己烯雌酚对人肝微粒体内CYP3A4和CYP2C9酶的抑制作用

目的:体外实验考察己烯雌酚(DES)对细胞色素P450 3A4(CYP3A4)和细胞色素P450 2C9(CYP2C9)活性的抑制作用,以评佑DES通过抑制这两个重要的细胞色素P450(CYP)亚型而引发药物-药物相互作用的可能性.方法:混合人肝微粒体与不同浓度的DES(或阳性抑制剂),CYP3A4或CYP2C9的探针...     

Metabolism of myclobutanil and triadimefon by human and rat cytochrome P450 enzymes and liver microsomes

Metabolism of two triazole-containing antifungal azoles was studied using expressed human and rat cytochrome P450s (CYP) and liver microsomes. Substrate depletion methods were used due to the complex array of metabolites produced from myclobutanil and triadimefon. Myclobutanil was metabolized more rapidly than triadimefon, which is consistent with metabolism of the n-butyl side-chain in the former and the t-butyl group in the latter compound. Human and rat CYP2C and CYP3A enzymes were the most active. Metabolism was similar in microsomes prepared from livers of control and low-dose rats. High-dose (115 mg kg-1 day-1 of triadimefon or 150 mg kg-1 day-1 of myclobutanil) rats showed increased liver weight, induction of total CYP, and increased metabolism of the two triazoles, though the apparent Km appeared unchanged relative to the control. These data identify CYP enzymes important for the metabolization of these two triazoles. Estimated hepatic clearances suggest that CYP induction may have limited impact in vivo.     

Identification of cytochrome P450 isoforms involved in the metabolism of loperamide in human liver microsomes

Objective: The purpose of the present study was to elucidate the cytochrome P450 (P450) isoform(s) involved in the metabolism of loperamide (LOP) to N-demethylated LOP (DLOP) in human liver microsomes. Methods: Three established approaches were used to identify the P450 isoforms responsible for LOP N-demethylation using human liver microsomes and cDNA-expressed P450 isoforms: (1) correlation of LOP N-demethylation activity with marker P450 activities in a panel of human liver microsomes, (2) inhibition of enzyme activity by P450-selective inhibitors, and (3) measurement of DLOP formation by cDNA-expressed P450 isoforms. The relative contribution of P450 isoforms involved in LOP N-demethylation in human liver microsomes were estimated by applying relative activity factor (RAF) values. Results: The formation rate of DLOP showed biphasic kinetics, suggesting the involvement of multiple P450 isoforms. Apparent K_m and V_max values were 21.1 M and 122.3 pmol/min per milligram of protein for the high-affinity component and 83.9 M and 412.0 pmol/min per milligram of protein for the low-affinity component, respectively. Of the cDNA-expressed P450 s tested, CYP2B6, CYP2C8, CYP2D6, and CYP3A4 catalyzed LOP N-demethylation. LOP N-demethylation was significantly inhibited when coincubated with quercetin (a CYP2C8 inhibitor) and ketoconazole (a CYP3A4 inhibitor) by 40 and 90%, respectively, but other chemical inhibitors tested showed weak or no significant inhibition. DLOP formation was highly correlated with CYP3A4-catalyzed midazolam 1-hydroxylation (r_s=0.829; P<0.01), CYP2B6-catalzyed 7-ethoxy-4-trifluoromethylcoumarin O-deethylation (r_s=0.691; P<0.05), and CYP2C8-catalyzed paclitaxel 6-hydroxylation (r_s=0.797; P<0.05). Conclusion: CYP2B6, CYP2C8, CYP2D6, and CYP3A4 catalyze LOP N-demethylation in human liver microsomes, and among them, CYP2C8 and CYP3A4 may play a crucial role in LOP metabolism at the therapeutic concentrations of LOP. Coadministration of these P450 inhibitors may cause drug interactions with LOP. However, the clinical significance of potential interaction of LOP metabolism by CYP2C8 and CYP3A4 inhibitors should be studied further.     

Metabolism of 3-methylindole by porcine liver microsomes: responsible cytochrome P450 enzymes.

The role of different cytochrome P450 enzymes on the metabolism of 3-methylindole (3MI) was investigated using selective chemical inhibitors. Eight chemical inhibitors of P450 enzymes were screened for their inhibitory specificity towards 3MI metabolism in porcine microsomes: alpha-naphthoflavone (CYP1A1/2), 8-methoxypsoralen (CYP2A6), menthofuran (CYP2A6), diethyldithiocarbamate (CYP2A6), 4-methylpyrazole (CYP2E1), sulphaphenazole (CYP2C9), quinidine (CYP2D6), and troleandomycin (CYP3A4). The production of 3MI metabolites was only affected by the presence of inhibitors of CYP2A6 and CYP2E1 in the microsomal incubations. In a second experiment, a set of porcine microsomes (n = 30) was analyzed for CYP2A6 content by protein immunoblot analysis and for their coumarin 7-hydroxylation activity (CYP2A6 activity). Both CYP2A6 content and enzymatic activity were found to be highly and negatively correlated with 3MI fat content. The results of the present study indicate that the CYP2A6 porcine ortholog plays an important role in the metabolism of 3MI and that measurement of CYP2A6 levels and/or activity could be a useful marker for 3MI-induced boar taint.     

Characterization of the metabolism of fenretinide by human liver microsomes, cytochrome P450 enzymes and UDP-glucuronosyltransferases

BACKGROUND AND PURPOSE

Fenretinide (4-HPR) is a retinoic acid analogue, currently used in clinical trials in oncology. Metabolism of 4-HPR is of particular interest due to production of the active metabolite 4′-oxo 4-HPR and the clinical challenge of obtaining consistent 4-HPR plasma concentrations in patients. Here, we assessed the enzymes involved in various 4-HPR metabolic pathways.

EXPERIMENTAL APPROACH

Enzymes involved in 4-HPR metabolism were characterized using human liver microsomes (HLM), supersomes over-expressing individual human cytochrome P450s (CYPs), uridine 5′-diphospho-glucoronosyl transferases (UGTs) and CYP2C8 variants expressed in Escherichia coli. Samples were analysed by high-performance liquid chromatography and liquid chromatography/mass spectrometry assays and kinetic parameters for metabolite formation determined. Incubations were also carried out with inhibitors of CYPs and methylation enzymes.

KEY RESULTS

HLM were found to predominantly produce 4′-oxo 4-HPR, with an additional polar metabolite, 4′-hydroxy 4-HPR (4′-OH 4-HPR), produced by individual CYPs. CYPs 2C8, 3A4 and 3A5 were found to metabolize 4-HPR, with metabolite formation prevented by inhibitors of CYP3A4 and CYP2C8. Differences in metabolism to 4′-OH 4-HPR were observed with 2C8 variants, CYP2C8*4 exhibited a significantly lower V_max value compared with *1. Conversely, a significantly higher V_max value for CYP2C8*4 versus *1 was observed in terms of 4′-oxo formation. In terms of 4-HPR glucuronidation, UGTs 1A1, 1A3 and 1A6 produced the 4-HPR glucuronide metabolite.

CONCLUSIONS AND IMPLICATIONS

The enzymes involved in 4-HPR metabolism have been characterized. The CYP2C8 isoform was found to have a significant effect on oxidative metabolism and may be of clinical relevance.     

大黄素对大鼠肝脏CYP450酶的诱导研究

目的 探讨大黄素对大鼠肝脏细胞色素P450酶(CYP450)及其主要亚型的影响。方法 20只雄性SD大鼠, 随机分成4组, 每组5只, 分别为溶剂对照组, 170、500和1 500 mg/kg大黄素染毒组, 大黄素蒸馏水混悬后连续经口给药16 d, 结束后次日取大鼠肝脏组织制作微粒体, 分别采用CO还原差示光谱法、分光光度法及化学发光法检测大鼠肝脏微粒体总CYP450水平, 红霉素脱甲基酶(CYP3A)、氨基比啉-N-脱甲基酶, CYP1A、CYP2B和CYP2E1酶活性变化。结果 大黄素连续经口给药16 d, 能够引起大鼠肝脏微粒体总CYP450显著升高、可轻度诱导CYP3A、CYP1A、CYP2E1和CYP2B酶, 500 mg/kg剂量组最明显。结论 大黄素对大鼠肝脏中CYP3A、CYP1A、CYP2B和CYP2E1酶均有诱导作用。     

Characterization of cytochrome P450s mediating ipriflavone metabolism in human liver microsomes

Ipriflavone, a synthetic flavonoid for the prevention and treatment of osteoporosis, has been reported to be extensively metabolized in man to seven metabolites (M1–M7). This study was performed to characterize the human liver cytochrome P450s (CYP) responsible for the metabolism of ipriflavone. Hydroxylation at the β-ring to M3, O-dealkylation to M1 and oxidation at isopropyl group to M4 and M5 are major pathways for ipriflavone metabolism in three different human liver microsome preparations. The specific CYPs responsible for ipriflavone oxidation to the active metabolites, M1, M3, M4 and M5 were identified using a combination of correlation analysis, immuno-inhibition, chemical inhibition in human liver microsomes and metabolism by expressed recombinant CYP enzymes. The inhibitory potencies of ipriflavone and its five metabolites, M1–M5 on seven clinically important CYPs were investigated in human liver microsomes. Our results demonstrate that CYP3A4 plays the major role in O-dealkylation of ipriflavone to M1 and CYP1A2 plays a dominant role in the formation of M3, M4 and M5. Ipriflavone and/or its five metabolites were found to inhibit potently the metabolism of CYPs 1A2, 2C8, 2C9 and 2C19 substrates.     

Metabolism of bilirubin by human cytochrome P450 2A6

The mouse cytochrome P450 (CYP) 2A5 has recently been shown to function as hepatic “Bilirubin Oxidase” (Abu-Bakar, A., et al., 2011. Toxicol. Appl. Pharmacol. 257, 14-22). To date, no information is available on human CYP isoforms involvement in bilirubin metabolism. In this paper we provide novel evidence for human CYP2A6 metabolising the tetrapyrrole bilirubin. Incubation of bilirubin with recombinant yeast microsomes expressing the CYP2A6 showed that bilirubin inhibited CYP2A6-dependent coumarin 7-hydroxylase activity to almost 100% with an estimated K_i of 2.23 μM. Metabolite screening by a high-performance liquid chromatography/electrospray ionisation mass spectrometry indicated that CYP2A6 oxidised bilirubin to biliverdin and to three other smaller products with m/z values of 301, 315 and 333. Molecular docking analyses indicated that bilirubin and its positively charged intermediate interacted with key amino acid residues at the enzyme's active site. They were stabilised at the site in a conformation favouring biliverdin formation. By contrast, the end product, biliverdin was less fitting to the active site with the critical central methylene bridge distanced from the CYP2A6 haem iron facilitating its release. Furthermore, bilirubin treatment of HepG2 cells increased the CYP2A6 protein and activity levels with no effect on the corresponding mRNA. Co-treatment with cycloheximide (CHX), a protein synthesis inhibitor, resulted in increased half-life of the CYP2A6 compared to cells treated only with CHX. Collectively, the observations indicate that the CYP2A6 may function as human “Bilirubin Oxidase” where bilirubin is potentially a substrate and a regulator of the enzyme.     

In vitro metabolism of nobiletin,a polymethoxy-flavonoid,by human liver microsomes and cytochrome P450

1. Cytochrome P450 enzymes (CYPs) in the liver metabolize drugs prior to excretion, with different enzymes acting at different molecular motifs. At present, the human CYPs responsible for the metabolism of the flavonoid, nobiletin (NBL), are unidentified. We investigated which enzymes were involved using human liver microsomes and 12 cDNA-expressed human CYPs.

2. Human liver microsomes metabolized NBL to three mono-demethylated metabolites (4′-OH-, 7-OH- and 6-OH-NBL) with a relative ratio of 1:4.1:0.5, respectively, by aerobic incubation with nicotinamide adenine dinucleotide phosphate (NADPH). Of 12 human CYPs, CYP1A1, CYP1A2 and CYP1B1 showed high activity for the formation of 4′-OH-NBL. CYP3A4 catalyzed the formation of 7-OH-NBL with the highest activity and of 6-OH-NBL with lower activity. CYP3A5 also catalyzed the formation of both metabolites but considerably more slowly than CYP3A4. In contrast, seven CYPs (CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6 and CYP2E1) were inactive for NBL.

3. Both ketoconazole and troleandomycin (CYP3A inhibitors) almost completely inhibited the formation of 7-OH- and 6-OH-NBL. Similarly, α-naphthoflavone (CYP1A1 inhibitor) and furafylline (CYP1A2 inhibitor) significantly decreased the formation of 4′-OH-NBL.

4. These results suggest that CYP1A2 and CYP3A4 are the key enzymes in human liver mediating the oxidative demethylation of NBL in the B-ring and A-ring, respectively.

     

Characterization of triptolide hydroxylation by cytochrome P450 in human and rat liver microsomes

Triptolide, the primary active component of a traditional Chinese medicine Tripterygium wilfordii Hook F, has a wide range of pharmacological activities. In the present study, the metabolism of triptolide by cytochrome P450s was investigated in human and rat liver microsomes. Triptolide was converted to four metabolites (M-1, M-2, M-3, and M-4) in rat liver microsomes and three (M-2, M-3, and M-4) in human liver microsomes. All the products were identified as mono-hydroxylated triptolides by liquid chromatography-mass spectrometry (LC-MS). The studies with chemical selective inhibitors, complementary DNA-expressed human cytochrome P450s, correlation analysis, and enzyme kinetics were also conducted. The results demonstrate that CYP3A4 and CYP2C19 could be involved in the metabolism of triptolide in human liver, and that CYP3A4 was the primary isoform responsible for its hydroxylation.     

Influence of rifampicin on the expression and function of human intestinal cytochrome P450 enzymes

AIMS: To investigate the potential induction by rifampicin of intestinal CYP2C8, CYP2C9, CYP2D6 and CYP3A4 using preparations of human enterocytes. METHODS: Using a multilumen perfusion catheter shed human enterocytes were collected from 6 healthy subjects before and after 10 days of 600 mg day(-1) oral rifampicin administration. The protein expression of CYP2C8, CYP2C9, CYP2D6 and CYP3A4 as well as that of CYP3A4 mRNA was determined using Western blotting and RT-PCR, respectively. RESULTS: CYP3A4 mRNA expression in shed enterocytes increased from 74.6 +/- 44.2 to 143.2 +/- 68.4 a.u. (P < 0.05, 95% CI: 21.8-115.3). Expression of CYP2C8 and CYP2C9 increased from 5.1 +/- 0.9 to 10.4 +/- 2.3 pmol mg(-1) protein (P < 0.01, 95% CI: 2.8-7.7) and from 4.2 +/- 1.4 to 5.7 +/- 1.1 pmol mg(-1) protein (P < 0.01, 95% CI: 0.6-2.4), respectively. No significant difference in CYP2D6 expression before and during rifampicin intake was observed. Rifampicin administration also resulted in a significant induction of CYP3A4 protein (34.1 +/- 10.7 vs. 113.9 +/- 31.1 pmol mg(-1) protein (P < 0.001, 95% CI: 51.8-107.6)). Ex vivo incubation of enterocyte homogenates with verapamil resulted in a significantly increased production of the metabolites formed via CYP3A4 (D-617: 125.9 +/- 118.8 vs. 277.2 +/- 145.5 pmol min(-1) mg(-1) protein (P < 0.05, 95% CI: 30.1-272.5); norverapamil: 113.0 +/- 57.9 vs. 398.4 +/- 148.2 pmol min(-1) mg(-1) protein (P < 0.05, 95% CI: 47.2-523.6)). CONCLUSION: Our findings indicate that shed enterocytes are a useful tool to study the expression, regulation and function of drug metabolizing enzymes. Induction of intestinal CYP2C8 and CYP2C9 might contribute in part to rifampicin - mediated drug interactions, in addition to their hepatic counterparts and intestinal and hepatic CYP3A4.     

中国人肝微粒体中细胞色素P450酶含量和活性的个体间差异(英文)

目的:比较不同中国人肝微粒体中几种重要细胞色素P450(CYP)的酶含量和活性。方法:运用West-ern斑点分析和光密度扫描,对17个汉族、17个壮族和8个苗族受试者肝微粒体中的细胞色素P4501A2(CYP1A2)、2C9及3A4进行定量;非那西丁、甲磺丁脲、异喹胍和奥美拉唑分别用于体外测量CYP1A2、2C9、2D6及3A4的活性。结果:CYP1A2、2C9及3A4的含量和活性具有很大的个体间变异,另外CYP2D6的活性在各样本间也有很大差异;CYP3A4(32％)是中国人肝微粒体中含量最丰富的CYP,CYP2C9(19％)和CYP1A2(16％)的含量也很可观;除了CYP1A2的含量和活性具有一定的种族和性别差异外,未发现其它CYP具有种族和性别差异;CYP1A2、2C9和3A4的酶蛋白含量分别和它们的活性具有很好的相关性。结论:我们的结果为在中国人中进行药物代谢研究提供了非常有价值的信息。     

Inhibitory effect of medroxyprogesterone acetate on human liver cytochrome P450 enzymes

Objective Medroxyprogesterone acetate (MPA), frequently used in contraception and chemotherapy, was involved in a report of drug-drug interaction (DDI) when co-administrated with phenytoin, doxifluridine and cyclophosphamide. In order to clarify the mechanism of such interaction, an in vitro study was undertaken to evaluate MPA’s potential to inhibit cytochrome P450 (CYP) enzymes.Methods Inhibitory effects of MPA on seven CYPs, including CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2D6, CYP2E1 and CYP3A4, were conducted in human liver microsomes. Time- and NADPH-dependent inhibitions were also tested. DDI potential was predicted according to the [I]/K _i value.Results MPA was found to inhibit CYP2C9 and CYP3A4; half inhibition concentration (IC₅₀) was 16.1 μM and 31.5 μM, respectively. Slight inhibition was observed on CYP1A2, CYP2A6, CYP2C8 and CYP2D6 with IC₅₀ of more than 100 μM. MPA exhibited activation rather than inhibition on CYP2E1. Further study revealed that MPA showed a noncompetitive inhibition on CYP2C9 and a competitive inhibition on CYP3A4 with K _i of 9.0 μM and 36 μM, respectively. In addition, MPA was not a mechanism-based inhibitor to any of seven isoforms tested. By using predicted concentration of MPA in liver, [I]/K _i was estimated to be 0.24 and 0.06 for CYP2C9 and CYP3A4, respectively. The concentration of phenytoin co-administrated with MPA was calculated to increase by 24%.Conclusion Based on our results, MPA can possibly cause clinically relevant DDI via the inhibition of CYP2C9.     

Identification of the human cytochrome P450 enzymes involved in the in vitro metabolism of artemisinin

AIMS: The study aimed to identify the specific human cytochrome P450 (CYP450) enzymes involved in the metabolism of artemisinin. METHODS: Microsomes from human B-lymphoblastoid cell lines transformed with individual CYP450 cDNAs were investigated for their capacity to metabolize artemisinin. The effect on artemisinin metabolism in human liver microsomes by chemical inhibitors selective for individual forms of CYP450 was investigated. The relative contribution of individual CYP450 isoenzymes to artemisinin metabolism in human liver microsomes was evaluated with a tree-based regression model of artemisinin disappearance rate and specific CYP450 activities. RESULTS: The involvement of CYP2B6 in artemisinin metabolism was demonstrated by metabolism of artemisinin by recombinant CYP2B6, inhibition of artemisinin disappearance in human liver microsomes by orphenadrine (76%) and primary inclusion of CYP2B6 in the tree-based regression model. Recombinant CYP3A4 was catalytically competent in metabolizing artemisinin, although the rate was 10% of that for recombinant CYP2B6. The tree-based regression model suggested CYP3A4 to be of importance in individuals with low CYP2B6 expression. Even though ketoconazole inhibited artemisinin metabolism in human liver microsomes by 46%, incubation with ketoconazole together with orphenadrine did not increase the inhibition of artemisinin metabolism compared to orphenadrine alone. Troleandomycin failed to inhibit artemisinin metabolism. The rate of artemisinin metabolism in recombinant CYP2A6 was 15% of that for recombinant CYP2B6. The inhibition of artemisinin metabolism in human liver microsomes by 8-methoxypsoralen (a CYP2A6 inhibitor) was 82% but CYP2A6 activity was not included in the regression tree. CONCLUSIONS: Artemisinin metabolism in human liver microsomes is mediated primarily by CYP2B6 with probable secondary contribution of CYP3A4 in individuals with low CYP2B6 expression. The contribution of CYP2A6 to artemisinin metabolism is likely of minor importance.     

Metabolism of steroids by cytochrome P450 2C9 variants

CYP2C9 is a human microsomal cytochrome P450c (CYP). Much variation in CYP2C9 levels and activity can be attributed to polymorphisms of this gene. Wild‐type CYP2C9 and ten mutants were co‐expressed with NADPH‐cytochrome P450 reductase in Escherichia coli. The hydroxylase activities toward steroids were examined. CYP2C9.2, CYP2C9.3, CYP2C9.4, CYP2C9.16, CYP2C9.28, CYP2C9.48 and CYP2C9.52 had higher testosterone 6β‐hydroxylation than CYP2C9.1. CYP2C9.4 showed higher progesterone 6β‐hydroxylation activity than CYP2C9.1. CYP2C9.28 and CYP2C9.48 showed higher progesterone 11α‐hydroxylation activity than CYP2C9.1. CYP2C9.48 showed higher progesterone 16α‐hydroxylation activity than CYP2C9.1. CYP2C9.2, CYP2C9.3, CYP2C9.16 and CYP2C9.30 had higher estrone 16α‐hydroxylation activity than CYP2C9.1. CYP2C9.3 had higher estrone 11α‐hydroxylation activity than CYP2C9.1. CYP2C9.39 and CYP2C9.57 showed similar activities to CYP2C9.1. These results indicate that the substrate specificity of CYP2C9.39 and CYP2C9.57 was not changed, but CYP2C9.2, CYP2C9.3, CYP2C9.4, CYP2C9.16, CYP2C9.28, CYP2C9.30, CYP2C9.48 and CYP2C9.52 showed different hydroxylation activities toward steroids compared with CYP2C9.1.     

心肌缺血再灌注损伤对大鼠肝细胞色素P450酶代谢的影响

目的探讨心肌缺血再灌注状态下,大鼠肝代谢功能和相关的氧化/抗氧化能力变化。方法雄性SD大鼠随机分为5组,除假手术组外,制备在体心肌缺血再灌注模型,并于缺血40min、再灌注15,60和180min分别处死大鼠,检测血浆丙氨酸转氨酶(ALT)和天冬氨酸转氨酶(AST)活性,肝匀浆丙二醛(MDA)含量、超氧化物歧化酶(SOD)活性;以红霉素N-脱甲基酶、五氧基异噁唑O-脱乙基酶和苯胺羟化酶法为探针测定肝细胞色素P450(CYP)3A,CYP2B1和CYP2E1催化功能;RT-PCR法检测肝Ⅰ相药物代谢酶CYP3A1,CYP2B1/2,CYP2E1,以及Ⅱ相解毒酶NAD(P)H醌氧化还原酶(NQO1)及其上游因子NF-E2相关因子(Nrf2)mRNA水平。结果再灌注60 min,肝匀浆MDA含量升高(P<0.05),SOD活力下降(P<0.01);再灌注180 min时,血浆ALT和AST活性升高(P<0.05)。Nrf2基因于再灌注60 min时显著激活(P<0.05),下游因子NQO1 mRNA于再灌注180 min时明显上调(P<0.05)。CYP3A催化功能和mRNA水平分别于再灌注60和180 min开始明显降低(P<0.05);CYP2B1/2 mRNA和催化功能水平分别于再灌注15和180 min开始明显降低(P<0.05);CYP2E1催化功能无明显改变。结论大鼠心肌缺血再灌注可引起肝组织氧化应激及并导致功能损伤。在再灌注早期,具有抗氧化功能的NQO1在转录水平显著上调,其机制可能与上游因子Nrf2被激活相关;CYP3A和CYP2B催化功能在转录和(或)转录后水平明显下调。     

Inhibitory effects of polyphenols on human cytochrome P450 3A4 and 2C9 activity

Polyphenols present in foods and supplements may contribute to human health by preventing cardiovascular diseases and cancer. Drug–food or drug–herb interactions have recently come into focus but, except for some phytochemicals, few components of food or herbs participate in such interactions. In this study, we systematically evaluated the inhibitory effects of 60 polyphenols and related compounds on human cytochrome P450 (CYP) 3A4 and CYP2C9 activity by in vitro assay to investigate whether some polyphenols induce drug interactions. In addition, the kinetics of potent CYP inhibitors was investigated by Lineweaver–Burk plot analysis. Three coumarins and 12 flavonoids significantly suppressed CYP3A4 or CYP2C9 activities. Lineweaver–Burk plot analysis indicated that apigenin and its dimer amentoflavone and imperatorin displayed a mixed type of inhibition on CYP3A4 or CYP2C9. Among the inhibitors, amentoflavone was the most potent inhibitor of CYP3A4 and CYP2C9 activities with IC₅₀ values of 0.07 and 0.03 μM, respectively. The K_i value of amentoflavone was significantly lower than that of the CYP2C9 inhibition positive control sulfaphenazole. These findings suggest that some dietary polyphenols may have the potential to inhibit the metabolism of clinical drugs.