Stereoselective metabolism of racemic carvedilol by UGT1A1 and UGT2B7, and effects of mutation of these enzymes on glucuronidation activity

Carvedilol, an alpha- and beta-adrenergic blocking drug, is mainly metabolized by CYP2D6, UGT1A1, UGT2B4 and UGT2B7. This drug is administered orally as a racemic mixture of R(+)- and S(-)-enantiomers. It has been reported that CYP2D6 prefers metabolizing S-carvedilol to R-carvedilol stereoselectively. On the other hand, stereoselective metabolism of carvedilol by UGTs is still unclear. Moreover, we have reported that patients with chronic heart failure who had polymorphism in CYP2D6, UGT1A1 and/or UGT2B7 had lower metabolic activity and oral clearance than did patients with no polymorphism. The aim of this study was to clarify stereoselective metabolism of carvedilol by UGT1A1 and UGT2B7 and to determine by using a recombinant enzyme-introduced mutation whether genetic mutation in UGT1A1 and UGT2B7 causes reduction in metabolic activity for carvedilol. A glucuronidation assay using human liver microsomes and recombinant UGT1A1 and UGT2B7 expressed in HeLa cells demonstrated that UGT1A1 prefers metabolizing R-carvedilol to S-carvedilol. On the other hand, UGT2B7 prefers metabolizing S-carvedilol to R-carvedilol. Moreover, G71R mutation of UGT1A1 reduced both affinity and capacity but did not affect stereoselective metabolism. On the other hand, both A71S and H268Y mutations of UGT2B7 reduced capacity but did not affect affinity and, as a result, the efficiency of metabolism was remarkably reduced. However, as in the case of UGT1A1, neither of the mutations affected stereoselective metabolism.     

Stereoselective Glucuronidation of Carvedilol in Human Liver and Intestinal Microsomes

Background and Purpose: Carvedilol is used clinically as a β-adrenoceptor antagonist for the treatment of chronic heart failure and is primarily metabolized into glucuronides by UDP-glucuronosyltransferase (UGT). In this study, the stereoselective glucuronidation of carvedilol by the human liver and intestinal microsomes was examined using racemate and enantiomers. Methods: Carvedilol glucuronidation activities at substrate concentrations of 1-1,000 μmol/l in human liver and intestinal microsomes were determined by high-performance liquid chromatography with fluorescence detection, and the kinetic parameters were estimated. Results: The activities of S-glucuronidation toward racemic and enantiomeric carvedilol in liver microsomes were higher than those of R-glucuronidation at all substrate concentrations examined. In intestinal microsomes, the activities of S-glucuronidation from racemic and enantiomeric carvedilol at ≤100 μmol/l substrates were higher than those of R-glucuronidation, whereas the glucuronidation activities at ≥200 μmol/l substrates exhibited the opposite stereoselectivity (R > S) compared with those at ≤100 μmol/l substrates. The activities of R- and S-calvedilol glucuronidation from racemate and enantiomers in the liver and intestinal microsomes were decreased at substrate concentrations of ≥100 or 200 μmol/l, and the kinetics at substrate concentrations of 1-100 and 1-1,000 μmol/l fitted with Michaelis-Menten and substrate inhibition models, respectively. The stereoselectivities of CL(int) values for carvedilol glucuronidation followed by Michaelis-Menten and substrate inhibition models were R < S for liver microsomes and R ≈ S for intestinal microsomes. Conclusion: These findings demonstrate that the stereoselectivity of carvedilol glucuronidation was different between human liver and intestinal microsomes, and suggest that the difference is due to the tissue-specific expression of UGT isoforms involved in the glucuronidation of carvedilol.     

Glucuronidation of icaritin by human liver microsomes,human intestine microsomes and expressed UDP-glucuronosyltransferase enzymes: identification of UGT1A3, 1A9 and 2B7 as the main contributing enzymes

1.?Icaritin is a natural flavonoid with anti-osteoporosis activity. This study aimed to characterize icaritin glucuronidation by pooled human liver microsomes (HLM) and pooled human intestine microsomes (HIM), and to determine the contribution of individual UDP-glucuronosyltrans-ferase (UGT) enzyme to icaritin glucuronidation.

2.?Glucuronidation rates were determined by incubating icaritin with uridine diphosphate glucuronic acid (UDPGA)-supplemented microsomes. Kinetic parameters were derived by appropriate model fitting. Relative activity factors and activity correlation analysis were performed to identify main UGT isoforms.

3.?UGT1A3, 1A7, 1A8, 1A9 and 2B7 were mainly responsible for catalyzing the formation of two glucuronides (G1 and G2). Icaritin 3-O-glucuronidation (G1) was significantly correlated with Chenodeoxycholic acid (CDCA) glucuronidation (r?=?0.787, p?=?0.002), propofol glucuronidation (r?=?0.661, p?=?0.019) and Zidovudine (AZT) glucuronidation (r?=?0.805, p?=?0.002). Similarly, icaritin 7-O-glucuronidation (G2) was also correlated with CDCA glucuronidation (r?=?0.640, p?=?0.025), propofol glucuronidation (r?=?0.592, p?=?0.043) and AZT glucuronidation (r?=?0.661, p?=?0.019). In addition, UGT1A3, 1A9 and 2B7 contributed 37.5, 33.8 and 21.3% for G1 in pooled HLM, respectively. Also, UGT1A3, 1A9 and 2B7 contributed 34.3, 20.0 and 8.6% for G2 in pooled HLM, respectively.

4.?Icaritin was subjected to significant glucuronidation, wherein UGT1A3, 1A7, 1A8, 1A9 and 2B7 were main contributing enzymes.     

Characterization of hepatic and intestinal glucuronidation of magnolol: application of the relative activity factor approach to decipher the contributions of multiple UDP-glucuronosyltransferase isoforms

Magnolol is a food additive that is often found in mints and gums. Human exposure to this compound can reach a high dose; thus, characterization of magnolol disposition in humans is very important. Previous studies indicated that magnolol can undergo extensive glucuronidation in humans in vivo. In this study, in vitro assays were used to characterize the glucuronidation pathway in human liver and intestine. Assays with recombinant human UDP-glucuronosyltransferase enzymes (UGTs) revealed that multiple UGT isoforms were involved in magnolol glucuronidation, including UGT1A1, -1A3, -1A7, -1A8, -1A9, -1A10, and -2B7. Magnolol glucuronidation by human liver microsomes (HLM), human intestine microsomes (HIM), and most recombinant UGTs exhibited strong substrate inhibition kinetics. The degree of substrate inhibition was relatively low in the case of UGT1A10, whereas the reaction catalyzed by UGT1A9 followed biphasic kinetics. Chemical inhibition studies and the relative activity factor (RAF) approach were used to identify the individual UGTs that played important roles in magnolol glucuronidation in HLM and HIM. The results indicate that UGT2B7 is mainly responsible for the reaction in HLM, whereas UGT2B7 and UGT1A10 are significant contributors in HIM. In summary, the current study clarifies the glucuronidation pathway of magnolol and demonstrates that the RAF approach can be used as an efficient method for deciphering the roles of individual UGTs in a given glucuronidation pathway in the native tissue that is catalyzed by multiple isoforms with variable and atypical kinetics.     

Stereoselective oxidation and glucuronidation of carvedilol in human liver and intestinal microsomes

The aim of the present study was to investigate the mechanism for the stereoselective presystemic clearance of carvedilol. We examined the oxidation and glucuronidation of carvedilol in human liver microsomes (HLM) and human intestinal microsomes (HIM). The oxidation of carvedilol in HLM and HIM was evaluated in the presence of NADPH, whereas glucuronidation was evaluated in the presence of UDP-glucuronic acid. Oxidation of S-carvedilol in HLM and HIM was greater than that of R-carvedilol. In addition, the oxidation of R-carvedilol in HLM was inhibited by quinidine, whereas that of S-carvedilol was inhibited by both quinidine and furafylline. On the other hand, R- and S-carvedilol oxidation in HIM was inhibited by ketoconazole. Glucuronidation of S-carvedilol in HLM and HIM was also higher than that of R-carvedilol. These results suggested that cytochrome P450 (CYP) 2D6 and CYP1A2 are involved in the stereoselective oxidation of carvedilol in the liver, that CYP3A4 is involved in intestinal oxidation, and that glucuronidation in the liver and intestine is at least partly responsible for stereoselective presystemic clearance.     

Identification of UDP-glucuronosyltransferases responsible for the glucuronidation of darexaban, an oral factor Xa inhibitor, in human liver and intestine

Darexaban maleate is a novel oral direct factor Xa inhibitor, which is under development for the prevention of venous thromboembolism. Darexaban glucuronide was the major component in plasma after oral administration of darexaban to humans and is the pharmacologically active metabolite. In this study, we identified UDP-glucuronosyltransferases (UGTs) responsible for darexaban glucuronidation in human liver microsomes (HLM) and human intestinal microsomes (HIM). In HLM, the K(m) value for darexaban glucuronidation was >250 μM. In HIM, the reaction followed substrate inhibition kinetics, with a K(m) value of 27.3 μM. Among recombinant human UGTs, UGT1A9 showed the highest intrinsic clearance for darexaban glucuronidation, followed by UGT1A8, -1A10, and -1A7. All other UGT isoforms were inactive toward darexaban. The K(m) value of recombinant UGT1A10 for darexaban glucuronidation (34.2 μM) was comparable to that of HIM. Inhibition studies using typical UGT substrates suggested that darexaban glucuronidation in both HLM and HIM was mainly catalyzed by UGT1A8, -1A9, and -1A10. Fatty acid-free bovine serum albumin (2%) decreased the unbound K(m) for darexaban glucuronidation from 216 to 17.6 μM in HLM and from 35.5 to 18.3 μM in recombinant UGT1A9. Recent studies indicated that the mRNA expression level of UGT1A9 is extremely high among UGT1A7, -1A8, -1A9, and -1A10 in human liver, whereas that of UGT1A10 is highest in the intestine. Thus, the present results strongly suggest that darexaban glucuronidation is mainly catalyzed by UGT1A9 and UGT1A10 in human liver and intestine, respectively. In addition, UGT1A7, -1A8, and -1A9 play a minor role in human intestine.     

Enantiomer Selective Glucuronidation of the Non‐Steroidal Pure Anti‐Androgen Bicalutamide by Human Liver and Kidney: Role of the Human UDP‐Glucuronosyltransferase (UGT)1A9 Enzyme

Bicalutamide (Casodex^®) is a non‐steroidal pure anti‐androgen used in the treatment of localized prostate cancer. It is a racemate drug, and its activity resides in the (R)‐enantiomer, with little in the (S)‐enantiomer. A major metabolic pathway for bicalutamide is glucuronidation catalysed by UDP‐glucuronosyltransferase (UGT) enzymes. While (S)bicalutamide is directly glucuronidated, (R)bicalutamide requires hydroxylation prior to glucuronidation. The contribution of human tissues and UGT isoforms in the metabolism of these enantiomers has not been extensively investigated. In this study, both (R) and/or (S)bicalutamide were converted into glucuronide (‐G) derivatives after incubation of pure and racemic solutions with microsomal extracts from human liver and kidney. Intestinal microsomes exhibited only low reactivity with these substrates. Km values of liver and kidney samples for (S)bicalutamide glucuronidation were similar, and lower than values obtained with the (R)‐enantiomer. Among the 16 human UGTs tested, UGT1A8 and UGT1A9 were able to form both (S) and (R)bicalutamide‐G from pure or racemic substrates. UGT2B7 was also able to form (R)bicalutamide‐G. Kinetic parameters of the recombinant UGT2B7, UGT1A8 and UGT1A9 enzymes support a predominant role of the UGT1A9 isoform in bicalutamide metabolism. Accordingly, (S)bicalutamide inhibited the ability of human liver and kidney microsomes to glucuronidate the UGT1A9 probe substrate, propofol. In conclusion, the present study provides the first comprehensive analysis of in vitro bicalutamide glucuronidation by human tissues and UGTs and identifies UGT1A9 as a major contributor for (R) and (S) glucuronidation in the human liver and kidney.     

Udp-glucuronosyltransferase 2b7 is the major enzyme responsible for gemcabene glucuronidation in human liver microsomes.

The predominant metabolic pathway of gemcabene in humans is glucuronidation. The principal human UDP-glucuronosyltransferases (UGTs) involved in the glucuronidation of gemcabene were determined in this study. Glucuronidation of gemcabene was catalyzed by recombinant UGT1A3, recombinant UGT2B7, and recombinant UGT2B17, as well as by human liver microsomes (HLM). Gemcabene glucuronidation in recombinant UGTs and HLM followed non-Michaelis-Menten kinetics consistent with homotropic activation, but pharmacokinetics in humans were linear over the dose range tested (total plasma C(max), 0.06-0.88 mM). Gemcabene showed similar affinity (S(50)) for recombinant UGTs (0.92-1.45 mM) and HLM (1.37 mM). S-Flurbiprofen was identified as a more selective inhibitor of recombinant UGT2B7-catalyzed gemcabene glucuronidation (>23-fold lower IC(50)) when compared with recombinant UGT1A3- or recombinant UGT2B17-catalyzed gemcabene glucuronidation. The IC(50) for S-flurbiprofen inhibition of gemcabene glucuronidation was similar in HLM (60.6 microM) compared with recombinant UGT2B7 (27.4 microM), consistent with a major role for UGT2B7 in gemcabene glucuronidation in HLM. In addition, 5,6,7,3',4',5'-hexamethoxyflavone inhibited recombinant UGT1A3 and recombinant UGT2B17-catalyzed gemcabene glucuronidation (with 4-fold greater potency for recombinant UGT1A3) but did not inhibit gemcabene glucuronidation in HLM, suggesting that UGT1A3 and UGT2B17 do not contribute significantly to gemcabene glucuronidation. Reaction rates for gemcabene glucuronidation from a human liver bank correlated well (r(2)=0.722, P<0.0001; n=24) with rates of glucuronidation of the UGT2B7 probe substrate 3'-azido-3'-deoxythymidine. In conclusion, using the three independent experimental approaches typically used for cytochrome P450 reaction phenotyping, UGT2B7 is the major enzyme contributing to gemcabene glucuronidation in human liver microsomes.     

Predominant contribution of UDP-glucuronosyltransferase 2B7 in the glucuronidation of racemic flurbiprofen in the human liver.

Flurbiprofen is a nonsteroidal anti-inflammatory drug used as a racemic mixture. Although glucuronidation is one of its elimination pathways, the role of UDP-glucuronosyltransferase (UGT) in this process remains to be investigated. Thus, the kinetics of the stereoselective glucuronidation of racemic (R,S)-flurbiprofen by recombinant UGT isozymes and human liver microsomes (HLMs) were investigated, and the major human UGT isozymes involved were identified. UGT1A1, 1A3, 1A9, 2B4, and 2B7 showed glucuronidation activity for both (R)- and (S)-glucuronide, with UGT2B7 possessing the highest activity. UGT2B7 formed the (R)-glucuronide at a rate 2.8-fold higher than that for (S)-glucuronide, whereas the other UGTs had similar formation rates. The glucuronidation of racemic flurbiprofen by HLMs also resulted in the formation of (R)-glucuronide as the dominant form, which occurred to a degree similar to that by recombinant UGT2B7 (2.1 versus 2.8). The formation of (R)-glucuronide correlated significantly with morphine 3-OH glucuronidation (r = 0.96, p < 0.0001), morphine 6-OH glucuronidation (r = 0.91, p < 0.0001), and 3'-azido-3'-deoxythymidine glucuronidation (r = 0.85, p < 0.0001), a reaction catalyzed mainly by UGT2B7, in individual HLMs. In addition, the formation of both glucuronides correlated significantly (r = 0.99, p < 0.0001). Mefenamic acid inhibited the formation of both (R)- and (S)-glucuronide in HLMs with similar IC(50) values (2.0 and 1.7 muM, respectively), which are close to those in recombinant UGT2B7. In conclusion, these findings suggest that the formation of (R)- and (S)-glucuronide from racemic flurbiprofen is catalyzed by the same UGT isozyme, namely UGT2B7.     

Evaluation of 3'-azido-3'-deoxythymidine, morphine, and codeine as probe substrates for UDP-glucuronosyltransferase 2B7 (UGT2B7) in human liver microsomes: specificity and influence of the UGT2B7*2 polymorphism.

UDP-glucuronosyltransferase 2B7 (UGT2B7) is involved in the glucuronidation of a wide array of clinically important drugs and endogenous compounds in humans. The aim of this study was to identify an isoform-selective probe substrate that could be used to investigate genetic and environmental influences on glucuronidation mediated by UGT2B7. Three potential probe substrates [3'-azido-3'-deoxythymidine (AZT), morphine, and codeine], were evaluated using recombinant UGTs and human liver microsomes (HLMs; n = 54). Of 11 different UGTs screened, UGT2B7 was the principal isoform mediating AZT glucuronidation, morphine-3-glucuronidation, and morphine-6-glucuronidation. Codeine was glucuronidated equally well by UGT2B4 and UGT2B7. Enzyme kinetic analysis of these activities typically showed higher apparent Km values for HLMs (pooled and individual) compared with UGT2B7. This difference was least (less than 2-fold higher Km) for AZT glucuronidation and greatest (3- to 6-fold higher Km) for codeine glucuronidation. Microsomal UGT2B7 protein content correlated well with AZT glucuronidation (rs = 0.77), to a lesser extent with morphine-3-glucuronidation (rs = 0.50) and morphine-6-glucuronidation (rs = 0.51), but very weakly with codeine glucuronidation (rs = 0.33). Livers were also genotyped for the UGT2B7*2 (H268Y) polymorphism. No effect of genotype on microsomal glucuronidation or UGT2B7 protein content was observed. In conclusion, although both AZT and morphine can serve as in vitro probe substrates for UGT2B7, AZT appears to be more selective than morphine. Codeine is not a useful UGT2B7 probe substrate because of significant glucuronidation by UGT2B4. The UGT2B7*2 polymorphism is not a determinant of glucuronidation of AZT, morphine, or codeine in HLMs.     

Identification and characterization of human UDP-glucuronosyltransferases responsible for xanthotoxol glucuronidation

1.?Xanthotoxol is a furanocoumarin that possesses many pharmacological activities and in this study its in vitro glucuronidation was studied.

2.?Xanthotoxol can be rapidly metabolized to a mono-glucuronide in both human intestine microsomes (HIM) and human liver microsomes (HLM); the structure of the metabolite was confirmed by NMR spectroscopy.

3.?Reaction phenotyping with 12 commercial recombinant human UGTs, as well as with the Helsinki laboratory UGT1A10 that carry a C-terminal His-tag (UGT1A10-H), revealed that UGT1A10-H catalyzes xanthotoxol glucuronidation at the highest rate, followed by UGT1A8. The other enzymes, namely UGT1A3, UGT1A1, UGT1A6, UGT1A10 (commercial), and UGT2B7 displayed moderate-to-low reaction rates.

4.?In kinetic analyses, HIM exhibited much higher affinity for xanthotoxol, along with high V_max and mild substrate inhibition, whereas the kinetics in HLM was biphasic. UGT1A1 (high K_m value), UGT1A10-H (low K_m value), and UGT1A8 exhibited mild substrate inhibition.

5.?Considering the above findings and the current knowledge on UGTs expression in HIM, it is likely that UGT1A10 is mainly responsible for xanthotoxol glucuronidation in the human small intestine, with some contribution from UGT1A1. In the liver, this reaction is mainly catalyzed by UGT1A1 and UGT2B7.

6.?Glucuronidation appears to be the major metabolic pathway of xanthotoxol in human.     

Differential modulation of UDP-glucuronosyltransferase 1A1 (UGT1A1)-catalyzed estradiol-3-glucuronidation by the addition of UGT1A1 substrates and other compounds to human liver microsomes.

Previous results demonstrating homotropic activation of human UDP-glucuronosyltransferase (UGT) 1A1-catalyzed estradiol-3-glucuronidation led us to investigate the effects of 16 compounds on estradiol glucuronidation by human liver microsomes (HLM). In confirmation of previous work using alamethicin-treated HLM pooled from four livers, UGT1A1-catalyzed estradiol-3-glucuronidation demonstrated homotropic activation kinetics (S(50) = 22 microM, Hill coefficient, n = 1.9) whereas estradiol-17-glucuronidation (catalyzed by other UGT enzymes) followed Michaelis-Menten kinetics (K(m) = 7 microM). Modulatory effects of the following compounds were investigated: bilirubin, eight flavonoids, 17alpha-ethynylestradiol (17alpha-EE), estriol, 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP), anthraflavic acid, retinoic acid, morphine, and ibuprofen. Although the classic UGT1A1 substrate bilirubin was a weak competitive inhibitor of estradiol-3-glucuronidation, the estrogens and anthraflavic acid activated or inhibited estradiol-3-glucuronidation dependent on substrate and effector concentrations. For example, at substrate concentrations of 5 and 10 microM, estradiol-3-glucuronidation activity was stimulated by as much as 80% by low concentrations of 17alpha-EE but was unaltered by flavanone. However, at higher substrate concentrations (25-100 microM) estradiol-3-glucuronidation was inhibited by about 55% by both compounds. Anthraflavic acid and PhIP were also stimulators of estradiol 3-glucuronidation at low substrate concentrations. The most potent inhibitor of estradiol 3-glucuronidation was the flavonoid tangeretin. The UGT2B7 substrates morphine and ibuprofen had no effect on estradiol 3-glucuronidation, whereas retinoic acid was slightly inhibitory. Estradiol-17-glucuronidation was inhibited by 17alpha-EE, estriol, and naringenin but was not activated by any compound. This study demonstrates that the interactions of substrates and inhibitors at the active site of UGT1A1 are complex, yielding both activation and competitive inhibition kinetics.     

Identification of the UDP-glucuronosyltransferase isoforms involved in mycophenolic acid phase II metabolism.

Mycophenolic acid (MPA), the active metabolite of the immunosuppressant mycophenolate mofetil is primarily metabolized by glucuronidation. The nature of UDP-glucuronosyltransferases (UGTs) involved in this pathway is still debated. The present study aimed at identifying unambiguously the UGT isoforms involved in the production of MPA-phenyl-glucuronide (MPAG) and MPA-acylglucuronide (AcMPAG). A liquid chromatography-tandem mass spectrometry method allowing the identification and determination of the metabolites of mycophenolic acid was developed. The metabolites were characterized in urine and plasma samples from renal transplant patients under mycophenolate mofetil therapy and in vitro after incubation of mycophenolic acid with human liver (HLM), kidney (HKM), or intestinal microsomes (HIM). The UGT isoforms involved in MPAG or AcMPAG production were investigated using induced rat liver microsomes, heterologously expressed UGT (Supersomes), and chemical-selective inhibition of HLM, HKM, and HIM. The three microsomal preparations produced MPAG, AcMPAG, and two mycophenolate glucosides. Among the 10 UGT isoforms tested, UGT 1A9 was the most efficient for MPAG synthesis with a K(m) of 0.16 mM, close to that observed for HLM (0.18 mM). According to the chemical inhibition experiments, UGT 1A9 is apparently responsible for 55%, 75%, and 50% of MPAG production by the liver, kidney, and intestinal mucosa, respectively. Although UGT 2B7 was the only isoform producing AcMPAG in a significant amount, the selective inhibitor azidothymidine only moderately reduced this production (approximately -25%). In conclusion, UGT 1A9 and 2B7 were clearly identified as the main UGT isoforms involved in mycophenolic acid glucuronidation, presumably due to their high hepatic and renal expression.     

Involvement of human hepatic UGT1A1, UGT2B4, and UGT2B7 in the glucuronidation of carvedilol.

Carvedilol ((+/-)-1-carbazol-4-yloxy)-3-[[2-(o-methoxyphenoxy)ethyl]amino]-2-propanol) is metabolized primarily into glucuronide conjugates. In the present study, we identified the human UDP-glucuronosyltransferase (UGT) isoforms involved in the glucuronidation of carvedilol by thin-layer chromatography using microsomes from human liver or insect cells expressing recombinant UGT isoforms. We observed two forms of carvedilol glucuronides, namely G1 and G2, in hepatic microsomes. The glucuronidation of carvedilol was catalyzed by at least three recombinant UGT isoforms: UGT1A1, UGT2B4, and UGT2B7. UGT2B4 formed both G1 and G2, whereas UGT1A1 and UGT2B7 were responsible for the formation of glucuronide G2 and G1, respectively. The enzyme kinetics for carvedilol glucuronidation by UGT1A1, UGT2B4, and UGT2B7 in addition to human liver microsomes were examined by Lineweaver-Burk analysis. The values of Km and Vmax for human liver microsomes were 26.6 microM and 106 pmol/min/mg protein for G1, and 46.0 microM and 44.5 pmol/min/mg protein for G2, respectively. The Km values for UGT1A1, UGT2B4, and UGT2B7 for G1 and G2 (22.1-55.1 microM) were comparable to those of the liver microsomes, whereas the Vmax values were in the range of 3.33 to 7.88 pmol/min/mg protein. The Km and Vmax/Km values for UGT2B4 and UGT2B7 for G1 were similar, whereas UGT2B4 had lower Km and higher Vmax/Km values for G2 compared with those of UGT1A1. These results suggest that G1 formation is catalyzed by UGT2B4 and UGT2B7, whereas G2 is formed by UGT2B4 and UGT1A1. These three hepatic UGT isoforms may have important roles in carvedilol metabolism.     

Quantitative prediction of in vivo inhibitory interactions involving glucuronidated drugs from in vitro data: the effect of fluconazole on zidovudine glucuronidation

AIMS: Using the fluconazole-zidovudine (AZT) interaction as a model, to determine whether inhibition of UDP-glucuronosyltransferase (UGT) catalysed drug metabolism in vivo could be predicted quantitatively from in vitro kinetic data generated in the presence and absence bovine serum albumin (BSA). METHODS: Kinetic constants for AZT glucuronidation were generated using human liver microsomes (HLM) and recombinant UGT2B7, the principal enzyme responsible for AZT glucuronidation, as the enzyme sources with and without fluconazole. K(i) values were used to estimate the decrease in AZT clearance in vivo. RESULTS: Addition of BSA (2%) to incubations decreased the K(m) values for AZT glucuronidation by 85-90% for the HLM (923 +/- 357 to 91 +/- 9 microm) and UGT2B7 (478-70 microm) catalysed reactions, with little effect on V(max). Fluconazole, which was shown to be a selective inhibitor of UGT2B7, competitively inhibited AZT glucuronidation by HLM and UGT2B7. Like the K(m), BSA caused an 87% reduction in the K(i) for fluconazole inhibition of AZT glucuronidation by HLM (1133 +/- 403 to 145 +/- 36 microm) and UGT2B7 (529 to 73 microm). K(i) values determined for fluconazole using HLM and UGT2B7 in the presence (but not absence) of BSA predicted an interaction in vivo. The predicted magnitude of the interaction ranged from 41% to 217% of the reported AUC increase in patients, depending on the value of the in vivo fluconazole concentration employed in calculations. CONCLUSIONS: K(i) values determined under certain experimental conditions may quantitatively predict inhibition of UGT catalysed drug glucuronidation in vivo.     

Glucuronidation of edaravone by human liver and kidney microsomes: biphasic kinetics and identification of UGT1A9 as the major UDP-glucuronosyltransferase isoform

Edaravone was launched in Japan in 2001 and was the first neuroprotectant developed for the treatment of acute cerebral infarction. Edaravone is mainly eliminated as glucuronide conjugate in human urine (approximately 70%), but the mechanism involved in the elimination pathway remains unidentified. We investigated the glucuronidation of edaravone in human liver microsomes (HLM) and human kidney microsomes (HKM) and identified the major hepatic and renal UDP-glucuronosyltransferases (UGTs) involved. As we observed, edaravone glucuronidation in HLM and HKM exhibited biphasic kinetics. The intrinsic clearance of glucuronidation at high-affinity phase (CL(int1)) and low-affinity phase (CL(int2)) were 8.4 ± 3.3 and 1.3 ± 0.2 μl · min(-1) · mg(-1), respectively, for HLM and were 45.3 ± 8.2 and 1.8 ± 0.1 μl · min(-1) · mg(-1), respectively, for HKM. However, in microsomal incubations contained with 2% bovine serum albumin, CL(int1) and CL(int2) were 16.4 ± 1.2 and 3.7 ± 0.3 μl · min(-1) · mg(-1), respectively, for HLM and were 78.5 ± 3.9 and 3.6 ± 0.5 μl · min(-1) · mg(-1), respectively, for HKM. Screening with 12 recombinant UGTs indicated that eight UGTs (UGT1A1, UGT1A6, UGT1A7, UGT1A8, UGT1A9, UGT1A10, UGT2B7, and UGT2B17) produced a significant amount of glucuronide metabolite. Thus, six UGTs (UGT1A1, UGT1A6, UGT1A7, UGT1A9, UGT2B7, and UGT2B17) expressed in human liver or kidney were selected for kinetic studies. Among them, UGT1A9 exhibited the highest activity (CL(int1) = 42.4 ± 9.5 μl · min(-1) · mg(-1)), followed by UGT2B17 (CL(int) = 3.3 ± 0.4 μl · min(-1) · mg(-1)) and UGT1A7 (CL(int) = 1.7 ± 0.2 μl · min(-1) · mg(-1)). Inhibition study found that inhibitor of UGT1A9 (propofol) attenuated edaravone glucuronidation in HLM and HKM. In addition, edaravone glucuronidation in a panel of seven HLM was significantly correlated (r = 0.9340, p = 0.0021) with propofol glucuronidation. Results indicated that UGT1A9 was the main UGT isoform involved in edaravone glucuronidation in HLM and HKM.     

Glucuronidation of the dietary fatty acids, phytanic acid and docosahexaenoic acid, by human UDP-glucuronosyltransferases.

Linoleic acid has recently been shown to be glucuronidated in vitro by human liver and intestinal microsomes and recombinant UGT2B7. In the present study, the dietary fatty acids (FA), phytanic acid (PA), and docosahexaenoic acid (DHA) have been used as substrates for human UDP-glucuronosyltransferases (UGTs). Both compounds were effectively glucuronidated by human liver microsomes (HLM; 1.25 +/- 0.36 and 1.12 +/- 0.32 nmol/mg x min for PA and DHA, respectively) and UGT2B7 (0.71 and 0.53 nmol/mg x min). Kinetic analysis produced relatively low K(m) values for PA with both HLM and UGT2B7 (149 and 108 microM, respectively). The K(m) for DHA glucuronidation by HLM (460 microM) was considerably higher than that for UGT2B7 (168 microM), suggesting the involvement in microsomes of other UGT isoforms in addition to UGT2B7. Glucuronidation of PA and DHA by gastrointestinal microsomes from 16 human subjects was determined. In general, both PA and DHA were glucuronidated by gastric and intestinal microsomes, and activity toward both substrates was lowest in the stomach, increased in the small intestine, and lower in the colon. However, there were large interindividual variations in UGT activity toward both substrates in all segments of the intestine, as has been seen with other substrates. Thus, PA and DHA are effective in vitro substrates for human liver, gastric and intestinal microsomes, and glucuronidation may play a role in modulating the availability of these FA as ligands for nuclear receptors.     

In vitro glucuronidation of the antibacterial triclocarban and its oxidative metabolites

Triclocarban (3,4,4'-trichlorocarbanilide; TCC) is widely used as an antibacterial in bar soaps. During use of these soaps, a significant portion of TCC is absorbed by humans. For the elimination from the body, glucuronidation plays a key role in both biliary and renal clearance. To investigate this metabolic pathway, we performed microsomal incubations of TCC and its hydroxylated metabolites 2'-OH-TCC, 3'-OH-TCC, and 6-OH-TCC. Using a new liquid chromatography-UV-mass spectrometry method, we could show a rapid glucuronidation for all OH-TCCs by the uridine-5'-diphosphate-glucuronosyltransferases (UGT) present in liver microsomes of humans (HLM), cynomolgus monkeys (CLM), rats (RLM), and mice (MLM). Among the tested human UGT isoforms, UGT1A7, UGT1A8, and UGT1A9 showed the highest activity for the conjugation of hydroxylated TCC metabolites followed by UGT1A1, UGT1A3, and UGT1A10. Due to this broad pattern of active UGTs, OH-TCCs can be efficiently glucuronidated in various tissues, as shown for microsomes from human kidney (HKM) and intestine (HIM). The major renal metabolites in humans, TCC-N-glucuronide and TCC-N'-glucuronide, were formed at very low conversion rates (<1%) by microsomal incubations. Low amounts of N-glucuronides were generated by HLM, HIM, and HKM, as well as by MLM and CLM, but not by RLM, according to the observed species specificity of this metabolic pathway. Among the human UGT isoforms, only UGT1A9 had activity for the N-glucuronidation of TCC. These results present an anomaly where in vivo the predominant urinary metabolites of TCC are N and N'-glucuronides, but these compounds are slowly produced in vitro.     

Stereoselective glucuronidation of propranolol in human and cynomolgus monkey liver microsomes: role of human hepatic UDP-glucuronosyltransferase isoforms, UGT1A9, UGT2B4 and UGT2B7

The stereoselective glucuronidation of propranolol (PL) in human and cynomolgus monkey liver microsomes, and the roles of human hepatic UDP-glucuronosyltransferase (UGT) isoforms involved in the enantiomeric glucuronidation of PL using recombinant UGT enzymes were investigated. In Michaelis-Menten plots, R- and S-PL glucuronidation by human liver microsomes showed sigmoidal kinetics whereas the kinetics of enantiomeric PL glucuronidation by cynomolgus monkey liver microsomes was monophasic. The Km, Vmax and CLint values of cynomolgus monkey liver microsomes were generally higher than the S50, Vmax and CLmax values of human liver microsomes in R- and S-PL glucuronidation. The glucuronidation of R- and S-PL was catalyzed by at least 3 UGT isoforms: UGT1A9, UGT2B4 and UGT2B7. Michaelis-Menten plots for R- and S-PL glucuronidation by UGT1A9 were monophasic, whereas the kinetics of UGT2B7 showed sigmoidal curves. Enantiomeric R-PL glucuronidation by UGT2B4 showed sigmoidal kinetics, whereas S-PL glucuronidation displayed monophasic kinetics. UGT1A9 showed remarkable stereoselectivity in Vmax and CLint values of R-PL < S-PL. These findings demonstrate that the profiles of enantiomeric PL glucuronidation in human and cynomolgus monkey liver microsomes are largely different and suggest that the human hepatic UGT isoforms UGT1A9, UGT2B4 and UGT2B7 play distinctive roles in enantiomeric PL glucuronidation.     

In vitro characterisation of human renal and hepatic frusemide glucuronidation and identification of the UDP-glucuronosyltransferase enzymes involved in this pathway

In order to gain insights into the renal and hepatic glucuronidation of frusemide (FSM), this study: (i) characterised the kinetics of FSM glucuronidation by human liver microsomes (HLM) and human kidney cortical- (HKCM) and medullary- (HKMM) microsomes, and (ii) identified the human UDP-glucuronosyltransferase enzyme(s) involved in this pathway. HLM, HKCM and HLMM efficiently glucuronidated FSM. FSM glucuronide (FSMG) formation followed Michaelis-Menten kinetics in all tissues. While the mean K(m) for FSMG formation by HKMM (386 +/- 68 microM) was lower than the K(m) values for HLM (988 +/- 271 microM) and HKCM (704 +/- 278 microM), mean V(max)/K(m) values were comparable for the three tissues. A panel of recombinant UGT enzymes was screened for the capacity to glucuronidate FSM. UGT 1A1, 1A3, 1A6, 1A7, 1A9, 1A10 and 2B7 metabolised FSM. Of the renally and hepatically expressed enzymes, comparison of kinetic parameters suggests a predominant role of UGT1A9 in FSM glucuronidation, although UGT1A1 may also contribute to FSMG formation by HLM. Consistent with these observations, the UGT1A selective inhibitors phenylbutazone and sulfinpyrazone decreased FSMG formation by HLM, HKCM and HKMM by 60-80%, whereas the UGT2B7 selective inhibitor fluconazole reduced FSM glucuronidation by < or =20%. The ability of HKCM and HKMM to form FSMG supports the proposition that the kidney is the main organ involved in FSM glucuronidation in vivo, although a role for hepatic metabolism remains a possibility in renal dysfunction. The data further demonstrate the potential importance of both the medulla and cortex in renal drug metabolism and detoxification.