Liang-Liang Zhu

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.

Potent and selective inhibition of magnolol on catalytic activities of UGT1A7 and 1A9

Human exposure to magnolol can reach a high dose in daily life. Our previous studies indicated that magnolol showed high affinities to several UDP-glucuronosyltransferases (UGTs) This study was designed to examine the in vitro inhibitory effects of magnolol on UGTs, and further to evaluate the possibility of the in vivo inhibition that might happen. Assays with recombinant UGTs and human liver microsomes (HLM) indicated that magnolol (10 µM) can selectively inhibit activities of UGT1A9 and extra-hepatic UGT1A7. Inhibition of magnolol on UGT1A7 followed competitive inhibition mechanism, while the inhibition on UGT1A9 obeyed either competitive or mixed inhibition mechanism, depending on substrates. The Ki values for UGT1A7 and 1A9 are all in nanomolar ranges, lower than possible magnolol concentrations in human gut lumen and blood, indicating the in vivo inhibition on these two enzymes would likely occur. In conclusion, UGT1A7 and 1A9 can be strongly inhibited by magnolol, raising the alarm for safe application of magnolol and traditional Chinese medicines containing magnolol. Additionally, given that UGT1A7 is an extra-hepatic enzyme, magnolol can serve as a selective UGT1A9 inhibitor that will act as a new useful tool in future hepatic glucuronidation phenotyping.

Investigation of UDP-glucuronosyltransferases (UGTs) Inhibitory Properties of Carvacrol

UDP‐glucuronosyltransferases (UGTs), the most important phase II drug metabolizing enzymes (DMEs), could metabolize many drugs and various endogenous substances including bilirubin, steroid hormones, thyroid hormones, bile acids and fat‐soluble vitamins. Evaluation of the inhibitory effects of compounds on UGTs is clinically important because inhibition of UGT isoforms could not only result in serious drug–drug interactions (DDIs), but also induce metabolic disorders of endogenous substances. The aim of the present study was to investigate the inhibitory effects of carvacrol on major UGT isoforms. The results showed that carvacrol could inhibit the activity of UGT1A9 with negligible effects on other UGT isoforms. When 4‐methylumbelliferone (4‐MU) was used as a nonspecific probe substrate and recombinant UGT enzymes were utilized as an enzyme resource, the inhibition of UGT1A9 was best fit to the competitive type and the inhibition kinetic parameter (Ki) was calculated to be 5.7 µ m. Furthermore, another specific probe substrate, propofol, was employed to determine the inhibitory kinetics of UGT1A9, and the results demonstrated that the inhibitory type was noncompetitive. The inhibition kinetic parameter (Ki) was determined to be 25.0 µ m. Because this substrate‐dependent inhibition of UGT1A9 might confuse the in vitro–in vivo extrapolation, these in vitro inhibition kinetic parameters should be interpreted with special caution. Copyright

Identification of cytochrome P450 (CYP) isoforms involved in the metabolism of corynoline, and assessment of its herb-drug interactions.

Corynoline, an isoquinoline alkaloid isolated from the genus Corydalis, has been demonstrated to show multiple pharmacological effects including inhibition of acetylcholinesterase, inhibition of cell adhesion, fungitoxic and cytotoxic activity. The present study focused on its metabolism and metabolism‐based herb–drug interactions. After corynoline was incubated with human liver microsomes (HLMs) in the presence of NADPH, two metabolites (M‐1 and M‐2) were formed. Chemical inhibition experiments and assays with recombinant CYP isoforms showed that CYP2C9 was mainly involved in the formation of M‐1 and CYP3A4 mainly catalysed the production of M‐2. Among seven major CYP isoforms tested, corynoline showed strong inhibitory effects on the activities of CYP3A4 and CYP2C9, with an IC50 of 3.3 ± 0.9 µm and 31.5 ± 0.5 µm, respectively. Kinetic analysis showed that inhibition of CYP3A4 by corynoline was best fit to a noncompetitive manner with Ki of 3.2 µm, while inhibition of CYP2C9 by corynoline was best fit to a competitive manner with Ki of 6.3 µm. Additionally, corynoline exhibited time‐dependent inhibition (TDI) toward CYP3A4. The inactivation kinetic parameters (KI and kinact) were calculated to be 6.8 µm and 0.07 min‐1, respectively. These data are of significance for the application of corynoline and corynoline‐containing herbs. Copyright

Determination of propofol UDP-glucuronosyltransferase (UGT) activities in hepatic microsomes from different species by UFLC-ESI-MS

Propofol O-glucuronidation has been used as probe reaction to phenotype UGT1A9 activity in human liver, thus a sensitive and specific method for determination of propofol O-glucuronide (PG) is urgently desirable. In the current study, a new LC-ESI-MS method for determination of PG in hepatic microsomes from human (HLM), monkey (CyLM), dog (DLM), minipig (PLM), rat (RLM) and mouse (MLM) was developed and validated using 4-methylumbelliferyl-β-d-glucuronide as an internal standard (IS). PG and IS was separated by a Shim-pack XR-ODS column (100 mm × 2.0mm, 2.2 μm, Shimadzu) under gradient conditions with the mobile phase of acetonitrile and water containing 0.2% acetic acid (v/v). The mass spectrometric detection was performed under selected ion monitoring (SIM) for PG at m/z 353 and IS at m/z 351. The assay exhibited linearity over the range 0.05-30 μM for PG with the correlation coefficient of 0.9995. The intra- and inter-day precision was less than 7.2%, with accuracy in the range 93.8-107.5%. The developed method was successfully used for characterizing interspecies and human individual differences in the O-glucuronidation activity towards propofol, as well as investigating inhibitory effects of androsterone and phenylbutazone on propofol O-glucuronidation in HLM.

Inhibitory effects of sanguinarine on human liver cytochrome P450 enzymes

Sanguinarine (SAG) has been recognized as an anticancer drug candidate. However, the drug-drug interactions (DDI) potential for SAG via the inhibition against human cytochrome P450 (CYP) enzymes remains unclear. In the present study, the inhibitory effects of SAG on seven major human CYP isoforms 1A2, 2A6, 2E1, 2D6, 2C8, 2C9 and 3A4 were investigated with human liver microsomes (HLM). The results showed that SAG was a potent noncompetitive inhibitor of CYP2C8 activity (Ki=8.9 μM), and competitive inhibitor of CYP1A2, CYP2C9 and CYP3A4 activities (Ki=2.7, 3.8 and 2.0 μM, respectively). Furthermore, SAG exhibited time- and NADPH-dependent inhibition towards CYP1A2 and CYP3A4 with KI/kinact values of 13.3/0.087 and 5.58/0.029 min(-1) μM(-1), respectively. Weak inhibition of SAG against CYP2E1, CYP2D6 and CYP2A6 was also observed. In vitro-in vivo extrapolation (IV-IVE) from HLM data showed that more than 35.9% of CYP1A2, CYP2C9, CYP2C8 and CYP3A4 activities in vivo could be inhibited by SAG, suggesting that harmful DDIs could occur when SAG or its medical preparations are co-administered with drugs primarily cleared by these CYP isoforms. Further in vivo studies are needed to evaluate the clinical significance of the data presented herein.

Selectivity for inhibition of nilotinib on the catalytic activity of human UDP-glucuronosyltransferases

Abstract 1. Nilotinib, a tyrosine kinase inhibitor, could potently inhibit SN-38 glucuronidation mainly catalyzed by UDP-glucuronosyltransferase (UGT) 1A1. This study was designed to investigate whether nilotinib can be used as a selective inhibitor of UGT1A1 in human liver. 2. Assays with recombinant UGTs indicated that nilotinib could strongly inhibit the activity of UGT1A1 and decreased the activity of extra-hepatic UGT1A7 to a much lesser extent. The inhibition on 4-methylumbelliferone (4Mu) glucuronidation by recombinant UGT1A1 obeyed competitive inhibition mechanism, with a kinetic constants (Ki) value of 0.17 μM. Assays with human liver microsomes (HLM) demonstrated that nilotinib could selectively inhibit estradiol-3-O-glucuronidation (E2-3-O-glucuronidation), a probe reaction of UGT1A1. Kinetic studies displayed that the inhibition on E2-3-O-glucuronidation followed non-competitive inhibition model, different from the inhibition on 4Mu glucuronidation. The Ki values were calculated to be 0.14 and 0.53 μM, depending on the enzyme sources of recombinant UGT1A1 or HLM, respectively. 3. Given that UGT1A7 is an extra-hepatic enzyme, this study indicates that nilotinib can be used as a selective inhibitor of UGT1A1 in human liver.

Substrate-dependent modulation of the catalytic activity of CYP3A by erlotinib

Aim:To ascertain the effects of erlotinib on CYP3A, to investigate the amplitude and kinetics of erlotinib-mediated inhibition of seven major CYP isoforms in human liver microsomes (HLMs) for evaluating the magnitude of erlotinib in drug-drug interaction in vivo.Methods:The activities of 7 major CYP isoforms (CYP1A2, CYP2A6, CYP3A, CYP2C9, CYP2D6, CYP2C8, and CYP2E1) were assessed in HLMs using HPLC or UFLC analysis. A two-step incubation method was used to examine the time-dependent inhibition of erlotinib on CYP3A.Results:The activity of CYP2C8 was inhibited with an IC50 value of 6.17±2.0 μmol/L. Erlotinib stimulated the midazolam 1′-hydroxy reaction, but inhibited the formation of 6β-hydroxytestosterone and oxidized nifedipine. Inhibition of CYP3A by erlotinib was substrate-dependent: the IC50 values for inhibiting testosterone 6β-hydroxylation and nifedipine metabolism were 31.3±8.0 and 20.5±5.3 μmol/L, respectively. Erlotinib also exhibited the time-dependent inhibition on CYP3A, regardless of the probe substrate used: the value of KI and kinact were 6.3 μmol/L and 0.035 min−1 for midazolam; 9.0 μmol/L and 0.045 min−1 for testosterone; and 10.1 μmol/L and 0.058 min−1 for nifedipine.Conclusion:The inhibition of CYP3A by erlotinib was substrate-dependent, while its time-dependent inhibition on CYP3A was substrate-independent. The time-dependent inhibition of CYP3A may be a possible cause of drug-drug interaction, suggesting that attention should be paid to the evaluation of erlotinibs safety, especially in the context of combination therapy.

Reversible inhibition of three important human liver cytochrome p450 enzymes by tiliroside

Tiliroside, an active flavonoid extensively found in many medicinal plants including Helichrysum italicum, Geranium mexicanum and Helianthemum glomeratum, has been demonstrated to exert multiple biological effects including antiinflammatory, antimicrobial, antioxidant and antitumor activities. Cytochrome P450 (CYP) enzymes play an important role in the Phase I oxidation metabolism of a wide range of xenobiotics and inhibition of CYP isoforms might influence the elimination of drugs and induce serious adverse drug response. The inhibition of seven CYP isoforms (CYP3A4, CYP1A2, CYP2A6, CYP2D6, CYP2C9, CYP2C8 and CYP2E1) by tiliroside was investigated using in vitro human liver microsomal incubation assays. The results showed that tiliroside strongly inhibited the activity of CYP3A4 (IC50 = 9.0 ± 1.7 μm), CYP2C8 (IC50 = 12.1 ± 0.9 μm) and CYP2C9 (IC50 = 10.2 ± 0.9 μm) with other CYP isoforms negligibly influenced. Further kinetic analysis showed that inhibition of these three CYP isoforms by tiliroside is best fit to a competitive way. The Ki value was calculated to be 5.5 μm, 3.3 μm, 9.4 μm for CYP3A4, CYP2C9 and CYP2C8, respectively. The relatively low Ki values suggested that tiliroside might induce drug–drug interactions with many clinically used drugs which are mainly metabolized by these three CYP isoforms. Therefore, attention should be given to the probable drug–drug interaction between tiliroside‐containing herbs and substrates of CYP3A4, CYP2C9 and CYP2C8. Copyright

The role of serum albumin in the metabolism of Boc5: Molecular identification, species differences and contribution to plasma metabolism

Boc5, the first nonpeptidic agonist of Glucagon-like peptide-1 receptor, has been recognized as a potential candidate for treatment of diabetes. However, the metabolic behaviors of this novel molecule in both human and experimental animals remain unclear. This study aimed to explore the metabolic behaviors of Boc5 in biological preparations from human, pig and rat. Boc5 was found to be very stable in liver microsomes of human, pig and rat, but it can be degraded to two metabolites in plasma from all three species, via the successive hydrolysis of the C-22 esters. Chemical inhibition studies using selective esterase inhibitors and assays with purified enzymes suggested that Boc5 hydrolysis in human was totally mediated by human serum albumin (HSA) rather than esterases. ESI-TOF-MS/MS analysis revealed that Lys525 of HSA could be modified by treatment with Boc5, strongly suggesting the pseudo-esterase activity of albumin. Studies on species differences in this albumin-mediated metabolism showed large species differences in degradation rate of Boc5, the half lives of Boc5 in plasma from three various species varied from 23.5 h to 83.1h, but they were much closer to the half lives of Boc5 in corresponding serum albumins, implying the predominant role of serum albumin in plasma metabolism of Boc5. Additionally, the effects of various ligands including fatty acids and several drugs with unambiguous binding sites on HSA, on the pseudo-esterase activity of HSA, were also investigated using both experimental and molecular modelling studies. These results showed that the binding of various ligands to HSA could significantly affect the pseudo-esterase activity of HSA towards Boc5, due to the ligand-induced conformation changes of HSA.