Yan-Yan Zhang

Time-dependent inhibition (TDI) of CYP3A4 and CYP2C9 by noscapine potentially explains clinical noscapine–warfarin interaction

AIMS To investigate the inhibition potential and kinetic information of noscapine to seven CYP isoforms and extrapolate in vivo noscapine-warfarin interaction magnitude from in vitro data. METHODS The activities of seven CYP isoforms (CYP3A4, CYP1A2, CYP2A6, CYP2E1, CYP2D6, CYP2C9, CYP2C8) in human liver microsomes were investigated following co- or preincubation with noscapine. A two-step incubation method was used to examine in vitro time-dependent inhibition (TDI) of noscapine. Reversible and TDI prediction equations were employed to extrapolate in vivo noscapine-warfarin interaction magnitude from in vitro data. RESULTS Among seven CYP isoforms tested, the activities of CYP3A4 and CYP2C9 were strongly inhibited with an IC(50) of 10.8 +/- 2.5 microm and 13.3 +/- 1.2 microm. Kinetic analysis showed that inhibition of CYP2C9 by noscapine was best fit to a noncompetitive type with K(i) value of 8.8 microm, while inhibition of CYP3A4 by noscapine was best fit to a competitive manner with K(i) value of 5.2 microm. Noscapine also exhibited TDI to CYP3A4 and CYP2C9. The inactivation parameters (K(I) and k(inact)) were calculated to be 9.3 microm and 0.06 min(-1) for CYP3A4 and 8.9 microm and 0.014 min(-1) for CYP2C9, respectively. The AUC of (S)-warfarin and (R)-warfarin was predicted to increase 1.5% and 1.1% using C(max) or 0.5% and 0.4% using unbound C(max) with reversible inhibition prediction equation, while the AUC of (S)-warfarin and (R)-warfarin was estimated to increase by 110.9% and 48.9% using C(max) or 41.8% and 32.7% using unbound C(max) with TDI prediction equation. CONCLUSIONS TDI of CYP3A4 and CYP2C9 by noscapine potentially explains clinical noscapine-warfarin interaction.

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.

Identification and Characterization of Human UDP-Glucuronosyltransferases Responsible for the In Vitro Glucuronidation of Daphnetin

Daphnetin has been developed as an oral medicine for treatment of coagulation disorders and rheumatoid arthritis in China, but its in vitro metabolism remains unknown. In the present study, the UDP-glucuronosyltransferase (UGT) conjugation pathways of daphnetin were characterized. Two metabolites, 7-O-monoglucuronide daphnetin (M-1) and 8-O-monoglucuronide daphnetin (M-2), were identified by liquid chromatography/mass spectrometry and NMR when daphnetin was incubated, respectively, with liver microsomes from human (HLM), rat (RLM), and minipig (PLM) and human intestinal microsomes (HIM) in the presence of UDP-glucuronic acid. Screening assays with 12 human recombinant UGTs demonstrated that the formations of M-1 and M-2 were almost exclusively catalyzed by UGT1A9 and UGT1A6, whereas M-1 was formed to a minor extent by UGT1A3, 1A4, 1A7, 1A8, and 1A10 at a high substrate concentration. Kinetics studies, chemical inhibition, and correlation analysis were used to demonstrate that human UGT1A9 and UGT1A6 were major isoforms involved in the daphnetin glucuronidations in HLM and HIM. By in vitro-in vivo extrapolation of the kinetic data measured in HLM, the hepatic clearance and the corresponding hepatic extraction ratio were estimated to be 19.3 ml/min/kg b.wt. and 0.93, respectively, suggesting that human clearance of daphnetin via the glucuronidation is extensive. Chemical inhibition of daphnetin glucuronidation in HLM, RLM, and PLM showed large species differences although the metabolites were formed similarly among the species. In conclusion, the UGT conjugation pathways of daphnetin were fully elucidated and its C-8 phenol group was more selectively catalyzed by UGTs than by the C-7 phenol.

Cycloartane triterpenoids from Cimicifuga yunnanensis induce apoptosis of breast cancer cells (MCF7) via p53-dependent mitochondrial signaling pathway

The present study was carried out to investigate the antitumor activity of five cycloartane triterpenoids isolated from Cimicifuga yunnanensis on the breast cancer cell line MCF7 and its corresponding drug resistant subline R‐MCF7, including cimigenol‐3‐O‐β‐d‐xylopyranoside (compound 1), 25‐O‐acetylcimigenol‐3‐O‐β‐d‐xylopyranoside (compound 2), 25‐chlorodeoxycimigenol‐3‐O‐β‐d‐xylopyranoside (compound 3), 25‐O‐acetylcimigenol‐3‐O‐α‐l‐arabinopyranoside (compound 4) and 23‐O‐acetylcimigenol‐3‐O‐β‐d‐xylopyranoside (compound 5). The results showed that compounds 2–5 have relatively high antitumor activity on both MCF7 and R‐MCF7 cells. The involvement of apoptosis as a major cause of cycloartane triterpenoids‐induced cell death was further confirmed. The results of RT‐PCR showed that compounds 2–5 increased the expression of p53 and bax, which led to the loss of mitochondrial potential and then resulted in the activation of caspase‐7. These findings collectively demonstrated that compounds 2–5 induced apoptosis of MCF7 via p53‐dependent mitochondrial pathway. 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

Interactions between human cytochrome P450 enzymes and steroids: physiological and pharmacological implications

Background: Steroids are groups of biologically active molecules with a wide variety of physiological and pharmacological functions. Human cytochrome P450 (CYP) enzymes present in probably every tissue are found responsible for biosynthesis and catabolism of steroids, which could result in either active or inactive metabolites. In addition, exposure to endogenous and exogenous steroids that causes modulation of CYP activities may substantially affect the pharmacokinetic behavior of a given drug. Objective: This article summarizes our current understanding of the ability of steroids to act as substrates, inhibitors or heteroactivators for human CYP enzymes, with a specific focus on their functional consequences. Methods: In the current review, we compare the mechanisms and regulation of CYP-mediated biotransformation of steroids, and in particular examine the diverse tissue distributions and biological roles of individual CYPs. Conclusion: Metabolic instability of steroids in the presence of CYPs not only affects the magnitude and duration of their actions but may also alter the profiles of their physiological, pathological, pharmacological and toxicological effects in relevant organs.

Chemotaxonomic study of medicinal Taxus species with fingerprint and multivariate analysis.

Species delimitation in Taxus has been controversial and it is very difficult to distinguish yew materials by their morphological characters. In this paper, a valid HPLC fingerprinting method coupled with multivariate analysis was used to define a framework for Taxus species identification and classification. Fingerprint-based similarity was employed for a chemotaxonomic study by hierarchical clustering analysis (HCA) and principal component analysis (PCA). Based on the PCA loadings, twelve chemical constituents were selected as chemotaxonomic markers which can be used to establish a more practical classification. Finally, eight studied species could be divided into six well-supported groups and most samples can be assigned to the correct species. Additionally, twelve markers were tentatively identified by LC/MS.

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.

Taxane's Substituents at C3′ Affect Its Regioselective Metabolism: Different in Vitro Metabolism of Cephalomannine and Paclitaxel

To investigate how taxanes substituents at C3′ affect its metabolism, we compared the metabolism of cephalomannine and paclitaxel, a pair of analogs that differ slightly at the C3′ position. After cephalomannine was incubated with human liver microsomes in an NADPH-generating system, two monohydroxylated metabolites (M1 and M2) were detected by liquid chromatography/tandem mass spectrometry. C4″ (M1) and C6α (M2) were proposed as the possible hydroxylation sites, and the structure of M1 was confirmed by 1H NMR. Chemical inhibition studies and assays with recombinant human cytochromes P450 (P450s) indicated that 4″-hydroxycephalomannine was generated predominantly by CYP3A4 and 6α-hydroxycephalomannine by CYP2C8. The overall biotransformation rate between paclitaxel and cephalomannine differed slightly (184 vs. 145 pmol/min/mg), but the average ratio of metabolites hydroxylated at the C13 side chain to C6α for paclitaxel and cephalomannine varied significantly (15:85 vs. 64:36) in five human liver samples. Compared with paclitaxel, the major hydroxylation site transferred from C6α to C4″, and the main metabolizing P450 changed from CYP2C8 to CYP3A4 for cephalomannine. In the incubation system with rat or minipig liver microsomes, only 4″-hydroxycephalomannine was detected, and its formation was inhibited by CYP3A inhibitors. Molecular docking by AutoDock suggested that cephalomannine adopted an orientation in favor of 4″-hydroxylation, whereas paclitaxel adopted an orientation favoring 3′-p-hydroxylation. Kinetic studies showed that CYP3A4 catalyzed cephalomannine more efficiently than paclitaxel due to an increased Vm. Our results demonstrate that relatively minor modification of taxane at C3′ has major consequence on the metabolism.