肝细胞微粒体的制备和细胞色素P450氧化酶活性测定

目的：为测定人肝细胞微粒体细胞色素Ｐ４５０氧化酶的活性。方法：用差速离心法制备３例人肝细胞微粒体。结果：细胞色素Ｐ４５０的含量为０．５２３±０．００５ｎｍｏｌ·ｍｇ－１；细胞色素ｂ５为０．２８５±０．０２５ｎｍｏｌ·ｍｇ－１；氨基比林Ｎ－脱甲基酶的活力为０．５±０．６ｎｍｏｌ·ｍｇ－１；乙基吗啡Ｎ－脱甲基酶活力为０．９８±０．０８ｎｍｏｌ·ｍｇ－１。结论：Ｐ４５０酶活性影响因素较多，个体差异大。临床用药时应考虑患者的个体情况。     

Metabolic capabilities of cytochrome P450 enzymes in Chinese liver microsomes compared with those in Caucasian liver microsomes

AIM

The most common causes of variability in drug response include differences in drug metabolism, especially when the hepatic cytochrome P450 (CYP) enzymes are involved. The current study was conducted to assess the differences in CYP activities in human liver microsomes (HLM) of Chinese or Caucasian origin.

METHODS

The metabolic capabilities of CYP enzymes in 30 Chinese liver microsomal samples were compared with those of 30 Caucasian samples utilizing enzyme kinetics. Phenacetin O-deethylation, coumarin 7-hydroxylation, bupropion hydroxylation, amodiaquine N-desethylation, diclofenac 4′-hydroxylation (S)-mephenytoin 4′-hydroxylation, dextromethorphan O-demethylation, chlorzoxazone 6-hydroxylation and midazolam 1′-hydroxylation/testosterone 6β-hydroxylation were used as probes for activities of CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A, respectively. Mann-Whitney U test was used to assess the differences.

RESULTS

The samples of the two ethnic groups were not significantly different in cytochrome-b₅ concentrations but were significantly different in total CYP concentrations and NADPH-P450 reductase activity (P < 0.05). Significant ethnic differences in intrinsic clearance were observed for CYP1A2, CYP2C9, CYP2C19 and CYP2E1; the median values of the Chinese group were 54, 58, 26, and 35% of the corresponding values of the Caucasian group, respectively. These differences were associated with differences in Michaelis constant or maximum velocity. Despite negligible difference in intrinsic clearance, the Michaelis constant of CYP2B6 appeared to have a significant ethnic difference. No ethnic difference was observed for CYP2A6, CYP2C8, CYP2D6 and CYP3A.

CONCLUSIONS

These data extend our knowledge on the ethnic differences in CYP enzymes and will have implications for drug discovery and drug therapy for patients from different ethnic origins.     

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.

     

甲基原薯蓣皂苷对人肝微粒体中7种CYP450酶活性的影响

目的研究甲基原薯蓣皂苷对CYP450酶的7种亚型酶活性的影响。方法将MPD和CYP450酶7种亚型的特异性探针底物咖啡因(CYP1A2)、右美沙芬(CYP2D6)、甲苯磺丁脲(CYP2C9)、s-美芬妥因(CYP2C19)、氯唑沙宗(CYP2E1)、香豆素(CYP2A6)及咪达唑仑(CYP3A4)与人肝微粒体进行孵化反应,采用HPLC和LC-MS/MS法测定对应的7种代谢产物(1,7-二甲基黄嘌呤、去甲右美沙芬、4-羟基甲苯磺丁脲、4-羟基美芬妥因、6-羟基氯唑沙宗、7-羟基香豆素和1-羟基咪达唑仑)的浓度,通过与对照组比较,确定MPD对以上7种酶活性的影响。结果MPD在1~10μmol.L-1时对7种酶均无明显抑制作用,在100μmol.L-1时对CYP2D6有抑制趋势,但对其他6种酶无抑制作用,均无统计学意义(P>0.05)。结论MPD在与以上6种酶(CYP1A2、CYP2E1、CYP2C19、CYP3A4、CYP2C9和CYP2A6)代谢的药物联合用药时,发生药物相互作用的可能性较小。     

豆腐果苷对人肝微粒体CYP450酶体外抑制作用研究

目的:通过评价豆腐果苷在体外对人肝微粒体CYP450酶的7种亚型酶活性的影响,预测服用豆腐果苷可能出现的食物-药物及药物-药物代谢的影响。方法:将豆腐果苷与CYP450酶7种亚型的特异性探针底物咖啡因(CYP1A2)、右美沙芬(CYP2D6)、甲苯磺丁脲(CYP2C9)、S-美芬妥因(CYP2C19)、氯唑沙宗(CYP2E1)、香豆素(CYP2A6)及咪达唑仑(CYP3A4)与人肝微粒体进行孵育反应,采用HPLC和LC-MS/MS法测定对应的7种代谢产物(1,7-二甲基黄嘌呤、去甲右美沙芬、4-羟基甲苯磺丁脲、4-羟基美芬妥因、6-羟基氯唑沙宗、7-羟基香豆素和1-羟基咪达唑仑)的浓度,与对照组比较,确定豆腐果苷对以上7种亚酶活性的影响。结果:豆腐果苷在1～100μmol.L-1时对7种酶的抑制作用均无明显统计学意义(P>0.05)。结论:豆腐果苷可能不会引起有临床意义的CYP450酶抑制现象的发生。     

Rapid determination of enzyme activities of recombinant human cytochromes P450, human liver microsomes and hepatocytes

Cytochrome P450 (CYP) substrates that yield fluorescent metabolites were used for rapid screening of drug metabolism activities of 13 recombinant human cytochromes P450, human liver microsomes and human hepatocytes. Reproducible results were obtained using a fluorescent plate reader (CytoFluor) more expediently than those generated using conventional HPLC methods. Typically, results for 96 samples were obtained with the plate reader in less than 10 min as opposed to 15-35 min/sample required by conventional HPLC. The fluorescent substrates used to measure CYP activities were as follows: 3-cyano-7-ethoxycoumarin (CEC) for CYP1A1, CYP1A2, CYP2C9 and CYP2C19; 7-ethoxyresorufin (7-ER) for CYP1A1, CYP1A2 and CYP1B1; 3-[2-(N,N-diethyl-N-methylammonium)ethyl]-7-methoxy-4-methylcoumarin (AMMC) for CYP2D6; dibenzylfluorescein (DBF) for CYP3A4, CYP3A5 and CYP2C8; 7-methoxy-4-trifluoromethylcoumarin (7-MFC) for CYP2E1, CYP2B6 and CYP2C18; and coumarin for CYP2A6. The chemical inhibition and correlation data indicated that the following substrates can be used as specific functional probes for individual cytochrome P450 present in human liver microsomes: coumarin for CYP2A6 (r=0.82), AMMC for CYP2D6 (r=0.83) and DBF for CYP3A4 (r=0.92). The fluorescent plate reader was found to be useful for the rapid assessment of CYP activities (positive control) in both intact cells and subcellular fractions.     

Inhibition of cytochrome P450 enzymes by thymoquinone in human liver microsomes

The aim of the present study was to investigate the potential effect of thymoquinone (TQ) on the metabolic activity of four major drug metabolizing enzymes in human liver microsomes, namely cytochrome P450 (CYP) 1A2, CYP2C9, CYP2D6 and CYP3A4. The inhibition of CYP enzymatic activities by TQ was evaluated by incubating typical substrates (phenacetin for CYP1A2, tolbutamide for CYP2C9, dextromethorphan for CYP2D6, and testosterone for CYP3A4) with human liver microsomes and NADPH in the absence or presence of TQ (1, 10 and 100?µM). The respective metabolite of the substrate that was formed was measured by HPLC. Results of the presented study presented that the metabolic activities of all the investigated CYP enzymes, viz. CYP1A2, CYP2C9, CYP2D6 and CYP3A4, were inhibited by TQ. At 1?µM TQ, CYP2C9 enzyme activity was maximally inhibited by 46.35%, followed by CYP2D6 (20.26%)?>?CYP1A2 (13.52%)?>?CYP3A4 (12.82%). However, at 10?µM TQ, CYP2C9 enzyme activity was maximally inhibited by 69.69%, followed by CYP3A4 (23.59%)?>?CYP1A2 (23.51%)?>?CYP2D6 (11.42%). At 100?µM TQ, CYP1A2 enzyme activity was maximally inhibited by 81.92%, followed by CYP3A4 (79.24%)?>?CYP2C9 (69.22%)?>?CYP2D6 (28.18%). The IC₅₀ (mean?±?SE) values for CYP1A2, CYP2C9, CYP2D6 and CYP3A4 inhibition were 26.5?±?2.9?µM, 0.5?±?0.4?µM, >500?µM and 25.2?±?3.1?µM, respectively. These findings suggest that there is a high probability of drug interactions resulting from the co-administration of TQ or herbs containing TQ with drugs that are metabolized by the CYP enzymes, particularly CYP2C9.     

In vitro metabolism of an estrogen‐related receptor γ modulator,GSK5182, by human liver microsomes and recombinant cytochrome P450s

GSK5182 (4‐[(Z)‐1‐[4‐(2‐dimethylaminoethyloxy)phenyl]‐hydroxy‐2‐phenylpent‐1‐enyl]phenol) is a specific inverse agonist for estrogen‐related receptor γ, a member of the orphan nuclear receptor family that has important functions in development and homeostasis. This study was performed to elucidate the metabolites of GSK5182 and to characterize the enzymes involved in its metabolism. Incubation of human liver microsomes with GSK5182 in the presence of NADPH resulted in the formation of three metabolites, M1, M2 and M3. M1 and M3 were identified as N‐desmethyl‐GSK5182 and GSK5182 N‐oxide, respectively, on the basis of liquid chromatography‐tandem mass spectrometric (LC‐MS/MS) analysis. M2 was suggested to be hydroxy‐GSK5182 through interpretation of its MS/MS fragmentation pattern. In addition, the specific cytochrome P450 (P450) and flavin‐containing monooxygenase (FMO) isoforms responsible for GSK5182 oxidation to the three metabolites were identified using a combination of correlation analysis, chemical inhibition in human liver microsomes and metabolism by expressed recombinant P450 and FMO isoforms. GSK5182 N‐demethylation and hydroxylation is mainly mediated by CYP3A4, whereas FMO1 and FMO3 contribute to the formation of GSK5182 N‐oxide from GSK5182. The present data will be useful for understanding the pharmacokinetics and drug interactions of GSK5182 in vivo. Copyright © 2014 John Wiley & Sons, Ltd.     

Metabolism and effect of para-toluene-sulfonamide on rat liver microsomal cytochrome P450 from in vivo and in vitro studies

AIM: To study the in vivo and in vitro metabolism and the effect of para-toluene-sulfonamide (PTS) on cytochrome P450 enzymes (CYP450). METHODS: Total CYP450 and microsome protein content were determined after iv pretreatment of rats with PTS. CYP-specific substrates were incubated with rat liver microsomes. Specific CYP isoform activities were determined by using HPLC. CYP chemical inhibitors added to the incubation mixture were used to investigate the principal CYP isoforms involved in PTS metabolism. The effect of PTS on CYP isoforms was investigated by incubating PTS with specific substrates. RESULTS: The groups treated with 33 and 99 mg/kg per d PTS, respectively, had a total CYP content of 0.66+/-0.17 and 0.60+/-0.12 nmol/mg. The K(m) and V(max) were 92.2 micromol/L and 0.0137 nmol/min per mg protein. CYP2C7, CYP2D1 and CYP3A2 might contribute to PTS metabolism in the rat liver. The inhibitory effects of sulfaphenazole and ketoconazole on PTS metabolism were shown to have a mixed mechanism, whereas PTS metabolism was inhibited noncompetitively by quinidine. PTS had little effect on the activities of the selected CYP isoforms. CONCLUSION: Generally speaking, it is relatively safe for PTS to be co-administered with other drugs. However, care should be taken when administering PTS with CYP inhibitors and the substrates of CYP2C, CYP2D and CYP3A.     

Identification of the human cytochrome P450 isoforms mediating in vitro N-dealkylation of perphenazine

AIMS: To identify the human cytochrome P450 (CYP) isoforms mediating the N-dealkylation of the antipsychotic drug perphenazine in vitro and estimate the relative contributions of the CYP isoforms involved. METHODS: cDNA-expressed CYP isoforms were used to identify the isoforms that are able to mediate the N-dealkylation of perphenazine, which is considered a major metabolic pathway for the drug. Using human liver microsomal preparations (HLM), inhibition studies were carried out to establish the relative contributions of the CYP isoforms involved in the N-dealkylation reaction. RESULTS: CYP isoforms 1A2, 3A4, 2C8, 2C9, 2C18, 2C19 and 2D6 were able to mediate the N-dealkylation of perphenazine. Reaction velocities and their relative abundance in HLM suggested that CYP1A2, 3A4, 2C19 and 2D6 were the most important contributors to N-dealkylation. Apparent Km values of CYP1A2 and CYP2D6 were in the range 1-2 microM, and Km values of CYP2C19 and CYP3A4 were 14 microM and 7.9 microM, respectively. Ketoconazole inhibition of N-dealkylation mediated by a mixed HLM indicated that CYP3A4 accounted for about 40% of perphenazine N-dealkylation at therapeutically relevant concentrations.The contribution of the CYP isoforms 1A2, 2C19 and 2D6 amounted to 20-25% each as measured by the percentage inhibition obtained by addition of furafylline, fluvoxamine or quinidine, respectively. HLM-mediated N-dealkylation of perphenazine accounted for 57% of the total amount of substrate consumed during incubation. CONCLUSIONS: The present in vitro study suggests that CYP isoforms 1A2, 3A4, 2C19 and 2CD6 are primarily involved in the N-dealkylation of perphenazine. The relatively modest role of CYP2D6 is at variance with in vivo studies, which indicate a greater contribution of this isoform. Alternative metabolic pathways, corresponding to 43% of the HLM-mediated metabolism of the drug, may depend more strongly on CYP2D6.     

Genotoxicity evaluation for single‐walled carbon nanotubes in a battery of in vitro and in vivo assays

The genotoxicity of single‐walled carbon nanotubes (SWCNTs) was determined using a battery of genotoxicity assays, comprising a bacterial reverse mutation test, an in vitro mammalian chromosomal aberration test and a mammalian erythrocytes micronucleus test. SWCNTs had no mutagenicity in S. typhimurium TA98, TA100, TA1535 or TA1537, or in E. coli WP2uvrA, in the absence or presence of metabolic activation. SWCNTs did not increase the number of structural or numerical chromosomal aberrations after short‐term or continuous exposure. In the micronucleus test using CD‐1 mice, SWCNTs did not affect the proportion of immature erythrocytes, the total proportion of erythrocytes or the number of micronuclei in immature erythrocytes. SWCNTs appear not to pose a genotoxic risk. Copyright © 2012 John Wiley & Sons, Ltd.     

甘草次酸在人细胞色素CYP450中体外代谢研究(英)

甘草根是中医临床常用解毒草药, 其活性成分甘草次酸主要是通过肝脏代谢。本文研究了人肝微粒体以及人源性CYP450s对甘草次酸的体外代谢影响, 以及甘草次酸对几种CYP450酶活性的影响。实验结果表明, 甘草次酸体外主要代谢酶为CYP3A4。体外药代动力学参数K_m, V_max和CL_int分别为18.6 μmol·L⁻¹, 4.4 nmol·mg⁻¹(protein)·min⁻¹和0.237 mL·mg⁻¹(protein)·min⁻¹。体外抑制试验显示, 50 μmol·L⁻¹甘草次酸可以抑制CYP2C19、CYP2C9、CYP3A4酶的活性, 其抑制率可高达50%以上。     

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.     

酮康唑对大鼠肝脏CYP450酶系的影响*

目的:观察酮康唑对大鼠肝细胞色素P450及其主要亚型的影响。方法:Sprague-Dawley大鼠用140,280,420μmol.kg-1.d-1酮康唑连续灌胃7 d,测定肝脏微粒体中总CYP450含量和CYP1A1,1A2,1B1,2B1/2,2E1和3A亚型活性。结果:不同剂量酮康唑给药后大鼠肝脏脏器系数、CYP1A1和1B1亚型活性明显增高(P<0.05,P<0.01);总CYP450含量和CYP3A活性显著降低(P<0.01);低剂量的酮康唑抑制CYP1A2和CYP2B1/2亚型的活性,高剂量却出现了诱导作用(P<0.05,P<0.01)。各剂量组对CYP2E1均无明显影响。结论:酮康唑对大鼠肝脏CYP450及主要药物代谢亚型CYP1A1,1A2,1B1,2B1/2和3A有影响,临床长期用药或与经肝脏CYP450代谢的药物联合应用时要注意监测血药浓度和肝脏功能,防止药物代谢减缓出现蓄积中毒或药物代谢加快而降低药效。     

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.     

丹参注射液对人肝微粒体酶CYP2C9,CYP2C19,CYP2D6体外抑制作用的研究

目的 评价丹参注射液在体外对人肝微粒体酶CYP2C9、CYP2C19、CYP2D6活性的影响。方法 将丹参注射液与CYP450酶3种亚型(CYP2C9、CYP2C19、CYP2D6)的特异性探针底物甲苯磺丁脲、奥美拉唑、右美沙芬与大鼠肝微粒体进行孵育,采用LC-MS/MS法测定对应3种代谢产物4-羟基甲苯磺丁脲、5-羟基奥美拉唑、右啡烷的浓度,求算出IC₅₀。结果 丹参注射液对CYP2C9、CYP2C19和CYP2D6的IC₅₀值均>50 μg/mL。结论 丹参注射液对人肝微粒体酶CYP2C9、CYP2C19、CYP2D6没有抑制作用。     

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.     

A convenient method to discriminate between cytochrome P450 enzymes and flavin-containing monooxygenases in human liver microsomes

 Liver microsomes are a frequently used probe to investigate the phase I metabolism of xenobiotics in vitro. Structures containing nucleophilic heteroatoms are possible substrates for cytochrome P450 enzymes (P450) and flavin-containing monooxygenases (FMO). Both enzymes are located in the endoplasmatic reticulum of hepatocytes and both need oxygen and NADPH as cofactors. The common method to distinguish between the two enzyme systems is to use the thermal inactivation of FMO and to inhibit P450 completely with carbon monoxide, N-octylamine or N-benzylimidazole. In the literature no indication could be found that the heat inactivation of FMO does not affect any of the human P450 enzymes or that the overall P450 inhibitors inhibit the different human P450 enzymes sufficiently and do not affect the FMO. The effect of N-benzylimidazole and heat inactivation was tested on specific activities of seven P450 enzymes in human liver microsomes, 1A2, 2A6, 2C9, 2C19, 2D6, 3A4/5, and 2E1, using methoxyresorufin O-demethylation, coumarin 7-hydroxylation, (S)-warfarin 4-hydroxylation, (S)-(+)-mephenytoin 4-hydroxylation, dextrometorphan O-demethylation, oxidation of denitronifedipine, and chlorzoxazone 6-hydroxylation respectively. The sulfoxidation of methimazole (MMI) was used as a specific probe for the determination of FMO activity. Methimazole sulfoxidation was compared with the well known assay for FMO metabolism, the formation of N,N-dimethylaniline (DMA) N-oxide, to be confirmed as an exclusively FMO mediated reaction. The participation of P450 and FMO in the sulfoxidation of four sulfur containing pesticides, ametryne; terbutryne, prometryne and methiocarb was investigated using human liver microsomes. All four reactions were demonstrated to be catalysed predominantly by cytochrome P450. Received: 13 March 1996/Accepted: 20 June 1996     

The novel antifungal agent PLD-118 is neither metabolized by liver microsomes nor inhibits cytochrome P450 in vitro

PLD-118 is a novel, oral antifungal drug, under development for the treatment of Candida infections. Possible metabolism of PLD-118 by rat, dog and human S9 liver homogenates and inhibition of human cytochrome P450 (CYP) enzymes were investigated. PLD-118 (10 and 100 microM) incubated for 0-60 min with S9 fractions and NADPH was determined by HPLC, using the Waters AccQ.Tag method after derivatization of amino acids to stable, fluorescent derivatives. CYP assays were performed using pooled human liver microsomes with substrates, selective towards human CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A, incubated at concentrations around the Km. Incubation mixtures were preincubated with PLD-118 (0.1-100 microM) or control inhibitor for 5 min. No metabolism of PLD-118 was detected with rat and dog S9 fractions. A small (8%) decrease in PLD-118 at 100 microM (not detected at 10 microM) with human microsomes was considered to be biologically irrelevant. PLD-118 did not inhibit any of the tested CYPs. PLD-118, at concentrations up to 100 microM, is not metabolized by rat, dog or human liver S9 homogenates and does not inhibit human CYPs in vitro, suggesting little likelihood for interaction of PLD-118 with drugs metabolized by these enzymes.