Inhibition of UGT2B7 Enzyme Activity in Human and Rat Liver Microsomes by Herbal Constituents.

The co-use of conventional drug and herbal medicines may lead to herb-drug interaction via modulation of drug-metabolizing enzymes (DMEs) by herbal constituents. UDP-glucuronosyltransferases (UGTs) catalyzing glucuronidation are the major metabolic enzymes of Phase II DMEs. The in vitro inhibitory effect of several herbal constituents on one of the most important UGT isoforms, UGT2B7, in human liver microsomes (HLM) and rat liver microsomes (RLM) was investigated. Zidovudine (ZDV) was used as the probe substrate to determine UGT2B7 activity. The intrinsic clearance (Vmax/Km) of ZDV in HLM is 1.65 µL/mg/min which is ten times greater than in RLM, which is 0.16 µL/mg/min. Andrographolide, kaempferol-3-rutinoside, mitragynine and zerumbone inhibited ZDV glucuronidation in HLM with IC50 values of 6.18 ± 1.27, 18.56 ± 8.62, 8.11 ± 4.48 and 4.57 ± 0.23 µM, respectively, hence, herb-drug interactions are possible if andrographolide, kaempferol-3-rutinoside, mitragynine and zerumbone are taken together with drugs that are highly metabolized by UGT2B7. Meanwhile, only mitragynine and zerumbone inhibited ZDV glucuronidation in RLM with IC50 values of 51.20 ± 5.95 µM and 8.14 ± 2.12 µM, respectively, indicating a difference between the human and rat microsomal model so caution must be exercised when extrapolating inhibitory metabolic data from rats to humans.

Inhibitory Effects of Baicalein Derived from Japanese Traditional Herbal Medicine on SN-38 Glucuronidation.

PURPOSE: The chemotherapeutic agent irinotecan is hydrolyzed to its active form SN-38 by human carboxyesterases, but SN-38 is converted into the inactive form SN-38G by hepatic UDP-glucuronosyltransferases (UGTs). The aim of the present study was to evaluate the inhibitory effects of two b-glucuronidase-treated Japanese traditional herbal medicines (kampo), Hange-Shashin-To (TJ-14) and Sairei-To (TJ-114) on SN-38 glucuronidation, and the deglycosylation of baicalin (BG) and glycyrrhizic acid (GL) derived from TJ-14 and TJ-114 to form their respective aglycones, baicalein (BA) and glycyrrhetinic acid (GA). METHODS: The inhibitory effects of b-glucuronidase-treated TJ-14 and TJ-114 on SN-38 glucuronidation by human liver microsomes were examined. BA and GA, which were enzymatically converted from BG and GL present in TJ-14 and TJ-114, were examined in the same manner. Furthermore, the enzymatic activities were measured by using recombinant UGT1A1 and UGT1A9 isoforms instead of human liver microsomes. BA, GA, SN-38, and their glycosides/glucuronides were analyzed with an LC-MS system. RESULTS: As regards the linear initial reaction rate, SN-38 glucuronidation by human liver microsomes was significantly inhibited by the addition of b-glucuronidase-untreated TJ-14 and TJ-114, but was more strongly inhibited by the addition of b-glucuronidase-treated TJ-14 and TJ-114. The results of LC-MS analysis and pharmacokinetic studies suggested that BA is the main inhibitor of SN-38 glucuronidation. In the Dixon plot, BA showed competitive inhibition of SN-38 glucuronidation, and the inhibition constant was 8.70 ± 3.24 mM. Previous reports, studies of recombinant UGT isoforms indicated that SN-38 glucuronidation was mainly catalyzed by UGT1A1. CONCLUSIONS: These findings strongly suggested that SN-38 glucuronidation is inhibited by BA. BA could act as a pharmacokinetic regulating factor associated with SN-38 glucuronidation. This article is open to POST-PUBLICATION REVIEW. Registered readers (see "For Readers") may comment by clicking on ABSTRACT on the issue's contents page.

Glucuronides as Potential Anionic Substrates of Human Cytochrome P450 2C8 (CYP2C8).

Glucuronidation is in general considered as a terminal metabolic step that leads to direct elimination of drugs and generally abolishes their biological activity. However, there is growing evidence to suggest that glucuronides can be ligands of human CYP2C8, making CYP2C8 distinct from the other CYP isoforms. Several classes of glucuronide conjugates, which include acyl glucuronides, ether glucuronides, N-glucuronides, and carbamoyl glucuronides, have been shown to be substrates or time-dependent inhibitors of CYP2C8. Although the structures of CYP2C8-glucuronide complexes have not been determined, the structural features of CYP2C8 active site support its binding to anionic and bulky ligands like glucuronides. As interaction perpetrators with CYP2C8, glucuronides of gemfibrozil and clopidogrel showed marked clinical drug-drug interactions (e.g., with cerivastatin and repaglinide), which are more than expected from the parent drug. This review summarizes glucuronides as CYP2C8 ligands and the active-site structural features of CYP2C8 that allow potential binding to glucuronides.

Metabolism of kurarinone by human liver microsomes and its effect on cytotoxicity.

CONTEXT: Kurarinone, the most abundant prenylated flavonoid in Sophora flavescens Aiton (Leguminosae), is a promising antitumor therapeutic. However, it shows significant hepatotoxicity. Furthermore, how kurarinone is metabolized in humans remains unclear. OBJECTIVE: The objective of this study is to investigate kurarinone metabolism in human liver microsomes (HLMs) and the role of metabolism in kurarinone-induced cytotoxicity. MATERIALS AND METHODS: The UDP-glucuronosyltransferase isoforms (UGTs) involved in kurarinone glucuronidation were identified using chemical inhibitors (100-1000 µM phenylbutazone; 10-100 µM ß-estradiol; 10-100 µM 1-naphthol; 10-500 µM propofol; and 100-1000 µM fluconazole) and recombinant human UGTs. Kurarinone (2-500 µM) was incubated with HLMs and UGTs (0.5 mg/mL) for 15 min to determine enzyme kinetic parameters. The IC50 value of kurarinone (10-200 µM) was evaluated in a HLMs/3T3 cell co-culture system. RESULTS: Kurarinone is extensively converted to two glucuronides (M3 and M4) in HLMs. M3 formation was catalyzed by multiple UGT1As, with UGT1A3 showing the highest intrinsic clearance (120.60 mL/min/mg). M4 formation was catalyzed by UGT1A1, UGT2B4, and UGT2B7. UGT1A1 showed the highest intrinsic clearance (60.61 mL/min/mg). The kinetic profiles of the five main UGTs and HLMs fit substrate inhibition kinetics, with Km values ranging from 5.20 to 46.52 µM, Vmax values ranging from 0.20 to 3.06 µmol/min/mg, and Ksi values ranging from 25.58 to 230.30 µM. The kurarinone IC50 value was 93 µM in the control group, 102 µM in HLMs with NADPH, and 160 µM in HLMs with UDPGA. DISCUSSION AND CONCLUSION: Kurarinone glucuronidation is a detoxification pathway. This information may help to elucidate the risk factors regulating kurarinone toxicity.

Inhibition of genistein glucuronidation by bisphenol A in human and rat liver microsomes.

Genistein is a natural phytoestrogen of the soybean, and bisphenol A (BPA) is a synthetic chemical used in the production of polycarbonate plastics. Both genistein and BPA disrupt the endocrine system in vivo and in vitro. Growing concerns of altered xenobiotic metabolism due to concomitant exposures from soy milk in BPA-laden baby bottles has warranted the investigation of the glucuronidation rate of genistein in the absence and presence (25 µM) of BPA by human liver microsomes (HLM) and rat liver microsomes (RLM). HLM yield V(max) values of 0.93 ± 0.10 nmol · min(-1) · mg(-1) and 0.62 ± 0.05 nmol · min(-1) · mg(-1) in the absence and presence of BPA, respectively. K(m) values for genistein glucuronidation by HLM in the absence and presence of BPA are 15.1 ± 7.9 µM and 21.5 ± 7.7 µM, respectively, resulting in a K(i) value of 58.7 µM for BPA. Significantly reduced V(max) and unchanged K(m) in the presence of BPA in HLM are suggestive of noncompetitive inhibition. In RLM, the presence of BPA resulted in a K(i) of 35.7 µM, an insignificant change in V(max) (2.91 ± 0.26 nmol · min(-1) · mg(-1) and 3.05 ± 0.41 nmol · min(-1) · mg(-1) in the absence and presence of BPA, respectively), and an increase in apparent K(m) (49.4 ± 14 µM with no BPA and 84.0 ± 28 µM with BPA), indicative of competitive inhibition. These findings are significant because they suggest that BPA is capable of inhibiting the glucuronidation of genistein in vitro, and that the type of inhibition is different between HLM and RLM.

Inhibitory effect of flavonoids on sulfo- and glucurono-conjugation of acetaminophen in rat cultured hepatocytes and liver subcellular preparations.

A large group of flavonoids was investigated for inhibitory effects on sulfo- and glucurono-conjugation of acetaminophen when added to rat cultured hepatocytes and liver subcellular preparations. The flavonoids inhibited the production of both sulfate and glucuronide conjugates in the cultured cells, with potencies that depended on the specific flavonoid. Among the flavonols, quercetin, kaempferol and galangin were much more effective than myricetin and morin. Flavones including luteolin, apigenin and chrysin were as effective as the corresponding three flavonols above. The inhibition of conjugation by other simple flavones such as 3-, 5-, 7- and 3',4'-OH flavones, and by catechins such as epicatechin and epigallocatechin, was very weak. These data suggest that the presence of both C5 and 7 hydroxyl substitutions on the A-ring in the flavone structure is required for effective inhibitory activity. The effect of flavonoids on sulfo- and glucurono-conjugation was also examined by incubating acetaminophen with isolated liver cytosolic and microsomal preparations, respectively. The active flavonoids in the cells remarkably inhibited the sulfation, but not glucuronidation, in cell-free enzymatic preparations in vitro. The mechanism of inhibition of conjugation by flavonoids in cultured hepatocytes is not likely to depend on the direct inhibition of sulfo- and glucurono-transferase activity by flavonoids.

Hepatobiliary excretion of berberine.

Berberine is a bioactive herbal ingredient isolated from the roots and bark of Berberis aristata or Coptis chinensis. To investigate the detailed pharmacokinetics of berberine and its mechanisms of hepatobiliary excretion, an in vivo microdialysis coupled with high-performance liquid chromatography was performed. In the control group, rats received berberine alone; in the drug-treated group, 10 min before berberine administration, the rats were injected with cyclosporin A (CsA), a P-glycoprotein (P-gp) inhibitor; quinidine, both organic cation transport (OCT) and P-gp inhibitors; SKF-525A (proadifen), a cytochrome P450 inhibitor; and probenecid to inhibit the glucuronidation. The results indicate that berberine displays a linear pharmacokinetic phenomenon in the dosage range from 10 to 20 mg kg(-1), since a proportional increase in the area under the concentration-time curve (AUC) of berberine was observed in this dosage range. Moreover, berberine was processed through hepatobiliary excretion against a concentration gradient based on the bile-to-blood distribution ratio (AUC(bile)/AUC(blood)); the active berberine efflux might be affected by P-gp and OCT since coadministration of berberine and CsA or quinidine at the same dosage of 10 mg kg(-1) significantly decreased the berberine amount in bile. In addition, berberine was metabolized in the liver with phase I demethylation and phase II glucuronidation, as identified by liquid chromatography/tandem mass spectrometry. Also, the phase I metabolism of berberine was partially reduced by SKF-525A treatment, but the phase II glucuronidation of berberine was not obviously affected by probenecid under the present study design.

Can xipamide or tacrolimus inhibit the glucuronidation of mycophenolic acid in rat liver slices?

The objective of this study was to investigate the effect of tacrolimus (Tac) and xipamide (X) on mycophenolic acid (MPA) glucuronidation in precision-cut rat liver slices. To assess a possible effect of these two drugs, the influence of the anti-inflammatory drug niflumic acid (NA)--a well-known inhibitor for MPA glucuronidation in human liver microsomes--was used as a standard. MPA and its main metabolite mycophenolic acid glucuronide (MPAG) were determined by means of high-performance liquid chromatography. MPA glucuronidation rate showed a significant linear correlation (p = 0.012) with MPA concentrations from 15.61 up to 124.88 microM in the medium. That means, the enzyme(s) responsible for the glucuronidation of MPA worked far below Km-value. With all MPA concentrations tested, neither the addition of Tac (31.30 nM) nor of X (28.25 nM) influenced the glucuronidation of MPA. In comparison, NA at a concentration of 70.92 nM showed a marked inhibitory effect (by 72%). The present pilot-study indicates that precision-cut rat liver slices are a suitable in vitro model to characterize the glucuronidation of MPA to its primary metabolite MPAG and interferences with other substances.

Characterization of autoantibodies against uridine-diphosphate glucuronosyltransferase in patients with inflammatory liver diseases.

Uridine diphosphate glucuronosyltransferase (UGT) was identified as an antigenic target in a subgroup of liver-kidney microsomal autoantibodies and was termed LKM-3. To evaluate the nature of LKM-3 antibodies, we screened sera from 80 untreated patients with autoimmune hepatitis (AIH) type 1 and 2, primary biliary cirrhosis (PBC), AIH/PBC, hepatitis C virus (HCV) infection, and 12 healthy individuals (controls) against 7 recombinant human UGT isoenzymes (UGT1A1, UGT1A4, UGT1A6, UGT1A7, UGT1A9, UGT1A10, and UGT2B7). Autoantibodies reacting against various UGT isoenzymes were observed in sera from 3 of 18 AIH type 2 and 1 of 27 of the HCV patients. The anti-UGT-positive sera from AIH type 2 patients revealed the strongest immunoreactivity against UGT1A1, the main UGT-isoform involved in the bilirubin glucuronidation. Additionally, these sera were able to block UGT-mediated substrate glucuronidation in vitro. The prevalence for UGT1A1 was shown by 2 independent techniques: (1) UGT1A1 was identified as the main antigen by Western blotting. Preabsorption of sera with UGT1A1 prevented reaction against all tested UGT-isoforms. (2) In vitro immunoinhibition experiments showed that glucuronidation of the anticancer drug flavopiridol by UGT1A1 was more strongly inhibited than its UGT1A9-mediated biotransformation. In contrast, the serum from the HCV-patient reacted predominately with UGT1A6, and moreover, the immunoreactivity pattern was different from that of the AIH group. To summarize, we show the subtype preference of antibodies against UGT1A1 in a subgroup of AIH type 2 patients. These autoantibodies inhibit UGT-mediated glucuronidation in vitro, but it is unlikely that anti-UGT antibodies will have a marked effect on the patients capacity for drug biotransformation, as serum bilirubin levels in patients remained within the normal range.