Characterization of bropirimineO-glucuronidation in human liver microsomes

Assessment of UDP-glucuronosyltransferase catalyzed formation of Picroside II glucuronide in microsomes of different species and recombinant UGTs

This study compared the hepatic glucuronidation of Picroside II in different species and characterized the glucuronidation activities of human intestinal microsomes (HIMs) and recombinant human UDP-glucuronosyltransferases (UGTs) for Picroside II. The rank order of hepatic microsomal glucuronidation activity of Picroside II was rat > mouse > human > dog. The intrinsic clearance of Picroside II hepatic glucuronidation in rat, mouse and dog was about 10.6-, 6.0- and 2.3-fold of that in human, respectively. Among the 12 recombinant human UGTs, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 catalyzed the glucuronidation. UGT1A10, which are expressed in extrahepatic tissues, showed the highest activity of Picroside II glucuronidation (K(m) = 45.1 μM, V(max) = 831.9 pmol/min/mg protein). UGT1A9 played a primary role in glucuronidation in human liver microsomes (HLM; K(m) = 81.3 μM, V(max) = 242.2 pmol/min/mg protein). In addition, both mycophenolic acid (substrate of UGT1A9) and emodin (substrate of UGT1A8 and UGT1A10) could inhibit the glucuronidation of Picroside II with the half maximal inhibitory concentration (IC(50)) values of 173.6 and 76.2 μM, respectively. Enzyme kinetics was also performed in HIMs. The K(m) value of Picroside II glucuronidation was close to that in recombinant human UGT1A10 (K(m) = 58.6 μM, V(max) = 721.4 pmol/min/mg protein). The intrinsic clearance was 5.4-fold of HLMs. Intestinal UGT enzymes play an important role in Picroside II glucuronidation in human.

Biotransformation of the chemopreventive agent 2',4',4-trihydroxychalcone (isoliquiritigenin) by UDP-glucuronosyltransferases

2',4',4-trihydroxychalcone (isoliquiritigenin), a chalcone found in licorice root and shallots, exhibits antioxidant, estrogenic, and antitumor activities. To complement our previous studies concerning the phase 1 metabolism of isoliquiritigenin, the phase 2 transformation of isoliquiritigenin by human hepatocytes and pooled human liver microsomes (HLMs) was investigated using liquid chromatography/tandem mass spectrometry and UV absorbance. Five glucuronides were detected corresponding to monoglucuronides of isoliquiritigenin and liquiritigenin, but no sulfate conjugates were observed. The UDP-glucuronosyltransferases (UGTs) involved in the formation of the major glucuronide conjugates were identified using recombinant human UGTs in combination with liquid chromatography/mass spectrometry. UGT1A1 and UGT1A9 were the major enzymes responsible for the formation of the most abundant conjugate, isoliquiritigenin 4'-O-glucuronide (MG5), with Km values of 4.30+/-0.47 and 3.15+/-0.24 microM, respectively. UGT1A1 and UGT1A10 converted isoliquiritigenin to the next most abundant phase 2 metabolite, isoliquiritigenin 2'-O-glucuronide (MG4), with Km values of 2.98+/-0.8 and 25.8+/-1.3 microM, respectively. In addition, isoliquiritigenin glucuronides MG4 and MG5 were formed by pooled human intestine and kidney microsomes, respectively. Based on the in vitro determination of a 25.3-min half-life for isoliquiritigenin when incubated with HLMs, the intrinsic clearance of isoliquiritigenin was estimated to be 36.4 ml/min/kg. These studies indicate that isoliquiritigenin will be conjugated rapidly in the liver to form up to five monoglucuronides.

Characterization ofN-Glucuronidation of 4-(5-Pyridin-4-yl-1H-[1,2,4]triazol-3-yl) pyridine-2-carbonitrile (FYX-051): A New Xanthine Oxidoreductase Inhibitor

In humans, orally administered 4-(5-pyridin-4-yl-1H-[1,2,4]triazol-3-yl) pyridine-2-carbonitrile (FYX-051) is excreted mainly as triazole N(1)- and N(2)-glucuronides in urine. It is important to determine the enzyme(s) that catalyze the metabolism of a new drug to estimate individual differences and/or drug-drug interactions. Therefore, the characterization and mechanism of these glucuronidations were investigated using human liver microsomes (HLMs), human intestinal microsomes (HIMs), and recombinant human UDP-glucuronosyltransferase (UGT) isoforms to determine the UGT isoform(s) responsible for FYX-051 N(1)- and N(2)-glucuronidation. FYX-051 was metabolized to its N(1)- and N(2)-glucuronide forms by HLMs, and their K(m) values were 64.1 and 72.7 microM, respectively; however, FYX-051 was scarcely metabolized to its glucuronides by HIMs. Furthermore, among the recombinant human UGT isoforms, UGT1A1, UGT1A7, and UGT1A9 catalyzed the N(1)- and N(2)-glucuronidation of FYX-051. To estimate their contribution to FYX-051 glucuronidation, inhibition analysis with pooled HLMs was performed. Mefenamic acid, a UGT1A9 inhibitor, decreased FYX-051 N(1)- and N(2)-glucuronosyltransferase activities, whereas bilirubin, a UGT1A1 inhibitor, did not affect these activities. Furthermore, in the experiment using microsomes from eight human livers, the N(1)- and N(2)-glucuronidation activity of FYX-051 was found to significantly correlate with the glucuronidation activity of propofol, a specific substrate of UGT1A9 (N(1): r(2) = 0.868, p < 0.01; N(2): r(2) = 0.775, p < 0.01). These results strongly suggested that the N(1)- and N(2)-glucuronidation of FYX-051 is catalyzed mainly by UGT1A9 in human livers.

Inhibitory potential of nonsteroidal anti-inflammatory drugs on UDP-glucuronosyltransferase 2B7 in human liver microsomes

OBJECTIVE

A number of nonsteroidal anti-inflammatory drugs (NSAIDs) are subject to glucuronidation in humans, and UDP-glucuronosyltransferase (UGT) 2B7 is involved in the glucuronidation of many NSAIDs. The objective of this study was to identify a NSAID with potent inhibitory potential against UGT2B7 using liquid chromatography with tandem mass spectrometry (LC-MS/MS).

METHODS

A rapid screening method for detecting the inhibitory potential of various drugs against UGT2B7 was established using a LC-MS/MS system. The effects of nine NSAIDs (acetaminophen, diclofenac, diflunisal, indomethacin, ketoprofen, mefenamic acid, naproxen, niflumic acid, and salicylic acid) against UGT2B7-catalyzed 3'-azido-3'-deoxythymidine glucuronidation (AZTG) were investigated in human liver microsomes (HLM) and recombinant human UGT2B7.

RESULTS

Mefenamic acid inhibited AZTG most potently, with an IC(50) value of 0.3 microM, and its inhibition type was not competitive. The IC(50) values for diclofenac, diflunisal, indomethacin, ketoprofen, naproxen, and niflumic acid against AZTG were 6.8, 178, 51, 40, 23, and 83 microM, respectively, while those for acetaminophen and salicylic acid were >100 microM. The IC(50) values for NSAIDs against AZTG in recombinant human UGT2B7 were similar to those obtained in HLM.

CONCLUSION

The method established in this study is useful for identifying drugs with inhibitory potential against human UGT2B7. Among the nine NSAIDs investigated, mefenamic acid had the strongest inhibitory effect on UGT2B7-catalyzed AZTG in HLM. Thus, caution might be exercised when mefenamic acid is coadministered with drugs possessing UGT2B7 as a main elimination pathway.

In vitro inhibitory effects of non-steroidal anti-inflammatory drugs on 4-methylumbelliferone glucuronidation in recombinant human UDP-glucuronosyltransferase 1A9--potent inhibition by niflumic acid

The inhibitory potencies of non-steroidal anti-inflammatory drugs (NSAIDs) on UDP-glucuronosyltransferase (UGT) 1A9 activity were investigated in recombinant human UGT1A9 using 4-methylumbelliferone (4-MU) as a substrate for glucuronidation. 4-MU glucuronidation (4-MUG) showed Michaelis-Menten kinetics with a Km value of 6.7 microM. The inhibitory effects of the following seven NSAIDs were investigated: acetaminophen, diclofenac, diflunisal, indomethacin, ketoprofen, naproxen and niflumic acid. Niflumic acid had the most potent inhibitory effect on 4-MUG with an IC50 value of 0.0341 microM. The IC50 values of diflunisal, diclofenac and indomethacin were 1.31, 24.2, and 34.1 microM, respectively, while acetaminophen, ketoprofen and naproxen showed less potent inhibition. Niflumic acid, diflunisal, diclofenac and indomethacin inhibited 4-MUG competitively with Ki values of 0.0275, 0.710, 53.3 and 69.9 microM, respectively, being similar to each IC50 value. In conclusion, of the seven NSAIDs investigated, niflumic acid was the most potent inhibitor of recombinant UGT1A9 via 4-MUG in a competitive manner.

In vitro inhibitory effects of non-steroidal antiinflammatory drugs on UDP-glucuronosyltransferase 1A1-catalysed estradiol 3beta-glucuronidation in human liver microsomes

The inhibitory potencies of non-steroidal antiinflammatory drugs (NSAID) on UDP-glucuronosyltransferase (UGT) 1A1-catalysed estradiol 3beta-glucuronidation (E3G) were investigated in human liver microsomes (HLM). Inhibitory effects of the following seven NSAID were investigated: acetaminophen, diclofenac, diflunisal, indomethacin, ketoprofen, naproxen and niflumic acid. Niflumic acid had the most potent inhibitory effect on E3G with an IC50 value of 22.2 microM in HLM. The IC50 values of diclofenac, diflunisal, indomethacin for E3G were 60.9, 37.8 and 51.5 microM, respectively, while acetaminophen, ketoprofen and naproxen showed less potent inhibition. Diclofenac inhibited E3G non-competitively with a Ki value of 112 microM in HLM. The IC50 value of diclofenac for 4-methylumbelliferone glucuronidation in recombinant human UGT1A1 was 57.5 microM, similar to that obtained for E3G using HLM. In conclusion, niflumic acid had the most potent inhibitory effects on UGT1A1-catalysed E3G in HLM among seven NSAID investigated.

ACYL GLUCURONIDATION OF FLUOROQUINOLONE ANTIBIOTICS BY THE UDP-GLUCURONOSYLTRANSFERASE 1A SUBFAMILY IN HUMAN LIVER MICROSOMES

Acyl glucuronidation is an important metabolic pathway for fluoroquinolone antibiotics. However, it is unclear which human UDP-glucuronosyltransferase (UGT) enzymes are involved in the glucuronidation of the fluoroquinolones. The in vitro formation of levofloxacin (LVFX), grepafloxacin (GPFX), moxifloxacin (MFLX), and sitafloxacin (STFX) glucuronides was investigated in human liver microsomes and cDNA-expressed recombinant human UGT enzymes. The apparent Km values for human liver microsomes ranged from 1.9 to 10.0 mM, and the intrinsic clearance values (calculated as Vmax/Km) had a rank order of MFLX > GPFX > STFX > > LVFX. In a bank of human liver microsomes (n = 14), the glucuronidation activities of LVFX, MFLX, and STFX correlated highly with UGT1A1-selective beta-estradiol 3-glucuronidation activity, whereas the glucuronidation activity of GPFX correlated highly with UGT1A9-selective propofol glucuronidation activity. Among 12 recombinant UGT enzymes, UGT1A1, 1A3, 1A7, and 1A9 catalyzed the glucuronidation of these fluoroquinolones. Results of enzyme kinetics studies using the recombinant UGT enzymes indicated that UGT1A1 most efficiently glucuronidates MFLX, and UGT1A9 most efficiently glucuronidates GPFX. In addition, the glucuronidation activities of MFLX and STFX in human liver microsomes were potently inhibited by bilirubin with IC50 values of 4.9 microM and 4.7 microM, respectively; in contrast, the glucuronidation activity of GPFX was inhibited by mefenamic acid with an IC50 value of 9.8 microM. These results demonstrate that UGT1A1, 1A3, and 1A9 enzymes are involved in the glucuronidation of LVFX, GPFX, MFLX, and STFX in human liver microsomes, and that MFLX and STFX are predominantly glucuronidated by UGT1A1, whereas GPFX is mainly glucuronidated by UGT1A9.

Identification of common polymorphisms in the promoter of the UGT1A9 gene: evidence that UGT1A9 protein and activity levels are strongly genetically controlled in the liver

OBJECTIVES

Polymorphisms in UDP-glucuronosyltransferases (UGTs) can influence detoxifying capacities and have considerable therapeutic implications in addition to influence various (patho)physiological processes. UGT1A9 plays a central role in the metabolism of various classes of therapeutic drugs in addition to carcinogens and steroids. The great interindividual variability of UGT1A9-mediated glucuronidation remains poorly explained, while evidence for its genetic origin exists.

METHODS

The proximal UGT1A9 promoter was screened for polymorphisms by sequencing and, the contribution of single nucleotide polymorphisms (SNPs) to the variability of UGT1A9 protein levels and activity was evaluated.

RESULTS

We confirmed the presence of the -109 to -98 T10 polymorphism and found ten novel SNPs that generated a diversity of haplotypes in two independent populations. In a panel of 48 human liver microsomes, the UGT1A9 expression varied by 17-fold and was significantly correlated with SNPs -275, -331/-440, -665 and -2152. The base insertion T10 reported to increase reporter gene expression in HepG2 cells [] was not linked to -275 and -2152 SNPs and was not associated with changes in UGT1A9 protein levels. Compared to wild-type individuals, there were statistically significant higher glucuronidating activities in livers with the -275 and -2152 using mycophenolic acid and propofol as UGT1A9 substrates, indicating an extensive glucuronidator phenotype associated with these variants.

CONCLUSIONS

This is the first study to demonstrate that naturally occurring sequence variations in the UGT1A9 promoter are informative in predicting the levels of protein and glucuronidating activity, providing a potential mechanism for interindividual variation in UGT1A9-mediated metabolism.