate, rat liver microsomes) = 44.99 - 1.79(delta Hact), r2 = 0.86; In(rate, human CYP2E1) = 46.99 - 1.77(delta Hact), r2 = 0.97 (rates are in nmol of product per min per nmol of cytochrome P450 and energies are in kcal/mol). Correlations were also obtained for five inhalation anesthetics (enflurane, sevoflurane, desflurane, methoxyflurane, and isoflurane) for both in vivo and in vitro metabolism by humans: In[F(-)]peak plasma = 42.87 - 1.57(delta Hact), r2 = 0.86. To our knowledge, these are the first in vivo human metabolic rates to be quantitatively predicted. Furthermore, this is one of the first examples where computational predictions and in vivo and in vitro data have been shown to agree in any species. The model presented herein provides an archetype for the methodology that may be used in the future design of safer chemicals, particularly hydrochlorofluorocarbons and inhalation anesthetics.